Friends,

I have a cooling fan issue with my 1987 951. The cooling fans will come on after 10 or 15 minutes of driving, even when driving on a highway at a steady 60 mph. Once the fan comes on, it will run at the upper maximum speed until the radiator is almost cooled down to the ambient temperature. It will run, even though the temperature gauge is just a millimeter above the first (lowest) white line. The relay in the fuse/relay box is scalding hot while the fan runs. I've replaced the temperature sender in the radiator.

Driving today, I was driving at about 60 mph and could tell that the fans were running (the voltmeter drops when the fans come on). I pulled over and I could put my hand on the top of the radiator it was that cool. Despite the cool radiator, the fans were running at their highest speed. The air coming off of them was also cool. I pulled the relay (again, it was almost too hot for me to grab it with my delicate hands) and the fans stopped. I drove the rest of the way home at 60 mph without the car running hotter than it had with the fans running.

All of this sounds like the temp sender in the radiator is coming on too early (too low a temp) then staying on for too long. I tested the one that was in the car when I bought it, found that it tested fine, but replaced it anyway. The new one displays the same behavior.

Is the relay "sticking" on? From what I can see there's no reason for the fans to come on in the first place. The relay has a date stamp on it for July 3rd, 1991.

Has anyone else had this problem?

Perry

 

You don't have a low temp sending unit do you? Or maybe it's not matched with the thermostat. Next time it happens check if the temp switch is turning on. If not then maybe the relay is turning on by itself. Definitely not normal for the fans to run at highway speed.

 

Stooge Moe,

I've thought the same thing; that the fan stayed on because the thermostat was closed. Both the thermostat and the radiator sensor are the stock temps, so the system should work.

The strange thing is that both fans were running at their fast rate even though the radiator was cool. After I finish driving, the fan stays running for a looooong time. I've stuck my hand down in between the radiator and the engine and the air that the fans are pulling past the radiator is barely above ambient. It sounds like the fan sensor is bad but this is the second one in since my ownership.

I'm assuming that the fan sensor in the radiator sends a low current signal to the relay that then switches on the high power circuit to send power to the fans. Can I test for that signal at the fuse/relay panel? Is the relay supposed to run that hot or does that indicate that it's defective?

Thanks!

Perry

 

 

MAGK944,

Thank you. As a manifestation of my laziness, I am finding it very difficult to access the connector for the radiator temp sensor. Can I bridge the connections in the relay panel? The wiring diagram shows that the TS terminal triggers the low speed circuit and the TF triggers the high speed circuit. I should be able to test for continuity to ground at these terminals to see if the respective circuits are being grounded as the temp increases. If they are not triggering the relay but the relay is still turning on the fans, that must mean that the relay is bad.

The wiring diagram also show that the relay has a diode. Could that be bad allowing "feedback" from some other circuit to energize the fans?

Perry

 

 

No, the diode could not cause that. It is just there to prevent voltage spikes when the magnetic field collapses in the relay coil.

Agree with MAGK944 about checking for permanent ground on the TF pin. I'm having a tough time thinking how the relay could get hot. Even if you energize the relay coil for hours, it doesn't draw that much current. There should be little resistance in the relay circuit to the fan itself. How hot does it get? Like too hot to even touch? Or just warm? I'm afraid your relay is going to end up being bad, which blows, because it is the most expensive relay in the car.

 

Perry, following this thread closely and hope you'll report your findings.

I'm also having a fan problem. Mine is that the fans come on as soon as I turn the key to the ACC on position and then cycle between high and low speed endlessly every 10 or so seconds. This happens with a cold engine. As soon as I turn the key, on go the fans. They don't come on high, they cycle between high and low.

It seems like it may be a related problem, slightly different symptoms, so I'll look forward to hearing your solution.

Regards,

 

Moe, I'll just guess there's no way to take the relay apart and repair it? I only ask because I just finished taking the electric window switches apart and repairing those with the help of PaulD.

 

 

Hmm. You may be right about that. My ex-wife is good with analog (she did power supplies for HP back in the day), but we don't talk much anymore.

 

What is that a picture of? There aren't any transistors or ICs in there. At least not in the one I took apart from my car. There are only four relays (and diodes).

Pretty easy to take apart. Just bend the metal can carefully all the way around and lift off. You're not likely to repair anything in there. The best you could do is clean the contacts which are probably badly burnt. They take the full current to the fans when they close. After what, thirty some years now, they're likely at the end of their life anyway.

 

So, let me get this; are you suggesting that maybe shooting them down with De-Oxit and hitting a few burnt looking contact points, maybe re-soldering the coils, might breath some new life into them?

Because I can do that. Replacing the inductors would be a bit harder, but I can see that too as long as I have the specs for them.

 

 

unplug the high pressure switch at the AC receiver and see if the fans cycle off.