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Ask Jerry Newton Your Own Question
Jerry Newton
Jerry Newton, Cadillac Technician
Category: Cadillac
Satisfied Customers: 4616
Experience:  ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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AC compressor does not see signal to turn on can manually

Customer Question

AC compressor does not see signal to turn on can manually turn it on with scanner when engine is off key on
Submitted: 1 year ago.
Category: Cadillac
Expert:  Jerry Newton replied 1 year ago.

Do you know how to check for ACM trouble codes? If not, I can walk you through it.

Customer: replied 1 year ago.
please do
Expert:  Jerry Newton replied 1 year ago.

Sit in the driver's seat, key on engine off. Press and hold OFF and WARMER buttons on climate control panel until everything lights up on the dash, then release the buttons.

After a few seconds, the instrument panel will start displaying trouble codes from all systems. Have paper and pencil handy, there could be a lot of them. Write them down and let me know what they are. When it's done, it will say PCM? At that point, you can just shut the key off.

Customer: replied 1 year ago.
done that 3 times, no ACM codes..... the CCP uses a serial data line to communicate with the ACM,and it's come down to the CCP not sending an a/c command to the ACM..So no a/c request sent to the PCM from the ACM.
Expert:  Jerry Newton replied 1 year ago.

If that were the case, it would seriously be the first time in my 25 years of working on Cadillacs that I've seen it happen. Suffice to say that the serial data message is almost certainly being sent.

Scan tool data would reveal whether the PCM is receiving the AC request, and whether the PCM is disabling the AC for some reason. Do you have a good scan tool that can read this data?

Customer: replied 1 year ago.
I haven't seen this in my 36 years .
I am using a Snap on Modus.PCM is not receiving AC request with key on engine off functional test of the AC clutch circuit can use scanner to have PCM turn on clutch off and on.When engine running AC is not turning on even though you command it at the CCP.
Expert:  Jerry Newton replied 1 year ago.

Can you see ACM data on your scan tool? Are the high and low pressure sensors reading in the correct range? These two sensors are in the liquid line on the firewall, and they are both inputs to the ACM, not to the PCM. A failure of one of these circuits or sensors would cause the ACM to not send a request.

Customer: replied 1 year ago.
Could you tell me the resistance of the sensors?
Expert:  Jerry Newton replied 1 year ago.

I don't have a chart that tells me that. I can tell you that they are 5V reference variable resistors, so if you had a scan tool, you would be able to see the pressure reading on each sensor.

Short of that, as with any 5V sensor, the operating range should be .5-4.5v, so you could use a multimeter to test for appropriate voltage at each sensor.

Also notable is that if the outside air temp reading is skewed (if it reads cold when it's not cold out) the AC can be disabled for that reason as well. You can see the outside air temp on the HVAC control panel.

Customer: replied 1 year ago.
Can the 2 sensors data be accessed thru the onboard display?Instrument panel inputs shows all buttons on display function however rear defogger button shows ( buttons are on momentary )I am leaning towards the 2 aensorsI have scan tool snap on modus updated to 2012