Thread: Air con wiring advice needed?

Only lasted a day. Back to where I started again!

The air con guy tested it and said it's fine mechanically and it's an electrcial fault and he could only test the fridge side. He said there was intermittent power going to the compressor apparently, 3 spd fan runs fine...... And thats what it does. Fan runs correctly and constantly, but then compressor will cut out for 5 to 10 minutes, air gets warm and ventually comes back on again. From other posts I'd read I thought it must have been icing up or something but the fridge guy says it's not that either.
I am an Electrician but not the auto type which usually gets me into more trouble than it's worth on vehicles. Anyway, he must have fiddled around with something because it came good for a day, worked perfectly and now it's back to intermittently shutting on and off.

I was hoping the problem may have been as simple as a loose relay connection but it looks by the wiring diagram if the power relay was faulty the 3 speed fan wouldn't work either. So the likely culprit will either be the connection plug off the thermo amp, the thermo amp itself. What do you guys think? Rgds Craig

it'll be one of a few things

1 the evaporator freezing up cooling stops the refirigerant gets no heat from the air (ice is an insulator) so stays liquid in hte evaporator and the High pressure cut out is taking effect, the same thing happens if its been over gassed.

if you're ABSOLUTELY sure that its not the evaporator icing up then bypass all the electrics and feed the compressor directly from the battery with the engine running and see how long it blows cold air for

having a set of aircon gauges will make diagnosing easier but you need to manually measure the temp of your piping

it should be hot going into the condensor from the compressor
cooler but still warmish going from the condensor to the TX valve
Cold coming out of the TX to the evaporator and
cool going from the evaporator to the compressor.


you may find that the if the TX valve has had a smack during removal/install that its not working properly Or the system has contaminates in it and something is blocked.

Definitely appears to be electrical. Spoke to the fridgy this morning and he put guages etc on the lines, tested all that size. He reckons it came good when he started playing with the wiring near the fuses , relays. Ran perfectly for a day then **** itself again

in that case...

try the manual bypass trick on the compressor with a 12v 10w bulb wired up to the normal wiring.

IF the light goes out but the AC is still pumping out nominal cold air then its time to follow the wiring backwards from the clutch connection.

Dont forget that this can also be a symptom of the compressor clutch or the wiring on the way out and this will be revealed by the compressor stopping with the bypass wiring still in place but the light staying on.

By the way mate, If I do connect the compressor directly, and it still cuts out that means it is icing up eh? I wont do any damage without the high / lo pressure switch in circuit? The air con guy told me that it's very rare they play up.

So where do you connect the 1w bulb mate? Accross the compressor terminals?
Sorry mate, clearly I don't have a clue :-(

So if it ices up, the compressor wont be receiving power anyway eh? because the pressure switch will cut off power to compressor? So I'd have to wire the light before the pressure switch to see if it's losing the feed somewhere before the pressure switch. If that's the case I know it's wiring. Couple of dmba$$ questions...

Where is the pressure switch? next to the compressor?

Are there relays / fuses etc mounted in the underdash unit itself and if so can you get to them? The air con guy thought the air con relays / fuses where mounted with the other fuses under the dash in the fuse box

ok, find the compressor it will have a connector on it.

disconnect that compressor

wire a light from one (if its the 2 pin type) of the connectors to a light and then to earth. Turn on the ignition and the AC the light should light, if it doesnt try the other pin. If its a single pin wire a 10w light in place from it to earth.

this light tells you what the AC system is telling the compressor to do.

now go straight from the battery + to the compressor and you should get a nice healthy clunk as it engages.

start the engine and within about 30 seconds or so you should be getting cold air.

At high idle (call it 1K RPM) with the doors open and the fans on the lowest speed you should see the light go off, when it does ramp the fan speed up and back off the engine RPM's and you should see the light come back on this means that the system is correctly cycling the the compressor. so Excluding the TX valve sticking (which happens but is rare) its going to be an intermittant electrical fault.

IF the light stays on but the cold air stops then its an intermittant fault in the refrigeration system that is pretending to be an electrical fault OR the compressor clutch is dodgy OR the compressor is knackerd.

I understand (I'm an electrcian but don't have a clue about DC!)
So the compressor will be hotwired, which means if all good on the mechanical side should stay cool indefinitely.

The light is attached to the signal coming from the thermo amp which would normally power the compressor, cycling on and off. I can't do any damage running the compressor like this (over riding the pressure switch?)

Makes perfect sense and even if I don't fix it myself will be able to point an auto elec in the right direction

By the way, if I'm looking for an electrical fault, where are the relays etc normally mounted. On the wiring diagram say they're in the underdash unit. Are they in a place where u can get to them or do I need to drop the under dash unit down.

Looks like there's a main power wire running to the thermo unit off the fan switch. If there was a fault anwhere past there the fan would still run but compressor could be off. Basic likely places to look appear to be the plug / harness to the thermo amp, and the connections from there out to the pressure switch / compressor. Unless it's a crook thermo amp?