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Pete, Technician
Category: Car
Satisfied Customers: 27852
Experience:  17 years automotive training and experience.
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318i: I have a 93 318i bmw 1.8 ac compressor is always on hook

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I have a 93 318i bmw 1.8 ac compressor is always on hook up scanner check data pids ac compressor says on all the time and the ac switch is off, relay cycles really fast only stops when engine stops running as soon engine starts it begins to cycle really fast and its in relation with rpm, power arcs across contacts compressor always on with all controls for ac off. Disconnected ac control module pids show ac compressor clutch still on. Engine ecm looks dirty and has markers writing on it do not know how to check if correct ecm for vehicle.
My name is XXXXX XXXXX I am a professional here at Just Answer. I noticed that your question was not getting a response and thought I would see if you still need help with this.

It may be this ECM at fault here as it does sound like it was replaced,usually the markings can be from salvage yards in most cases.

The best way to find out for sure if this ECM is compatible in this vehicle is to contact a dealers parts department over the phone and have all of your vehicles info handy and give them the part number above that is on the ECM and they can check to see if it is one that fits this vehicle.

If it is not and doesn't belong in this vehicle then it would need replaced with one that does belong in the vehicle.The incorrect ECM can cause this fault here.

Also if there is an ac relay clicking this may be shorted internally and causing this although the ecm should show off if it was the relay at fault so this can be the ecm itself at fault here.

Thanks Pete
Pete and 9 other Car Specialists are ready to help you

I am following up with you to see if you still needed help with your question you had asked.If my help didn't satisfy your question please let me know so we can continue.

Thanks Pete
Customer: replied 4 years ago.

vehicle will come back this coming weekend for me to continue I will let you know thank you for your help and sorry it took me long to reply.

Your welcome.Let me know if you need anything else.

Thanks Pete