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Hello, all. The car in question is 1993 940 Wagon with manual climate control (no climate control, I should say, just old-fashioned knobs). The A/C compressor does not kick in at all. The charge seems to be ok, but I just never see or hear the clutch kick in. Needless to say, there is no cooling. I have read on the FAQ about the PC board on the controller having a problem with weak traces. Took the control unit apart this morning and was disapointed to find that the PCB is fine, no sign of burnt traces or loose solder joints. Too bad, because I am very comfortable fixing loose, cold and otherwise bad solder joints or broken traces or anything else that requires a soldering iron. So, how do I troubleshoot this? I looked at the compressor and saw only one wire going to it. Should I check voltage on that? How do I characterize the problem? Is it mechanical or electrical? I am pretty certain that the charge is good in the system, but will appreciate advice on how to make sure. Before quiting the A/C was intermittent cutting in and out, which I think is normal in really hot weather. This winter the car had an encounter with a deer, which destroyed the condenser and after replacement and charging the system, the mechanic said he did not see voltage at the compressor. I did not verify this, but just deferred it to warmer months (now). Then I read about the controller PCB problem and thought that I had a handle on the issue since the symptoms seemed to match. This morning proved me wrong and I am on a quest once again.
Appreciate in advance any and all wisdom.
--
Vladimir. '98 S70 base, 5-speed manual - his, '93 945 (165K miles and rolling on...) - hers,
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Dear Vladimir,
Good a.m. and may this find you well. In regard to failed solder joints on the PCB, a break - if there is one - often is too small to be seen with the unaided eye. Indeed, it can be microscopic,or close to it.
The only way to be sure, that there is not an invisible break in a solder joint, is to reflow all of the solder connections between:
(a) the four pins of of the main relay (the small[ 3/4" / 19mm long], black plastic box close to the main connector) that connect the relay to the circuit board
(b) the 12 or 14 pins of the main connector, that go into the circuit board.
If, after reflowing the solder - use a 25-watt iron - there is no improvement, then:
(a) the problem is elsewhere in the system, see Rule 308's excellent suggestions
(b) the relay - a switch - may have failed.
Hope this helps.
Yours faithfully,
spook
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If you nailed a deer and the condensor was replaced, the wires to the high pressure sensor etc up front may have been broken or not re connected properly. check your email I've sent you a file that may help chase it down
--
-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '86 740 GLE turbo diesel, '82 Mercedes 300SD for sale
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Let's start with jumping the pressostat at the accummulator and see what that does. If the compressor kicks in you have one of two problems, you are low/out of refrigerant or you have a bad pressostat, I have seen both. If it fails to kick in at that point then you have one of two problems there, either you are not getting enough current to the pressostat in the first place (bad solder joint on the PCB) or you have a break in the wire somewhere between the pressostat and the compressor. If it does not engage I would jumper a wire from the fat red B+ wire on the back of the alternator to the compressor lead right there at the compressor. It should fire up, if it does go see if it will blow cold. If it does blow cold this indicates that you have enough refrigerant in the system to close the pressostat and make things work and that your problem is somewhere in the control side. If it does not engage then you have an open in the compressor clutch or the ground cable for it is broken.
I have seen all of the options I have laid out here happen. Bad clutches, broken ground wire, bad pressostats, faulty PCB's, no refrigerant. In fact I had one of the fellow board members in my shop the other day and while stuffing in his pre-pump I gave him a quick down and dirty diagnosis on his a/c, bad solder joint on the PCB. Well he went home and fixed that but still no a/c. He came back to the shop and paid for diagnosis and I went ahead and diagnosed the rest of his problem, bad pressostat and it is a bit low on refrigerant. So as you can see these problems can exist on multiple levels, follow the guide lines I laid out above and see if you can't narrow it down.
Mark
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Thank you for a great answer. I have not solved the problem yet, but I am getting somewhere now.
I have done some experimenting based on the suggestions I got and here's what I found:
1. When I jumped the pressostat harness, the compressor clutch kicked in for a short time and then stopped and I have not been able to make it go by bypassing the pressotat again. I also measured the resistance of the pressostat itself and got a reading of 0.3 ohms (pretty much zero), which would indicate that the pressostat is closed.
2. When jumping from alternater positive terminal directly to the compressor, the clutch kicks in and stays on, but the cooling produced is minimal (but it's there). I checked the pressure in the low-side and it's around 45 psi while the system is operating. So, now I am thinking that I do indeed have a control problem as indicated by not kicking the compressor clutch AND possibly a bad/weak compressor as indicated by whimpy cooling when compressor is forced to pump. Or I just have an improperly charged system (is that possible? Air bubbles?)
Please comment on my logic. I am an EE and HVAC is kind of unchartered territory for me. I will try to chase down a control problem, but it seems even with the compressor engaged, the cooling is poor.
--
Vladimir. '98 S70 base, 5-speed manual - his, '93 945 (approaching 200K miles and rolling on...) - hers
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