C280 Coolant Temp High at Idle - Page 2 - PeachParts Mercedes-Benz Forum

Arthur Dalton · #16 10-20-2002, 10:31 PM

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This mark is the 100C index [ all marks are 20C apart]
From what I read here, you may want to verify the gauge/ sensor values [ ohms/temp] to first see if that is a good reading to use ..
Here is that info: [ single wire sensor-read from terminal to ground].

60C-110 ohm
80C-67 ohm
100C-38 ohm

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This is the blue 2 pole ECT for the high fan. The cut-in spec calls for 105-107C
Here is spec for that sw.
20C-5.0K-8.0K
60C-900-1800
85C-460-650
100C -300-400
So, you are in the 100C range

As you can see, the specs on yours at 105-107 should be lower
for trip [ I have found most 104s to have a trip point cut-in of about 265, They do vary, and that is why spec calls for the 105C-107C spread.
You really want to get a resistance reading at 105-107 [ after verifying gauge accuracy]
That will tell more [ I am inclined to agree with Jims thoughts of a high reading sensor..]
If not, it could also be the control unit out of spec..[ long-shot]
By manually getting the fan to run from the ECT sensor means that the rest of the circuit has to be intact and operating, leading to diagnosis of an out of wack sensor as that is the ONLY trigger for high fan. [ nothing to do w/low fan]
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Yes, the resistance values trip the A/C PB and that trips current to the K-9 [ high speed] relay primary coil circuit. Which pulls the relay contactor in for fan load [ the fan fuse in on this load side of the relay]

The other speed is activated by the high pressure sensor at the reciever/drier of the a/c system. This is K-10 relay [ green]and does not read any resistance values as it is triggered via high side pressure [ on/off] only. The lower speed is accomplished with a dropping resistor behind the left headlamp. [ one will notice that this relay is called the "pre-resistor" relay...
The easy test here is to jumper this high side pressure switch and watch for low fan.
If no fan w/jumper , than check fuse on K10 relay, dropping resistor, etc...BUT, if the low does run w/jumper, before condemning the switch you can have a possible slightly low freon level [ not enough freon
for high side to reach cut-in pressure .]
On 104s, remember that the high and low fan circuits are independent of each other [ thats why different relay, switchs, etc. ]. but they do share the same fan motor/motors....[ which is where all the confusion comes from]

oldsouth · #17 10-20-2002, 11:42 PM

QUOTE]Thermostat will start opening at 85 to 89, ect, ect, ect.[/QUOTE]

Do you know the above specs for a 1995 S-350 diesel??

#18 10-21-2002, 12:05 AM

I'm using the 71C thermostat. Yes, in hot weather heavy traffic, the aux fan (single unit) kicks in at high speed to prevent the coolant temp from creeping to the top! It also keeps the temp from large fluctuations.

Quote:

Originally posted by jsmith

what thermostat are you using? the stock 87C one won't even be starting to open when your aux fans cut in. in summer or in heavy traffic i have to believe that your aux fans will be on all the time at high speed.

suginami · #19 10-21-2002, 12:16 PM

I've never heard of a 71 degree t-stat, and in my humble opinion, can't believe it's a good idea to use one.

71 is waaaay too cold for this car. Your car is probably running awfully rich as a result, and it's probably hard on your catalytic converter, as well.

Behind the Star · #20 10-21-2002, 03:38 PM

The blue relay with the 15 amp fuse is for the low speed aux fan.
Check that fuse.
If both relays are activating high speed, look for the wires to have been attached together at the preresistor.
Brian

vince · #21 10-21-2002, 07:14 PM

The winner is -- Brian at Behind the Star. The fuse was blown on K9/1. It should have been the obvious thing to check, but I was busy trying to sort out the method of operation first, so I had not checked the fuses. Anyway, that's my excuse and I'm sticking to it. I want to thank all the others who have helped me understand this system. Normally I would pull out my manual and study the problem, but this is the modern world of M/B. I did get my hands on a Model 202 Introduction Manual. It shows K9/1 as the Stage 1 relay and K10 as the Stage 2 relay. From a chart in the Intro Manual, both relays respond to the ECT B10/8 and/or refridgerant discharge pressure (as conditioned by the A/C Pushbutton unit). This may be different that some of the other cars with the M104 engine, judging by some of the responses. What was happening when I jumped out the ECT was in effect telling the system that things were very hot (0 ohms) and thus the K10 was operating the fans in high speed (Stage 2). Now the fans are cycling on (in Stage 1) at about 95-98C and off about 90-93C. Of course, the sensor for the guage could be off a bit as Dalton suggests. The only thing I need to verify now is the Stage 2 on/off points. Thanks again to all of you.

#22 10-21-2002, 09:55 PM

You've not heard of it probably because you are not located in a tropical country. This is a common part readily available in many of the Merc parts dealer here. Anyhow FYI, my car is NOT running rich and as I've mentioned in my other posts, my temp is consistently kept between 80C to <90C in all conditions.

Quote:

Originally posted by suginami
I've never heard of a 71 degree t-stat, and in my humble opinion, can't believe it's a good idea to use one.

71 is waaaay too cold for this car. Your car is probably running awfully rich as a result, and it's probably hard on your catalytic converter, as well.

Arthur Dalton · #23 10-21-2002, 11:03 PM

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Vince,
I did some research on the C class vs the E [124/104] aux fan circuits and sure enough, there has been some modifications to the system with the Cs..
As you now have found, the differences are :
The C class circuit mod is that they now have the aux fan come on low speed via ECT sensor at 100C , with a cut-out of 95C.
The low also comes on via high side pressure a/c sw at 16 bar, cut out at 13 bar.
They also now have changed the high fan ETC to cut in at 115C, cut-out at 107.... So they have come to the conclusion that aux fan ECT cut-in should be lowered [ which is what the resistor mod on the 124 accomplished] and that the low fan should be sufficient, but in extreme heat, [ 115] , they add the high fan.

It is also noted that they also added high fan to the a/c circuit
at 20 bar cut-in, 17 bar cut-out..

So the Cs have the advantage of using 2 different speeds for both coolant temps and a/c high side pressure with 2 different cut-in/cut-outs...a welcomed modification...
Kinda makes sense now....

Arthur Dalton · #24 10-22-2002, 12:12 AM

Hi T ..
I think they just came up with a better idea and when they changed the a/c controls, it was a good time to incorporate better control on the fans using basically the same parts except for the a/c controller..
Kinda makes one think we 124/104 temp complainers were on the right track..
I always thought the a/c high side would benefit from more air
[ read high fan ] as that always seemed to be the starting point of temp rise [ a/c usage at low vehicle speed in high ambients]

As far as the 124/104 mod, I have had excellent results with a high fan cut-in just over 100C with a cut -out around 96/97
This is only shaving 5-6 degress C off spec., but doesn't interfere with normal operation as my temps never reach cut-in til the system is burdened...so minimum fan use with adequate protection.
But that is owner opionion/taste...

I think the high fan resistor mod for the single high speed may be a better set-up than the Cs low fan 100C, high fan 115, but
my guess is they have addressed the issue well and seeing that they have made improvements over the earlier system makes the mod look like not such a bad idea after all.....

oldsouth · #25 10-22-2002, 12:16 AM

Well, the only mod I have made to mine is to take out the resistor and now I only get high speed fan even when it calls for low speed. This runs it at high speed when the a/c needs it and also cools the engine coolant off more. Without the egr, I can tell the engine runs a little warmer than it did before the egr mod.

Arthur Dalton · #26 10-22-2002, 12:23 AM

If I were in a hot climate, I would not hesitate to do the same...

More air- the better...

vince · #27 10-22-2002, 06:30 AM

I think part of the reason for the change on the C class is the fans themselves. The C has two fans that cover the entire width of the radiator. (My 1999 E430 has a clever dual fan with one motor and a belt drive for the second fan). My 1988 E class had one fan in the center. I don't know if that changed later. I think the greater coverage allowed M/B to lower the fan speed and still have adequate airflow. This significantly reduces the electrical load under most circumstances. I have never seen the fans come on at Stage 2 on this car, even in the summer heat here in Virginia, when the system is working right. That is one improvement I have noticed since the C class came out. Much better management of cooling system and A/C loads on the later cars. Anyway, thanks again to everyone. Its fun to learn something AND fix a problem.

Arthur Dalton · #28 10-22-2002, 08:45 AM

The earlier 104s also have the two fan set-up, but I think the Cs fans and low speed have been modified a bit as they are different part #s than the 124/104.
Prob a little faster low speed, as they also changed the resistor part #. That would indicate a speed change..
That would account for the large difference in temp specs between the high and low cut-in of the ECT specs and why you hardley ever see a high fan.

As you say, the fan management is the biggie and one will notice that even later models have finally dumped the viscous for an electric puller...Think that was way over-due....

suginami · #29 10-23-2002, 12:37 AM

Man, this thread has been a great learning experience for me.

Thanks Arthur, Vince, Takafumi, et al., for all the great info.

periap · #30 10-20-2007, 08:49 AM

Hi everyone! Just an update - I have a 1995 C280 with only 63k miles and have been having the same coolant temp problems since I purchased the car a year ago. I went through all of the normal updates: new thermostat, flush and drain, even purchased from JimF the "cool harness" - but the real symptom never changed.

There was another post where someone mentioned that in the low speed fan circuit for his 1994 C220 - there was a relay that controled this circuit in the section "behind" the fuse box on the drivers side of the engine compartment. This relay has a "piggy-back" fuse installed at the factory as a 15A fuse. The indication was that the fans draw over 20A at times and that a 25A or 30A fuse should be placed here.

When I looked - that was my problem - a simple fuse replacement would have solved the problem.

Here are the symptoms:
1) At some temp the low speed fan circuit should turn on the fans and they should run continuously until the temp gets low enough to turn them off - or - the high speed circuit kicks in.
2) If your fans only turn on and of in high speed and typically stay on only for 20 seconds or so until extreme high temps are reached (110-120 degrees C) and you never see a slow speed fan condition. CHECK THIS FUSE
3) Most of the time the temp stays between 85-95 degrees C when operating at speeds of 30 MPH or more, but then in stop and go traffic - or low speed driving - temps can move up to over 100 degrees C and stay there until "something changes" (no clear indication of what made the temp go down). CHECK THIS FUSE