A/C works intermittently

Tiny
JOE WRIGHT2
  • MEMBER
  • 2010 CADILLAC CTS
  • 3.6L
  • V6
  • 2WD
  • AUTOMATIC
  • 175,000 MILES
Intermittent A/C, almost never works on start up. But most days after driving it from 1-100 miles on long distance, it may come on. Sometimes when I stop, or turn off the car, it might come on. Usually after it comes on, it’s good for the day. If I turn the car off, when I restart it, it usually continues to cool, but sometimes doesn’t. In rare instances, it may stop cooling while driving. I don’t know of any codes, but I used a Zeus, once to troubleshoot, and could make it cool just fine. Then afterwards, when I started it to drive home, no A/C. I’m was just hunting and pecking with the Zeus, as it wasn’t mine. I pretty sure that it’s not getting a signal all the time at the A/C relay, as I can jump it out and clutch engages. I do A/C heating for over 40 years, but not a lot of new car systems. So I am at a loss. I tried replacing the TCM 20824578 module, but no change.
Friday, March 12th, 2021 AT 1:29 PM

23 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
With your experience we will get this fixed. Let's start with the voltages that you had.

With the relay removed.

Pin 30 should have battery voltage
Pin 87 should have battery voltage only if the relay is closed. To test the compressor just jump 30 to 87 with the engine on and the compressor should come on.
Pin 85 should have battery voltage
Pin 86 should have ground when the AC is turned on. To test this you need to put your red meter lead on battery positive and the black lead on this terminal with the relay removed. Then start the vehicle and turn the AC on. Your meter should have battery voltage. This means the ECM can ground the relay properly which closes the other side of the relay and turns the compressor on.

Not sure you need it but here is a guide that will get us on the same page with terms and how the automotive system operates. It is basically the same as housing HVAC but the components are slightly different.

https://www.2carpros.com/articles/car-air-conditioner-not-working-or-is-weak

What I would suggest is to start with after checking the voltages above is checking pressures just to make sure they are correct.

https://www.2carpros.com/articles/re-charge-an-air-conditioner-system

Below is a chart of temperature and pressures so we can see if they are correct.

Let me know what the pressures are with the AC off and then with it on. Jump the relay if needed to get the compressor to come on to check it.

If all this checks out then we can test the pressure sensor but I suspect we will find something here.
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Saturday, March 13th, 2021 AT 3:48 PM
Tiny
JOE WRIGHT2
  • MEMBER
  • 81 POSTS
These are the readings I have looking down on the fuse box from right side of fender. The 30 and 86 terminals are closest to the fender after inverting relay back into fuse box. The t30 and t87 positions really aren't a problem for me. It's t86 that should be reading to ground, with calling for ac Eng On and relay removed, as far as I can tell. But its reading 3+ volts to ground.
I will check pressures tomorrow, but what would cause voltage on t86?
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Saturday, March 13th, 2021 AT 5:26 PM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
I think you have 30 and 87 backwards because the wiring diagram shows 30 is the feed from the fuse and 87 feeds the compressor the voltage when the relay is closed. However, I am not too concerned with that at this point.

Let's focus on the 86 because that should not have voltage on it. That is a ground control with the ECM. However, we need to make sure we are measuring it properly. See that wiring diagram I posted previously. You should not be checking this to ground. You should be putting your meter on 12 volts and then the ground lead of your meter needs to go on this terminal. That way when the ECM grounds the circuit your meter will show 12 volts.

Does that make sense?
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Saturday, March 13th, 2021 AT 6:01 PM
Tiny
JOE WRIGHT2
  • MEMBER
  • 81 POSTS
I understand the concept, but if I have 14 volts on one side of the relay coil and nothing to ground from the ECM, then something from that point back to the ECM or further must be the problem, correct?
It baffles me that I would have 3+ volts on a circuit that only is a ground circuit. L have read across the fuse box terminals with relay removed, and ENG/On. AC/On. From both 87(14vdc) to 86.85(14vdc) to 86 and get 0 volts.
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Saturday, March 13th, 2021 AT 7:37 PM
Tiny
JOE WRIGHT2
  • MEMBER
  • 81 POSTS
My thinking is the ECM is not always sending a ground signal as it should. I would like to see more detailed schematics for the circuitry, so I might trace the wiring, as I am in tune to wiring schematics more than just reading help. I thought the ECM was under the fuse box somewhere, am I wrong? That's what I have read, in everything I have tried to research.
Thanks for all your help and patience.
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Saturday, March 13th, 2021 AT 7:43 PM
Tiny
JOE WRIGHT2
  • MEMBER
  • 81 POSTS
Can you show me where the ECM is located exactly?
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Sunday, March 14th, 2021 AT 8:14 AM
Tiny
JOE WRIGHT2
  • MEMBER
  • 81 POSTS
These are the results, 86 to grnd = 3.2vdc
86 to 85=10.40vdc
86 to pos= 10.40vdc
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Sunday, March 14th, 2021 AT 10:45 AM
Tiny
JOE WRIGHT2
  • MEMBER
  • 81 POSTS
Actual termination points relay removed.
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Sunday, March 14th, 2021 AT 10:51 AM
Tiny
JOE WRIGHT2
  • MEMBER
  • 81 POSTS
So I noticed theses relays are reversible, so you can't put them in wrong, just the numbers reverse.
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Sunday, March 14th, 2021 AT 11:22 AM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Okay. You appear to be on it. 86 to positive and then the A/C turned on only has 10 volts, then you have either resistance in the wiring to the ECM from the relay that is requiring the 3 volts (which you are seeing in the voltage check of 86 to ground), or the ECM is faulty.

There will be a small amount of back feeding through the ECM but you should have very close to battery voltage when testing it this way.

I would just test the resistance between the relay terminal and ECM connector and make sure it is less a half ohm of resistance. If so, it is okay.

Then just jump the relay and make sure the compressor runs and you can check the pressures do this as well. Clearly don't let the compressor run continuously to build the pressure too high but as long as it can get them close to proper levels, then we know the compressor is operating and Freon level is okay.

At that point, I would replace the ECM.

Below is the process for the ECM.
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Sunday, March 14th, 2021 AT 5:52 PM
Tiny
JOE WRIGHT2
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  • 81 POSTS
Okay, but don't the press sensors, and ambient air sensors have something to do with the ECM signals?
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Sunday, March 14th, 2021 AT 6:37 PM
Tiny
JOE WRIGHT2
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  • 81 POSTS
I have stated before that when the compressor runs, it cools just fine.
I can make the clutch make and break by jumping 87 and 30, so I know that is ok! I was under the impression t hat the ECM gets signals from the pressure and ambient sensors. Which in turn allow the ECM to send the ground signal to the relay.
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Sunday, March 14th, 2021 AT 6:43 PM
Tiny
KASEKENNY
  • MECHANIC
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The ambient sensors do have an input to the HVAC control but the PCM is not looking at them to determine anything. The PCM in this system is simply a slave module which means it only turns the compressor on and off. They have the PCM control the compressor because if it needs to cut the compressor for other reasons then the want the PCM to have that control.

The main sensor that determines on/off is the pressure sensor. If the pressure is high then it opens and doesn't allow voltage to the PCM which then cuts the signal to the compressor.

So it only controls the on/off so you are correct that if it comes on and works correctly then the compressor and pressures are most likely just fine. If it were me, I would be putting a PCM in it at this point based on this info.
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Tuesday, March 16th, 2021 AT 6:41 AM
Tiny
JOE WRIGHT2
  • MEMBER
  • 81 POSTS
So the AC safeties are in the same circuit as the clutch? ( N/O relay contact) The PCM is only grounding the relay coil? Also, forgive me, but l cant find the PCM. Where the heck is it? Lol, I don't see it anywhere!
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Tuesday, March 16th, 2021 AT 6:48 AM
Tiny
JOE WRIGHT2
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  • 81 POSTS
Am I looking for the PCM or ECM? The schematic you sent, says ECM controls the ac. I think the PCM is in the trunk? The ECMis under the air box?
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Tuesday, March 16th, 2021 AT 6:53 AM
Tiny
JOE WRIGHT2
  • MEMBER
  • 81 POSTS
You stated both PCM and ECM, so which one needs to be replaced?
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Tuesday, March 16th, 2021 AT 6:54 AM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Sorry. ECM and PCM are basically the same thing. Engine control module vs Powertrain control module. The difference is if this module controls just the engine systems or all of the powertrain.

You have an ECM. I attached the info below on how to replace it.
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Tuesday, March 16th, 2021 AT 7:34 AM
Tiny
JOE WRIGHT2
  • MEMBER
  • 81 POSTS
There's no ECM at that location, I have a 3.6 v-6. I believe that is for the v-8.
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Tuesday, March 16th, 2021 AT 9:19 AM
Tiny
AUTODOCTOR24
  • MEMBER
  • 1 POST
I have the same exact scenario you mention. 2010 Cadillac CTS4 Premium 3.6L AWD - bought it used.
No A/C, When A/C or Defrost is turned on - Clutch doesn't engage and hence compressor doesn't spin

Compressor Clutch is OK - clicks when I jump the relay.
Radiator Fan is OK but doesn't turn on - Fan Runs if I jump 30-87 pins on FAN1 and/or FAN2 relay.

On the FAN1 relay as well as A/C compressor relay, when front Defrost is on,
86 to grnd = 3.5vdc
86 to 85=8.5vdc
86 to pos= 8.5vdc
Battery = 14vdc (when engine is running)

So, it seems to me some sensor's input is being taken by ECM to give lesser voltage at 86 of both Fan1 and Compressor - relays. Other relays, I don't see this issue. That tells me that there is some other sensor whose inputs are causing this 3.5v offset - could it be the ambient temperature sensor (ATS), or the evaporator temperature sensor (ETS)? If the (faulted) ETS is sensing very low temperature, then A/C wouldn't kick in.
I jumped A/C relay to force the compressor run for 1 minute - it ran and both the high and low pressure lines were warm - so there is R134a for sure. Radiator Fan was not spinning, so the refrigerant couldn't get cooled enough in the condensor - or may be pressure is low.

Everything else electrical in the vehicle works fine.

BTW, the PCM (or ECM) is under the fuse box - bit hard to unseat the fusebox and reveal the underside of it to see if there is any corrosion - I loosened it a bit and checked out - it seemed good to whatever extent I could see.

Any help?

Joe Wright2 -Did your issue get solved?
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Thursday, August 3rd, 2023 AT 11:46 AM
Tiny
JC WRIGHT4
  • MEMBER
  • 3 POSTS
The problem was the high pressure control was defective, I no longer have this car.
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Thursday, August 3rd, 2023 AT 12:30 PM

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