Help! '87 elect A/C problem

[geekinavette]

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.

Yeah that compressor ain't gonna kick in with only 2V applied! Something is very wrong on the ground side. Is that supposed to be a direct ground connection (switching happens on the 12V side), or is it a switched ground?

 

[boomdriver]

Its switched....the path is completed in the power module, but that has to have the "yes" command from the control head....as far as I can see. The ECM is involved mostly as a relayed thing, not as a control.
The new info that I found last night suggest that there is more to it than all that.....with the vent programmer being part of the command loop.
What SHOULD happen is a 12V signal is initiated by the control head (the auto button) and then the system runs a check list to decide what to do. Whats the temp inside vs outside? is the engine running? what is the blower status? If the stars align, it gets a "yes" signal and allows a ground to complete back in the power module. If there is a "no" signal, or an open circuit it will not complete that controlled ground path. So, the power to the compressor is ON all the time. Control is thru the grounds like every other C-4 engine management servo.
My drawings are pretty useless. They do not even show the programmer or the detail of the control head, just a note that its there. All I have is the main harness drawing with the C-68 option. The details are pretty vague.
I located a reman control head for $149...that takes about a week to get right now. I have a programmer, and as soon as weather permits I'll install that and see what happens.
I know more of what its NOT than what the problem is...Its not wiring or basic grounds. All is intact. It's not the power module. Tried that. its not ECM related,. traded that. I thought that it had to be a control issue, and by process of elimination end up with the control head OR the programmer and the article written about the programmer is surprising to me. Wish I had my FSM near here...

 

[geekinavette]

Well sounds like you have a good handle on it. Without diagrams I don't think I can really be of any more help...though would if I could!

Good luck with it and keep us posted!

 

[boomdriver]

lol...thanks. I have learned more in the last 3 days than I had in the last 20yrs regarding the a/c controls....I appreciate everyones comments and suggestions.

Here is the link to the article that I mentioned. Interesting "how to" reading. Note the date it was written.1999

C-68 Electronic A/C System

 

[vetteoz]

A clarification
The programmer then applies a low amperage, 8-12 volt signal to the high pressure switch. If the system pressure is under 430 psi, the switch is closed and the signal continues to the pressure cycling switch. If the system pressure is over 25 psi, the contacts are closed and the signal is sent to the PCM where it is recorded as a request for compressor engagement.
In the 87, the line through the pressure switches is to ground, not to the ECM. When the head unit sees ground on the pressure switches it signals the ECM through a seperate wire

Have you seen
http://forums.corvetteforum.com/1568603057-post3.html

 

[boomdriver]

yes, Thats one of the things that I was uncertain about in that article, the route of the pressure switch series. The C-60 (manual-air) has a signal to the cycling switch, while the C-68 is a series of grounds.

I have control voltage in a switch on one side. That must be the fan switch as nothing else makes sense. This switch is 1" away from the press/cycling switch. Two different color wires in the fan switch. I was incorrect earlier in assuming that there was an ECM link that activated cooling fans...its direct by the switch whenever pressure builds.

The programmer, as a part of the command system is completely new to me....I had never consider a defect there until now. My cheap skematics do not even include the programmer. This is why they call them "cartoons".

However, clearing the memory by way of battery disconnect did nothing to help. I need to know if the diode in the comp plug was part of the 87 system. GM made so many slight changes during the 80's that its difficult to assume anything stayed the same for 2 years... Knowing the old compressor ran right since 1998 without a diode is bugging me a bit.

The control head link is interesting. Thats why I have 2 of them..! lol..
But because both are old, and have been repaired multiple times I'll get the rebuilt/exchange. I have had to repair one while using the other. The display flicker is fairly common and my harsh suspension only accellerates the damage. My digital dash is next....I've had to solder that as well...
I won't solder the boards together mostly because once thats done, in a series of pins like this, its forever. Desoldering is possible with braid, but its a PITA to do...
What I did was tighten the fit in the socket by taking narrow strips of stick-on folder labels and placing them on the opposite (non-contact) side of the tab with the contacts on the main board where it fits to the display board. It may take 2 or 3 to make the blade tight enough to fit the socket and not vibrate loose again. This makes it very snug. The advantage here is that this is infinately repairable. IF it ever comes loose, peel the old strips off and add more. I would imagine that there are some tapes that might also work but they have to be cut to fit. The stick on labels can be cut with sissors and then the backing peeled away and they are a form of coated paper thats pretty durable.

 

[vetteoz]

That must be the fan switch as nothing else makes sense. This switch is 1" away from the press/cycling switch. Two different color wires in the fan switch. I was incorrect earlier in assuming that there was an ECM link that activated cooling fans...its direct by the switch whenever pressure builds.

Strange thing is that switch is only noted on the coolant fan schematics ; not on the A/c ones even though it is part of A/c operation

Knowing the old compressor ran right since 1998 without a diode is bugging me a bit. .

The FSM notes the diode is there to take out the induced voltage when the clutch coil is de energised.
"Induced voltage can damage the clutch control circuit in the BCM"

Will see if I can find someone to scan the FSM schematics for the compressor control.Might make more sense than what you are working with

 

[geekinavette]

The FSM notes the diode is there to take out the induced voltage when the clutch coil is de energised.
"Induced voltage can damage the clutch control circuit in the BCM"

That is precisely what the diode is there for. That solenoid is nothing more than a big 'ol inductor...and inductors store current (just like how a capacitor stores voltage). When the solenoid is released that current will want to go somewhere. The diode provides a "safe" low impedance path for that current to be discharged.

 

[G Winter]

That is precisely what the diode is there for. That solenoid is nothing more than a big 'ol inductor...and inductors store current (just like how a capacitor stores voltage). When the solenoid is released that current will want to go somewhere. The diode provides a "safe" low impedance path for that current to be discharged.

What happens if that diode is in backwards ? Will it create a bypass.Just a thought.

Glenn

 

[boomdriver]

thanks Vetteoz, I appreciate it.
If what you are saying is true, I MAY have 2 bad control head boards....or the programmer, there ain;t much else left.

G, because the clutch coil is the same thing either way, its the same turning the plug around. Its like plugging into a long wire....either end is acceptable and yeilds the same result. What matters is which way the diode is placed between the 2 wires...it has to allow flow from green to black. and block black to green. A check valve.

 

[geekinavette]

If the diode is installed backwards the aforementioned stinky smoke will be released from it.

 

[geekinavette]

What matters is which way the diode is placed between the 2 wires...it has to allow flow from green to black. and block black to green. A check valve.

Woahh hold on...

Let's make sure this is right:

The diode has two leads, one is Anode one is Cathode (marked with a small silver band on one end). The Cathode (silver band) connects to 12V, Anode to ground.

Is this how the diode is connected?

 

[vetteoz]

The diode has two leads, one is Anode one is Cathode (marked with a small silver band on one end). The Cathode (silver band) connects to 12V, Anode to ground.
Is this how the diode is connected?

That is how it is shown in FSM

 

[boomdriver]

yes, the band is on the green (pos) wire. Its a factory plug that I snipped from a harness that was pulled from a wreck. I salvaged most of the plugs in the harness for under-hood, to replace damaged plugs. That was easier than a whole harness replacement. GM had wiring issues in the 80's and had some DOT problems due to insulation shrinking away or hardening before its time from exposure to petroleum based products, in the engine bay.

How the diode is mounted INSIDE the plug between the wires is what matters. Which way the plug sits on the clutch coil contacts does not matter. Thats going to be the same path either way.

No work done today. Too wet, too cold and too tired !

 

[vetteoz]

. GM had wiring issues in the 80's and had some DOT problems due to insulation shrinking away or hardening

Wouldn't happen

 

[boomdriver]

....yeah, like that.

fan & fuel relay?

 

[geekinavette]

NAPA still carries a lot of the older weatherpack connectors and terminals. Not sure about those relay connectors tho.

 

[boomdriver]

I have seen some after market packages of various plug sockets that are generic by the mfg.
Its pretty random as to whats hanging on the rack, but most stores will have a catalog that will list which plugs or connectors they can order. Worse case, just get them from a vette salvage yard. I know that J&D has a PILE of C-4 harnesses that you can pick thru and they will sell you whatever you find at a reasonable price. That way you can get a good pig-tail along with the plug.

 

[vetteoz]

NAPA still carries a lot of the older weatherpack connectors and terminals. Not sure about those relay connectors tho.

I changed all my engine bay non weatherpak connectors and use the later style relays
Mike at EFI Connection has most of the pigtails at a good priice
EFI Connection

 

[boomdriver]

I need to check that out. I had the worst luck in past yrs with chain store relays. Some better plug/relays would be nice.

PS.
There is a small silver relay thats in the main harness near the a/c programmer thats above the gas pedal. it has a large orange and a lg purple wire with 3 other small gauge wires.
Anyone know what this is or what system its part of? The programmer cable splits off the main harness and this relay splits off of that.

I traded programmers yesterday and got no results. The rebuilt control head will arrive next mon-tue. Hopefully that will solve the a/c problem so I can move on to needed mechanical repairs. BTW, that a/c programmer was not mounted with the intention of ever being serviced... One screw to remove and it has to be the most awkward screw in the entire car.