95 Volvo 940 Temperature Gauge won't read cooling fan off

[larrybum]

I did a head gasket replacement and found the car with poor acceleration. The temp gauge was reading zero, so I replaced the coolant sensor. Resistance on the old sensor read normal, so I suspect this was not the problem. Found a strange vacuum disconnected and this seemed to perk up acceleration, but gauge is still at zero and the cooling fan won't turn on.

My problem is that the temperature gauge sits at zero and the cooling fan on the radiator will not come on. I don't know if the fan has a relay or not, but it seems to me the problem is the device that controls both of these items.

Looking at diagrams, the coolant sensor goes to both the ignition and the engine control modules. Fuses seem ok. Which of these two modules is the most likely culprit? This 940 is a non-turbo version.

Any other ideas?

[MadeInJapan]

I don't know that particular car but what comes to mind is the thermostat...did you replace it with the temp sensor?

[larrybum]

Well, since the temp gauge was reading zero and the sensor is on the hot side of the cooling water, the gauge should be showing hot. I agree that the cooling fan would not be coming on probably if the thermostat was blocking flow to the radiator. The cooling water is HOT and under pressure. I'll check to see if the radiator itself is getting hot or if it remains cold, indicating a blocked thermostat. What I'm suspecting is that whatever sends the signal to turn on both the cooling fan and operate the gauge is at fault. I'll look into the termostat question.

[larrybum]

The thermostat is not the problem. The radiator is hot, the engine overheats after a long time at idle due to the cooling fan not running. I found the fan relay very near the battery and jumpered the 12v to each of the red and green leaqds going to the fan. The fan runs on both speeds. So, it apppears that the control signal to turn it on is missing. Also, the temp gauge is reading zero. The coolant sensor has been replaced and the old one I removed shows a good resistance value, so that was never the issue.

I need a recommendation as to which computer module to replace given this situation. The ignition module or the engine control module?

[billofdurham]

It is possible that I am coming in here a little late, but I have been away for a few days.

What resistance did you get on the temp sensor and at what temp?

Have you checked for fault codes, particularly 1-2-3 on the fuel injection? This is Coolant Temp Signal. If the code appears, you could have a wiring fault.

Bill.

[larrybum]

Sorry for the long delay also in responding. I'm getting no codes from anything on this car...neither A1 nor A2 testing gives me a code. The check engine light is on.

I ordered and put in a used fuel control module and, the first time I started the car, the gauge read and the cooling fans came on. We then attached the module to the side of the car (had to unplug and replug) and then, when we turned it back on, the temp gauge was at zero and the cooling fan would not com back on. - We found one of the braided ground wires at the back of the engine was not attached. There was a second braided wire right beside it, but I remember removing only one to do the head gasket repair. Not sure where it attaches but it is in the same area as the first one (firewall to the engine head).

I'm suspicious that somehow the second computer module blew due to some wiring problem.

I found some detailed wiring diagrams for the 940 and it seems to show TWO temp sensors. One going to the gauge and one going to the engine control module. The ECM one is labelled 7/16. THe one for the gauge is 7/7. This is really puzzling since I saw no listing for anything but one coolant sensor for this engine (not turbo).

I'm still looking into how to interpret all this info. I'm pretty good at reading diagrams, but I'm not sure completely how to interpret what I'm seeing. Having no temp and no coolant seems to point to a sensor problem since that should control both. The resistance on the old sensor was about 3K at room temp. The new one read about the same. ( I read term 1 to term 2, not to ground).

Still chasing. The car is still parked until we get this resolved somehow. Haven't yet given up, but appreciate any counsel you can provide on this. Next thing I'm doing is going over the connectors with a fine toothed comb and reconnecting the ground to SOMETHING back there.

[billofdurham]

I'm getting no codes from anything on this car...neither A1 nor A2 testing gives me a code. The check engine light is on.

Are you not even getting 1-1-1? If the CEL is on and you are getting no codes there is a problem with your diagnostic box. You can find a trouble shooting section in: https://www.matthewsvolvosite.com/chat/p ... c&start=12

Scroll down to pages 5/6.

As far as I am aware there should only be one braided earth (ground) wire from the engine to the frame. Perhaps the other is one that was replaced at some time. There would be no harm done connecting both.

If you look carefully at your wiring diagrams you will see three temp sensors. One is the ambient temp sensor for the ECC. One is the ambient temp sensor for the gauge, not the engine temp gauge but the gauge showing the outside temp. The third is the engine coolant temp sensor which does feed into the ECM. The third is the important one in your case.

The temp coolant sensor resistance readings are done over the two terminals. Depending on how warm your room was, the readings would appear to be correct. In fact, at 68F (20C) the readings should be 2280 - 2720 ohms.

Whilst you are checking your connectors pay special attention to the earth (ground) connections on the intake manifold. If these are poor they can cause all sorts of problems including a malfunction of the self diagnostic unit.

Bill.

[larrybum]

Bill,

Let me clarify a typo: The check engine light is NOT on. Both A1 and A2 show 1-1-1 codes.

From a post I read on the birckboard last night, there are two engine temp sensors on a 95 940 w/o turbo: I confirmed this by scouring over my wiring diagrams. Both sensors are under the intake manifold screwed into the head. From the front of the engine, the first temp sensor goes directly to the instrument panel temp gauge for the engine. The next one has one lead fed to the ECU and the other lead fed to the Ignition computer.

I'm studying these wiring diagrams closely and can finally confirm most of this. I was surprised that there were two sensors for engine temps. So this failure is really surprising since I expected to find one source of both maladies. Since the ECU coolant sensor has nothing to do with the temp gauge, then that has to be the other one that I have so far ignored causing the gauge reading problem.

So, with the info I have in hand, I'm going to see if I can make progress on this. With no code showing from either the original nor the replacement ECU computer, I think the new coolant sensor I installed is working fine. I'm going to check in detail the wiring and grounds. No doubt the ECU has nothing to do with being at fault.

If I get this solved, I'll post the results.

[larrybum]

Well....at last. Here's the secret to my confusion:

This problem all began with an overheat due to a blown head gasket. The trip the cylinder head took to the machine shop to be milled resulted in damage to one of the temperature sensors still screwed into the head.

When the temp gauge would not register, I assumed a failed temp sensor, but thought there was only one...the coolant temp sensor. Actually, the coolant temp sensor was fine. The gauge sensor had been damaged.

I replaced the coolant temp sensor (furthest one from the front of the engine head) and measured the resistance. The replacement was about 3k. The one I took out was about 3k.

At this point, the car would overheat at idle and the cooling fans were not coming on.

Well, the only thing left in my mind was the ECU which actually turned on the fans via a ground to a relay (I had verified the relay/fans/etc would work if given the right signal)

I replaced the ECU with a used one from ebay to no effect...the car was still overheating and the fans weren't coming on.

I did some more investigation and found that the NEW coolant sensor was not showing about four times the resistance it should have. The two legs of the sensor should each have shown 1.5K to ground. One side went to the ECU; the other to the ignition module. One side now showed 1.5k while the other showed 5k. Somehow, its behavior had changed since I had installed it. There was no damage I could tell to the sensor physically, but it was bad. That gave no code since the resistance was within range still but it made the ECU think the engine temp was about 0 F which made the engine run richer and apparently caused the car to overheat at idle (in addition to not turning on the fans).

I switched back to the original sensor (still had it). Now, I could not get the car to overheat at idle nor would the fans come on at idle. I had to get it in traffic and put some stress on it, but the cooling fans came on at that point. The engine never overheated.

So, the ultimate happened to me: I replaced a working sensor with one which quickly failed and gave me another symptom. It drove me right up the wall since I'd then marked the sensor off my list of causes of the problem even though a new problem had been introduced.

Thanks for all the assistance on this forum. It helped me at least find some resources to help figure this out. I'm in the process of reselling the ECU.

[billofdurham]

Thank you for the update. It may prove helpful to others in the future.

Hope the car will now behave as any well bred Volvo should.

Bill.