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Discussion Starter · #1 ·
I have a 1998 volvo S70 non-turbo. The compressor clutch relay is not getting activated. With the engine running and the a/c off, i have 12 volts to the coil side of the relay. But the negative wire going to the ecm has no connection. If I turn the key off the negative wire going to the ecm then connects to ground.
 

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Do you have power to the low pressure switch under the hood by where the A/C lines go into the car?
 

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Then do you have power at the compressor all the way to the clutch?
 

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Discussion Starter · #5 ·
NO, I sure don't. I've got power to the relay, and i also have power to the coil side of the relay. I can pull the relay out, and apply voltage to the relay and it works.
 

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If you have power at the low pressure switch but not at the compressor then I would look at the wiring.The relay should control the power to the low pressure switch.

There is also a switch in the back of the compresor that might be bad.
 

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Discussion Starter · #7 ·
I have to the compressor clutch relay. I can jumper the relay and everything cools good. The relay test good on the bench, but it's not getting any signal on the coil side of the relay.
 

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Has anyone figured this out yet? I have gone through the same process.
1)Checked Freon OK
2) Checked voltages OK
3) Bench check relay OK
4) Jump relay contacts compressor works, Cold air comes out.
5) Tried working ECC compressor wilnot engage.
6) Themostat control works fine for Heater.
Stopped looking last fall, its spring and I am motovated to get this fixed.
Thanks
Al

98 S70
92 240
 

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Hi,

Just joined the forum looking for a solution to the same problem. I've managed to obtain a circuit diagram.

As already stated power to the compressor is fed via the relay in the fuse box.

The relay is energised not by the voltage being applied to the relay (That should be permanently there), rather its the job of the ECU to ground the relay thus completeing the circuit, causing the relay to switch and therefore energising the clutch on the compressor.

The ECU is supplied with signals from the two pressure switches. The low pressure switch is mounted near the bulkhead. This can be easily checked by removing the connector and shorting it out. If the switch is faulty (Or the pressure is low) the compressor should kick in.

I think my problem is with the high pressure switch, mounted at the front of the engine near the fill valve. The switch is a three pin device. I have no data on how the internals of the switch are configured, so I cant figure out how to test the switch, or bypass it. I dont want to risk blowing the ECU!! I would guess as this is safety critical, the ECU is expecting a short circuit over two of the pins. This would make it fail safe.

Putting an ohm meter across the three pins suggests all three pins are open circuit to each other. But is this correct?

Anyone else able to check theirs.....?
 

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Hi Jace,
Sounds like we are at the same point. On one board, It was suggested to reseat the ECU with the power off. I did this, with the battery detached, did not help. Are you using a schematic from Alldata? On the high pressure switch Pin 1 connects to ground thru the ECU, Pin 3 looks to be V+ and Pin 2 is at ECU reference (signal ground). I agree jumping pins 2 and 3 should work. They should be at the same potential as pins 1 and 2 on low pressure switch. I will check it this evening.
Al

Ok Just checked it out, the above assumptions were correct. Pin 1, Hi Press, is chassis ground, Pin 2 is a sig ground (no voltage across 1&2) Pin 3 has a +5 VDC. Also checked voltages to the low pressure pins. Voltages are different, that has +12VDC between pin 1 &2. Also checked from the Hi press pin 2 to Lo press pin 2 and they are at the same potential. Also on the schematic the device on the high pressure is refered to as a sensor the low pressor is a switch. I don't think shorting it will work or at least be good for it.
Al
 

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Yeah I agree. I've just replaced the high pressure sensor with a second hand spare. Made no differecne


As you say bypassing the relay works fine, plenty of cold air.

I'm happy the low pressure switch is ok. So I'm stuck now. Looks like maybe a broken wire/connector from a sensor/switch, or the wire from the relay to the ECM. I need to get to the plug on the ECM with a meter.

How do you get into the ECM? The box on my V70 appears to be locked shut by a steel band that goes all the way around.

Do I have to get an angle grinder to this
 

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By lifting the metal band it will pop the ECU out of the socket, lift it to the center. Thats assuming you have the plastic cover off. I just ordered a high pressure sensor, I think that will do it. Were you able to change your sensor without evacuating the R-134?
Good luck, I'll let you know how things turn out.
Al
 

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OK, I've fixed it.

The metal band I'm reffering to is an anti-tamper device to prevent access to the ECM. Cant even get the plastic lid off until this has been removed.

I guess this is a UK 'thing'. The band has been removed, open the box, re-seat the ECU and now the compressor is cycling again.


I didn't stop there though. I found that if you emptied the system of refrigerant, and then topped it up again, the compressor refuses to work. I had to bypass the relay again. I guess forcing the compressor on helps to dsitribute the new gas charge and equalise the pressure around the system.

Put the relay back in and remove the bypass all is well.

In relation to the high pressure switch I found you can replace it without losing the charge.

I reckon I had the two faults (ECM connecton and h/p sensor), that helps me to justify the expense of the sensor.

So just need to take the car back to the garage to get some more refgrigerant and the jobs done.
 
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