Test the sensor w/ohm meter.
Here is a temp/ohm chart for calibration

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

Most likely suspect is STICKING OPEN thermo.. don't replace w/whaler..

Peter's advise is what I'd use. If you follow his logic, the air is going to be pushed out because all escape routes will be shut off.

M.B.DOC once posted a technique used on some earlier 103 motors. The front end of the car was elevated to get the air out. The expansion tank cap was of course removed.

My experience with 103 motors has been that trapped air causes overheating. If coolant can get to the t-stat, it won't open and allow coolant to flow. Coolant then starts boiling in the engine block.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Hi Arthur, I just looked, the temp sending unit has only one pole? How do I meaure ohm on that one pole. I was thinking about dip it to hot water to observe the changing ohm.

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The ohms reading is taken from the one pole to the case [ ground]
You can do it several ways, but the Easiest is to get the engine close to one of the specs on the chart and pull the wire and take an ohm value to see if it relates to the chart..
The Best way it is to get a 1% tolerance resistor [ RShack} of the proper value
on the chart and just hook the wire that goes to the sender to it and ground the other end of rsistor to the engine ground. I have one of these in my tool box, just to save time and you do not have to run the engine to do the test.. you are simply subbing a known chart value and the gauge has to respond to that value . the advantage to this test is you now testing BOTH the gauge and the sensor.....
Your water boil will work too, but then you have to remove it and then monitor the water in the pan , bla, bla..
this is why the laser temp units are so popular..............

Great Idea Arthur. Let me rephase it so I am clear. I will need a 67 ohm resister and I connect one end to the cable and the other to any metal part on the car. All I need is to turn the key half way to activate all gauges w/o running the car. If the gauge shows 80C, my sending unit is bad. If the gause still shows 60C, my gauge is shot.

BTW, I already got the T-stat out and it is at fully closed status. Boiling water test showed normal open and close for both stages.

Man, after doing all these work on this piece of *&^%$#@$%, I will be a decent mechanic. Probably make more money that way.

Correct ..

You can also get a trimming resistor and set it to each chart resistance value [ [as measured w/ohm meter] and check the full range of the units, but that is over-kill...

Are you sure?

I have a similar problem with my M103: warms quickly to 60C (mark between 40 and 80), runs like this practically all the time, only in stop & go traffic raises to 80C.

I supposed the gauge did not show correct temp - if I open the heat, it will blow hot air after 5-10 minutes of running. If coolant was cold, this should be impossible...

When I rotated the fan by hand, with engine cold and stopped, it rotated "somehow sticky", i.e. not fully free, but neither rock-hard as if clutch was fully engaged.

My coolant was highly diluted during summer - put in distilled water.

Any ideas?

~Nautilus

__________________
1990 260E Sportline (that's 300E 2.6 for our American friends) -> sold
2001 E320 4Matic Elegance -> my Dad's daily drive
2005 Seat Leon FR 1.8T

[QUOTE=Nautilus]Are you sure?

1987 260E Overheating

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Put in a new Behr T-stat and flushed the system with distilled water. Drove the car out to get some beer and could not get the gauge to go over 60. Everything is exactly the same as before changing the T-stat. The good news is that I flushed out the "green" coolant but the bad news is that car remains problematic after $$ spent.

Between the tip of that single pole sensor and its base, I got 245 ohm. Should I just change the sensor?

In another forum, someone showed me a four pin senor for coolant temperature. Is there more than one sensor for engine temperature?

Look at what Benzmac had to say in this thread:


1986 300E coolant temp. sensor!

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1A70KFNW8&year=1991&make=MB&model=560-SEL-001&category=P&part=Water+Temperature+Sender

If you got 245 ohms , you are on the aux. fan switch, not the gauge sensor.

This is the fan sw., the other I listed is the gauge sensor

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1A70KFNW8&year=1991&make=MB&model=560-SEL-001&category=G&part=Auxiliary+Fan+Switch

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Yes
You will know if you have the gauge sensor cuz the gauge will stop working when you have it unplugged...
....do this when warm so you know there is some gauge reading

hello Arthur, I am pretty sure I got the 245 ohm from the gauge sending sensor. It looks exactly like the P4036-12281. When gauge was reading 60 C, I unplug the cable from this unit and the gauge goes directly to 0. I believe I got another two poles sensor which should be the fan switch. Now I need to find the sensor for EZL and CIS-E.

Please help.

OK

If the gauge went to zero, that is it and you have a bad sensor...[ providing you got the reading from THAT sensor and not the aux fan 2 pole]
get a new one ..I see they are only $15.

The other sensor [ 2 wire] is your aux fan switch.. Fast Lane show this to also be a single wire one, but they are mistaken.. I looked up your car on the fan schematic and it takes the 2 wire thermistor one like all the '91s do...


The cis/ezl sensor is the 4 prong one .. nothing to do with the temp gauge...