Auxillary Fan relay W124 Japanese '89 update
 

Can anybody here identify which of these relays provides the feed for the dual auxillary fans on My 1989 W124 260e? Car is RHD Japanese version.
here is a foto of the relay box located behind the fuse panel from underneath the bonnet. My auxillary Fan is No Go neither with over heat or if the AC is turned on. Tanks in advance Non of the relays has the obligatory fuse on the top. (Cruise control doesn't work either)
http://farm4.static.flickr.com/3082/...b3dbf05e8d.jpg

My auxiliary fan was not working and it was due to the Resistor that is forward on the drivers side of the engine compartement on my 1988 300E. Sadly they replaced the fuse and just coincidentally the Fan froze up.

I would check your fan to make sure that it is not froze up.

The relays have position markers under them. The high fan relay is in position "B"
Low fan is "C"
There will be the square relays on the left in pic.

The fuses are #7 for relay coils and fuse D for Power.

A quick test for fan circuits:
Key ON
Pull the connector off the blue coolant sensor at therm. housing..that should get HIGH fan

Jumper pigtail wire harness at a/c drier at end of pigtail. that will give you LOW fan.
Report those findings.

Nothing happens on this test.
Fans are O.K. - connected battery direct to fan wiring and fans operate. I am checking the power distribution with a multimeter.

OK

I am assuming you tested the fuses I mentioned ???

Behind the headlamp [ don't know which b/c you have RHD], there is aux fan Resistor..the wires burn off,,check that for the next test.

http://catalog.peachparts.com/RenderScriptTemplate.epc?_cmd=epccat_VehicleAAA&cookieID=2GN17VPXQ2GO19XIJQ&yearid=1989%40%401989&ma keid=MB%40%40MB%40%40X&modelid=260%2DE%2D001%40%40260E&catid=G%40%40Cooling+System&mode=PA&subcatid= G5040@@Aux%2E+Fan+Resistor&source=www.peachparts.com&clientid=catalog.mercedesshop

Resistor is fine and connections to it a fine.
Checked potentiual to the relay holders - fine in both relays.
Relays work fine. Tested outside car wioth seperate power supply and checked contacts with multimeter.

I assume that the temperature sensor sends a signal to the ECU which in turn operates the relay which supplies 12 volts to the fan on the high speed side. I also assume that the air dryer contact also supplies a signal to the ECU which operates the relay for the low speed
Assumption being the mother of all .....

Wrong on both assumptions.

The coolant sensor is a termistor and it is measured by The Control Panel. That triggers the relay.
The a/c high side pressure is a simple On/Off relay circiuit.

So, there is the relay sensor side and the relay load side.

The first test is to unplug the blu coolant temp termistor w/key ON. That defaults to high fan.

The second test is to jumper the pigtail at a/c high side pressure sw and that triggers relay directly for low fan. { Make sure you are jumping the pigtailed sw, not the other spade connector one]

So, if you do the two sensor test and no fan, the next test is to bring/jumper battery positive 12v to the Resistor . On one side you should get low fan , the other high.
If yes , then you have a feed problem. [ usually the fuses I had you check.]

One fuse is for load [ relay contacts] and one is relay coil side. [ sensor activated]
So, the next test is to do the sensor test again and listen closely for relay activation [ clicking] That test coil side of relay. If yes , then sensors circuit is working and the feed is bad on the load/fan side. That can be then tested by taking the relays out and jumping the sockets at 30 and 87. That will give direct fans w/o sensor circuits.
You can also test at 30 of the sockets for fuse 12v feed.......

Hi Arthur, Your doing a great job here.
Firstly My car is a two fan jobbbie.

(a) Shorted out 30 and 87 on both relays - Fans operate on both - High and low speed.

(b) Removing temp sensor wires from engine does not make the fans operate. However by shorting the two leads they fans do operate.

(c) Sorting the Pig tails on top of the air dyer will not cause the fans to operate. The relay doesn't appear to "click" and even if it did I have checked the relay and it operates O.K. and the contacts are O.K - (external test)

Conclusion
(1) The engine temperature sensor maybe cactus. (another Assumption - the fans should come into play when the engine temp gets to around 105 degrees c)
(2) Some wiring problem in between the pigtails and the relay - if there is a direct connection.

Fotos below Just to make sure I'm looking at the right pigtails.

http://farm4.static.flickr.com/3061/...b8257b3327.jpghttp://farm4.static.flickr.com/3106/...50f325cde2.jpg

After more tests I have found that the two tails which are connected to the red unit on th side of the dryer when shorted will operate the fans at low speed. Are these tails on the wrong item?

http://farm4.static.flickr.com/3046/...71eed31b_o.jpg

<<Firstly My car is a two fan jobbbie.>>

As your First post stated..............and I also know that b/c I have your schematic up on a wall here. That is how I diagnose over the net and why I have sequenced test procedures for you to perform while I am tracing your systems circuit out.

OK

I can see your problem is your understanding of "PIGTAIL"
Re-read my post warning notation about the difference between the 2 sensors on the drier.
A pigtailed sensor is a sensor that has a length of wire with the connectors on the end of that wire [ pigtail wire]..

So , your first picture is the sensor for the compressor and only has spade connectors. That is the one I posted not to use.
The aux fan sensor I wanted jumped has a pigtail lead with the connectors on the end [second picture ]

Here is the correct sensor:

http://catalog.peachparts.com/RenderScriptTemplate.epc?_cmd=epccat_VehicleAAA&cookieID=2GN17VPXQ2GQ0OFOTO&yearid=1989%40%401989&ma keid=MB%40%40MB%40%40X&modelid=260%2DE%2D001%40%40260E&catid=R%40%40Climate+Control&mode=PA&subcatid =R1055@@A%2FC+Temp%2E+Switch&source=www.peachparts.com&clientid=catalog.mercedesshop

If one has a low fan jumping THAT sensor at the end of the pigtail ,[ which you now have concluded] then your problem is most likely the refrigerant level is slightly LOW, thereby not allowing the high side pressure to reach the cut-in spec of that sensor. Low fan ONLY runs when there is a high thermal load on the a/c condenser. The only other possible is a bad sw , but that is highly unlikely b/c low Refrig is a 99% diagnosis. If one sees bubbles'foam in the drier eyesight, you are low. [ or gauges on High side will tell]

OK
Back to the High Fan
Is this the sensor you have??

http://catalog.peachparts.com/RenderScriptTemplate.epc?_cmd=epccat_VehicleAAA&cookieID=2GN17VPXQ2GQ0OFOTO&yearid=1989%40%401989&ma keid=MB%40%40MB%40%40X&modelid=260%2DE%2D001%40%40260E&catid=R%40%40Climate+Control&mode=PA&subcatid =R1075@@A%2FC+ACC+Temp+Sensor&source=www.peachparts.com&clientid=catalog.mercedesshop

Arthur,

Those two links to the relevant parts are correct.

(a) After checking, the AC has minimal Gas in it.

(b) I will replace the engine temp sensor. Even with the engine over 115 degrees C the resistance to ground remains over 750Kohms on both legs to ground.

Three problems I have:
(1) Engine overheats on idle - Thermostat replaced, New radiator, most likely viscous fan + loose fan belt.
(2) No low speed auxiliary fan
(3) No high speed auxiliary fan

Thanks to Arthur, Items (2) and (3) now look like being cleared up.:)

On "b", there should be NO resistance to ground...that is a thermistor with a return circuit, so it does not use a casing ground on the sensor.. I am assuming you are ohming across the sensor with it unplugged.
If you have a casing ground on the sensor, the sensor is bad ...if NO , then I can give you the Temp/Ohms chart for sensor calobration verification...

Another tip I will give you is many guys bridge the pigtail sensor right where I had you jumper test it with a two wire cord that they bring back into the cabin with a toggle switch...that gives the operator a manual over-ride for additional air-flow from the low fan any time one deems it needed..[ like towing or extreme traffic conditions, etc]..this over-ride does nor affect the a/c low fan circuit, but gives you that added air control manually..something to consider.

One of the most common mis-diagnosed complaints I see with engine overheat when car is at idle or in traffic w/ac on is the low fan does not come ON simply b/c the refrig is low...but it is border-line low, meaning you have a/c cooling , but b/c you do not have quite enough to trigger the low fan when the car is not moving, the coolant temp rises by passive heat load exchange from the a/c condenser to the RADIATOR ...so guys see an over- heat condition and start all kinds of thermostat and radiator changes ,water pumps, etc,...when the proper diagnosis is the refrig level is slightly low, causing no low fan b/c of cut-in spec of sensor...simple as that..no airflow over condenser/radiator when it is needed the most...even some Techs miss that one.............

I will make connect a circuit as you have suggested - i.e. parallel the sensor with a manual overide switch in the cabin.

There is no short to earth on the sensor so maybe I need your resisitance chart to check out the sensors operating range.

I am in the southern hemisphere. It's now spring coming out of winter. The car will overheat at idle without the AC being switched n. Even at ambient outside temperatures of 12 - 15 degreeC.

<The car will overheat at idle without the AC being switched>

That would be a coolant system problem.

If you changed the thermo, I would suggest you burp the system of any posssible air lock.. also see if the air-flow aid of your cabin sw drops the temp when the condition is present.............

Hi Arthur,

By switching the heating up to high and turning the interior fans full blast, this reduces the engine temperature - drastically - down towards normal operating temp (87c).

I've tried two other thermostats - one brand new (off market Wahler) The other OEM so new it still shines. On freeway the temperature is too low - way under 80 degrees C. On idling will gradually creep up to 115+. I assumed (assumption being the mother of all screwups) that a new thermostat would solve this problem.

Maybe I need to get an original Behr thermostat.

The radiator is a new Behr unit. System has been bled. P.O. put green elcrappo fluid in. Had to change it too the new MB blue stuff. Another assumption suspected that P.O. had caused a problem with his cheap coolant.

I would check the coolant ratio..you want 60/40, h20/AF...too much a/f will not take the heat away.

I like Behr thermos and no other. Also check for debries between the rad/cond...
Does the viscous coast when you turn engine off ????

The viscous fan is tight and seems to spin O.K in concert with the engine. Doesn't carry on spinning after engine turned off. The revs are a bit low in drive - 550 rpm. However even when in Park (700rpm) the system will overheat - quite promptly.

I will order a Behr thermostat and install it before doing anything else to the cooling system.

Bye the bye. The P.O. installed a Satnav system. He has done something to the hall sensor on the speedo to get a required signal to the satnav GPS system. I can do without SatNav - I have a street directory in my head.
Another assumption - this will be the reason that the cruise control is not working. I have seen previous posts where the hall sensor also affects the KE injection system - I don't know if this is a fact.

Try a thermo..

If you can get the temp down w/heater, that is thermo indication.
If you wire that low fan up w/cabin sw and that holds the temp at idle, then the viscous is suspect.

Hi Arthur,
I have 2 " new" thermostats. 1 Erzatz, - Made in ????. The other supposed to be OEM, labeled made in Germany, cost US$18.00.

Both don't work properly.:mad:

Went to Stealership bought a third thermostat with the 3 pointed star, the part number and made in Germany stamped on it. Cost US $55.00.:eek:
It works perfectly.;)
On freeway with ambient outside temperature of 12 degrees C engine temperature sits at 87 degrees C . 15minutes at idle - temperature never gets close to the 100degree C mark.:D

Curious, I bought all three into the house popped them into a saucepan and heated them in water to boiling point. All reacted the same in this static test.:rolleyes:
In the car it's a different matter.

The only way to truely test a thermo is in the car and that is always the first step on an over-heat conditrion on a Benz..and I prefer the Berh b/c of many years experience with them..
Most think a thermo is a thermo is a thermo....I commend you in not brushing off the changeout on the assumption that it could not be the proper diagnosis just b/c you had changed yours.

I would still do the toggle mod..just for that extra manual convienience..
..and you can add a bridging resistor on the blu sensor to have the high fan cut=in at a lower temp for a Buck. That is a good idea if you are in a tropical type climate. I prefer the cabin switch cuz I am then in control and I am one of those guys who actually watch the gauges ...Imagine that ...:)

Update.

Had a warm day today. Temperature hit 22 degrees C. Engine overheating again.:mad:

Checked viscous fan when engine very hot - fan blades spin freely. Checked viscous fan when cold - fan blades are hard to turn. Is it possible that the fan unit can be installed backwards?:confused:

I obtained another auxiliary fan temperature sensor I specified one to operate at 100 degrees C. Unfortunately it does not work. as required. The high speed fan runs continuously wether the engine is hot or cold.


As Arthur has pointed out, removing the wiring from the sensor should cause the fan to operate at high speed - correct. This would infer that the sensor initially starts at a lower resistance when cold and gets to a higher resistance when very hot. There must be a balance circuit that operates the high speed fan relay.

The red unit has a resistance to ground on both terminals. As it gets hotter the resistance decreases.

here is a foto of the two units. The blue one is the original.

http://farm4.static.flickr.com/3215/...b7d992507d.jpghttp://farm4.static.flickr.com/3292/...72dc3c688b.jpg

>>As Arthur has pointed out, removing the wiring from the sensor should cause the fan to operate at high speed - correct. This would infer that the sensor initially starts at a lower resistance when cold and gets to a higher resistance when very hot. There must be a balance circuit that operates the high speed fan relay.
>>


Incorrect.. but close ..you just have it backwards.
The sensor is a NTC ..Negative Temp Coefficient Thermistor...
That simply means that as the temp INCREASES, the resistance Decreases..
So, when that sensor gets to approx 250 Ohm, it triggers the fan driver circuit in the A/C Control Panel Module N/22. And that N22 triggers the relay for the fan motor.
The reason my test for high fan circuit is to remove the blu sensor plug is b/c that same circuit has a DEFAULT trigger in case the sensor or wiring fails/opens, the fan comes ON by default in order to protect the engine in an overheat condition coupled with a bad sensor..just a safety circuit thay have added in there ..and knowing that, I use that as a test for the fan circuit b/c it is so simple to do. By unplugging the sensor . I am forcing the DEFAULT ciruit in the N22 module ..that's all. It is a Tech Trick for immediate diagnosis of high fan circuit integrity.
The RED sensor you have will not work, but as I stated earlier , you can bridge the blu sesnor with a resistor to lower the cut-in temp of that sensor.
There is a member here that sells them as a plug-in, but you can make one for a Buck. By parallel bridging the sesnors thermistor, you are just changing the calibration factor relationship of temp/Resistance , which will make the fan come on quicker [ lower coolant temp]...I prefer the cabin sw I discussed in an earlier post.
If you have an ohm meter , I will post the temp/Ohms chart for the sensor to check it.
If the fan does not come ON @ 105/107 C , but does come On with the unplug test, you need a blu sensor or you have an out of calibrated trigger circuit in N22.
A general test I use is I have a couple of resistor plugs I made up..one is a 100 ohm amd one is a 500 ohm. With the 500 ohm across the plug in place of the sensor, you should have no fan, but when you replace that with the 100 ohm, you should have fan. That is just a general ON/OFF N/22 test.

The "blue" sensor, call a varistor, should have a resistance of about 5K ohms "cold". It will decrease resistance as the coolant warms and about 300 ohms, that should trigger the fan ckt to come on.

The failure mechanism of a varistor is that it creeps "UP" in resistance value, so instead attaining 300 ohms @ 100C, it only gets to, say 500 ohms. This value will NOT trigger N22 to turn on the Aux Fans.

I sell a device (Cool Harness) that will allow the Aux Fans to come on at designated temperature values. For it to work properly, you must have a properly operating varistor so test it using an ohmmeter and see what value it is. At 100C (or there abouts), it should be around 300 ohms, if not, replace it with a new "blue" one.

The blue sensor measures above 5K ohms cold, but not much less when hot.

I went to the Stealership to see about a new one. After consulting the computer with the part numbers as seen above on the picture, They informed me that one needs to be sourced from Singapore (MB Asia's warehouse.) - a weeks wait.
In the mean time I've installed a switch inside that bridges the low speed fan switch at the AC dryer.
The AC is a R12 unit. It's low on gas. So some buckeroos need to be spent to change out the required components to R34 etc.:rolleyes:

Highly recomend staying w/R12 if at all possible...might just need some topping off.

The sensor is definately bad. You can also put a toggle on that circuit until you get the new part, but in that circuit, you want the sw to OPEN for high fan...should be able to get by untiil the parts arrives with the low fan cabin anyway...

Sensor testing chart:
Temp C - Ohms
20 ......... 5-8K
60 ....... 900-180
80 ....... 460-650
100 ...... 300-400

*Approx , as there was a production change on the sensors on that chassis, so use your part#

Those sensors don't fail often but when it does, the resistance rises.

Agree w/ AD; wouldn't add any switches. Get a new sensor from Phil; pn here is: R1075-26553 - about $30.

AD: don't agree with the tolerance numbers. At 100C, for example, the sensor needs to have a tighter tolerance than 100 ohms (300 to 400 ohms). Needs to be 20 - 30 ohms max.

For example: the stated 'trigger' values for a 3 stage system is:
Switching Point (deg C): 100 ------- 107 ------ 115 degs C
CTS resistance ( ohms): 310 ------- 250 ------ 200 ohms

If the sensor is 100degs "high" then the engine coolant temp could be close to 120C . . . not a good thing!

He has single stage sytem and those tol are taken from the Benz CD Chart for THAT chassis. And the tol are much closer as one approaches the ohm values at 105/107C.
That may be why there was a production change for that sensor.

And the added note for approx values for simple
approx sensor testing was added cuz I knew you were coming..................
after the fact.

No sir . . . it's the same part for MY car and Ivan's car(!) and a ton of other MB cars. Your 'explanation' doesn't hold water . . .

It makes no difference what car it's installed into . . the varistor has the same specs b/c it's the same part.

Of course, are you saying that Fastane is incorrect?? Don't think so. . .

Didn't say the sensor was different..I said the CHART was from the BENZ chart. You want to argue with the CHART ???? ...be my quest.

And I listed the approx value so he could test the sensor for approx value just to see if his sensor was even working . as part of my diagnosis.
And the diagnosis is done and his problem is fixed.

Here is the rest of the chart from THAT BENZ CHARTfor THAT sensor. I merely listed up to 100C for his info to test the sensors approx ohms,which we now know does not change regarless of temp..which was the purpose of the info to begin with.

110C- 230/290 ohm
120C -180/220
130 -135/175.

Not my Chart or specs ..Benzs Chart....
Stop your barking ..

If there was, the PN would have to change . . it didn't!. It would effect all MB cars using it.

Since I've tested a lot of these in boiling water (100C) and they all were within 20 ohms. The readling may have been even better b/c of experimental errors.

Keep a civil tongue in your head. :mad:

Get Lost..
You want to jump on my diagnostic threads, then you do the diagnostics on the next ones ..this is not the first time you pull this on me and other Techs.
Barking up the wrong tree once again.
We were doing aux fan sesnor resistor bridging way before you came along .....

You want to argue with your findings and a Chart that I take out of the Benz Manuals, feel free...but I don't care about YOUR experiments .. What I care about is diagnosing fellow members problems in as simplistic a way as possible w/o being an idiot over the exact tolerance factors and specs of a sensor. I leave that to you.
If he was having a fan come on at 110 instead of 105, THEN I would get down to the fine points and calibration of the sensor...but his problem was NO FAN, so we went about the diagnosis just fine.
You take the next POST and then I will Critique your diagnosis ...How's that, ?????
You did the same thing back when I put out the led tool..telling everyone it would not work cuz it needed a dropping resistor, not realizing I already considered that and purposely had a part# for a 12v led ...remember that attack? I still do....you were real cool that time too.................and wrong.
Forget it....do some diagnostic for a change.

You're a good 'tech' and I'm the first to shout your abilities. But this is not about "you", it's about helping Ivan. So I guess anybody that helps Ivan other than you is . . jumping in on your diagnostic threads". Wow, sorry, thought we ALL were here to help.

We both helped Ivan. I gave him the sensors readings that allowed him to quickly diagnose the problem (post #23 and #24). Certainly you had 'worked-the-problem' up. Excellent work!

It was not any of the above that was questioned, it was your statement about " . . production change to that sensor" to which you have not responded to (other than to rant!). You are obviously incorrect since the same sensor is used in a myriad of MB autos.

If a change were made to that sensor, then it would have logistics and configuration control problems that would cost mega-bucks. What companies do when faced with that problem, is to 'pay-extra' so that the part remains the same. The vendors end up "throwing" marginal sensors away at test b/c they don't meet specs. Thus it costs more to produce, but that's cheaper than having to change all of the 'documents' for the using autos. And there are a lot of documents!

So rant away about whatever you wish if that makes you happy but it's unbecoming to you.

Ok ...I will buy some of that...
..but don't think or claim for a second that aux fan sensors did not change on 124 chassis cuz it would be too costly, or other BS yada-yada, because there were 4 aux fan sensor changes on 124 chassis.

..and my rub with this whole fiasco is your disagreement with a temp chart that I posted from my Benz Data......argue with them. The chart was posted for Ivan b/c he asked for it ..tell him it is incorrect.
It's always a pleasure , Jimbo....

Not sure what you mean but if you mean that some test procedure's "limits" were changed, that could well be.

That is always desirable than making a 'source' part (such as the sensor) change. MB is certainly aware of the cost of changing a sensor's specs b/c of what I detailed in the previous post. . . . ah, but changing a test-procedure's limits only effects the vehicle in question so it's a minor cost problem. If that's what you're referring to, that explains the mis-communication.

I'm not here to give you any problems and I don't want any myself. So we can agree to disagree . . . . if we must.

Pleasure here also . . . Ivan's car is fixed . . and we're almost :) . . .

The local stealerships computers only list the production change variation - and then list a Behr product. The unit I have is a VDO unit - not apparently installed in the Australian versions of the W124 - (mine is a grey Japanese import.)

R12 gas is officially banned in Australia (as in the rest of the "Western world" - still produced and used in many third world places). There are some places where R12 is still available. (I have a line on one of these places). I will see what I can do.

As an aside my car - grey Japanese import should have been converted to R34 as part of its import licensing. (Must have slipped throuigh the radar so to speak)

To order the sensor from Fastlane is unfortunately overly costly from Australia -conversion rates and then expensive postage.

When I recieve the new sensor I will wire and internal switch with 250 ohm resistor in parrallel with it.

Recharge AC
New viscous fan
New blue sensor

My tech's shop uses Freeze 12 if I'm not mistaken . . works well w/ R12 substute.

Don't do that . . unless you want your Aux Fans to run all the time . . . just replace the sensorand see that everything works as it should.

After you're sure all is ok, you can add a resistor across the sensor. Details shown in MENU#18 of the values to use. Most use the "92 C" resistor.

<<When I recieve the new sensor I will wire and internal switch with 250 ohm resistor in parrallel with it.>

I suspected a possible difference in sensors for Asisan version, but be aware that the 250 ohm resistor as a bridge is not the correct value
you are going to need..what you are going to be looking for is a resistor that drops the TOTAL of the sensor and the resistor to approx 250 ohms. [ cut-in ] That 250 is not the bridge , so you mis-read that. You will know that when you get the new sensor and see what that value actually is.
The reason for the parrallel vs series is a series resistor will increase the total sensors circuit resistance, but a par. circuit of resistor and sensor will DROP the total R , thereby triggering the High Fan on sooner/low temp. Reasoning here is that the Sensors R drops as temp increases..that factor is what allows for the concept of the R bridging. Parrallel R always drop the R factor lower than the smallest R in the group. So, you are dropping the R/T ratio even further, getting a faster cut-in fan .What your final R needed can be calculated when we see what you have to work with new sensor . An est will be around 1200-1400 ohms, so don't go buy any 250 ohm resistor yet.
That info is all in Jims page.
My preference for yor chassis [ IL-6 engine] is to drop the circuit to a point where the fan cut-in is approx 100C , down from the 105/107 they use. That gets you the added jump on temp rise w/o having the fans on too much.
That also gets you a cut-out of a few degrees less. The V8s suffer from running hotter and some guys prefer to trigger the fan earlier, but that is owner call. Where you live/ambients is also a factor ..I find the 6 engine seldom requires a high fan, even when a bridge mod @100C is installed..just the nature of the beast.

You can see why I like the cabin switch..I am the deciding Factor and I am ZERO OHMS ....:)
It is a good mod . and many have done it..It is also another cheap $$$ improvement, so that makes it even more tempting.
Be interested in the actual sesnor they send for that car..as I for one know little about what they did w/asian exports..
See ya

Jim F
I see we were answering at the same time , so I leave it there........

I was going to put the 250 ohm resistor in series with the manual switch so that I don't put a dead short across the sensor. I didn't intend on altering the resistance of the sensor at all. Manual switch for emergencies only - after I replace the original sensor.

Meanwhile The red sensor I bought will go into my W190 2.6. This car is a U.K. version. There is no auxiliary sensor at all in this car. The Auxiliary fan only operates off the air con. Maybe the auxiliary fans were not needed in the U.K. due to ambient weather there. - certainly required in Australia - in particular where I live - summer temperatures often above 40 degrees C (over 100 degrees f)

Doing that will also screw up the system . . if you put it in series, the N22 controller will see it as the sensor resistance + 250 ohms. So at 100C when the sensor is supposed to be around 300 ohms, it will "see" 550 ohms and the aux fans won't come on.

You can only put the resistor in parallel with the sensor (ie across the leads) and the 'trick' is to get the correct resistor. Typically the value varies from 1.1K ohms to 1.8K ohms. Depending on the value chosen, you will turn the Aux Fans on early, from 92 - 93C to 98 - 99C.

I personally don't like adding "switches" but some think it's the only way to go since you can control the fans. I'd rather see the 'system' make it's own decision as to when to come on and go off.

Oops . . didn't read what you said . . . You said "manual switch". If your manual switch is basically a "short", it still may not work depending on where you place the switch in relation to the N22 wiring and the value of the resistor + stray resistance in the wiring . So use a 100 ohms to be sure.

He is talking about the temporary switch until he gets the sensor...........

Ok. . . . I'm sorry . . . . I was talking about a new sensor installed. . . . but added a explanation at the end.

Yeah , I read it the same way first too..

I had told him he could put a temporary toggle to open the high fan sensor circuit to Default while waiting on the new sensor.
He is not interested in modifying the new one [ or at least until he gives it a go]

A low fan toggle mod only requires shorting/closing the high side pressure sw, whereas a high fan toggle would require just the opposite [ open the circuit in order to default fan activation]..
He just wants to add the series R for when the toggle is closed for no fan , but I would up that to 500 ohm just to make sure N/22 does not read that as trip................cutting it close...
....this would be w/o a sensor in the circuit, where the series R would be the component replacement of the sensor with a constant R factor higher than N/22 trigger, holding that circuit in NO FAN mode until the toggle is OPENED ...that would result in High Fan thru the default feature of N/22.
Simple.

For those interested in the manual over-ride cabin sw, ..the main reason for using the low fan circuit vs high fan is b/c you just have to hook your toggle ACROSS the high side pressure sw to make it work. This is as simple as it gets and there are 2 distinct advantages to using this point in that circuit for the switch ...the first being that you are still using the entire factory circuit
for fan activation [ minus the high side sw], meaning you still have a relay circuit and the protection of both the relays coil side fuse and the fans load side fuse...they are already there , plus the R15 Dropping resistor .... so this jumper/toggle uses the stuff that is already there ......you just add the switch.......:) The other nice about this is that when you are NOT using the manual switch, your fan circuit is right back to STOCK FACTORY confiquration..no change at all...that is why I refer to it as a low fan system OVER-RIDE vs just a manual low fan circuit. You are simply over-riding the pressure switch b/c you are in parrallel with it. The rest of the circuit could care less....
The high side requires a little more wiring and is not as convienient to get the wires to, whereas the high side sensor pigtail is staring right at ya...[ on the drier/reciever]
Another plus is you can have BOTH the manual for low fan [ which is actually all that is needed] and the bridging Resistor to lower the high fan sensor cut-in...best of both and all for a couple of Bucks.............[the best part]
Guys here if FL love the manual sw b/c they do a lot of boat and camper towing...comes in real handy b/c you get the jump on a condition that you already know is going to present itself.

For Ivan:
If you were by chance talking about a series 250 Ohm R in the low fan manual switch circuit I mentioned, that is not needed and would in effect only lower the 12v feed to the relays coil side ..not what you would want to do..the relay actuating coil wants a full 12v ... I do not think that is what you meant, but I write this note just in case..............
What many do not realise is the fact that the low fan, even tho it is the a/c systems acting fan, has nothing to do with the N/22 modules circuitry..it is just a simple direct 12v relay circuit using a dropping resistor R15 for low speed control and a pressure sw for trigger in the event the high side gets a thermal load on it requiring some additional air-flow....the N/22/circuit sees none of this and only operates high fan circuit, with the blu sensor being N/22s temp input and it is N/22 that controls the high fan relay [ along with compressor cut-out]
Two SEPERATE circuits, with seperate relays, fuses, resistor, wiring, and purpose ..but sharing the same fan motor/s at the end of each circuit.
One high, one low............................

Viscous fan, new sensor, R12 gas, coolant flush, front of engine M103 shock absorber, fix small vacuum leak in heater switch, all coming this Wednesday.:)

As I went to bed, I thought . . crap . . I forgot his sensor is running around 5K so adding a switch to "open" the lines is the easiest but is not a thing that I would do b/c you're breaking into MB wiring. To me, that's "verboten".

I totally agree that low-speed "switch" is the best way; you get low fans which will do a good job; it's easier to do (wire in parallel) and you hardly know they are on.

Hey..you agree ????
Let me Print this one out ....

AD . . . you'da man . . :D

I know this is an old post but can someone take a pic or draw a diagram on how to wire in the cabin low fan switch? Thanks

Year and model please.

1987 300TD (wagon) Thanks :)

bump