tdenneson

My A/C compressor clutch will not engage when I turn on the A/C.

The system is adequately charged. The compressor clutch is ok because it kicks in when I jump it from the battery and cold air comes out of the dash vents. There must therefore be something wrong with the wiring or a relay/switch somewhere.

The A/C switch on the dash tests out ok. The condensor fan runs and the up idle kicks in when I hit the switch. Based on what I've read in other posts, the condensor fan would not engage if the switch were bad.
The low pressure switch is also good because I'm getting power at the wiring harness plugs at both the condensor fan and A/C clutch relays. I tested the relays by switching them, assuming these are interchangeable and the fan still runs but the compressor clutch doesn't. I assume that means the relays are ok, right?

Assuming I'm right about that, here's where I'm stuck: If I unplug the wiring harness from the A/C relay and jump the red wire from the compressor to the black wire (w/yellow line) while the A/C is on, the clutch kicks in. When I take a 12V test light to the black/yellow wire, the light glows bright red. When I test the white wire, I get nothing. When I test the second red wire (w/ blue line), the light glows, but noticeably DIMMER than when testing the black/yellow wire. I'm not sure what this means. I read in the Haynes manual that relays work by having a lower voltage signal used to switch the circuit open/closed. I wonder if the relay is working fine, but something is sending a signal from the other end of the red (w/blue line) wire to trip the relay so no power gets through to the compressor.

There are other posts discussing a bad ECU/ECM ground as a possibility or an evap sensor. I find it hard to believe that the ECU or ground would be the culprit, especially after reading a number of people's posts where replacing the ECU did nothing. I am intrigued by the evap sensor/switch lead and plan on taking a look at that. Does anyone have any other ideas? Any help would be MUCH appreciated at this point. I've been trying to fix this for a couple years now with no luck and I don't think I can stand another sweltering commute. It doesn't help either that this is a BLACK coupe with a charcoal/black interior and my commute is over an hour both ways.

Thanks in advance!

99accordFX

Have u checked the high and low pressure switches?

tdenneson

Thanks for the response!

Yes. There is only one pressure cutout switch as far I can see, which, according to the wiring diagram I have, is tripped when too low or too high. I've ruled that out because the condensor fan runs with the A/C on. If the high/low pressure switch was tripped, wouldn't the condensor fan cut out too?

Before I recharged the system (which I was able to do while jumping the relay terminals and pressure switch), the fan wouldn't come on unless I jumped the pressure switch. Now that it's charged, the fan comes on on it's own but the clutch still doesn't engage, which makes me think that the pressure switch is ok.

After acquiring a wiring diagram, I've discovered that the red/blue wire giving the dim signal I described above is coming directly from the ECU. Not sure what this means though. Thoughts?

tdenneson

I found a wiring diagram online for my A/C system, and it indicates that the white wire going to the A/C compressor relay should be "hot at all times." When I put a test light on the white wire at the A/C relay with the engine off, I get nothing (for that matter, I get nothing from the white wire when the ignition is on). When I put a test light on the white wire at the condensor fan relay with the ignition off, I DO get a signal. The white wire, according to the wiring diagram, comes from the 15 AMP fuse under the hood. There's a junction from which it splits to the condensor fan relay and A/C clutch relay. The white wire is obviously fine up until the point where it splits to the condensor fan. Somewhere between that junction and the compressor relay there's a break. If I can't find the break, I could probably run or splice a new wire to the compressor relay plug. I'll keep ya'll posted on my progress. Later!


Modified by tdenneson at 3:14 AM 7/24/2008

tdenneson

I was able to splice the white wire for the A/C relay into the white wire going to the condensor fan relay. This succeeded in getting juice going to the A/C relay through the white wire and the fans still work as well. So I'm set there. However, the compressor is STILL NOT ENGAGING.

The ECU is still sending a positive signal to the relay to cut off power to the compressor. It should be sending a ground instead. In desperation, I tried cutting the Red/Blue ECU wire and grounding the end coming back from the relay to the chassis. This caused the compressor to kick in. One small problem, however: when I turn the A/C off at the dash, the compressor keeps running anyway! So, not a good solution. It needs the ECU signal not only to cycle the compressor on or off, but to actually stop the damn thing when you turn the A/C off. So, my next move is to check the evaporator thermostat. If that's not it, then I might have to look at the ECU itself after all. Other than that, I am out of ideas! I'll keep everyone posted.

tdenneson

I'm pretty sure now that the evaporator thermostat is not the problem. I was able to get at it behind the glove box and tried hotwiring it a couple of different ways. I jumped the two blue wires while the harness was plugged in but nothing happened. I also cut the double black wire going from the thermostat into the evaporator box and jumped those. The fans kicked in, but the compressor did not (the fans already run with the connector plugged in anyway). So, I twisted the wires back together, taped them up, and plugged the connector back in.

Is this adequate to test the thermostat? I assume the thermostat works by simply completing the circuit if the temperature is within a certain range. I'm at a loss and not sure what to do at this point. The only thing I can think of is that the ECU might be at fault, but that just doesn't seem likely. I don't want to blow a bunch of money on an ECU just to find out. Anyone else have any ideas?

tamex

What was your finding my friend?

jdmcertified909

ever think its the switch in the dash that turns on your a/c???? youve tested everything else except that lol

Spaceballsthelunchbox

2 0 0 8

Turbo SiR

I just fixed the AC on my 94 LX. Everything worked and the clutch engaged when I jumped it at the relay. My problem was the red and blue wire that provides a ground to the system did not work. I ended up splicing into the wire with a toggle switch and ran the wire to a chassis ground. It works great. I am able to leave the ac switch on and run the cooling fans with the compressor off if i need a little boost to get up a hill or starting off. My system was working just like yours is before i grounded the red and blue wire. Hope this helps.