Is high engine temp really all about AC? - PeachParts Mercedes-Benz Forum

ksing44 · #1 07-11-2007, 05:48 AM

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.

Arthur Dalton · #2 07-11-2007, 10:38 AM

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...

ksing44 · #3 07-11-2007, 11:35 AM

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!

Arthur Dalton · #4 07-11-2007, 11:52 AM

>

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...

Pete Geither · #5 07-11-2007, 12:15 PM

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.

Arthur Dalton · #6 07-11-2007, 12:57 PM

Quote:

Originally Posted by Pete Geither

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.

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 .

ksing44 · #7 07-11-2007, 01:03 PM

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.

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

JimF · #8 07-12-2007, 10:51 AM

Quote:

Originally Posted by ksing44

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.

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.

ksing44 · #9 07-12-2007, 11:13 AM

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

Arthur Dalton · #10 07-12-2007, 11:51 AM

>>

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 .

juribe2 · #11 07-12-2007, 01:09 PM

Quote:

Originally Posted by Arthur Dalton

>>

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.

Arthur Dalton · #12 07-12-2007, 01:29 PM

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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.

JimF · #13 07-12-2007, 05:54 PM

Quote:

Originally Posted by ksing44

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.
Thanks for the information Jim,

Ken

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!!

ksing44 · #14 07-21-2007, 12:57 PM

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

naidd · #15 08-23-2007, 10:28 AM

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).