Coolant temp running up..........

[no-blue-screen]

Okay,

This is rather complicated scenario, but I don't want to completely overhaul the cooling system as a fix....so here is what is going on:

1982 300D Turbo with 158k on the clock

Problem: The coolant temp is running up higher than it does normally. It hangs around the 3rd hash mark (inbetween the middle mark and the last mark which is labeled 120). It only does this with the A/C on and it only seems to happen at highway speeds.

I am thinking the fan clutch is bad, but I am not sure. I went on a 1 hour drive yesterday afternoon and ran the A/C on max the entire way and I noticed it would hover around that third hash mark and sometimes fall slightly below and then rise again and then fall again. It is my understanding that normal operating temp is 90C, and that operating above this could cause damage to the engine.

Here is what I have inpected so far:

1) The aux fan on the condenser is operating properly

2) The coolant level is normal and there are no coolant leaks.

3) There seems to be a slight wobble in the pulley that the fan clutch is attached to, but again there is no coolant leaking and no wierd sounds that would indicate a failing water pump.

What I am thinking is that the fan clutch is bad, and this is preventing the fan from spinning at full speed. I have the old 6-blade fan on this car and I have already purchased the newer 9-blade fan and a new behr fan clutch.

Given these symptoms and that it only happens with the A/C on and driving on the highway, what do you experts have to say?

[plurkje]

Check the visco fan with the engine running, open the hood and listen, around 2500RPM you should hear the fan engage. If not you'll need a new one (expensive!).
About the temperature; I think it's pretty normal although it should not climb up when driving uphill.

[00CLK208]

My son 95 C280 had the same high temp gage (when the A/C is on) it's on upper half all the time, the mechanic replaced the A/c fan clutch take care of it. Your diagnostic is good bet (before replaced the a/c fan clutch, I can hear & see the fan kick in), the a/c ran a lot quieter after the clutch is replaced. I can hear the different when I pick up the car.

[no-blue-screen]

Thanks for the replies. I have also been googling this subject and it appears the fan clutch is the problem. Thanks again.

[jmas2k]

My '81 300D ran hot at highway speeds also. Diesels generate more heat when pulling loads at faster speeds using more power. The large air flow at around 65MPH usually eliminates the fan & clutch. I found the old radiator had lost its efficiency due to mineral build up internally. Put in a new Radiator (about $200) and Temp ran normally afterwards.......

[no-blue-screen]

Well, in my situation it is only happening when the vehicle is operated a highway speeds and then it quickly cools down once I pull off the highway and start driving under say 40-45 mph. If it were the radiator, I don't think it would only be a problem at highway speed but it would rather be a constant problem. Anyone else wanna chime in?

[Boardmonger]

Well replace the thermometer first, that is probably the culprit. With the new fan and clutch and thermomerter the temps ought to stay lower.

My 83 turbo runs between 80-95 and just over 100 on a long hard grade while my euros stick to 90 unless I am going up a big hill or something.

[no-blue-screen]

That is a good idea. That third hash mark, is that 100? I am assuming it is because the last marked hash is 80 and the third hash mark is between the 80 and the last hash which is 120.

[Boardmonger]

I've assumed it is about 100 or so.

First is 40, then about 55 or so, followed by 80, then about 100 and finishing at 120.

[no-blue-screen]

Thanks again to all who replied. Will probably work on it this weekend and let you all know the results.