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Old 12-21-2001, 12:33 AM
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95 E320 cooling problem?

Hi all,

I have a problem that I've posed in several other forums and so far no one can figure it out so I thought I would give it a try with everyone here.

I have a 95 E320 with 140k miles on it. My temp. gauge runs around the 100'C mark after the car is warmed up. I think I've got it narrowed down to either a faulty gauge or a faulty temp. sensor after replacing everything in the cooling system. What I need to help further my troubleshooting is an owner with a 95 E320 that has the 96 wiring harness on it. Mercedes did that shortly after the production of the 95's started. To quickly tell if you have the 96 wiring harness, look at the 2 sensors located near the thermostat housing, one will be a 4 pole sensor and one will be a single pole sensor. All prior wiring harness' had a 2 pole sensor which controlled the aux. fans and a single pole sensor that goes to the gauge in the car.

If you have a car like this or are knowledgelable about this setup please e-mail me with the resistance of the single pole sensor when cold (mine runs around 550ohms@70'F) and when hot (56 ohms on mine, 100'C on gauge). As some of you can see these figures in no way reflect what is stated in the service manual, so my guess is either I'm being sold the wrong temp. sensor or my gauge going bad on me.

Just so everyone knows why I think the engine is not running hot, I used a laser temp. sensor on my car and my wifes 2000 C230. Both cars had very similiar temperatures at the radiator and pretty much everywhere I pointed it, except her gauge reads around 82'C and mine is reading 100'.

Thanks in advance all,


95 E320 w/140k miles
00 C230 Special Edition w/30K miles
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Old 12-21-2001, 09:13 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
The ECT(engine coolant temp) sensor has been the same for almost 30 years!!
Almost all cars use this sensor until the late 90's, even the same color wire- GREEN.
The values are:
110ohm = 60C
67ohm = 80C
38ohm = 100C
BY these figures the gauge is incorrect!!
The sending unit is telling the gauge to read around 86C the way I figure it.
Possibly could add a 10 ohm resistor to the circuit.
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Old 12-22-2001, 06:05 PM
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Thank you very much M.B. Doc. You have helped me tremendously. Today I used a digital temp. sensor layed up against the cooling fins, where the hot fluid enters the radiator, and the hottest temp. I could get was 181'F. My gauge was reading just under the 100'C mark. The reading from the temp. sensor was 48 ohms at that time too!! That does not correlate correctly to the gauge or the temp. I'm getting with the digital sensor. My guess is the sensor is a little off as well as the gauge. I bought a 10 ohm resistor today and will put in tomorrow.

Thanks for your help, your the only one that has been able to help me with this.


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Old 12-22-2001, 07:14 PM
Registered User
Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,804
I have taken readings on the same car.
I get 40 ohms @ 100C
55 ohms @ 90

Also, you want a reading at the temp sending unit location/thermo-housing . The reading at the radiator has already dissipated heat.

As you may have noticed , the temp readings in relation to the resistor changes are non- linear, so a fixed resistor value to change the reading will only accomplish this at That specific reading.
In other words, the other readings will be off as the fixed
resistor/thermistor combo effect is different at different temps.

You also want to make sure that you are taking your resistance readings with the gauge wire disconnected from the sensor.
[to eliminate incorrect R readings]
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Old 12-22-2001, 08:11 PM
Registered User
Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,804
As an after-thought.
To accurately test the gauge, simply purchase a couple of 1% tol. resistors [ R.Shack-couple of $- May have to do some combinations to get correct ohms] of the proper ohms by MB Docs temp/resistance chart and hook them up instead of the sensor . Turn the key on and see what the gauge says. That will tell all.
Guessing at temps and varying resistance values of the sensor/thermistor is hard to nail it.

*** footnote
As Dale has mentioned , there is a change as of 8/94 in the other ETCs on the E320s 124s. They have combined the FI-ECU temp unit with the aux fan one into a four prong unit.
So for those interested in the Jim F/IBenz aux fan mod., the new aux fan pins on the 4 prong will be 2&3. This is where the mod resistor will parellel. This ETC has nothing to do with the above gauge sensor circuit.
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Old 12-23-2001, 04:24 PM
Posts: n/a
Thanks Arthur for your response, I'm amazed at how much I've learned from you and M.B. Doc in just a few responses. I guess I'm not sure now what to do with this cooling problem, if it exists.

I was wondering if there is a way to get a reading from the 4 pole sensor and convert it to degrees to see if it is seeing the same temperatures as the single pole unit is? That would either confirm or deny the possibilty of a cooling problem to begin with. Or is there a way to rig up an aux. temp. sensor that will give me the exact water temp.?

Thanks again for taking the time to respond to me earlier. By the way, have you ever been to Naples, Me.? I see you live in Milford of which I've been through years ago. I been in Atlanta for over 14 years now but I grew up in Naples.

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Old 12-23-2001, 06:02 PM
Registered User
Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,804
Have not been to Maine yet. This is as far North as I get. We would be
Flat-Landers to the Maine folks...

On the E320 temps..
There is a chart for the FI ecu ETC that would give you the temp./ohms to make a comparison, but I think a known resistor in the gauge sensor circuit would tell you the acccuracy of the gauge. From there you will know if you need a sensor or not [ I would just change the sensor as they do get out of wack with age and they are only 10-12 $$$]
I would also like to say that I don't find 95/100 C running temp to be out of line on E320s. The aux fan is not called for until 107.
The thermo is 89C, but that is start to open temp. Full open is
around 96C.
If you check the archives, you will notice that many of the members have done a mod on the aux fan cut in to keep from going past 100.
I personally think the car run best at 95/100 and should cause no alarm.
I may also mention that the AF mixture of 50/50 seems to work the best for heat transfer.
If you would like the other ETC chart, email me and I will send it to you.
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