Aircon Troubleshooting

Evening Folks,

Was trying to find the fault with my aircon yesterday. When I hit the AC button it lights up but the compressor doesn't run. It will engage if I pull out the relay and stick a wire across it however, so compressor and all seems good.

I'm running a Link G4 ECU so I can plug in the laptop to see if the ECU gets the AC engage signal - it doesn't. I did the test on the AC ECU by holding in the recirculate button and Auto button while turning the ignition on and I get the fresh air flashing for abnormal refrigerant pressure. I can't take this to the garage to regas tho since compressor doesnt run.

I thought one of the buttons on the heater controls would send a signal to the ECU for aircon but after sticking a meter on the blue and red on the smaller G4 ECU loom connector I see nothing. This cable apparently is the signal from the AC ECU. I also tested each pin on the 20 and 14 pin connectors coming out of the back of the heater controls to see which one had a voltage change when I hit the AC button to make sure that even tho the light comes on its sending a signal but couldn't find anything.

Can anyone advise where I should be looking or testing to see where the fault is? Which wire from heater controls to AC ECU is the signal wire, which wire from AC ECU into G4 ECU is input for AC signal ect? I had hoped I could follow the signal through and see where it stops but I couldn't even find it from the control cluster. I'm a little stumped.

Thanks,

Johnny

Looking at some MKIV maps I have here, the ECU appears to receive a signal to say that the AC pump has been activated, doesn't look like there is an AC out.

EDIT: I think all the maps I am looking at may be from cars with AC removed

This AEM ECU pinout might be useful.

http://www.mkivsupra.net/vbb/attachment.php?attachmentid=84095&d=1226099581

Shows that on the stock loom 23A is for the AC clutch, 16A is climate control and 34A is the switch input for AC request

Thanks Wez, that's close to another pinout I have. PIN 34A looks like it is the signal wire telling my G4 to engage the AC clutch. If I was to splice into this and give it 12v I might get the green light on the ECU and see the compressor run. This would tell me that the fault is caused by the AC button on the heater control console or is something to do with the AC ECU

Found this on http://www.clublexus.com/forums/performance-and-maintenance/727672-2jzgte-wiring-harness-made-easy.html

Pin 34 - A/C , Air Conditioning (AC) Request Signal , Input , This pin is used to request that the AC be turned ON. Having an AC request input and a separate control output allows the engine ECU to turn the AC OFF under some conditions, such as hard acceleration. This pin is connected to battery voltage to request that the AC be turned ON. The AC request signal is usually generated by the climate control ECU, however connecting this pin to battery voltage will not guarantee that the AC is turned ON

Gonna try tapping into that wire and see what happens

It is reading 11.5v all the time with A/C on or off. Nightmare!

ac systems can be (and ideally are) charged with the compressor not running.

I have no idea how the wiring side of things work on the supra, but if it's showing abnormal refrigerant pressure it means the pressure switch is activated.

this means it won't put a feed to the compressor as it will knacker it being run with no refrigerant.

chances are the control circuit isn't putting a feed to your ac controls.

sounds like it needs a leak test / repair

Thanks Squiffy, wasn't sure on how AC systems are re gassed. Thought the compressor would have to be running to get the gas round the system. Looked over my settings tonight for the AC control on the Link and have it sorted now so the ECU sees the 12v input coming from the AC ECU and does what it needs to. However... the voltage coming to the Link G4 ECU pin 34 from the AC ECU is always 12v whether the AC is on or off, this doesn't seem right. If the AC ECU didn't want the system running due to a fault ect surely this should stick to 0v?

Can anyone confirm this?

You'll probably find that your compressor isn't starting up because the low pressure cut off is activating. First thing you need to check is that you actually have refrigerant in your system. You can buy pressure gauges and snap on valve couplings off eBay for around £50 or get a garage that specialises in a/c work to test if for you. If you have under 15psi the compressor wont start up, the system should be stood at around 50 - 70psi dependent on the ambient temperature.

If you find you are out of gas then you need to have a vacuum test to check for leaks. If you are down to around the cut off pressure it may just need a top up. They recon you should think of losing around 10% of the charge a year just through hoses even though they are barrier (plastic lined) hoses. You can buy R134a cylinders online or get a garage to do it all for you.

If you are filling refrigerant yourself always fill off the gas rather than liquid valve (if you feed in liquid refrigerant its certainly quicker but you risk a hydraulic lock in the compressor) into the inlet suction side (the service port with the 'L' on the cap - low pressure) . You wont be able to fill up the system until the compressor kicks in as that will effectively suck in refrigerant from the gas cylinder but you will get above the cut off pressure, sufficient to get the compressor up and running. Once you get to 20psi with the compressor running you should see liquid in the sight glass by the radiator. When this runs clear liquid (no bubble stream) you'll be just about right on for the correct charge. You can weigh in the gas but you need vacuum equipment to do that properly to start off from zero.

If you have no gas then it is probably a holed condenser that's your problem. Which is a bummer as the front of the car has to come off to get to that so it's not a quick fix. Best hope you are just running below the clutch cut off pressure.

PS - Don't force run the compressor without a charge. They have a low pressure cut off for a reason. You don't want to risk sucking in humid air from outside of the system and the refrigerant carries soluble lubricant around the system and to the compressor. a/c systems have a low pressure cut off because without it, you run the risk of pulling a vacuum on the inlet side of the compressor on a low charge system. Regard it as a safety indicator that you need to top off your system rather than something has gone wrong.

A aircon guy diagnosed mine as the pressure switch. I think from memory the pressure was very high? Anyway on his advice I replaced this with a used one, lower glove box out and some bits of the heater/aircon system behind too, then he came back and refilled and tested it and it's been fine since.

It is reading 11.5v all the time with A/C on or off. Nightmare!

The ecu might use a pull up resistor on that output.

If the ecu sees no pressure then it will not start the pump

Garage will regass with nitrogen first to check for leaks

Thanks Rider, that's a pretty detailed reply on how to check the system! I'll leave it disabled for now until I get the gas checked and clear any fault codes showing on the AC ECU first. Hopefully its just low pressure rather than nothing in there at all

I'm hoping I don't have to dismantle anything again Scooter! Abnormal refrigerant pressure doesn't tell me me much so will get a gauge on here and see what it says.

The Pullup resistor option is enabled on that digital input Chris so you could be on to something there. Would have thought the AC ECU would be sending the 12v signal to the G4 for the AC request so might have to double check that. I'll get any faults cleared on the AC system first to rule that out

Thanks for all the feedback guys will let you know how I get on

Update: Went and got R134 gas and a gauge with nozzle. Hooked it up with engine off to low side and got 0psi. Filled it to 20psi and did AC ecu check, no warning lights. Started car and turned on AC and full fan and left running for a few minutes when I checked again the gauge showed 0psi but the compressor was running and laptop showed AC request on

Continued to fill gas until pressure on low showed 10psi at idle with compressor running. When I revved the car to 1500rpm it went back to 0psi. Held car at 1500rpm and continued to fill gas until it showed 20psi

Air from the vents is ice cold when the AC is running. Finished off by setting the ECU to increase rpm by 150 when it sees an AC signal and to automatically disengage the AC clutch when throttle position is over 50%

Heading out a run tomorrow so will check pressure over the next while to see if it drops incase there is a leak somewhere in the system.

Thanks for the help guys

Update: Went and got R134 gas and a gauge with nozzle. Hooked it up with engine off to low side and got 0psi.

If your system was stood at 0psi you definitely have a leak somewhere. You can add a dye to the low pressure port inlet and use a uv lamp to see if you can track down the leak. If you are fortunate it'll be from a union that just needs tightening or an new sealing olive fitted. If you are unlucky it'll be the condenser or the evaporator. It's much more likely to be a holed condenser as that is at the front and liable to get road debris thrown at it.

You may have a small leak or you could have a big leak. I had a BMW that I could top up twice a year and maintain a leaky system. My Supra with a holed condenser would only run on charge for a few days so ended up costing almost £1,000 to get a garage replaced condenser supplied and fitted. There are also system sealers you can use but I'm no fan of those, figuring they are like the egg white in the radiator or sawdust in the axle kind of short term bodge.

PS - Should add for anyone handling refrigerant for the first time it is a refrigerant for a reason. Contact with the liquid form can cause severe chill burns so goggles and gloves are a must.