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I have replaced the evaporator in my 1992 VOlvo 850, non turbo with automatic AC, because of a confirmed coolant leak. I myself have done the dashboard removal and refitting, the replacement of the evaporator itself was done by an independent garage. I have driven the car with the dashboard disconnected and removed from my place to the garage and back, which may or may not be relevant, but I mention it just to be sure.
The problem now is, that after replacing the evaporator, reconnecting the dashboard and filling up the AC system, the compressor doesn't start. The compressor was replaced with a new unit two years ago, because the old one didn't start anymore due to the increaed gap in the magnetic "clutch". I have not checked any fault codes yet. Does anyone have a suggestion where to start looking for the fault?
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Make sure that the AC switch is on, the temp controls are on the coldest setting, the engine running, and then jumper the pressostat switch. Fixit2002 is correct, you cannot fill the system without the compressor running.
You should also check the diagnostic boxes and clear all of the codes you can, driving without the electrics in the dash connected can trigger all sorts of problems.
Klaus
--
Common sense trumps a PhD
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You can't really fill the system without getting the compressor to work. How was that done? You can jumper the pressure switch plug to see if the clutch esgages.
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I'm not sure if the system wass actually filled or the servicemen only tried to fill it and realized the compressor doesn't start. He just said I should check out the wiring on the AC unit (that I reconnected) before he tries anything else. He had the car connected to the pump, that fills the AC system for a while though (30 to 45 minutes).
I have tried connect the two ends of the connector, that attaches to what I believe is the pressure switch - the round 2 cm high component, attached on one of the AC pipes, approx. 5 cm from where it exits the firewall. (I hope that's what jumping the pressure switch plug means :). The compressor still didn't engage.
I read out the fault codes for AC (in module B1) and there are none reported. Also none reported for engine and ignition (A2 and A6). Also the AC unit itself shows no signs of failure (no blinking lights on rec and ac off).
I have checked the 3 connectors for the AC unit, and they are all connected securely as far as I can tell. Is there another connector, vital to the AC which I could have forgotten?
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Check the clutch connection on the compressor. There's a black wire coming out of the clutch that goes through a odd plug after a few inches. Disconnect the plug and jumper the wire to the plus side of the battery. The car doesn't need to be running. You should be able to activate the clutch. If it doesn't click connected directly to the battery it could be bad although the compressor itself also has to also be grounded to the engine block.
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As you suggested I jumpered the black clucth wire to the positive battery side, and that did activate the clutch - I could hear the clicking sound. That would probbably mean the clutch is OK and the problem is elswhere?
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I also had some difficulty getting my system to start after replacing a hose. I was only getting 40 psi in the system on the 12oz can of 134a, and the pressure switch won't let the compressor start until 45 psi. A heat gun on the can (underside) was needed to charge the system.
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If I understood KlausC and fixit2002 correctly, I bypassed the pressure switch when I jumped the pressure switch connector (plug), so the clutch should engage regardless of the pressure in system. And the other test proved, that the clutch itself is working.
So, still need help, what can I check beside the things I already tried?
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Check the AC relay?
--
Common sense trumps a PhD
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I sucessfuly fixed the fault. It was the faulty AC relay, and the replacement works all right. The AC control unit was also faulty, because it supplied unninterupted 12V current to the relay even when turning the AC off, so the compressor never shut down after putting in the replacement relay. I replaced the AC control unit with a used spare one and performed the damper motor readjustment procedure. Now the whole AC system works without any problems. I think the AC relay and AC control unit failures were however not connected to the evaporator replacement, I just haven't noticed the faults earlier, because the AC system was emtpy and I haven't used it during the winter months.
Thanks guys for your help and support, I never would have managed without it. And thanks KlausC for the manual, it's awesome.
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I checked the AC relay and it seems it was faulty. I had another used spare AC relay and I plugged it in and now the compressor runs constantly, even when I turn the switch on the control panel to AC off. I am not sure if the replacement relay is OK, but it showed no continuity on the Ohmmeter when unplugged and since the compressor is running when I plug the relay in, it obiouvly works, sort of. What bothers me is I cheked the other two wires, that connect to the relay, and give the impulse to switch it on or of, and one of the wires constantly has 12V, even when I turn the AC control board switch to AC off. This probbably shouldn't be the case, or should it?
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Now that you mention the AC relay, I replaced the cooling fan power stage a month or two ago, because the fan wasn't working. While doing that I also tampered with the relay located above the power stage. Actually, I think there were two relays there, and because I thought one of them has something to do with the fan not working I removed it and wanted to test it by connecting the two pins used for turning the relay on and of to 12V, and than check the continuity between the other two pins with an Ohmmeter. That might have damaged it, in any case my testing probbably wasn't such a good idea after all.
Now, before I burn anything else - the way I planned to test if the AC relay is the culprit, was to remove the AC relay and jumper the two pins in the relay housing, which the relay normally connects and disconnetcts, and at the same time jumper the pressure switch connector. If the clutch than engages with the AC on and the engine runnig, the relay is faulty. Or should I try it another way?
I don't have any wiring diagrams for the AC, I only have the Haynes manual and that only contains a diagram for the fan, so I'm not sure if there are two relys there or just one. I'll check that tomorrow and post my findings.
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Your clutch seems OK. If you have a voltmeter check the pressure switch plug connectors with the ac on and the engine running. You should find 12 volts to ground on one of the pins. If not that's your problem but I don't know where it comes from. Also with the engine off you should have continuity from one of the plug pins down to the clutch wire plug. All that pressure switch does is connect 12 volts to the clutch coil. Good luck....
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I cheked both pressure switch plug connectors to ground and I don't get 12v on any of the two connectors (AC on, engine running, cooling to max and fan on). That probbably means the fault is above that point. After that I didn't chek the continuity to the clutch, since it cannot work in any case, if there is no voltage to it from the pressure switch. I'll try the AC relay next, as Klaus C suggested.
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