My A/C has always been bad, and I have ignored it since last summer. Now the temps are high again, around 28C at noon. My car is dark gray, so the heat is horrible, it feels like a sauna (but without two Swedish blonds).

I have noticed that the cooling fan does not turn on automatically when the A/C is on, so I guess there is something wrong either with the ECT sensor or the fan relay. The temp indicator on the instrument cluster seems to be working ok, so I want to test the fan relay which is known to go bad.

The wiring diagrams show that a hot 12V line (power) goes into the relay, and that two signal lines (yellow and white/yellow) come from the ECU. I suppose the fan has two speeds, so each line is for a different speed. Two lines leave the relay and go to the fan (red and green), and the fan has its own ground line (black).

What is the correct way to test this? Just use a digital voltmeter and check yellow and white/yellow for 12V? Once that is ok then check red and green for 12V? What are the expected voltages?

Should I also check the fan ground impedance to make sure it is ok? Maybe remove the ground wire, redo the connection and solder it? How many ohms regarding chassis should I expect?

Thanks
Axel

 

The way I always test it is to probe one for the small wires in the 2 wire plug in the middle I believe it is with a test light. That grounds the circuit and makes teh relay and fan work.
Also a bad High pressure switch would make the fan not work with the A/C on.

 

Tech,

Would a bad high pressure switch make the compressor cycle every 10 seconds?

I reshimmed the clutch last year using your instructions. I also changed the low pressure switch, and the compressor relay behind the glove compartment is ok.

I don´t think the evaporator is leaking since I get cold air when the compressor is engaged. The R134a was charged last January and it is still holding.

I very seldom hear the engine fan working, even in summer when I am at a stop light.

 

Yes if the fan portion didn't work the High pressure will get to high and the switch will turn it off till the pressure drops back down.

 

Thanks for your help. I'll check it out and post my findings here.

 

Wouldn´t that burn the compressor clutch very quickly? On-off-on-off every ten seconds. Just a thought.

 

With a cold engine, I started the engine and turned on the defroster at max cold, max blower speed. Then I opened the hood and I could hear the compressor engaging and disengaging. No news from the engine fan at all.

The thick red wire with hot 12V is OK. The yellow and yellow/white both measured 0V.

The pins that you can see here both measured 12V too.
http://picasaweb.google.es/mammes/20...64656862077746

Ground reference for the measurements was the battery negative.

What is going on? Why are both signals that come from the ECU sending 0V. Am I measuring incorrectly?

Should I put some load on the lines like you are doing with the test lamp? Where exactly are you connecting the test lamp?

Are you sure the fan should turn on even if the engine is so cold? I think it should since otherwise the condenser won´t be able to work, but maybe I am wrong.

Thanks in advance.

 

I just grounded the test light on the engine.
Yes the fan will come on with the A/C on even with the car being cold.

 

You put the test lamp negative on the engine block, but where did you connect the positive?

 

What do you mean positive?
You have one of the 2 wire testlights?
I only use the single wire one.

 

Do you mean the "screwdriver" with a small lamp inside? In those you act as ground with your index finger at the end of the screwdriver.

 

Yes one like that.
When you probe the wire with it that adds ground and activates the relay.

 

Tech,

Sorry, I am having some trouble understanding your instructions. There are three connectors on the relay. The first one is hot 12V, the second one has two control signals from the ECU and the third one has the output to the fan. Are you saying that I should unplug the middle connector and use the test lamp on the open pins like the ones in the picture in the link?

http://picasaweb.google.es/mammes/20...60352751952706

Thanks
Axel

 

If you probe the wire with the plug still connected it will ground the circuit and turn the fan on.

 

Tech,

All the wires are insulated. How should I probe it?

Isn't there a more direct way of activating the relay?

What should happen if I unplug the middle connector and ground it?

When the engine is hot, the fan works and so does the A/C. But when the engine temp goes down then the engine fan goes off and the high pressure switch is triggered.

Unluckily I don´t have access to another relay to easily see if a swap will fix it.

Volvo parts here at least 3X US prices, otherwise I would have already replaced the relay.

Thanks

 

I unplugged the middle connector and grounded each one of the control pins to the engine block. The relay and fan worked perfectly in stage 1 and stage 2. I asked someone to sit inside the car while I forced stage 2 and she told me the A/C was blowing cold without any crzy compressor cycling. So it´s not the relay or the fan.

I took a look at the high pressure switch. I was able to find a short between two of the three wires. Some idiot apparently wanted to troubleshoot the A/C, so he stripped a bit of the insulation on the wires to connect a multimeter. The he just slapped some tape on to the exposed wires. This ghetto crap did not hold, the tape moved and both cables were shorted. I removed the short but the fan did not turn on. I understand the yellow and yellow/white ground wires that control stage 1 and 2 on the fan go straight to the ECU. There is some signal that is not being fed correctly to the ECU that´s telling it not to turn on the fan. How does the high pressure switch affect this circuit? How do I test for a faulty switch? Just jump it with a paper clip like I did with the low pressure switch? It has three pins, so which ones should I jump?

What about testing continuity between the ECU and the relay ground signals? Which stage is used to move air to the condensor? I suppose it's stage 1 to keep the amps as low as possible, right? Remember that the fan works ok when the engine heats up.

 

Tech,

Continuity between the relay connector and the ECU is apparently OK. The engine fan suddenly turned on for a couple of minutes in accordance with the defroster on/off. The engine was cold so I am sure that it was not the fan responding to a temperature reading.

I am suspecting that the high pressure switch is bad. When the fan was suddenly working, I pulled the connector from the switch and the fan kept on working. I understand that this event should be interpreted by the ECU as "high pressure triggered" and the compressor and fan should be shut off immediately. Well, that did not happen, instead the aircon kept on working, ignoring the high pressure signal.

 

HMMM I would try a new high pressure switch and see. Maybe something is working off the switch being bad.

 

How do I bypass the switch?

 

Bump for Tech. I want to be sure before I buy parts at rip-off prices. How can I test the switch without getting a new one?