My wife’s '04 Sienna is having an AC problem. The compressor comes on with the first start of the day and the AC blows nice chilly air. As you take that first drive of the day the air will switch to warm, then the compressor may kick back on or it may not, but then it stays off for the rest of the day. I’m not sure if this is because of the ambient temperature rising throughout the day or because of the engine parts heating up.
The low side pressure is 35 when the compressor is on.
I tried switching the MG-CLT Relay with the Horn Relay to see if it was bad but the horn still worked and the compressor still doesn’t come on.
I’ve been doing research and it seems like shops & dealers are really quick to recommend replacing the compressor so I was wondering if anyone had any ideas of anything else I could check on it.
It’s possible that the compressor clutch is bad and is failing to engage when it heats up. On some of these, the “air gap” is adjustable and/or the clutch assembly can be replaced without replacing the compressor, though it takes a special tool to remove it I believe, and a lot of shops will just say they can’t do it and try to upsell you a compressor.
You can try a little test. When the A/C quits, hold the shaft of a screwdriver near the compressor clutch and see if there is a magnetic field present. Do this very carefully, as with the engine running there are of course things spinning that can grab loose clothing, rip the screwdriver from your hand, and cause injury in a number of ways. Also, the magnetic field is powerful, so be prepared for it to grab the screwdriver and yank it towards the clutch. If there is a strong magnetic field present, the clutch is trying to engage and is either worn out or the air gap is too wide. If no magnetic field, then you know the system isn’t trying to engage the compressor for whatever reason. You can carefully rap on the assembly with the handle of the screwdriver and see if it makes the clutch engage too.
Since the air is nice and cold when it is actually working, I doubt you have problems with the system being undercharged.
If your car uses a “clutch cycling switch”, this may be wearing out and sticking in an open state when it heats up. I had a car that did exactly that.
Naturally, there could be other problems as well, such as the ‘computer’ not allowing the A/C to run due to it believing the car is overheating or similar.
There’s no way to tell much over the interwebs. I’d need system operating pressures and I’d also need to see the system when it’s malfunctioning. Your low pressure seems ok, but having just the low pressure is like telling you my blood pressure is 78. We need both the high and low to determine more.
Is the cooling fan working?
But from what you describe, I would say the compressor assembly is a likely culprit. Either the air gap on the clutch assembly is worn beyond spec or the coil is getting weak, either of which would be made worse by higher engine compartment temps. You can check this if you have a voltmeter. Check for power at the wire (I think it’s a single wire lead) leading to the compressor clutch when the system is malfunctioning. If you have battery voltage there and the clutch isn’t engaging you’ve found your problem.
I wouldn’t replace just the clutch and coil on a 10-12 year old car.
Other than agreeing about knowing what the high side pressures are and checking power at the clutch coil, it could be that to sort this out you may have to pay someone to go through the diagnostics if there are no issues with the above.
At one time A/C system controls were pretty simple and someone could get through it with a test light or VOM and not much else.
This car is likely similar to many others by using solar sensors, ambient temp sensors, room temp sensors, evaporator temp sensors, compressor control modules, and so on.
From the DIY standpoint it may require a factory service manual to be able to weed the problem out.
Update to this: It turned out to be the compressor clutch. I had it replaced with a new part.
Thanks for the feedback. My guess would have been icing of the evaporator first then probably the compressor clutch second or third down the line.