AC compressor question - ZR-1 Net Registry Forums

mlipmd · 06-04-2021

The '90 Zr-1 I just got probably hasn't had AC for quite a while and the PO told me it was probably just low on freon. When the AC is turned on, the compressor does not go on. It is not frozen, you can turn the inner shaft, but bypassing the low pressure switch connector will not get it to spin. Where can I apply 12V to see if I can activate the clutch? I don't want to fry something and I can't follow the wires coming out of the compressor on the drivers side as they disappear under the plenum. I heard there is a relay behind the battery but haven't looked into there yet.

I connected a can of refrigerant with a valve in the line and the static pressure measured with that, on the low side, was ~50psi with an ambient temp of ~85, so it's probably low on freon. I didn't measure high side pressure because I don't have a gauge set yet. You can't really add freon to the system if the compressor's not running and right now I don't know if the clutch or the compressor is any good. It's getting hot in FL now and I want the AC, but not really wanting to replace the entire system right now, and with everything packed in the engine bay, that looks like a bitch of a job.
Anybody have any suggestions on how I can test the clutch, where I apply 12v to the compressor? Or any other help?
And as an aside, the blower motor runs but very little air comes out of any of the vents. I pulled the blower control module and took a look inside and it's clean as a whistle in there, so that problem may be the vacuum/relay assembly tucked way up underneath the driver's dash.
So in addition to the fun of driving it, I have a new toy to play with for quite a while 'till I get all this sorted out.
I have ordered a FSM but it hasn't arrived yet and you guys know more than that book anyway.

jss06c6 · 06-04-2021

Disconnect the low pressure switch just outside the evaporator. Take a paper clip and short across the contacts on the plug. This will allow the compressor to kick on. Don't run it this way too long unless you're recharging the system. You should also see a "09" code when the system is low on freon.

Assuming it's still R12? If so, probably best to find someone that can service R12. It's going to be pricey though. Most have converted over to R134a.

Sent from my SM-G950U using ZR-1 Net Registry mobile app

jss06c6 · 06-04-2021

Your blower motor control module is also suspect since you have very little fan speed. They are notorious for degrading when they get old..

Sent from my SM-G950U using ZR-1 Net Registry mobile app

mlipmd · 06-04-2021

Thanks for the suggestions, but - I did that, compressor did not turn on. See my third sentence up above.
No codes when I interrogate the electronic control on the dash. Just get 00 when I press the 2 fan speed buttons. And nothing when I press the middle button between them.
Has already been converted to R134 by another PO because of the connector on it.

mlipmd · 06-04-2021

The fan speed I don't think is the issue, it changes from low to high with the control buttons. It just doesn't blow a significant amount of air out of the vents, either the dash, floor or defroster. There may be a diverter door stuck or that vacuum manifold control for the vents under the driver's dash is plugged. I may also possibly have a vacuum leak but the pumps are not running like a leak was present.
So I have 2 issues, the compressor won't go on, shorting or by passing the low pressure switch doesn't work and I need to know some other way to apply 12V to it to see if the clutch activates, and I need to figure out why there is very little air coming out of the HVAC vents. Something could also be plugged up, like a rat's nest in there somewhere but at least right by the blower and the module, it's clean in there.

Marc Haibeck · 06-05-2021

Grounding pin C16 on the ECM will directly activate the compressor clutch relay. If the compressor runs, your problem is that the ECM is not receiving an "AC Request" signal.

mlipmd · 06-05-2021

Marc - Thanks for that. I think I know where that is, at the light gray connector to the ECM, the 2nd from the left on the driver's side, a yellow wire. Just so I don't blow anything up and damage the ECM (which we just had repaired by you recently), do I ground that pin in the ECM directly after removing the connector, do I ground the corresponding socket in the connector after removing it from the ECM, or do I push something like a paper clip alongside the yellow wire leading to pin C16 leaving the connector in place to accomplish that? If the clutch activates, then good, the problem is elsewhere leading to that point. If the clutch won't activate, then it is kaput, right? Is that rebuildable or do I then have to replace the whole AC system? If the ECM is not getting the request signal, the problem could also be in the dash control module with something as stupid as dirty buttons. Thanks for your help.
Marc

Edit - my connectors to the ECM are all the same color-red.

mlipmd · 06-05-2021

OK, I got it fixed, I now have cold AC.
Basically the AC system is shut off by the low and high pressure sensors/switches if either are out of nominal readings. Bypassing the low pressure connector did nothing but applying 12V to the blue wire in the connector activated the electromagnetic clutch and that was a great sound to hear and great sight to see.
You cannot add freon to the system unless the compressor is running so I turned on the AC system with the engine running and left the 12V jumper in place to keep the compressor running 'till I filled the system to the correct pressure reading the gauge on the can (I know it's not as accurate as the manifold set reading both sides, but WTH).
When I removed my jumper cable energizing the compressor clutch and reconnecting the lead to the sensor, the clutch and compressor would not turn on, even though there was enough pressure in the system.
That's because the ECM has those bad memories of low pressure in it's brain and you have to re-educate it by wiping out it's memory. I did that by disconnecting the neg battery lead for 30 sec., reconnecting it, restarting the car, turning on the AC, and voila, the compressor and the AC fans runs as ordered and I have cold AC.
I did get the code 09 (low freon) on the AC panel on the dash before and it's gone now.
So right now, all's well with that part of my system and I have to wait and see how long the freon charge will last. Cold air blows out of the dash vents but not the volume I expect (fan speed does change) so one of these days I may investigate the distribution vacuum manifold under the driver's dash with the problem with the nipple connections that has been described elsewhere.
Nothing like getting a 31 yo car and getting EVERYTHING to work.

06-04-2021	#1
mlipmd Join Date: Mar 2021 Location: Crawfordville, FL Posts: 216	AC compressor question The '90 Zr-1 I just got probably hasn't had AC for quite a while and the PO told me it was probably just low on freon. When the AC is turned on, the compressor does not go on. It is not frozen, you can turn the inner shaft, but bypassing the low pressure switch connector will not get it to spin. Where can I apply 12V to see if I can activate the clutch? I don't want to fry something and I can't follow the wires coming out of the compressor on the drivers side as they disappear under the plenum. I heard there is a relay behind the battery but haven't looked into there yet. I connected a can of refrigerant with a valve in the line and the static pressure measured with that, on the low side, was ~50psi with an ambient temp of ~85, so it's probably low on freon. I didn't measure high side pressure because I don't have a gauge set yet. You can't really add freon to the system if the compressor's not running and right now I don't know if the clutch or the compressor is any good. It's getting hot in FL now and I want the AC, but not really wanting to replace the entire system right now, and with everything packed in the engine bay, that looks like a bitch of a job. Anybody have any suggestions on how I can test the clutch, where I apply 12v to the compressor? Or any other help? And as an aside, the blower motor runs but very little air comes out of any of the vents. I pulled the blower control module and took a look inside and it's clean as a whistle in there, so that problem may be the vacuum/relay assembly tucked way up underneath the driver's dash. So in addition to the fun of driving it, I have a new toy to play with for quite a while 'till I get all this sorted out. I have ordered a FSM but it hasn't arrived yet and you guys know more than that book anyway.

06-04-2021	#2
jss06c6 Join Date: Aug 2015 Location: Magnolia, Texas Posts: 884	Re: AC compressor question Disconnect the low pressure switch just outside the evaporator. Take a paper clip and short across the contacts on the plug. This will allow the compressor to kick on. Don't run it this way too long unless you're recharging the system. You should also see a "09" code when the system is low on freon. Assuming it's still R12? If so, probably best to find someone that can service R12. It's going to be pricey though. Most have converted over to R134a. Sent from my SM-G950U using ZR-1 Net Registry mobile app

06-04-2021	#3
jss06c6 Join Date: Aug 2015 Location: Magnolia, Texas Posts: 884	Re: AC compressor question Your blower motor control module is also suspect since you have very little fan speed. They are notorious for degrading when they get old.. Sent from my SM-G950U using ZR-1 Net Registry mobile app

06-04-2021	#4
mlipmd Join Date: Mar 2021 Location: Crawfordville, FL Posts: 216	Re: AC compressor question Thanks for the suggestions, but - I did that, compressor did not turn on. See my third sentence up above. No codes when I interrogate the electronic control on the dash. Just get 00 when I press the 2 fan speed buttons. And nothing when I press the middle button between them. Has already been converted to R134 by another PO because of the connector on it.

06-04-2021	#5
mlipmd Join Date: Mar 2021 Location: Crawfordville, FL Posts: 216	Re: AC compressor question The fan speed I don't think is the issue, it changes from low to high with the control buttons. It just doesn't blow a significant amount of air out of the vents, either the dash, floor or defroster. There may be a diverter door stuck or that vacuum manifold control for the vents under the driver's dash is plugged. I may also possibly have a vacuum leak but the pumps are not running like a leak was present. So I have 2 issues, the compressor won't go on, shorting or by passing the low pressure switch doesn't work and I need to know some other way to apply 12V to it to see if the clutch activates, and I need to figure out why there is very little air coming out of the HVAC vents. Something could also be plugged up, like a rat's nest in there somewhere but at least right by the blower and the module, it's clean in there.

06-05-2021	#6
Marc Haibeck Join Date: May 2007 Location: Addison IL Posts: 250	Re: AC compressor question Grounding pin C16 on the ECM will directly activate the compressor clutch relay. If the compressor runs, your problem is that the ECM is not receiving an "AC Request" signal.

06-05-2021	#7
mlipmd Join Date: Mar 2021 Location: Crawfordville, FL Posts: 216	Re: AC compressor question Marc - Thanks for that. I think I know where that is, at the light gray connector to the ECM, the 2nd from the left on the driver's side, a yellow wire. Just so I don't blow anything up and damage the ECM (which we just had repaired by you recently), do I ground that pin in the ECM directly after removing the connector, do I ground the corresponding socket in the connector after removing it from the ECM, or do I push something like a paper clip alongside the yellow wire leading to pin C16 leaving the connector in place to accomplish that? If the clutch activates, then good, the problem is elsewhere leading to that point. If the clutch won't activate, then it is kaput, right? Is that rebuildable or do I then have to replace the whole AC system? If the ECM is not getting the request signal, the problem could also be in the dash control module with something as stupid as dirty buttons. Thanks for your help. Marc Edit - my connectors to the ECM are all the same color-red. Last edited by mlipmd; 06-05-2021 at 02:50 PM.

06-05-2021	#8
mlipmd Join Date: Mar 2021 Location: Crawfordville, FL Posts: 216	Re: AC compressor question OK, I got it fixed, I now have cold AC. Basically the AC system is shut off by the low and high pressure sensors/switches if either are out of nominal readings. Bypassing the low pressure connector did nothing but applying 12V to the blue wire in the connector activated the electromagnetic clutch and that was a great sound to hear and great sight to see. You cannot add freon to the system unless the compressor is running so I turned on the AC system with the engine running and left the 12V jumper in place to keep the compressor running 'till I filled the system to the correct pressure reading the gauge on the can (I know it's not as accurate as the manifold set reading both sides, but WTH). When I removed my jumper cable energizing the compressor clutch and reconnecting the lead to the sensor, the clutch and compressor would not turn on, even though there was enough pressure in the system. That's because the ECM has those bad memories of low pressure in it's brain and you have to re-educate it by wiping out it's memory. I did that by disconnecting the neg battery lead for 30 sec., reconnecting it, restarting the car, turning on the AC, and voila, the compressor and the AC fans runs as ordered and I have cold AC. I did get the code 09 (low freon) on the AC panel on the dash before and it's gone now. So right now, all's well with that part of my system and I have to wait and see how long the freon charge will last. Cold air blows out of the dash vents but not the volume I expect (fan speed does change) so one of these days I may investigate the distribution vacuum manifold under the driver's dash with the problem with the nipple connections that has been described elsewhere. Nothing like getting a 31 yo car and getting EVERYTHING to work.