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New A/C compressor won't engage

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Old 06-18-2017, 12:33 AM   #1
The Thomas J
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Default New A/C compressor won't engage

My old A/C compressor was eating clutches but was working, blowing cold air, no leaks etc.. I purchased a brand new Denso from Rock auto and had my mechanic install the compressor a freon converter kit and, I needed a new alternator as well. My mechanic has informed me that he installed everything but the compressor won't engage. He said it's wired right, and they attempted to put freon in but it won't kick on. He told me that they believe it has something to do with the blower motor module which is an A/C delco part and he is having a hard time locating one. Does this sound right or is he screwing me over and just going to install a $20 low-pressure switch and charge me God knows how much $$$$.
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Old 06-18-2017, 12:53 AM   #2
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Doesn't sound right at all. After the system is evacuated you should be able to get at least 12oz of gas into the system even if nothing else works. That's usually enough to get the compressor to start for at least a few seconds or so.

The blower motor module is just that- it controls the blower speed. You didn't say but if you have the automatic a/c those usually fail "on" high. they aren't that hard to find but are pricy. ignore that if you have the manual system.

Anyway, compressor clutch problems usually aren't all that difficult to diagnose. You can even jump the low pressure switch easily if that's suspected. Hard to tell you anything are without the gory details.

So just my two...

Last edited by Tod Stiles; 06-18-2017 at 12:53 AM.
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Old 06-18-2017, 12:57 AM   #3
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Doesn't sound right at all. After the system is evacuated you should be able to get at least 12oz of gas into the system even if nothing else works. That's usually enough to get the compressor to start for at least a few seconds or so.

The blower motor module is just that- it controls the blower speed. You didn't say but if you have the automatic a/c those usually fail "on" high. they aren't that hard to find but are pricy. ignore that if you have the manual system.

Anyway, compressor clutch problems usually aren't all that difficult to diagnose. You can even jump the low pressure switch easily if that's suspected. Hard to tell you anything are without the gory details.

So just my two...
I have the automatic system.
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Old 06-18-2017, 01:19 AM   #4
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On early C68 systems the blower control module also controls the compressor clutch. You can tell if you have this configuration because all the cavities in the blower control module connectors will have wires in them.
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Old 06-18-2017, 09:35 AM   #5
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On early C68 systems the blower control module also controls the compressor clutch. You can tell if you have this configuration because all the cavities in the blower control module connectors will have wires in them.
Ok, now were getting some where. What would be the reason the new clutch isn't communicating with the module any more?
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Old 06-18-2017, 10:02 AM   #6
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If old worked, the new should work unless it is defective. If you look out on the web, you will see some bad reviews of Denso A/C compressors and some interesting notes that they partner with other mfg'ers and package them under their name. You may or may not get what you think you are getting. Same with Rock Auto. My son bought Moog parts from them and they shipped them from a fly by night auto in Cleveland Ohio. Weren't even Moog parts in the boxes, were Napa, still in the Napa bags and weren't even the right part. Then Rock Auto pulled their "you ordered the wrong part" crap.

If the old compressor is still around, if you plug in the electronics to it, you could see if it will fire up for a second or 2. The low pressure switch will be activated from the system in the car, just see if the old one will try to spin up. That should tell you if the new one has an issue.
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Old 06-18-2017, 04:12 PM   #7
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This is really easy to get a direction on.

Do 2 actions, why 2, for verification.

1 - Disconnect the wire to the clutch. Put 12 volts directly to the clutch wire and see if it engages.
No need to run the engine, just see if the center shaft of the compressor is locked to the outer pulley.

2 - While the car is running and AC on, measure the voltage to the clutch and see if it is 12 volts.

I would also make sure that no weird thing is going on like the other end of the clutch coil is not grounded. You can just make a resistive measurement for this to make sure.


Results, if there is no 12 volts to the clutch it's the system in the car. If you apply 12 volt the clutch and it does not engage, it's clutch related and might be bad.
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Old 06-18-2017, 06:34 PM   #8
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This is really easy to get a direction on.

Do 2 actions, why 2, for verification.

1 - Disconnect the wire to the clutch. Put 12 volts directly to the clutch wire and see if it engages.
No need to run the engine, just see if the center shaft of the compressor is locked to the outer pulley.

2 - While the car is running and AC on, measure the voltage to the clutch and see if it is 12 volts.

I would also make sure that no weird thing is going on like the other end of the clutch coil is not grounded. You can just make a resistive measurement for this to make sure.


Results, if there is no 12 volts to the clutch it's the system in the car. If you apply 12 volt the clutch and it does not engage, it's clutch related and might be bad.
Honestly, I feel like it's a bad ground. The mechanic clipped the harness on the new compressor and Re wired it to clean up the mess that was there from past owners. When I put the new clutch on I had it wired for power and wired for ground separately and it worked fine. I think he has the wires crossed.
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Old 06-18-2017, 07:42 PM   #9
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Honestly, I feel like it's a bad ground. The mechanic clipped the harness on the new compressor and Re wired it to clean up the mess that was there from past owners. When I put the new clutch on I had it wired for power and wired for ground separately and it worked fine. I think he has the wires crossed.
A bad ground is easy to check out.
I would follow my gut feeling, something's missed, usually simple.

Some mechanics fall on their face when it comes to electrical. Just donít where to start or how to troubleshoot basic electrical and donít even do the basic checks. Thing is, cars of today are more electrical and not mechanical as in the earlier years.
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Old 06-19-2017, 02:44 PM   #10
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I'd like to add a couple possibilities; there are several A/C system faults that can register in the PCM, and several that can register in the C68 System if so equipped. If any of these discrete faults register then your Compressor is locked out, to clear these you simply disconnect your Battery for a minute or so and retry. Another possibility is the "Air gap" at the Clutch assy its-self, check this with a feeler gauge. a good rule of thumb is it should be about the same thickness as a business card (010-020) in
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Old 06-19-2017, 04:36 PM   #11
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I'd like to add a couple possibilities; there are several A/C system faults that can register in the PCM, and several that can register in the C68 System if so equipped. If any of these discrete faults register then your Compressor is locked out, to clear these you simply disconnect your Battery for a minute or so and retry. Another possibility is the "Air gap" at the Clutch assy its-self, check this with a feeler gauge. a good rule of thumb is it should be about the same thickness as a business card (010-020) in
so, as I mentioned earlier there was a mess of wires under the hood and lumed that I thought was from the factory. It appears that a previous owner, and there were quite a few before the car got to me had an issue with the compressor not engaging because of the blower control module. Instead of replacing it he or she somehow was able to bypass it and run a hot wire with a relay off the climate control from the blower motor module. In the process leaving a hot wire and a small drain on the battery as well..My battery would die after a week or two if I didn't start or hook it up to the tender. This may have also shortneed the life of the alternator as well. So it looks like I may have a solved several issues by changing out the blower motor control module. When I went by today he was working on it and trying to un **** the fuckery prior to my ownership.

Neither one of us had an issue finding the module. However this went from like a $750 bill with parts to $1,200. Not really happy about it. I mean how could I be. But, it will be fixed and fixed right. As far as we can tell this is the only hatchet job that has been done to what was pretty well taken care of car.

fingers crossed that this solves the issues. I would appreciate any more thoughts and suggestions you may have.

Last edited by The Thomas J; 06-19-2017 at 04:38 PM.
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Old 06-20-2017, 09:30 PM   #12
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If it worked before replacing- did the mechanic disconnect the battery? If he didn't, as I didn't when I replaced the ac compressor, it did not engage- disconnected the battery for 5 minutes, hooked back up, charged and clutch engaged like it was supposed to. Altho this might be too easy....
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