Cooling fan won't turn on - wiring issue??

tomellis · Post by **tomellis** » 17 Dec 2008, 18:24

The cooling fan in my '94 850t is not turning on, either when engine is warm or when AC is turned on. I have already checked to make sure the fan motor is good by running power directly to the motor; it spools right up and blows hard. I thought the problem was the cooling fan relay, but after replacing with a new unit still no fan. I then checked the wiring coming into the relay. The power lead from the battery is hot. The relay is actuated when a ground circuit is completed through either of two wires through the middle plug. I ran a ground wire to each of the posts of the middle plug on the relay, and the fan worked perfectly (meaning the relay is also working correctly - should have checked that before putting in another unit). I may be wrong about this, but as I understand this system when the AC is on the fan should always be running; at least that has been my experience with this car in the past, and with other cars I have owned. But I may be wrong about this and will happily accept any corrections!

So it appears that there is an issue with the wiring to the center plug of the relay, with a ground circuit not being completed. The two sources that (I think) should activate the fan are the temperature switch and the AC. But at this point Haynes stops being of any use, since the diagrams there give me little or no information about where to look next. Since I make no claims of being an electrical expert it may be that I just don't understand how to read the diagrams!

Any ideas of where I need to look to trace the problem backwards? Since the fan is not activating by either the cooling switch near the thermostat, or via the AC circuit, it would seem there may be a common point that would be the logical place to look for a problem. Is there a fuse hidden somewhere that I should check? Another relay that may have failed?

If I can't find the actual problem I will try a short-term fix of wiring a lead to the fan from a circuit that is only hot when the car is running. At least that way I should be able to drive the car without worrying about overheating. But I'd much prefer to do the repair correctly, so please tell me what I need to do next.

Thanks in advance for any help!!

Japedo · Post by **Japedo** » 17 Dec 2008, 18:45

could be bad coolant temp sensor, or maybe a bad connection at the temp sensor. It's located under the thermostat housing on the passenger side of the engine.
I had the same problem, and that was the culprit.
Good luck.

Japedo
94 850 turbo
01 V70 XC

Post by **Ozark Lee** » 17 Dec 2008, 18:52

You have done some very good diagnostic work. At the end of the day the ECU controls the engine cooling fan. Since you have grounded the control wires I think you have verified that the relay and the fan are A-OK. I think my next step would be to yank the ECU and clean all of the edge connectors with contact cleaner and try to Ohm out the wires between the ECU socket and the cooling fan relay to verify continuity.

Schematics can be had here - it is a Polish site and it is very slow:

http://volvo.techno.org.pl/

Click on the Schematy link and it shows stuff in English. This version of the schematics is very cryptic but very precise - you need to run back and forth to the cheat sheet at the end of the file to figure out what component you are looking at. The great part of these schematics is that it shows very precise junction points and branching points and where they are located.

If everything checks out OK for continuity then you are looking at a new ECU. I see them by the dozen on eBay for cheap.

Good luck with it - let us know what you find.

...Lee

tomellis · Post by **tomellis** » 18 Dec 2008, 20:25

Had to put off the car work today; I woke up to find 4 inches of snow on the ground (and car). Not a usual occurrence in Seattle. Up to 8 inches now, but tomorrow is supposed to be clear. So once I dig the car out I'll pull the ECU and see what I find. I'll post again once I have something to report.

I did check out eBay and located several ECUs, so worst case I'll buy one there. And I located an actual service manual for the car through a link on this site, so I now have a good wiring diagram.

Thanks for the help!

Sidewinder · Post by **Sidewinder** » 19 Dec 2008, 10:17

Test or replace the temp sensor. If that does not solve the problem then you may have a problem like I had. My fan was doing the same thing. I found the ground wire in the factory harness was bad. I cut the ground a few inches from the plug and ran a new heavy gauge ground wire to a good chassis ground. I have not had any problems with my fan since. It was a very hard problem to diagnose as everything I tested worked fine by them selves but put them all together and nothing worked.

Arnold · Post by **Arnold** » 30 Dec 2008, 21:29

I am having the same problem and like Sidewinder I have tested everything and it all works by itself. I will try to track down this wiring harness ground. Where exactly is it located?

tomellis · Post by **tomellis** » 05 Jan 2009, 17:57

Finally was able to spend some time on the car today. I ordered a new temp sender, figuring even if it didn't fix the problem it was a cheap spare part. As soon as I disconnected the old sender from the harness, the fan motor turned on. My guess is that this means that the temp sender switch is usually in the closed position for "off" and opens when the engine is warm, turning the fan on. Why this would keep the fan from operating when the AC is running is still a mystery.

But the important thing is that when I replaced the temp sender my fan began working correctly. I let the engine warm up, and (for the first time in quite awhile) the fan turned on as expected.

Thank you to all of you who offered suggestions. If anyone knows more about the wiring of this car, and why the temp sender would also control the fan with the AC I'd love to hear the explanation.

Post by **MadeInJapan** » 05 Jan 2009, 20:02

It's a preventative measure...A/C will overheat your car without the fan running.

tomellis · Post by **tomellis** » 05 Jan 2009, 22:02

I understand why the fan should run while the AC is on; what I don't understand is why the fan circuit is apparently completely dependent on the temp sender. It would make more sense to me to have the AC fan control completely separate from the temp sender, so that even if the sender failed (as in my case) the fan would still run when the AC was activated, keeping the car from cooking itself. I imagine the car's designers had their reasons, I'm just curious about what those reasons were.

Post by **MadeInJapan** » 05 Jan 2009, 22:24

Gotcha...good question and idonno