Kinggs350

I have a 95 sc300 with AC issues. My old compressor kept running even with the AC off. It finally blew so I changed the compressor and pressure switch. I have not charged the system with freon yet but the compressor is kicking on even with the AC off. Could this be an issue with the AC control unit in the car? Or maybe constant power is going to the compressor? What is the best test to dignose why the compressor runs with no freon nor with the AC on?

Arsenii

Hello,

Sounds like your A/C Clutch Relay got cooked and shorted out, causing the Compressor to run at all times, try taking it out and checking if you have Continuity on the Switch Side, and Power coming in at the Coil Side.

The Relay is controlled by the Engine ECU apparently, which in turn is commanded by the Climate Control, seems like it was done this way to ensure that the Compressor Clutch would disengage if the Lockup Sensor would detect a mismatch between the Compressor and Engine RPM, aka the compressor got jammed and is currently chewing the belt to shreds.

Hope this helps and best of luck!

Kinggs350

Yes sir that did the job. Just removed the MG clt relay and the compressor stayed off. The relay was in poor shape and looked to be stuck open. Will get a new one from toyota or lexus soon. Thank you Arsenii!

Arsenii

Stuff like that usually doesn't happen by accident, could be that the Magnetic Clutch Coil was drawing more power than it should have, either because of internal damage or if it wasn't OEM. The point is, make sure to keep an eye on it for a while to ensure that you won't get a repeat.

Hope this helps and best of luck!

Kinggs350

Well I thought that had solved the issue but I guess not. Put a new relay in but the compressor clutch still remains to kick on once I start the engine with the AC off.

Here is what has been changed compressor, pressure switch, and relay.

Could the control unit be faulty? Maybe I need to start testing wiring!

Arsenii

Is the Clutch engaged when the Engine is off? You can check by trying to rotate the inner part of the Compressor by hand, it should spin freely if the Clutch is not engaged.

Try taking the relay out again and make sure that the Clutch is not engaged, if it is, then the Clutch is not adjusted correctly. If that's good, get a Test Light and put its leads across the Coil terminals of the Relay, that way you will know if the ECU is commanding the Clutch to be engaged.

If the Test Light goes On, you will likely have to start digging into the ECU issue, as it is the Engine ECU that commands the Clutch to be engaged, Climate Control only determines if the Clutch needs to be engaged, not turning it on.

Coming to think of it, one way to partially test the ECU may be to disconnect the Lockup Sensor from the Compressor, if the ECU is indeed commanding it to be Engaged, then it must Disengage the Clutch if the ECU doesn't see the RPM Signal from the Lockup Sensor.

Hope this helps and best of luck!

Kinggs350

Ended up not having enough time to dig in so took it to a mechanic. It is an issue with the ECU. Will have to replace the ECU. I did find a company that rebuilds ECU and am considering going that route as it seems to be cheaper.

Arsenii

Seems strange, did the mechanic give you justification behind it? I would still try to check the Signal to the Relay with a test light, if it shows signal, try disconnecting the Climate Control, if that doesn't fix it, disconnect the ECU itself, it shouldn't take too long.

Hope this helps and best of luck!

Kinggs350

Apologize got busy. So the mechanic said the relay is getting constant power. They assumed it is the ECU. I will do these test to see if it is the control unit. It kinda didn't make sense to me the ECU is the problem. I do agree with you Arsenii.

dwoods801

It’s also possible that a capacitor has overheated inside of your ECU, and that’s causing your unusual A/C issue. Not sure about your specific year and model, but many Toyota ECU’s from this era were made using liquid filled capacitors and after 20-30 years they start leaking, causing phantom problems. They can ofter be serviced or rebuilt for a reasonable price, if you can’t figure out another explanation, this could be your culprit.