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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...

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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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Old 06-23-2017, 07:47 AM   #13
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Update: I got the car back Tuesday afternoon. Everything was working however, the mechanic told me his system was detecting a leak they shot dye onto the system and didn't find anything on top of the motor. He checked the condenser as best he could without taking the covers off. The system is holding a charge but isn't ice cold.

I bought a thermometer and the best temp I saw was 60* day time, 50* at night. I was away the past two days and thw car sat in my driveway. I drove it work today and had the same temp readings so it is holding the charge. There was a lot of moisture on the dryer and lines which means low freon I gave it a shot of freon drove it around around bit and it's still at the he same temperature as before.

now what do I do? For the money in spent I want it to be ice cold.
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Old 06-23-2017, 10:12 AM   #14
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When the AC is running is the accumulator and fat pipes ice cold?
Is the compressor running and cycling normal and not cycling off every 5 seconds?

Is the temperature control set to full cold, blend door going fully to the cold position? If it is not the AC will not be cold.

For the money you spent I would go back to the mechanic and have him check it out
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Old 06-23-2017, 11:37 AM   #15
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Pipes are very cold. The compressor isn't cycling excessively. Temp is set full blast

Last edited by The Thomas J; 06-23-2017 at 11:38 AM.
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Old 06-23-2017, 05:00 PM   #16
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When the AC is running is the accumulator and fat pipes ice cold?
Is the compressor running and cycling normal and not cycling off every 5 seconds?

Is the temperature control set to full cold, blend door going fully to the cold position? If it is not the AC will not be cold.

For the money you spent I would go back to the mechanic and have him check it out
How do you check the Blend Door position?
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Old 06-23-2017, 05:22 PM   #17
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How do you check the Blend Door position?
I put the heat on and turned the temp up to 90* I hear the blend door moving and the feel the temperature change.

I put the freon can on again when I got home from work and the gauge on the can is showing that the system is full. It's right on the black line between green and red. I don't think it's leaking. It get's colder when I drive but not cold enough. It's still hanging in that 55* to 60* range. It's almost 90* here in NYC today and even with the sun beating down on me through the targa-top and my untinted windows I wasn't sweating which is good. But, I want it to be perfect.
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Old 06-23-2017, 06:46 PM   #18
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I put the heat on and turned the temp up to 90* I hear the blend door moving and the feel the temperature change.

I put the freon can on again when I got home from work and the gauge on the can is showing that the system is full. It's right on the black line between green and red. I don't think it's leaking. It get's colder when I drive but not cold enough. It's still hanging in that 55* to 60* range. It's almost 90* here in NYC today and even with the sun beating down on me through the targa-top and my untinted windows I wasn't sweating which is good. But, I want it to be perfect.
I think the blend door is inside the ductwork. If it only opens part way you wonít get full heat or AC or defrost. My defrost ducts are putting out air all the time, thatís why I ask. As I research why my AC isnít working in this COLO heat!

I have seen a photo of some really gummed up ductwork in a C4.
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Old 06-23-2017, 08:40 PM   #19
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How do you check the Blend Door position?
If it's anything like the later models I'm familiar with you can remove the blower control module from the top of the evaporator box and look inside with a flashlight (under hood). Set the fan to low and with key ON, (no engine) set to max heat then max cold. The door should move about 90* from end to end and should be able to see the door close off at full deflection.

It may need to be calibrated for full deflection. Check this thread last post for info.

https://www.corvetteforum.com/forums...r-1988-c4.html
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Old 06-26-2017, 03:58 PM   #20
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Thanks much PCOLT!
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