Help! '87 elect A/C problem

[boomdriver]

I just replaced my compressor because the old one scattered and caused a shaft seal leak. As long as there was gas, the electronic a/c system worked as it should.
Now, the new compressor will not engage even with a minimal gas charge and jumping the low pressure cut-out switch. I have 12+volts at the compressor plug , the low pressure switch plug has 5v on one side and zero on the other. Even with the low press switch jumped, the compressor clutch will not activate. The clutch coil has zero resistance and carries voltage.The clutch is free to move and the compressor rotates. Its brand new.Never been run or installed. The ONLY thing different is the new compressor, yet it will not operate. The diode in the plug is intact. Blower and everything else operates as it should.The underhood fuse is good.

I vaguely recall something about this that tells me that having voltage at the compresser is meaningless without the end of that circuit being managed by the ECM and power module. I am thinking that the power module is fried from lack of cooling when the defroster button was used directing power to the compressor. Although the comp circuit was interrupted, that may have still energized the module and created excessive heat or resistance. With no gas charge the switches prevent the compressor from running. But, that has been jumped and there IS a minimal gas charge on the system at this time.

Anyone have any thoughts? I'm going blind reading the electrical drawings and want to avoid spending $125 on a module based on a "best guess".

What would help would be knowing more about what the voltages should be coming from the PM. I'm not going to run a jumper to the compressor coil. Its not a +/- type of circuit. The current simply has to move thru the coil to energize it by creating a magnetic field. Connecting a pos/neg to the coil would melt it down with battery voltage/amperage, if I am reading this right.
Thanks !

 

[vetteoz]

The clutch has power , key on and is operated by going to ground at the blower control module.Could try grounding that wire and see if clutch engages
Although your module may be faulty ( not uncommon with age ) I doubt you fried it as it also controls the blower fan and operates when the system is in heat mode.
I recall from other posts the system must be charged for it to work

 

[tonylong]

I'm going to need to follow this, too.

 

[boomdriver]

The clutch has power , key on and is operated by going to ground at the blower control module.Could try grounding that wire and see if clutch engages
Although your module may be faulty ( not uncommon with age ) I doubt you fried it as it also controls the blower fan and operates when the system is in heat mode.
I recall from other posts the system must be charged for it to work

I'll look at the grounds in the morning,. The system is partially charged but cannot take anymore gas until the compressor will run, and the compressor low safety switch has been jumped(standard proceedure to charge) and yeilds no change.
The power module is mounted in the a/c evaporator cold side, so when it powers up the only air it gets is ambient temp air or the 40degree of the a/c system. Its not exposed to the heater side. But, it does get VERY hot when its energized, so without cold air blowing across it, that could have done it, or the wire to the compressor may have grounded at some point...Don;t know....
If I knew what the voltage was supposed to be at the module when working right, I could tell in 30 sec if that was the problem or not. It almost has to be.
The system is simple,. It has a control panel that directs or signals, there is the power module that sends the right voltage to the different parts, and there is the programmer that mixes air and operates the vents. The compressor is the last major part. Everything else like fans, cooling relays, are tapped off the main system.
So I'm kinda baffled on this one.....I will be looking at the blower grounds at sunrise.
Thanks for the idea.

 

[vetteoz]

The system is simple,. It has a control panel that directs or signals, there is the power module that sends the right voltage to the different parts, and there is the programmer that mixes air and operates the vents.

I am famillar with the system operation.What I was implying was that the BCM is not reliant on cold air from the evap core for it's heat sink cooling
As you correctly state the BCM is controlled by the head unit.It grounds the clutch wire to turn the compressor on and supplies power to the blower fan
The hi pressure cut off switch and the pressure cycling switch are in series and both have to be closed for the compressor to run
Terminal D ( green/ yellow ) on the BCM has 12V from the head unit. When the clutch is commanded to engage by the head unit it drops to nominal 6V causing the BCM to ground the control wire ( green ) from the clutch
Under normal conditions the two pressure switches maintain terminal C1 on the head unit at ground.If either opens the the head unit sees a rising voltage and applies 12 V to the BCM causing the clutch to disengage

 

[boomdriver]

THANKS!
Thats exactly what I needed to know. That tells me that my problem is either at the ECM or a bad module if there is the signal from the control head. I went thru this ordeal in the exact same place, same way right at 11 yrs ago during a drive thru visit to this city....and it was resolved but I cannot remember the exact details. It was the module after much trial & error repair by a local vette shop that will remain nameless in public.....

I knew that the 12+ V at the compressor plug was not right for operation. Something in my memory kept telling me that it should be half that. Also, the 12V is present with the head control off or set to auto...Next, the brown wire that ends up at the ECM has a value, but the ground for that circuit is open from the module. After more eyestrain last night studying the drawings, I was moving to the conclusion that the module was bad. Now I can check the signal from the head to see if the correct control voltage is present and confirm the diagnosis. I will also see if I can get the head to go into diagnostic mode and review that as well. I should see some kind of code if the module is bad. Still going to test the other 2 pressure switches & plugs, just to make sure the loop is intact..

I appreciate the assistance. Sure seems like these modules were around $75 a few yrs ago....now its $113 to $130 depending on where you buy or which Chinese mfg produced it.

 

[boomdriver]

UPDATE

after many hours of studying drawings, digging thru the harness, messing around with a meter AND the purchase of a new module, I'm still where I was yesterday with this.
I measured values on the new module and compared them to the old one. Not exactly alike but close enough to make me believe that the module is OK.
I changed circuit boards in the control head. No difference. I swapped ECM and Proms, The spare ECM & prom behave exactly as they should(different fan temps,etc).
All the refrigerent line switch plugs show to be alive, and carrying a signal.

Tommorow I'll crawl under it and look at all the grounds on the bellhousing...If there is no damage to be found there, its off to BillyBobs Auto-Electric on monday.
This is making me insane....

 

[vetteoz]

Should not be ECM related , only hookup to ECM is A/c on input signal ( blue on C6 ) to tell ECM A/c is operational so it commands the coolant fan on
There is no feedback from ECM to head unit

.I knew that the 12+ V at the compressor plug was not right for operation.

Not sure if we are talking about same terminal but compressor plug always has 12V, key on .Control as mentioned by going to ground through the module.
Grounding the control (green ) wire that goes to the module should engage the clutch to confirm that the compressor itself is operating correctly

 

[boomdriver]

Thanks again.
I am using a poor set of drawings, but these show that ECM cav B-8 as a signal for a/c clutch engaged and B-9 as open. Thats circuit 959 which is the red/blk wire on the module.
I believe this is the link that the ECM uses to kick the IAC motor up when the a/c is engaged. There is no other relay or connection to the IAC that I can see. I was looking to see if the IAC was part of the ground loops. It is not. All 4 IAC wires run to the ECM.

I suspect the module ground somewhere. I see that circuit 150 & 450 are ground and "power grounds" and all gather at the same place. Its a poor spot for the harness to be exposed right along the bellhousing to drop all the grounds out so they can be bolted down. Thats what I'll look at immediately after trying your suggestion with the green control wire. I understand exactly what you are telling me. Thats the same principle as jumping the cooling fans, by supplying an alternate ground path to the relays control wire.
I also notice that the IAC is not effecting the idle, so the signal there is not being relayed. That comes directly from the ECM, so I am hoping that it IS just a ground issue.The blower motor has its own ground by the housing.

You;re right about the compressor plug, that does maintain 12V...it was the control circuit that I was think of. Sorry about the confusion.

I lost my fiber-glass front air-dam a few days ago when a truck tire came apart right in front of me....that took off the fiber glass, then the whole mess went under the car and felt like an explosion....I am hoping that this damaged the ground wires that are about where the gas pedal is, on the bellhousing underneath. Nothing else was effected, so I have not looked.

I appreciate your input. This will end up being something simple. I can feel the embarrassment..:~)

 

[vetteoz]

I am using a poor set of drawings, but these show that ECM cav B-8 as a signal for a/c clutch engaged and B-9 as open. Thats circuit 959 which is the red/blk wire on the module.

I have the '87 FSM open in front of me.
It shows B8 (ECM ) as the above mention A/c on input signal (circuit #997 blue / white)

Circuit #959 (green / yellow) is the compressor clutch control circuit from head unit to module The one where the voltage drops to make module engage the clutch

Black/red wire at module ; is output to blower fan for speed control .Circuits #65 /965

What I have at module
2 pin plug
Dk green ;clutch control
Red ;12V via fusible link

4 pin plug
Ground
Green/yellow ;clutch on input
Brown ; blower control input
Blk / red ;blower control output

 

[vetteoz]

I believe this is the link that the ECM uses to kick the IAC motor up when the a/c is engaged. There is no other relay or connection to the IAC that I can see. I was looking to see if the IAC was part of the ground loops. It is not. All 4 IAC wires run to the ECM.

When the ECM sees the A/c on input ,it raises the idle and turns fans on through internal programming.There are no specific direct external connections other than the ones used for normal IAC and fan control.
If you were to access the programming you will see there is a setting for how many rpm to raise the speed when A/c on signal is seen

 

[vetteoz]

I suspect the module ground somewhere. I see that circuit 150 & 450 are ground and "power grounds" and all gather at the same place. Its a poor spot for the harness to be exposed right along the bellhousing to drop all the grounds out so they can be bolted down.

All the engine grounds terminate on a large bolt (3/4" head) directly above the oil filter.The only ground to bellhousing is neg cable from battery on pass side

 

[boomdriver]

my grounds were moved to a bellhousing bolt for some reason. The installer did that one himself, same guy that could not figure what the oil temp sender was for...

I have 12V where it should be and 5.9 in a couple other places. One brown wire on the modual only shows 2.3V...makes me wonder if thats broken the harness or if the ground for the module if off. I know this system has power everywhere and this one does. But power is half the issue...it needs good grounds to make use of any of that power.

Tomorrow Morning I will take your info and do some ground searching and get this thing straightened out. I'll also pull the harness out from around the ECM and visually inspect all the plug-ins. I really like the odds of a broken black wire underneath, that would about explain everything. I will get this sorted out tomorrow with your good info.
The ONLY thing that I have not looked at is the programmer. I seriously doubt there is a way for that to keep a circuit open and break my control ground. Programmer seems to be ok..

 

[boomdriver]

Sun...
This just keeps getting more bizzare by the minute.
last night I tried to recall everything that had changed since the last time the a/c worked. I realized that the old compressor was wired direct and did not have the factory plug with the diode. So I did that,....and started it and it RAN...but, the comp would not shut off.
I replaced the plug& diode to see if this was related and it would not engage. I then put the direct connection back and it would not run again. Still no shorts in the clutch coil, no resistance. I even tried the new module. No help.
I did try grounding the control wire from the module and that did nothing.

 

[G Winter]

I had an 89 Riv that you needed to take power off the ECM to get the compressor to kick in after loss of pressure in the AC system. Don't know if the Vetts are the same.

Glenn

 

[boomdriver]

This electronic a/c control is controlled by the keypad or button panel in the center dash. It does relay to the ECM for idle control, but the power is not thru the ECm. Its got its own source. It has grounds that go straight to frame, with a control ground going by the wiper motor, then the cruise control then to frame. For some reason this one is getting conflicting control signals...the thing runs the whole system, blower, indicator lites, vents, heat/cool vents, temp sensors, all just great. But, it will not allow the compressor signal to have a clean path. When it did run, I was unable to turn it off on the panel...
The pressure switches all have control voltage and the compressor has voltage, (12+ on the green and 10.5v on the black) but it acts like there is an open circuit somewhere in the control. It DID engage this morning when stone cold, but never again today. I pulled the harness out of the passenger dash and looked at that mess, and swapped power modules and even ECM's. Nothing. I am starting to think that I have a defective control head even though everything works except for the compressor. I had a different head but that one had a bad display board, so its value is in doubt. There must be a chip/relay in the control head circuit board that manages the compressor signals. I'll search for a new control head Monday and/or a trip to the auto electric shop to get their diaqnosis.
The arctic fronts that are reaching this far south make this more of a challenge than a necessity right now.

 

[geekinavette]

Can you post or email a copy of the wiring diagrams? (I have them for '84, but yours may be quite different)

 

[boomdriver]

Thats what I was just searching for....
I have some really poor drawings for 84 and up...but they have several pages, all the options written to make it confussing, and they shortcut so you have no idea where something is.

While I was searching I did find some good GM advise regarding elect a/c and according to this GM publication, the programmer up by the gas pedal does have as much to say about the compressor as the control head. SO thats getting looked at first thing,.

I;ll go back and find the link and post it for future a/c trouble shooting.

 

[geekinavette]

Well I have '84 diagrams...that setup is very simple.

What bugs me is a comment in your first post about one of the pressure switches measuring only 5V at one of the terminals. Both of those switches are simply in series with the compressor clutch solenoid...the only valid voltages on either pin of either switch are zero or battery voltage.

Also...that clutch solenoid is not a "zero resistance" device. It is indeed operated simply by applying battery voltage and ground across it. Most meters just don't measure 1 or 2 ohms very accurately.

Again...these statements apply to the '84 setup...I don't know what's the same for '87 and what isn't.

As a sidenote...the reason for that diode across the clutch coil leads is to shunt any "inductive kickback" energy to ground (that happens when the clutch is turned off). The compressor should not ever be operated without this diode...and of course don't ever connect it backwards! (the smoke in those things really stinks).

Bill

 

[boomdriver]

Thanks for the good drawing. The 87 electronic control is a little different. The line switches do have a seperate power source from the control head or programmer that is around 5v. The hi-press & cycling switch are series. The low press cut-out is series also from a different circuit.Its the cycling switch that has a control signal then it loops over to the hi-press switch. This is how it relays to the ECM for main cooling fan to come on and the idle kick-up when the a/c is engaged.
The urge to let the auto-electric shop fix it is getting stronger.The new info and detailed instruction that I found last nite have delayed that for now...

I am not 100% sure that with the electronic control, there is a diode thats supposed to be on the compressor plug....This is a replacement plug. The original was destroyed years ago when a belt came apart and wrapped itself around the compressor pulley. That took off the plug, one connector tab and some wire length. I had to solder a connector onto the clutch coil and use male/female connectors on the compressor. There was no diode installed then and it worked perfectly for 11 years. I need to look into that.

Another strange thing is the voltage difference on either side of the comp plug. green side has 12+ as it should for Batt voltage,. but the other side (black) is 10v.

Problem is, that no drawings of the electronic a/c control (opt-68) show which module does what. After several days of tracing and probing, last night I learn that the seemingly innocent programmer thats supposed to be for vent control also has some influence in the compressor engagement. Its stores trouble codes and has set operating parameters. if something is off, then it prevents the compressor circuit from completing just like a bad cycling switch. I have another programmer and will swap them out, if I can contort myself into a position where I can see the darn thing.
I could break down and just go pay someone to get it scanned......but that would'nt be any fun.