E320 Cooling Problems - PeachParts Mercedes-Benz Forum

earossi · #1 05-02-2002, 08:42 AM

I have a 95 E320 that contimues to have an overheating problem due to the auxilliary fans not coming on. I have researched this board on the problem and ways to troubleshoot it, and there are some discrepancies that I would like to have cleared up. First, my understanding is that both fans are off or on together (no single fan operation). Next the fans run on low speed when A/C line pressure trips an overpressure switch. The low speed is produced by a resistor which is in that circuit (I assume in series with the fans?). The fan high speed is triggered by a thermo switch located on the thermostat neck. There seems to be some confusion on how to test the thermo switch. Some posters say that merely removing the connector (which produces infinite resistance across the connector) will activate the fans. Other posters have suggested grounding or jumping the connector pins on both the A/C pressure switch or the thermoswitch. Whether it be the overpressure switch on the A/C or the thermoswitch.......do both switchs operate through the SAME relay? There are two connectors on the thermostat housing. One, a two wire connector, while the other appears to have 3 or 4 wires. Which one is the thermoswitch? Lastly, where in the wiring harness is the EASIEST place to jump battery voltage to test each fan? Sorry for the long post, but troubleshooting these fans seems pretty straightforward, but is proving otherwise. I would appreciate it if someone would confirm my understanding and provide some insight to solve my problem.

jsmith · #2 05-02-2002, 08:54 AM

if it's an overheating problem and the fans don't come on then you have to check the main sensor by the thermostat. note that the fans should come on at around 107 degrees. if you pull the connector off (you are right - infinite resistance) then it should come on. the sensor is the one that is painted blue.

the low speed fan sensor by the AC drier is tested by shunting the terminals. there is a fuse in the fuse box as well as a resistor under the driver side headlight that needs to be checked.

Arthur Dalton · #3 05-02-2002, 12:03 PM

I will add some to Jims post...

There is a mid-production change on the E320, 124.032 chassis fan circuit.
First, there are now 3 fuses to be concerned with.
1- Fuse #7 in the fuse box. -- relay feed.
2- Both fan speed relays are now fused, [ on top of each relay].
These fuses are fan feed [load].
These are located behind fuse box, under panel. [Remove 6 screws]
The one in slot "C" is high speed fan [K9, blue one]. This originally called for 15A., but has been increased to 20.
Slot "B" is low fan resistor relay. This is 30A. [ K10,green relay]
This is resistor series feed for low speed. [ thus the higher Amps for lower speed].
PS - the fan load is always both fans, They are wired parallel.
So, first check is Fuses. The trick of jumpering the a/c high pressure sw. is that if that brings on low fan, you know you have good fuses #7 and relay fuse. Plus you now know your fans themselves are good.
Now we get to the high speed. [ which has nothing to do with low].
This is triggered by the temp thermistor sensor at the thermostat housing when the coolant temp causes the resistance in the sensor to DROP to around 275-300 ohms. This is a negatve temp/resistance type sensor. [ a good test is ohm meter ---- I have a temp/ohm chart , if need be]
This is the blue , 2 wire sensor everyone talks about, BUT, there is a late change-over to a 4 wire/prong sensor in 95.
The early ones had 3 sensors
1- single wire on top of housing- This is gauge sens .
2- 2 wire, blue on top of housing - this is aux. fan sens.
3- 2 wire, 3 prong on side of housing- this is Engine Management control sens.
The late ones have changed over to a 4 wire/prong sensor that now combines both the ECU and Aux fan sensors. The operation is the same, but one needs to know that the pins on the new one for the fan circuit will be pins 2 & 3.
This may add to/ or help clear up some of the confusion.
The main thing to do after checking to see which sensor your E320 uses is to check all fuses and then jumper the a/c sw to see where you stand and go from there with the sensor test.....

Arthur Dalton · #4 05-02-2002, 12:06 PM

<>

Correction--- "Joes post"

Sorry Joe......

earossi · #5 05-02-2002, 01:19 PM

Joe and Arthur, thanks for the fast response. Since there are only TWO sensors on the thermostat housing, I guess that I can assume that I have the later version, which means that the aux fans are controlled by the 4 wire sensor. Since the thermoswitch is a thermistor, my assumption is that resistance DROPS as coolant termpature rises, correct? If this be the case, then pulling the sensor connector (which would produce infinite resistance across the pins) will NOT trigger the high speed fans as had been previously reported. If this is the case, then I should be able to jumper the two pins for the aux fans, and they should come on, right? I will pull the relay cover this evening and check fuses and also jumper the A/C switch to confirm that part of the circuit and fan integrity. If that proves out the fans (at low speed), can I jumpter the temp sensor pins to confirm high speed fan operation? Please corrrect me if any of my above assumptions are incorrect. Thanks in advance for the help.

Arthur Dalton · #6 05-02-2002, 04:30 PM

To be honest with you, I could never fiqure out why the high speed comes on when the sensor is disconnected, but I have noticed that this test does not allways work. It seems to work at certain temps and not others.
The resistance value drop is calibrated to turn on a switching device
[ maybe a gate or SCR, etc, - not sure] in the ECU. From there the relay gets the signal. It may be that the disconnect spikes the correct trip - maybe someone knows ???
Anyway, as a precaution ,I always use a 1/4 watt, 180 ohm resistor so I am sure of not screwing with the ecu with a direct short or open circuit.
The 180 is beyond the res/temp range for cut-in, so that seems to work well for the high speed test....

earossi · #7 07-04-2003, 11:08 PM

As you will note from the previous threads, I have been plagued with overheating on my 1995 E320. Thanks to all that responded, I was able to trouble shoot the system, and found a number of problems. The first one that was the most difficult to unravel was that the low speed and high speed relays had been reversed. This may have occured about a year ago when I had the car worked on because the alarm system had immobilized the car, and the mechanic did some wiring changes to bypass the alarm interlock.

Secondly, I apparently had a bad temperature sensor. Not very expensive to replace (about $40).

Lastly, when the above two items were corrected, only ONE of the auxxilliary fans came on! So, I apparently had burned up one of the fans sometime in the past.

Incidentally, unless there is some secret to replacing the aux fans on the 124 chassis.............it's a many step job! Even though the aux fans sit in front of the radiator........you cannot remove them from the front. Instead, you have to remove both the radiator and the A/C condenser.........and the fans then pull out from the rear. This is some terrible German engineering. I had to drain the radiator and depressure the A/C system to get the fans out.

At any rate, the deed is done and the car is running nicely now.

One question for the BB, though. Sometime ago, I recall a poster offering a custom plug with a resistor built in that reduces the trigger temperature for the aux fans from 107 degrees..........down to about 95 degrees. But, I can 't find the information from the archives. Would appreciate any info that would allow me to either fabricate the plug or purchase one. I get a little nervous when the temp gauge inches up to about 110 degrees before the fans come on!

ceward · #8 07-05-2003, 12:22 AM

Just as I was looking for more info on my issue I found this link to your need for the fans speed modification..

http://pages.prodigy.net/jforgione/MB_S500.html

b52rule · #9 07-08-2003, 03:34 PM

The car (95 E300d) usually runs around with the temp indicator at the top of the 8 (gauge) on the freeway and around town, summer or winter. If I turn on the AC it jumps well above the next mark on the gauge for a bit on the freeway, and around town it goes even higher and stays there to the point I think it turns off the AC. If I go up a prolonged grade on the freeway the temp climbs accordingly, but really moves if the AC is on. Freeway driving, over time will get the temp down to the 8 again, AC on or off. Any thoughts? Do have a problem?
I checked the engine driven fan after I parked it, and it is turning. I disconnected the temp sensor (blue plug) in the inlet to the engine block and both fans came on at a very high speed, when connected back the fans turn off. Is there a way to check the temp sensor since for as long as I have owned this car I have never seen those fans turn. I did check the relays, fuses, and resistor as mentioned earlier on thos post, they all seem fine.

Thanks,

B52Rule

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ceward · #10 07-08-2003, 04:07 PM

I had a similar issue which vanished once we recored the radiator.

This also occured on my Eurovan which had major issues keeping the engine cool when in town with the AC on, you could hear the secondary fans howling away like mad men, at all times when the AC was running.

Again after a recored rad the issues went away.

earossi · #11 07-08-2003, 04:24 PM

If you pulled the connector on the thermo switch and both fans came on, you have essentially proved out the fan circuit including the relay. If the engine is running hot at that time, the culprit is more than likely the thermoswitch. You can measure the resistance across the terminals..........but, to tell you the truth....the switch is very inexpensive.........less than $30. So, I would just replace it.

The part about your note that is confusing has to do with the temp gauge reading hot while driving at speed. At speed, air flow across the radiator should be more than enough to maintain low coolant temperatures. At speed, the viscous fan is freewheeling and the auxilliary fans should be off.

Overheating all the time is probably a defective thermostat...again, this is an inexpensive part to replace.

As another poster indicated, you could have a plugged radiator, though if you are running an anticorrosive antifreeze........plugged radiators are rare.

In summary, you could have more than one thing wrong.......as did I. When I finally sorted out my overheating problem, I found that the hi and lo speed relays had been reversed, the thermoswitch was bad, and one of the two auxilliary fans was defective!

b52rule · #12 07-08-2003, 09:10 PM

I think replacing the thermoswitch and thermostat are in order. I found the thermostat in Fast Lane, but not the switch. I sent in a parts request so we will see what happens.

Thanks,

B52rule

b52rule · #13 07-12-2003, 02:23 PM

If I disconnect the two prong pigtail at the top of the block, key on or off both fans come on high.

If I disconnect the blue with red trace leads to a relay below the drivers headlight the fans never come on. I can ground them out, connect to each other, connect the terminals, or just about any other combination you can think of............no fans at any time.

Any suggestions?

I did check all the fuses BTW.

b52rule · #14 07-14-2003, 02:33 PM

The 30 amp in the relay fuse is good, but the 15 amp fuse in the relay was blown, I replaced it and the fans come on at low speed, intermittently while the AC is running. This happened just after I fired it up after replacing the fuse. I'll take it out tomorrow and we will see what really happens.
Where is this resistor located?

B52Rule

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Arthur Dalton · #15 07-14-2003, 03:15 PM

<>

The reason the low [ a/c fan] was blown is b/c someone has switched relays or fuses..

The low fan is relay K10- and belongs in slot "B"... that is a 30A.,
not a 15 [ which will blow again]
The high fan relay K9 is in slot "C" and originally had a 15 , but
Benz has allowed a 25A replacement..

The dropping resistor for K10 is behind the left headlamp, but if you now have low a/c fan, the resistor is fine ...

Your problem for no fan was bad fuse..and under-rating was the possible cause of failure..
{ I am assuming chassis 124.131]