Is high engine temp really all about AC?
 

Well, it was in the mid 90s for a couple of days here in PA. Sure enough the Cool Harness V2 has the aux fans come on high speed at about 95 degrees and then they don’t turn off until you turn off the car. Too bad. In more normal temperatures, the aux fans never even come on at high speed, but in the very high heat it is on and never off.

The good news is that before the V2 install my car was doing pretty good with the heat. It seems that my fans running at low speed, from the AC pressure, keeps my car under 100 degrees with no problem. My aux fans were not responding to AC system pressure before, since my system was a bit low on charge. It still got cool in the car, but the charge was low. Now with a new full charge, the aux fans do come on low speed quite frequently and that seems to keep the car running cooler without going to the higher temperature that kicks in the high speed.

Now I am thinking that maybe the W124 would not run so hot if everyone had good AC systems with a full charge. The real weak link might be keeping a full charge in the AC system, since it seems everyone has issues with them. The AC system just leaks. Sooner or later it just happens. If you are lucky, it isn’t the evaporator. I am on my second leak in 10 years. The first was the expansion valve to receiver drier. This time it is the compressor.

Is it possible the Mercedes actually designed the car to stay cool in most normal situations by responding to AC pressure and that the high speed fan is really just a kind of emergency fail-safe kind of thing? It makes sense that you would have the AC on if it is hot, so why not depend on the AC? They also seem to have designed the car to use AC even in winter driving to control humidity. Maybe they just figured AC would always be on.

Check the archives for a few articles I wrote on that exact scenerio..
No low fan is one of the most common, mis-diagnosed , causes of high coolant temp complaints.
Once the high thermal load caused by a/c passive heat exchange from condenser to radiator starts to creep on you , it's over...
I tell guys their problem with high coolant temps w/ac is caused by a slightly low refrigerent level and and they think I am crazy. [ until I top it off and the fan comes on at spec]
FIRST check for this coolant temp condition is LOW FAN , Problem is, the test used is they jumper the high side pressure switch and the fan comes On ..So,sure enough, they wrongly condemn the Switch.
It is almost [99%] always the refrig level, not the switch.
I have only seen 2 sw go bad ..and one was physically broken, the other had a short and ran the low fan all the time.
The best system is to run 2 wires off the drier pressure sw to a cabin sw [ on/off] that shunts the system so you can get a low fan on Demand manually. This is great also for towing. If the sw is OFF , the system just goes back to stock configuration b/c the sw is in parellel with the circuit.
The resistor bridge at the coolant temp sensor/thermistor for High fan does lower the cut-in of that sensors 105/107C value, but in doing so, it also lowers the CUT-OUT ..sometimes to the point of never turning off the fan. In other words , it has a differentail . With proper refrig/low fan operation [ and a cabin sw], you are controlling/checking thermal loads on the eng temps way before they gets to these higher temps.
And w/cabin sw , there is no cut-in/cut-out differential , so you don't have to worry about a HIGH fan on all that time [ not good for fan motor longevity or charging system]. Basically , I use the correct thermistor value as a coolant temp safety cut-in rather than trying to controll the engine temps with the High Fan. So, I keep that cut-in/cut-out on the high range, as it's main purpose is engine protection at higher than normal temps.
I also keep them at a higher cut-in so they turn back OFF after they have checked the condition ..that simple requires different R values. If one is in constant hot/Southern climates, then you may just have to use a lower cut-in of high fan for temp control, but that is another story..the large V/8 engine seem to benefit here also.
I actually use BOTH systems.
An added note to all this is that armed with this info, it is also easy for one to see that their a/c system is low on refrig simple by noticing a NO LOW Fan condition..you are in reality using the high side pressure sw as a monitor for rerig level.. [ just a casual observation]
The manual sw is not needed to control the temp if low fan is working, but it is a neat add-on for manual /over-ride control, as it can also be used without the a/c being ON [ like towing,etc].. I have always has a cabin sw on all my Benzs over the years , just in case that one time comes when I don't like the rising temp gauge staring at me in a traffic jam and I can't do a thing about it [ except throw that switch... :)

Thank you for your brilliant explanation. I am also feeling a little good about myself since I kind of figured it out.

The bad news is that now I think I just have to learn how to rig myself a switch. Do you think I could do it, even if I am not really much of a mechanic? My biggest accomplishment so far was that I was able to adjust my own vacuum modulator.

Another thing, since I have you here. If I have “dye all over my compressor”, as stated by my indy, is it likely that I need a new compressor and that is isn’t just a little leak from a connection. From reading on the forums, it sounds like the clutches go bad in the compressor and if I wait too long I may get all kinds of debris in my system as the compressor self destructs. I am thinking I should just go ahead and replace the compressor sooner rather than later. Does that make sense? Bye the way, I have a 1995 E320 with 105K miles. Also, it sounds like I should do the receiver drier and the expansion valve when they do the compressor. I did replace those two items about 4 or 5 years ago, but I read that you should always do the drier with the compressor.

Thanks again for your help. I just love these forums!

<The bad news is that now I think I just have to learn how to rig myself a switch. Do you think I could do it>>

This is a very simple wiring job.

Just get a regular ON/OFF toggle switch and a little bracket to mount it under the dash .
This sw will have 2 terminals..hook a wire to each terminal and run the wires [ I use lamp cord b/c the 2 wires are in the single cord, makes it easy and neat] thru the firewall somewhere and route them over to the switch at the drier . This sw is the one with the 2 pigtail wires on it ..your chassis has two white connectors on the end of this pigtail..just hook one wire to one of the 2 pigtail wires. and the other to the other pigtail wire at these white connectors..that's it, you are done.
This set-up uses all the original low fan circuitry of the stock system [ relay/fuse/resistor/etc], so there is no need to do any crazy elecrtonic additions..just the sw. Meaning it is a very safe , fused system.
When mounting the sw in the cabin, I mount them so when the sw in the ON position, the toggle lever is facing foward, as in the direction of travel of the car..that way you always know the sw is On or OFF from the toggle lever position.. Once the key is OFF, you will get no fan, regardless....
Once you have this additional manual control over your fans operation, Murfey's Law says you will never need it...:)

Reading this thread I get the impression that the two electric fans run independently of each other,,, is this true ? And if so which is high and which is low ? I always though the two started and stopped at the same time. Perhaps this is why the 400E is running a bit hot these days.:confused:

Your confusion is very common with these two different systems.

Fan Systems/Explaination :

The two fans are wired in parellel and ALWAYS work in unison.

There are TWO different circuits ,HIGH FAN and LOW FAN.

Each circuit is INDEPENDANT of each other , EXCEPT, they SHARE the same FAN MOTORS.

There is high fan relay, fuse , sensor circuit.

There is a low fan relay, fuse ,sensor , and dropping resistor [ Referred to as R15/ Pre-resistor.]

The a/c circuit can NEVER trigger a High fan, nor can the coolant temp circuit ever trigger a LOW fan..however , if the coolant circuit calls for high fan, that will over-ride a low fan demand from the a/c low fan circuit, resulting in high fan, regardless of a/c demand.

So, if one looks at a schematic, you will note that each of these independant circuits MEET at a COMMON terminal at/after the dropping resistor. [ the two wire side of the resistor]
That is the point in the circuit where either circuit feeds the FAN MOTORS. That is where they now share the fan motors, regarless of which circuit has a demand signal from it's circuits sensor [ Coolant temp sensor for high fan, a/c pressure sw/sensor for low fan.]
The low fan is accomplished by feeding 12v to the R15 resistor BEFORE this common terminal, thereby dropping the voltage to get a low fan RPM. Whereas , the High fan circuit by-passes thus R!15, allowing a full battery voltage to reach the fan motors, resulting in a HIGH fan RPM....
..so, two different circuits Sharing the same , parellel wired fan motors.
If you have a fan motor turning slower than the other , or not turning at all, the problem is NOT in either high/low fan circuits, it is a faulty FAN MOTOR .

I am not an expert, but from what I have read and seen with my car it seems that both fans run together. They can run at low speed or high speed, however, and they really go like crazy when they are on high speed. If they come on and your temperature is not over 100 degrees, then that is probably the low speed. If you ever get stuck in traffic on a hot day and your AC system is a bit low, so your fans don't come on at the low speed, then your temp may go up over 100 and then the fans will come on at high speed. I suppose they can also can come on at high speed without having low AC pressure, but I think it might take rather extreme conditions to get over 100 if your low speed fans are coming on with regularity due to the pressure in the AC system.

Others here are real experts, so they may chime in if I am getting too carried away with my newly acquired expertise on this matter. :book:

Woops, I was too slow and a real expert answered while I was writing.

You are now as Expert as I....

It is all in reading this Great Forum. All the info is right here.

Thanks once again for the great info Arthur,,,,, I sure wish you lived in Western Pa.:D

I did do a Gig once in Weastern Pa..at least I think that is W. Pa ??

York, Pa.
A Club right in the center of town, across from a Noodle Resturant..Very nice, but years ago, so my menory is cloudy on the exact Name of the Place [ prob not even there anymore.]
I do remember those long tunnels going through the mountains on the Pa Turnpike..What a road trip that was ...:)
Next stop from there was Erie , Pa...Completely different type town, if I remember correctly.

My wife was raised on a small dairy farm right outside of York Pa. She is still a great worker, thanks to that training on the dairy farm. She is a great cook too!

York - East,,, Erie - West,,, about 100 miles north of Butler.

Another common failure is the resistor behind the driver side headlight. The resistor drops the voltage to create low fan speed. Have seen it queried more than a few times on the forum. As well, my low mileage/dealer maintained/Starmarked/owned by MB dealer management 124 came with a burned out resistor and no low speed fan operation. On this 1993 model the teltale clue was coolant temp rising to 105C, fans noisily engage, and coolant temp drops to ~95ish. Surprisingly, the A/C worked fine.

After replacement of the resistor and upgrading the wiring the low speed fans work great. The car sits on ~90C coolant temp and blows cold air all day long, no matter the outside temperature.

Since I know you're going to ask about the wiring upgrade. On the early 124 models a 2.5mm diameter wire supplied power to the resistor. For unknown reasons MB dropped this to 1.5mm on later models. The smaller wire tends to overheat and break near the resistor. Per a tip on this forum from Donnie, I replaced the entire wire from the low speed relay to the resistor and all has been well for several years.

- JimY

Yes , they dropped the wire size when they came out with the dual fans..but they under-estimated the current draw . They later changed the wire feed and also changed the fuse rating on low fan relay from 15 to 25 amp. [ the ones w/fuse on top of each relay]
Some guys in Tropical Climates take that same feed wire from the low relay to the R15 and use that to trigger another independent, higher amp circuit with another relay, which by-passes the R15 circuit altogether, allowing for High Fan for a/c condenser airflow. No need to change the wire from the 1.5 size b/c that now only feeds the coil side of the new relay. [ Low amps].
Never tried it, but I can see the advantage of a high fan for cond. airflow when one has the extreme thermal load conditions of Tropical type climates............................which lately, could be almost anywhere in the USA ...:)
Maybe we should just call that one the " Global Warming Modification TSB" ???
Here is the latest proto-type we are working on ..definate solution for
any Global Conditions , right into the next 10 years..

http://www.sl113.org/forums/uploaded/cees%20klumper/2005621181316_airco1.jpg

Got into the 400E tonite and found a VERY corroded,,, almost broken wire going to the resistor. Fixed the wire and as I ran the engine the low fans came on. I am sure the problem with the high temps was the wire to the resistor. Once again the forum saved me from a serious problem later on. MANY thanks to Arthur D. and Jim Y.
Now if I could only fix the idle surging on the 320,,,,

I think your last statement is what they had in mind; since they call it "climate-control", I think they wanted you to keep it on set for "70degs" (or whatever temp makes you happy) and then the fans will take care of keeping your engine cool. Since the A/C will be on, the fans will come on when needed.

Of course, you need to keep your A/C operational . . . but using your "ears", you can tell when something isn't correct . . . if the fans don't come on "low", on a HOT day, then something is wrong. *

If you ever "forget" what the key trigger points are for each separate mode operation, go to MENU#18 and look at the table, called "Aux Fans Activation", in the middle of the page. All the key 'numbers' are there. . .

* You can check the A/C's operational parameters by accessing the info via the control panel. Follow MENU# 15 to read-out these parameters esp "5" (a/c evap temp) and "7" (a/c compressor pressure). The latter will tell all . . . high vent temp or very low pressure means a low charge.

Hi Jim,

It is great to have you chime in on this. My friend's W210 has that kind of access to all kinds of information. The W124 does not have that available, as far as I know.

I also have a friend that wants to play with a slightly different resistor in your cool harness, to see if he can get the high-speed aux fan function to turn off at normal driving temperature after it kicks in. It may come on just a bit higher than 95 degrees, but still just short of say 100, so that the fans will still prevent excessive heat, but will also turn off when the car gets back down to 90 degrees. Right now, on a very hot day, my high-speed fans do not turn off after they come on with the cool harness V2. I think I still prefer that to seeing 105 degrees, but I also think I am interested in maybe just a slight tweaking of your cool harness. Maybe a resistor that kicks in a 97 or 98 degrees would let the things turn off. As I remember, you had a 97-degree cool harness in the old version. Did that one allow for the fans to turn back off on a very hot day, so it cycled more like with the stock system?

Thanks for the information Jim,

Ken

<if the fans don't come on "low", on a HOT day, then something is wrong. *
>>

This KEY wording here is : "..on a HOT day".

Aux fan is just that....an aux added fan to AID the system with increased air-flow when the sensors detect a "Higher than normal Thermal Load".
A/c is used in winters to aid in clearing windshield of humidity, but you will never see a low fan when it is doing so... b/c you have NO High Thermal Load.
Car travelling down the road at 45 MPH, a/c On , no aux low fan.. b/c it is not needed... NO high thermal load b/c travelling vehicle has plenty of airflow to keep condenser pressure in check. But come to a stop light or slow traffic and On comes fan..b/c of rise in Thermal load and Sensor Demand for more air-flow..
So, the word HOT here is the trick, coupled with low airflow..
Do not expect to see your low fan activated on a cool day just b/c your a/c is ON..it only comes on with Thermal Load Demand .. it is ONLY an aux airflow aid device.. mostly activated at idle, car at rest, hot conditions...and it is actually possible in cooler climates to never come on .

So, how can you actually determine if the low speed fans are doing it's job? With the car stoped and a/c on, you can see the fans runnig but when driving how can you tell if they did turned on at the right moment? I can actually hear when the hi speed fans turn on when temperature is like 110 or something but, can you hear the low speed fans when driving?

And the other question is, how can you test the resistor?

Thanks for this great info.

<<So, how can you actually determine if the low speed fans are doing it's job? With the car stoped and a/c on, you can see the fans runnig but when driving how can you tell if they did turned on at the right moment?>>

The correct Moment is if the high side pressure switch is calling for the aid of aux fan..if there is a very high thermal load , they come on, if not, they don't..so, if they come on at idle w/ac on on a hot day, they are fine.
When the car is moving, they may come on or may not come on..that is dependent on demand of high side pressure, which is determined by the intensity of thermal load. If they are called for to check pressure and they do not come on ,then the high side will get to a dangerous pressure and the compressor cut-out pressure switch will deactivate the compressor and you will have no a/c.
All fan activation is on pressure demand..the pressure sw does not know if the car is moving, what the temp are , or any other variable..it only knows that SOMETHING is making the high side pressure get out of spec, so it comes on to bring that pressure back to efficient working condition. [ that is what the airflow does..it drops the condensers pressure.]

The R15 resstor can be checked with an ohm meter or a test lamp on each side of the circuit..if you have low fan, you do not have a bad resistor..high fan does not use the R15.

Now I understand and know that my E320 has hi and low fans working ok.

thanks again Arthur.

I have a related question.

What is the correct way to put additional refrigerant into a 1998 air condition system? I believe this is R134.

I always tell everyone I am not an expert before I say anything, so please keep that in mind.

I was told that the correct way to add refrigerant is to pull a vacuum for some period of time and then add a specific measured amount, by weight, of the correct refrigerant. I was also told that measuring pressures, high or low pressure side, is not an accurate way to determine how much to add. I was told that the low side pressure is an especially poor diagnostic, which is what most DIYers do in their driveway.

For me, it is only about $60.00 to have it done correctly at my Indy.

I guess I picked a bad example for '95 owners but for the newer models ('96 and on), you can read the A/C operational parameters from the dash controller.

Press "REST" for 8 secs or until the display changes to "1" in the left window and "TEMP" in the right window.

Press left "AUTO" to advance to next step; press right "AUTO" to decrease one step.
Step #5 is evap temperature and #7 is compressor pressure in BAR. (14.7psi = 1 bar). So if you 'sort-of' remember the "Aux Fan Activation" tables, then you can tell by the pressure where the aux fans state should be.

Yesterday, I took a '02 E320 for a ride and pressed "REST"; sure enough the operational parameters read out. . it's a/c evap was 35F. . w/ the compressor at 15bar. 35F is . . . . cold!!

Cool Harness was great yesterday
 

It was great to stay cool in the traffic jam on the NJ Tpk yesterday on my home from New Brunswick NJ. There is always a jam where the truck and car routes converge. Usually I had to sit and watch my temperature climb to about 105 degrees before my high speed fans would kick in. Not anymore, since I have the Cool Harness V2! The fans do seem to stay on, once they are triggered at 95 degrees, but that seems much better than waiting to see 105 degrees. The reality is that if it is a hot day, the fans would have had to come back on again anyway. I suppose it is OK that they just stay on. I can't hear the fans at 80 miles an hour anyway. I think I would rather pay to replace my fan motors some day than run my car back and forth up to 105 degrees. Maybe now my head gasket leak won't get worse too fast. The car never went over 95 degrees. It was usually about 87 degrees when moving, just as it is supposed to be.

Anyway, I am a happy customer with my Cool harness V2.

Thank you JimF

Going back to Arthur's explanation of the HIGH and LOW fan systems, imagine this scenario:

- You have the manual switch in place but it is in the OFF position.
- Your A/C is OFF.
- The car heats up in stop and go traffic.

Will the HIGH fans still kick in when the car gets hot enough (105 degrees, I presume)? It would make sense since the HIGH and LOW fans are on different circuits, however, I am not sure if installing the manual switch puts it on the LOW fan circuit only.

Also, since the Cool Harness was also mentioned:

Does the original Cool Harness have the same problem as the V2 -- keeping the fans on HIGH once they are on? Is this a consistent problem with Cool Harness users or an isolated incident?

I am interested in applying both changes to my '89 190E 2.6 which has a dead A/C (leaky evap).

The hysterisis for the Aux Fan ckt is around 5 degs C; so if it goes on at 100C it should go off at 95C. Those MBs with multiple fan speeds have a slightly larger hysterisis value (on=115C; off=107C) at higher engine temps. All of this info is in MENU#18 in the "Aux Fans Activation".

If your car has dead A/C then it shouldn't get to 100+C; suggest that you follow the steps in MENU#17 in the "agua" colored table. The 5 steps should lower the engine temp so that the fans won't run all the time.

My car with A/C in 95F w/ 80% RH, doesn't go above 95C. On the highway, it's under 90C. So the A/F are off.

If the A/Fs don't go off, it's b/c the engine temp doesn't lower when the fans turn on, indicating something isn't right. The Cool Harness doesn't KEEP them on, your engine temperature does!

Also the original Cool Harness comes in three (3) ranges as shown on the CH table. I use the CH-92 but many prefer the CH-95 and/or CH-98. The numbers represent the trigger point for A/F cutin.

Jim, thank you for the clarification. It makes sense now.

My car also stays below 90C on the highway. In stop and go traffic without A/C and roughly 95F ambient temperature, I have seen the auxiliary fans come on several times during the last 2 weeks or so. It looks like they engage at around 105-110C. It is a rare occurrence and the car cools down quickly with the fans on, but having them engage earlier will give me some extra peace of mind. CH-92 would be my choice as well.


Update from 8/26/07:

I installed the Cool Harness today, went with a CH-95 instead. It works flawlessly. Fans engage at 95C and go off at around 87C. This is awesome!

Very hot 300TE with Low (& High??) cooling problem
 

Guys

I don't mean to takeover this thread, but I have been reading quite a few since my temp gauge went to 120+ in traffic a couple of days ago (ambient temp was 32C or about 95F). I tried to get the fans to come on by turning on the A/C but they refused, so I managed to lower it by turning down some side streets & getting some air through the radiator.

A/C works well & the belt driven fan clutch was renewed last year & seems to be working OK.

I've tried to do some tests and so far today (35C) I have had 1 fan come on high at about 95-100C with the A/C on a short trip (right side looking at it from the front - your driver side).

Later I removed the Temp guage connector (which looked a bit beaten up) and the same fan came on high, but nothing from the other one.

When I bridged the plug I heard relays switching near the fusebox & near the A/C drier but no Low fan came on - in either of the fans.

I also tried bridging the each side of the Low fan resistor with 12v - but nothing happened with either side fan. Though I may not have done this correctly?

Does this mean -
I have at least 1 fan that's dodgy;
I possibly have a Resistor that's stuffed;
Possible other problems...

Any help greatfully accepted.

PS - something else I was wondering is that when all of the R12 was replaced with R134a a few years back, how does this affect the psi readings from the A/C for the Low speed fan? Are they close enough?

Cheers

Stu

Is this a double post?