1979 240D temperature issue

Happy Holidays everyone, Here's a real message about a Mercedes as opposed to more offers for Prada, Rolexes and whatever!
My old 240 has developed an issue where its temperature goes up much
higher than it used to -- not overheating, but up to about 210 degrees Fahrenheit or so at times, though sometimes it will go back to normal at about 180. The system holds pressure just fine (no leaks) and the coolant level is full. I tried replacing the thermostat but there was no change.
I did discover the electric cooling fan is not working, though I don't think it ever has (I ran 12V directly to it and no go so I assume the motor is bad). Also, this happens even in cool weather at constant speed so I doubt it would be caused by the fan not coming on.
Any thoughts on what to try next or suspicions as to what might be wrong?
Also, anybody know where I can get just a motor for the cooling fan?
Thanks! Steve
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Il Tue, 14 Dec 2010 18:42:21 -0800, runbiodiesel ha scritto:

Merry Xmas!

What do you mean with thermostat? The temperature sensor or the automatic valve that opens coolant flow once trespassed a temperature limit? Usually I'd start from one of the two above. You could eventually disassemble the valve and "boil it" on the kitchen with some water, to see if it opens correctly, eventually checking with a thermometer the temperature threshold. It's usually in 70-80° C. Let me say 165-170° F.
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Hi!

Stefano
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It's pretty clear that by thermostat he means the one in the coolant flow path that every car has between radiator and engine. He said he already replaced it and it hasn't fixed the problem.

Two? What two? I have yet to see a car with a seperate temperature sensor and valve that controls coolant flow between the radiator and engine. Might exist somewhere in a new car, but in his car, there is only one thermostat and it's a self-opening valve that responds to temperature.

I hope by "disassemble" you really mean remove.

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Il Thu, 16 Dec 2010 06:26:16 -0800, snipped-for-privacy@optonline.net ha scritto:

Of course.
Maybe you're overlooking I'm Italian. It's not very easy for me to find the correct words in a different language, in a semantic space that I only use here (mechanical English, I'd say mech-american English for the vast majority of subscribers). So, pardon me if I'm naive in my choice of words and grammar. I'm just trying to help. My suggestion was on thermostatic valve, to be REMOVED (ok...) and tested in a pot of boiling water, visually inspecting if it opens totally. I can remember here a discussion a couple of years ago on two different defective valves.
And then to carefully check if the instrument and the sensor of the instrument work accurately.
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Ciao!

Stefano
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Thanks for the input trader4 and Stefano. Yes, indeed I meant I had replaced the mechanical thermostat that regulates coolant flow. I did test the old one after replacement and it wors fine. Oh well, it's a cheap fix.
Thanks for the advice abou the radiator. That makes sense. I found a brand new radiator on e-bay for $99 and it just arrived today. When the weather clears up a bit and I'm not contending with rain and mud I'll try the swap and see if that does it. It makes sense. I'll repost when I do and let you know how it goes.
While I'm at it, I'll see if I can find a cooling fan and replace that too.

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Ahye Karumba! $99? Amazing it's even available for a car that old. Boy did you get lucky!

You should also verify that the circuit that energizes it works. On my 300SD there are two temp sensors, one in the engine coolant, other at the receiver/ dryer for the AC. Either can turn it on via a relay.

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I had similar issue with my old 380SE.
I changed the thermostat... no change Changed the temperature semsor... no change. Changed the radiator... no change. Changed the fan clutch to newer type... no change.
Lastly, I had an issue with my odometer so I stuck in a different cluster and to my shock... the temperature gauge registered normal... The conclusion is that the temperature gauge does go bad that the reading is off... this is probably due to some 'extra' resistance... in the circuit... wire corrosion... etc. or the resistor on the termperature gauge going bad that it reads higher temperature.
In any case, make sure all your hoses are new... after all the car is over 20 years old by now... so it is time to change all the heater hoses while you are at it so it will last another 20 years... lol.
Happy holiday.
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Thanks for the suggestion, Tiger. I can't believe I didn't think of that since I had something similar happen on my sailboat which has an old Perkins diesel. I went through all kinds of repairs before finding out I had a bad gauge head (I had replaced the sender which made me think the thing was reading correctly). Anyway, to finally test the gauge theory I got an infrared temperature sensor that you just point at a surface and click to get temp readings. Turns out when the gauge was showing in the red the cylinder head temp held consistently at about 180, just right.
I'll try the same thing on the car and see if I can rule that out. Thanks again for the suggestion!

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Yes, infrared thermometer is the best thing in this case... You should aim at where the sensor is... and that is what the gauge should be reading. 180 degree is perfectly normal.
I remembered helping out another guy with 380SEL long ago... he did everything like I did... replace this and that... but he had a machanic suggestiing him to change this and that... costing him a fortune... I told him to stop and change the cluster I had... and no more after that... temperature is normal.
Make sure you aux fan is working... it is extremely important when running AC... it would mean whether you really got AC or not.
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