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Airconditioner does not tun on - 03 Corolla

The AC in my 03 Corolla stopped working. A Mechanic originally diagnosed it as low in Refrigerant - brought it up to full charge. Stopped working again in a couple of days. I took it to a different mechanic who confirmed that there was no leak and it had a full charge. He could make the compressor come on by shorting a connector (located near the compressor). Otherwise the compressor does not come on when the AC is turned on from the switch inside. The blower fan works OK. The fuse is OK. The relay works OK.
The mechanic tells me that the only thing could be that the PCU ( the computer?) has gone bad and that only the dealer can replace it and reprogram it. The estimate is that it will run around a $1000 to fix the AC problem.
It sounds a little far fetched to me. Any body care to give me your opinion on this or and alternate suggestion.
The car has done 120,000 miles and I am wondering if its worth spending that much money to get it fixed – that is if the PCU is the real problem.
Could it be something else? Is there a way to get a used PCU and have it installed by a mechanic other than the dealer?
Appreciate any suggestions

If the relays and pressure switches check out for the AC system then the only thing left that can cause this problem is a defective PCU. Call local auto recyclers to locate a replacement PCU for your vehicle. Then bring it to a mechanic to have it installed.


Jump the relay contacts, if the compressor comes on, then your relay is not as good as you think. Don’t go tot he dealer for this, they want $115 for it. You can get one for about $18 at Advance. AutoZone doesn’t carry it.

Keith, & Tester, thanks for your comments. I also read your suggestion on the same subject (July 2011). The mechanic tells me that the first thing he checked was the relay. In fact he temporarily replaced it while doing the diagnostics to be sure. He also checked the expansion valve, the blower , the refrigerant charge, and the fuse. They all checked out fine. According to him the relay is just not getting a signal and the system thinks that there is no refrigerant so the clutch doesn’t kick in. His conclusion is that the PCU is dead and that only the dealer is equipped to flash the correct program based on the car’s options, date and location of manufacture, etc. Of course the dealer’s shop will only sell a new PCU. Estimated cost ~$1000

Even if the relay is good, you or your mechanic still needs to jump the contacts to determine if the rest of the system is good and it truly is the PCM. Jumping the contacts is pulling out the relay and using a small jumper wire to short out the contacts (not the coil pins) in the relay box. If the AC comes on, then either it is a bad relay, a bad PCM or a broken wire between the PCM and the relay box, or a broken wire between the relay box and ground.

The next step is to turn on the AC with the relay out and check for voltage between the coil contacts. If no voltage between the coil contacts, then measure voltage between each relay coil contact and ground. If one of them has 12V, then check for resistance/continuity between the other one and ground. If you have 12V on one, continuity to ground on the other and jumping the contacts turns on the compressor, then the relay is bad and the one he substituted did not work either.

Thanks for your response.
Before I tried the measurements you described I bought a new relay (from O’Reilly for $14). The compressor still did not come on. So I followed your suggestions to diagnose for the trouble spot – or confirm what the guy at the shop told me. This is what I found.

With the engine running and the AC switch & Blower on, I removed the AC relay and jumped the || terminals and the compressor came on as expected. There was no voltage between the coil contacts =. However, there was approximately 13.5 volts between one of the coil contacts and ground. There was no continuity between the other coil contact and ground.

I suppose that means that there may be either a broken connection between the relay box and the ground or a broken connection from the PCM. or a bad PCM. Either one of them will be equally hard to locate. The more I look into this the more I think the shop mechanic was on the right track and maybe the PCM is dead. This guy also tried to by pass the PCM altogether. That way he made the clutch stay energized. However the clutch stayed energized continuously even with the engine off and the key out of the ignition !!! I don’t understand that - but its something to do with power flowing through the negative side of the circuitry.

What do you think.

I would probably wire in a new switch, and bypass the PCM… I dont see the plus side to spending $1000 to fix a problem a $5.00 switch and wireing will fix.

Before you jump on the PCM, check for voltage on the coil contact to ground that had the 13.5 volts, but this time check for it with the AC button turned off. If it is still there, then that means the PCM provides the ground. If its not, then the PCM provides the voltage. I doubt that the PCM is bad in either case, I still think its a wire.

I think that you can jump the relay like you tested and then turn off the ac by hitting the AC button or turning the fan off. If this works, you can just bypass the PCM this way. About the only thing you will lose is shutting off the AC during wide open throttle and if the engine overheats.

I lost my PDF of the FSM when my HD crashed, but you can download a free copy here. Your mechanic probably subscribes to AllData so he can look up the wiring and see exactly what the PCM provides and if you can bypass it.

or here

Thanks for the info. I’ll try and get a switch wired to by pass the relay box like you suggested and will write back on any results. Though I am afraid it may be getting beyond my capabilities to mess around with the electrical circuitry. Lets see how it comes out

His conclusion is that the PCU is dead

So he verified the pressure switches are doing the right thing and providing a good signal to the PCM? Don’t know your specific vehicle but one would assume that if the PCM controls the operation, it must also get input from the switches/sensors. If the input signals are not correct to the PCM, it won’t engage the compressor. Personally, I’d suspect the switch before the PCM…

TwinTurbo, I agree with you 100%. I’m trying to get him to try everything else before dropping a grand on a new PCM, and then having it not fix the problem.

Glad to let you all know that the A/C is fixed. The problem was in a bad thermistor. I was finally able to locate a REAL mechanic with patience to repair rather than replace. He got under the dash and located the discontinuity in the circuit from a bad thermistor and just by passed it. Everything seems to work well. What will I lose by not having the thermistor in-line?