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)

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.

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1999 Mercedes ML320
2002 Mercedes 230K

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.

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A Dalton

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&makeid=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

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A Dalton

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.......

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A Dalton

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.

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?

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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&makeid=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&makeid=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

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A Dalton

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.............

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A Dalton

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.

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.............

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A Dalton

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.