91 300CE M104, Coolant Level Light is On - PeachParts Mercedes-Benz Forum

pantherpaw · #1 06-05-2009, 03:24 PM

My 91 300CE (W124 with M104 engine) runs great, but the temperature is getting up a little higher than I like to see it when I'm in stop and go traffic. Sitting in traffic with, no A/C running, temp= 90 ~ 100 deg. C; which I assume is OK but higher than I like it to be. Sitting in traffic with A/C running, temp goes up over the 100 deg. C mark, but not all the way up to the RED MARK.

The Coolant Level Light comes on at times when hot, but goes back off again when I'm at cooler temps at cruising speeds.

I checked the level, and the coolant reservoir is full...What else could be wrong, or any advice would help greatly...

G-Benz · #2 06-05-2009, 03:46 PM

The coolant level sensor develops leaks over time that corrode the terminals and set off a "false positive" condition...in other words, the light on the dash tells you the car is low on coolant, but in fact it is not.

You can attempt to disconnect the sensor (just near the lower part of the coolant tank) and dry the contacts and clean them with contact cleaner. I've done this and it works for a couple of days, but the problem will resurface.

Replacement of the sensor is the only cure. An inexpensive part (around $40), and can be done in a few minutes. Since the tank has to be drained to do this, pick a day that you plan on flushing the coolant to perform the repair.

As far as your stop-and-go-traffic temps, it's considered normal by most on this forum, but I found that I stopped seeing the temps you describe after my headgasket replacement and top-end overhaul. Car now sits at 89C even in the severe Texas summers...90-92C with the A/C at full tilt and sitting in traffic...

pantherpaw · #3 06-05-2009, 04:02 PM

thanks, I've been doing some reading on the search forum tool and have learned alot...My engine has been well maintained over its life, but after reading your post and the others I found, I beleive my problem lies in the fans, the thermostat, or the water pump fins (worn)...I am going to check these three items and the coolant level sensor as you suggested...I will post my findings here later...

Cal Learner · #4 06-05-2009, 05:05 PM

Right David, the fans (both auxiliary and clutch fans), water pump and thermo are the places to start. Add to that, make sure there aren't leaves, twigs, etc caught in the condenser or between that and the radiator. Of course, you understand that the sensor issue is unrelated to the overheating issue, right? Just coincidence.

pantherpaw · #5 06-10-2009, 04:25 PM

Coolant level sender was replaced 20,000 miles ago in april 2007, my records indicate that the sensor has been replaced THREE TIMES in 17 years...the reservoir must be leaking like you guys suggested...I will check that next...

G-Benz · #6 06-11-2009, 11:35 AM

I will be replacing mine soon, and that will be sensor replacement number two (last replacement was four years ago). I have only replaced the coolant reservoir tank once, and that was due to a loss of copious amount of coolant showing up in pools underneath the tank, not the radiator or pump.

I've noticed that some replacement parts have shown up lately that are cheaper than OEM...but the tradeoff is that the quality is sometimes not on par with OEM, and thus, longevity is compromised.

If you aren't seeing a noticeable loss of coolant in your reservoir, then you don't have a leak, you have a bad sensor.

bsmuwk · #7 06-12-2009, 04:30 PM

If you're worried about your M104 reaching temperatures that make you uncomfortable, you could solder on a 1.1k ohm resister to your water temp. sensor. I've done this to my 92 300CE and haven't seen the temperature needle go anywhere near the 100 deg C mark ever since I've added the resistor. Instead of the aux. fans turning on at the normal 103.5 to 105 deg C range, they should turn on at around 96 deg C.

It's failsafe. If the resistor fails, everything will go back to normal operation.

Just thought this might be helpful if you're worried about your engine running super hot.

pantherpaw · #8 06-16-2009, 02:47 PM

Bravo...thats just what I was looking for; I've got a zillion resistors in my electrical toolbox. How, specifically, should it be wired? Should I cut the lead going to the sensor and wire the resistor in-line, or is it applied like a jumper across the leads by-passing the sensor?

Cal Learner · #9 06-16-2009, 04:31 PM

Quote:

Originally Posted by pantherpaw

Bravo...thats just what I was looking for; I've got a zillion resistors in my electrical toolbox. How, specifically, should it be wired? Should I cut the lead going to the sensor and wire the resistor in-line, or is it applied like a jumper across the leads by-passing the sensor?

You want to use a fixed value resistor, not thermistor, wired in parallel with the existing sensor. That will lower the overall resistance of the sensor signal and, in essence, you'll be fooling the CCU into thinking that the designed cut-in temp for fan operation has been reached before it really has been. Slick.