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89 Corvette C68 ac compressor won't engage.

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Old 07-07-2018, 08:33 PM
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Default 89 Corvette C68 ac compressor won't engage.

Hi Corvetteforum,
I have been working on my friends 89 Corvette and it has been beating me to death trying to figure out why I can't get the AC to work. When I started on the car someone had jury rigged it to get it to work. They installed a switch inside and powered the compressor directly. I removed the switch and repaired the wiring but I haven't been able to get it to engage the compressor. I have verified that the clutch and power side wiring is good because when I supply a ground to the ground side, the clutch engages. I verified that the refrigerant is good with my gauges, and the car cools off when I jumped the ground and engaged the compressor. I replaced the blower resistor module because I see on the wiring diagram that the ground appears to be supplied by the blower resistor module. I checked all the wiring from the resistor to the head unit and it all shows good, so I replaced the C68 head unit with a rebuilt one. I still can't get it to work. I checked the low pressure switch and it is good. When I unplug the high pressure switch the cooling fan turns on. At this point I'm not sure what to do next. Any ideas? Thanks, Jim
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Old 07-07-2018, 08:38 PM
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Also I meant to mention, I checked for any fault codes in the head unit and I show 00 in mode 8. So I guess that means there's no problem that it identifies.
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Old 07-07-2018, 08:57 PM
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Originally Posted by Shdwjim View Post
Also I meant to mention, I checked for any fault codes in the head unit and I show 00 in mode 8. So I guess that means there's no problem that it identifies.
I would start with whatever part of the hvac control calls for cooling. It seems to me you've every thing but that. Something has to say.. hey its warm in here please send cool air. It certainly would have to do with the temperature setting on the hvac control.
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Old 07-07-2018, 10:50 PM
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Is there something separate that you have to do to get it to call for the AC to engage? I don't see anything other than dialing the temperature down on the climate control setting on the head unit. I don't see anything to specifically engage AC.
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Old 07-07-2018, 11:08 PM
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Originally Posted by Shdwjim View Post
I have been working on my friends 89 Corvette and it has been beating me to death trying to figure out why I can't get the AC to work. When I started on the car someone had jury rigged it to get it to work. I replaced the blower resistor module because I see on the wiring diagram that the ground appears to be supplied by the blower resistor module.
Thanks for stepping up to help a friend! You've got a special challenge when someone's been in there messing with the wiring! Do you have the genuine Factory Service Manual (FSM) or a generic (and useless) book? I don't know the '89 wiring, but I would be very surprised if the blower resistor module has anything to do with the compressor ground.

I checked the low pressure switch and it is good.
Please tell us the procedure you used to check the low pressure switch. Do you get 12 v. on the low pressure switch when you dial down the temperature control?

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Old 07-07-2018, 11:54 PM
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I'm using wiring diagrams from Mitchell's Pro Demand. For the test to check the low pressure switch I checked the continuity through the switch to make sure it's not showing open and I also tried a jumper wire across the terminals on the wire connector in case there was some sort of intermittent thing going on with the switch.
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Old 07-08-2018, 12:01 AM
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Also, looking at the schematic, it appears that the head unit sends a signal to the blower resistor so it can pull in the ground. The ground side of the wiring for the compressor does go directly from the compressor to the blower resistor. I didn't believe it either until I started with this car. I have worked on quite a few AC systems in cars and this one has definitely been a learning experience. I just need to finish learning how fix it. Lol Thanks for your help.
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Old 07-08-2018, 12:28 AM
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The low pressure switch was something else that is different than what I have seen on other cars. The diagram calls it a pressure cycling switch. There's that switch and the high pressure cutout switch in the same circuit and if they both are seeing correct pressures it sends a ground signal back to the head unit. Or at least that's what the diagram is showing and that's how the car is set up. I didn't have to fix any of that wiring.
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Old 07-08-2018, 12:51 AM
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That's the way I read those switches for my '88, i.e. both must be closed.
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Old 07-08-2018, 01:25 AM
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Post the schematic and this forum will get you through this simple.problem.
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Old 07-08-2018, 02:18 AM
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I'll ask again: Do you get 12 v. on the low pressure/cycling switch when you dial down the temperature?

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Old 07-08-2018, 02:49 AM
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Old 07-08-2018, 02:53 AM
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Hot rod Roy, No I don't get 12 volts at the switch. The end after the switch goes to ground and the switches are both through switches. The other end goes to the head unit, so it appears that it must send a ground signal to the head unit.
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Old 07-08-2018, 08:55 AM
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Originally Posted by Shdwjim View Post
Hot rod Roy, No I don't get 12 volts at the switch. The end after the switch goes to ground and the switches are both through switches. The other end goes to the head unit, so it appears that it must send a ground signal to the head unit.
You should have voltage at the low side switch when you turn on the ac. Either 12 volts or maybe 5 volts. Probably 12. Don't expect it to light a test light, although it may. This voltage originates from terminal c1, a light green wire, at the ac/heater control, then passes through the high side switch, then passes through the low side switch to ground.

Reconnect everything, start the car, turn on the ac to its lowest temp setting, disconnect the low side switch, otherwise known as the cycling switch, both wires should be black if your schematic is right (its probably not right) test for voltage at the low side switch connector. Use a voltmeter and not a test light. If you don't have voltage, then go to the high side switch and repeat the test. If you don't have voltage at the high side switch connector, then backprobe terminal C1 at the control head. Make sure its a light green wire. Use a needle or something, don't try a jam a test lead into the body of the connector.

Based on what you have reported in your above posts, it seems like you have an open in the wiring between the control head and the high side switch.

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Old 07-08-2018, 09:47 AM
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Originally Posted by Shdwjim View Post
Hot rod Roy, No I don't get 12 volts at the switch. The end after the switch goes to ground and the switches are both through switches. The other end goes to the head unit, so it appears that it must send a ground signal to the head unit.
It will be helpful on this problem, and in the future, if you revise how you think about circuits that are wired this way.
While you are correct (in a very limited way) that the head unit detects when the circuit is grounded, there is no such thing as a ground signal.

There is only voltage, or no voltage because the circuit is grounded.

In this case, the head unit sends voltage through the circuit, and when that voltage finds a ground path (through the high and low side switches), only then will the compressor operate. Said another way, as long as there is voltage on that circuit, because there is no ground path (open switches or open wiring) the compressor will not operate.

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Old 07-09-2018, 03:16 PM
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Sorry I didn't get a chance to look at the car yesterday to check it out. I checked it today and I have a 5.11 volt reading at the sensor. I checked at both switches and I even tried bypassing the switches and gave it a good ground and the compressor still doesn't engage.
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Old 07-09-2018, 03:53 PM
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Originally Posted by Shdwjim View Post
Sorry I didn't get a chance to look at the car yesterday to check it out. I checked it today and I have a 5.11 volt reading at the sensor. I checked at both switches and I even tried bypassing the switches and gave it a good ground and the compressor still doesn't engage.
Ok that amount of voltage at those connectors sounds normal.
The next thing I would do, key on engine running, ac system on, temp at the lowest setting, test for voltage (with voltmeter) at terminal C, a dark green and yellow wire, at the blower control module. You should have voltage there.
If you have zero volts, connect a test light to battery positive, (so that you have 12 volts at the tip of the test light probe) now backprobe that same terminal C at the blower module. What happens ?
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Old 07-09-2018, 03:57 PM
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Originally Posted by Shdwjim View Post
Sorry I didn't get a chance to look at the car yesterday to check it out. I checked it today and I have a 5.11 volt reading at the sensor. I checked at both switches and I even tried bypassing the switches and gave it a good ground and the compressor still doesn't engage.
I assume you mean the high and low side switch connectors ?
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Old 07-10-2018, 07:05 PM
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Yes I checked at the high and low switch connectors. Dark green and yellow wire has 9.38 volts at the connector.
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Old 07-10-2018, 09:48 PM
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Originally Posted by Shdwjim View Post
Yes I checked at the high and low switch connectors. Dark green and yellow wire has 9.38 volts at the connector.
9.38 is a weird number. I would expect 12. There is also a splice where the wire changes colors from dark green and orange to dark green and yellow.
What is the voltage coming out of the control head at terminal c3 dark green and orange ?
You might try and backprobe terminal c at the blower module with a full12 volts to see what happens. Use a test light connected to battery positive.
You have to have a full 12 volts at the 2 wire connector terminal B red wire, it should be able to light a test light. You need a solid ground terminal D of the 4 wire connector black wire. It should also be able to light a test light if you connected the light to battery +. Double check all the female side connector terminals and make sure one of them isn't all wallowed out and not making contact.
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