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

vince · #1 10-19-2002, 10:29 AM

1995 C280. New water pump, viscous fan clutch about 20K ago. At idle coolant temp climbs slowly to mark above 80C on guage. With the climate control on it climbs somewhat higher, over the mark above 80C. Temp is fine when car is moving. NOTE: It is doing this even at ambient temps of 60F. To me it seems like an airflow problem. Question: Does the auxiliary fan come on at elevated coolant temps, or is the viscous fan clutch totally responsible for additional air flow in this case? The aux fan does come on intermittently, but it seems to be in response to air conditioning compressor high side pressure. If there is a switch to control the aux fan, where is it and at what temp should it activate? Is there a relay? I wish M/B would publish a manual!!!

JimF · #2 10-19-2002, 10:39 AM

than you can handle!

But here's quick check list:

1) cap: see that it's tight and not leaking. Replace if not sure.

2) mixture: AF/water ratio should be no more than 50/50. I like to use 40/60 and Watter Wetter. More info my page [www house below], menu #16

3) tstat: maybe it's stuck. Replace w/ 80C version.

4) rad: partially plugged. Either have it professionally cleaned or replaced.

Good Luck!

vince · #3 10-19-2002, 12:48 PM

All those things check out OK. I would think if the radiator was plugged the temp would go up as the load increased. That is not the case here.

jsmith · #4 10-19-2002, 02:10 PM

there's another thread addressing overheating issues with about 5 pages on it now. was the radiator checked for hot / cold spots with a non-contact ir device? only sure way to find out. all of these are discussed at length in the other thread...

suginami · #5 10-19-2002, 06:40 PM

Did anyone notice that he's worried about the temp gauge going above 80 degrees? His t-stat isn't even starting to open up at 80 degrees. It starts to open at about 87 degrees, and isn't even in the full open position until 102 degrees.

People usually get scared when the gauge goes above 100 degrees, but 80 degrees would be considered very cold for this engine, and if it's running at or below 80, you're car is probably running rich as a result.

I believe you have an 87 degree thermostat, and that is the temp that your t-stat starts to open. I'm sure your car should be running between 87 and 92 degrees most of the time, and will probably run about 100 degrees at an idle in hot weather. Here is what the CD says about the cooling system and temperatures on M104 engines:

Thermostat will start opening at 85 to 89
Thermostat will fully open at 102
Viscous fan clutch will cut-in at 96 to 104
Viscous fan safety cut-off will ocurr at engine rpm 4500
The same will ocurr at fan rpm 3250
Electronic fan (1st stage) will cut in at 16 bars of refrigerant pressure)
Electronic fan (1st stage) will cut out at 11 bars
Electronic fan (2nd stage) will cut-in at 107
Electronic fan (2nd stage) will cut-out at 100
A/C will go into 50%on, 50%off cycle (20 seconds interval) at coolant temp 121 to 123
A/C will go into emergency shut-off mode at 126 to 128
Pressure cap will open at bar 1.3 to 1.5
Coolant temp max is 130

Remember that water boils at 100 degrees celsius, but with a 50 / 50 mixture of water to coolant, your boiling point is raised to 120 degrees, and the car isn't considered to be overheating until the temp exceeds 130 degrees.

MB designed these cars to run hot, and 100 degrees is not even considered close to being hot.

Now, many of us on this forum (including me) doubt the wisdom of the brilliant German engineers for these cars, and carry out modifications to prevent the car from going much over 100 degrees.

I have installed the "cool harness" that JimF sells, and it causes the high speed auxillary fan to engage at 100 degrees, and runs until the temp reaches 82 degrees.

But I think something is wrong if your car runs below 80 degrees. Maybe your t-stat has been stuck in the open position and you aren't aware of it.

jsmith · #6 10-19-2002, 06:43 PM

i think he meant the mark over 80 degrees which is 100C...

suginami · #7 10-19-2002, 06:43 PM

After writing my brilliant and well thought out post, I noticed that he said the mark above 80 degrees.

Since the marks on the gauge are at 20 degree increments, this means that his car is running at or above 100 degrees.

Ignore my comments about running cold at 80 degrees, but everything else applies.

:p

#8 10-19-2002, 09:38 PM

IMO, this is a problem with the radiator having insufficient cooling at idle/low-speed driving. Most likely culprit would be the aux fan. Your aux fan high speed mode is activated somewhat late, though this is the design spec as mentioned in any earlier post. You might like to check all associated components for the aux fan. My aux fan high speed mode engages at 85C instead to prevent overheating during extended idle periods.

jsmith · #9 10-19-2002, 09:55 PM

Quote:

Originally posted by Dinky
.... My aux fan high speed mode engages at 85C instead to prevent overheating during extended idle periods.

your aux fan must be running all the time then. what temp does it cut out at?

#10 10-20-2002, 01:41 AM

It cuts out at 80C. Keeps the coolant temp constant between 80C to <90C.

jsmith · #11 10-20-2002, 11:48 AM

Quote:

Originally posted by Dinky
It cuts out at 80C. Keeps the coolant temp constant between 80C to <90C.

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 · #12 10-20-2002, 01:40 PM

Maybe when he refers to his auxilary fan coming on, he's referring to his low speed fan.

If his car never goes above 90 degrees, the high speed fan is never coming on.

Something seems off to me. 90 degrees is not even close to being hot for this car. It's barely warm. By looking at the cut-in and cut-out temps of the t-stats, viscous fan, low and high speed fans, these cars were made to run between 90 and 100, and even a little higher. It your car is never going over 90, it's not running at the temerature it was designed to run at.

By the way, the high speed and low speed fans are the same fan. There are not two separate fans. There are two separate electrical circuits going to the same fan, and it is simply different resistance that causes the fan to come on at the low speed or high speed.

vince · #13 10-20-2002, 06:51 PM

I have been tracing this car and trying to incorporate the info you folks have provided. I never said the car should run at 80C or below. The car runs around 90C on the road. If we are to believe the mark above the 80C mark is 100C (the guage does not seem to be linear - no Mercedes ever has been in my experience), then my car is climbing to 105 -110 without the auxiliary fan coming on (at any fan speed). I have no doubt that it will continue to climb in excess of 120C if allowed enough time and high ambient temps. Bringing on the auxiliary fan (manually) brings the temp down fast. I have figured out that all the temp switches are on the thermostat housing. One seems to be for the ECM. One is for the guage. The two pole switch seems to be B10/8 for the auxiliary fan. With the engine cold the switch reads 3,333 ohms. At 85-90C it reads 525 ohms. At 90-95C it reads 415 ohms. At 100-105 it reads 331 ohms. There are two relays (K9/1 & K10) in the back of the fuse box that respond and start the fan when I jump the leads at the B10/8 connector. BUT, they do not seem to be "smart" relays that can recognize the difference in resistance at the switch. I think the sensor B10/8 may feed it's signal the the A/C pushbutton unit, and from there the signal goes to the appropriate relay. So far, I can only get the fan to run in high speed. I really need info on the correct readings at the switch to determine if it is at fault, or further troubleshooting help if the switch isn't the problem.

JimF · #14 10-20-2002, 07:47 PM

it should be about 310 ohm at 100 -105C, so yours seems to be reading high, thus the fans would come on til about 120C, much too late.

Check my page, menu #17 for comparison resistor values of B10/8.

suginami · #15 10-20-2002, 08:06 PM

Vince,

JimF sales a "cool harness" that'll trick your high speed fan into coming on at 98 degrees, preventing the car from climbing up into the 100 range.

It's worth a look...