vetteoz
Well-known member
Wait until you work on the heater. Rumour has it they put the heater core on the assembly line and built the car around it ;squint:
Help! '87 elect A/C problem
- Thread starter boomdriver
- Start date
rlg
Well-known member
the relay on my 90 is what sends the signal to the ECM that there is a request fro A/C.
gmjunkie
Well-known member
- Thread starter
- #44
boomdriver
Well-known member
Rig,
really?
then we're gonna get a new relay and see if that does anything. I was wondering if that signal happened on the control head circuit board or if there was a relay that did that. Thanks.
yeah Vetteoz...I think thats why I sold my first c-4... heater core.
I looked, I evaluated, and I considered.
I got a "for sale sign" before the next winter....
really?
then we're gonna get a new relay and see if that does anything. I was wondering if that signal happened on the control head circuit board or if there was a relay that did that. Thanks.
yeah Vetteoz...I think thats why I sold my first c-4... heater core.
I looked, I evaluated, and I considered.
I got a "for sale sign" before the next winter....
rlg
Well-known member
Boom,
The relays are cheap, but you can test the one you have by checking the voltage one one wire then turning on the A/C and see if the voltage transfers to the other big wire.
On the 90 the tan wire is the one you ground for the Compressor(I think)
The relays are cheap, but you can test the one you have by checking the voltage one one wire then turning on the A/C and see if the voltage transfers to the other big wire.
On the 90 the tan wire is the one you ground for the Compressor(I think)
- Thread starter
- #46
boomdriver
Well-known member
Thanks.
I was thinking that I should be able to see a signal or voltage transfer with the a/c switched on/off. Might even hear a click...
I was thinking that I should be able to see a signal or voltage transfer with the a/c switched on/off. Might even hear a click...
- Thread starter
- #47
boomdriver
Well-known member
UPDATE..
well, the rebuilt control head did'nt do a darn thing towards making the compressor come on. It sure looks nice...about $150 worth, I guess.
What are the chances of getting a bad power module from the parts store? I got a new one,. tried it, and nothing changed. So I reinstalled the old one and took the new one back.
I am baffled now. There is not a single part of this system that has not been replaced or tested. With voltage always present at the compressor plug, that means the problem HAS to be in the ground that completes the path. Process of elimination takes me back to the power module, again. I can take power from the compressor plug to one side of the coil then ground the other pole on the coil and it will engage.
If no one here has any other ideas, I'm taking it to the auto-electric shop or some GM a/c qualified shop for diagnosis. I am <1" from giving up.
well, the rebuilt control head did'nt do a darn thing towards making the compressor come on. It sure looks nice...about $150 worth, I guess.
What are the chances of getting a bad power module from the parts store? I got a new one,. tried it, and nothing changed. So I reinstalled the old one and took the new one back.
I am baffled now. There is not a single part of this system that has not been replaced or tested. With voltage always present at the compressor plug, that means the problem HAS to be in the ground that completes the path. Process of elimination takes me back to the power module, again. I can take power from the compressor plug to one side of the coil then ground the other pole on the coil and it will engage.
If no one here has any other ideas, I'm taking it to the auto-electric shop or some GM a/c qualified shop for diagnosis. I am <1" from giving up.
rlg
Well-known member
Boom,
The path should be from the control head to the a/c heat programmer controller through the relay to the ECM to the loe pressure switch to the compressor.
PM me your phone number and maybe I can help if the 87 and 90 systems are the same.
The path should be from the control head to the a/c heat programmer controller through the relay to the ECM to the loe pressure switch to the compressor.
PM me your phone number and maybe I can help if the 87 and 90 systems are the same.
vetteoz
Well-known member
Not the way it is shown in '87 FSM
ECM does not play part in compressor operation; only fan
As Boom notes ;
compressor clutch has 12V ; key on.
Clutch coil is controlled by going to ground at the BCM which receives a clutch operating signal ( voltage drop 12V > 6V )) from the head unit .
There is a "A/c on " input signal from head unit to the ECM as well as the A/c coolant fan switch input to ECM
Have you been able to see the afore mentioned voltage drop on the compressor clutch control wire to the BCM ( green / wht ) when A/c turned on?
rlg
Well-known member
vetteoz,
OK, Thats why I made sure everyone knew that my working knowledge was for my 90.
OK, Thats why I made sure everyone knew that my working knowledge was for my 90.
- Thread starter
- #51
boomdriver
Well-known member
I do not recall if it was that wire where I had 6V w/a-c ON. I'll have to ck that again tomorrow afternoon.
Something is happening to the black side of the comp wire...after it leaves the clutch. With the connector cut off, green was 12+V and the blk was only 10V.
Black will be 12+ when the plug with diode is back on. SO, the black wire is going somewhere picking up some kind of voltage from the BCM?
I know the green w/yellow has its 12 from the con-head. Thats happening. new control head. better be happening !
So, Fri pm I'll check for the voltage drop and see if that is happening as signal to the BCM to complete the ground path. Thats the key...why is'nt the ground path being secured.
I went back and put the Ohm meter on the coil again and on the 200 scale it would show 4.3 or so.(found a GM site that said 4.5 was in range for that compressor.
On larger scales I got nothing...zero or near zero resistance. I checked each pole to the grounded body of the compressor and got no continuity. nothing there shorting the coil, or the coil open & tou8ching the conmpressor body. Since I KNOW that everything worked pre-compressor failure, this is really baffling. First thought was a new compressor with a bad coil. Looks like thats not the case. The only other thing that is different is the clean fusable link hot wires that bundle behind the battery. Thats all clean, tight and pretty. besides, if that were screwed up there would be something else not working.
Thats why I am back at the BCM or Power module. Thats where all the power comes from, and is managed, where the control head just directs how much to where. This has got to get resolved pretty quick. I got carpet coming, 2 new door panels complete and weather stripping most things this weekend. then PAINT.!
BUT, none of this laundry list of parts/projects gets done.....until the a/c works right.
Rig, I'll shoot you a PM shortly. vetteoz, you too.
Thanks for all the help,.
Is anyone planning on skipping school and going to the houston vette/chev expo here in march? Anyone thats close......go for it ! I;ll post more info on that in the coming days.
- Thread starter
- #52
boomdriver
Well-known member
3-18-thur
The war rages on...
The car now sits in an auto-electric shop with fresh eyes upon the problem.
I supplied them with the spare ECM, a different NEW power module, and the new control head. Along with the repair history in recent months.
This is what I DO KNOW by way of the last test that I did...
The pressure cycling switch has 4.5V on one side, 0v on the other(ground) side.
The Green/Yellow wire from the control head to the module has 12.9V when "auto" is on, and 4.5v when the control is turned OFF.
The black wire (ground or return) side of the clutch circuit....
I took the plug with the diode off and tested the 2 wires independently. With the plug off, the green always has 12.5+ volts.
The black wire will have 13.5V IF control is switched to "auto". When the control is switched to off, the black wire drops to zero volts, this is with the diode plug cut off and testing the 2 clutch wires seperate of each other.
If I plug in the green wire to the clutch, and run a ground to the other pole on the clutch, the clutch will react and a magnetic field is there.
I'm anxious to see what the REAL problem is. That 2nd new power module was purchased from a different parts store and my hope is that the first module that I bought, was simply a bad unit. I DID run the Ohm meter across the new module and this one has dramatically different resistance readings than the other new one did. However, before installing the $200 module and then owning it for good,(no return if plugged in) I wanted someone else to come to the same conclusions. I'd rather pay someone else and help them earn a living than to just guess and install another part that may or may not be needed.
We'll see in a few hours...
The war rages on...
The car now sits in an auto-electric shop with fresh eyes upon the problem.
I supplied them with the spare ECM, a different NEW power module, and the new control head. Along with the repair history in recent months.
This is what I DO KNOW by way of the last test that I did...
The pressure cycling switch has 4.5V on one side, 0v on the other(ground) side.
The Green/Yellow wire from the control head to the module has 12.9V when "auto" is on, and 4.5v when the control is turned OFF.
The black wire (ground or return) side of the clutch circuit....
I took the plug with the diode off and tested the 2 wires independently. With the plug off, the green always has 12.5+ volts.
The black wire will have 13.5V IF control is switched to "auto". When the control is switched to off, the black wire drops to zero volts, this is with the diode plug cut off and testing the 2 clutch wires seperate of each other.
If I plug in the green wire to the clutch, and run a ground to the other pole on the clutch, the clutch will react and a magnetic field is there.
I'm anxious to see what the REAL problem is. That 2nd new power module was purchased from a different parts store and my hope is that the first module that I bought, was simply a bad unit. I DID run the Ohm meter across the new module and this one has dramatically different resistance readings than the other new one did. However, before installing the $200 module and then owning it for good,(no return if plugged in) I wanted someone else to come to the same conclusions. I'd rather pay someone else and help them earn a living than to just guess and install another part that may or may not be needed.
We'll see in a few hours...
- Thread starter
- #53
boomdriver
Well-known member
SUCCESS
I gotta admit that this a/c problem that many folks helped with was getting the better of me. I am fairly familiar with the C-4 and usually work my way thru the problems with some training and some simple logic.
However, this time all that failed me....
We actually touched upon the problem early in the thread. I set up the struggle years ago...
The new a/c compressor refused to come on because the power module was bad, AND the DIODE in the plug.
I could swap modules all day long and it was not going to operate. The one time that it did operate for a few seconds was without the plug with that diode built in.
The diode was bad. It was back-feeding voltage backwards thru the system, which blew the power module. The diode is a useless piece of faulty engineering. Whats it supposed to protect? The power module? Power cannot come backward in the system and go anywhere with a dead module and the only way that can happen is with a bad diode. Its pointless.
How I elevated the frustration level was innocent enough. Years ago I started running mostly breakers instead of fuses. If there had been a fuse in that line, it would have blown days ago and I would have had a clue that there was a short to ground and been able to trace it. But, the breakers are the "cycling" type that break then reset, so I never got the news that the power wire to the clutch was shorted and getting power thru the wrong way. I saw a different voltage on one side of that circuit, but did not suspect anything in the plug because I never considered the diode being faulty or as anything that could do this.
So, the shop where I took it installed a fuse and watched it blow everytime the green/yellow was grounded. That sent him directly to the plug. The tech snipped the diode out, then was able to jump the circuit and determine that the module was now burned out. We replaced it with the new one that I bought a couple days ago and it operates perfectly.
Lesson learned, breakers may give you a 2nd chance at life in a circuit, but they can hide the evidence of a short circuit,. So I think I will go back to fuses and just keep a couple breakers for emergency stashed in the console. Blown fuses are a beautiful thing...
Big thanks to Vetteoz, Rig and everyone that offered suggestions and gave me their time and help with my attempt to diagnose this problem.
In re-tracing the steps, the plug with the diode was added at the same time as the compressor and I failed to believe that this item could have been at fault.I never checked just the diode. I kept looking for anything that had changed since the old compressor failed, and could think of nothing. That plug had changed. My car has not had a diode in that circuit since 1995...and likely never will. I bought a replacement plug when I ordered the new compressor because I wanted it to be as original as possible.
Like the tech at the shop said..."put it back in?....for what?"
Cost of repair: $75 in labor at auto electric shop. (believe in them)
Parts : $197 for BCM/power module @ carquest.
Aggrevation factor: 9.7
I gotta admit that this a/c problem that many folks helped with was getting the better of me. I am fairly familiar with the C-4 and usually work my way thru the problems with some training and some simple logic.
However, this time all that failed me....
We actually touched upon the problem early in the thread. I set up the struggle years ago...
The new a/c compressor refused to come on because the power module was bad, AND the DIODE in the plug.
I could swap modules all day long and it was not going to operate. The one time that it did operate for a few seconds was without the plug with that diode built in.
The diode was bad. It was back-feeding voltage backwards thru the system, which blew the power module. The diode is a useless piece of faulty engineering. Whats it supposed to protect? The power module? Power cannot come backward in the system and go anywhere with a dead module and the only way that can happen is with a bad diode. Its pointless.
How I elevated the frustration level was innocent enough. Years ago I started running mostly breakers instead of fuses. If there had been a fuse in that line, it would have blown days ago and I would have had a clue that there was a short to ground and been able to trace it. But, the breakers are the "cycling" type that break then reset, so I never got the news that the power wire to the clutch was shorted and getting power thru the wrong way. I saw a different voltage on one side of that circuit, but did not suspect anything in the plug because I never considered the diode being faulty or as anything that could do this.
So, the shop where I took it installed a fuse and watched it blow everytime the green/yellow was grounded. That sent him directly to the plug. The tech snipped the diode out, then was able to jump the circuit and determine that the module was now burned out. We replaced it with the new one that I bought a couple days ago and it operates perfectly.
Lesson learned, breakers may give you a 2nd chance at life in a circuit, but they can hide the evidence of a short circuit,. So I think I will go back to fuses and just keep a couple breakers for emergency stashed in the console. Blown fuses are a beautiful thing...
Big thanks to Vetteoz, Rig and everyone that offered suggestions and gave me their time and help with my attempt to diagnose this problem.
In re-tracing the steps, the plug with the diode was added at the same time as the compressor and I failed to believe that this item could have been at fault.I never checked just the diode. I kept looking for anything that had changed since the old compressor failed, and could think of nothing. That plug had changed. My car has not had a diode in that circuit since 1995...and likely never will. I bought a replacement plug when I ordered the new compressor because I wanted it to be as original as possible.
Like the tech at the shop said..."put it back in?....for what?"
Cost of repair: $75 in labor at auto electric shop. (believe in them)
Parts : $197 for BCM/power module @ carquest.
Aggrevation factor: 9.7
rlg
Well-known member
Glad you got it working and what seems to be a reasonable cost.
My problem with my 90 was the green wire from the ECM that fed the low pressure switch was cut. repaired wire, clutch came on.
My problem with my 90 was the green wire from the ECM that fed the low pressure switch was cut. repaired wire, clutch came on.
rlg
Well-known member
by the way it took 4 days for me to figure out what it was so dont feel to bad about the time element.
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