87 GN air conditioning issue

theman1990

theman1990

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Hello all! New to the forum and GN's, not new to cars at all. Searched already but didn't find an answer to this specific question...

Picked up a very well maintained 1987 Grand National from the Northwest. A/C was completely replaced about 4 years ago, car has 55k miles. When all the under hood components were replaced (have all receipts), it was converted to R134a and reportedly blew ice cold... Now I have it here in Florida and immediately found the AC lacking. Was cool then hot after starting. Compressor cycles on and off every 30-60 seconds and cycles quicker until it just stops engaging. It was low on coolant so I added a bottle of 34a and got the gauges out for 30's psi on low and high 00's on high (while it ran.) After it stops engaging it's 100/90 high/low. I cleaned all contacts with no change. Jumped the low pressure switch with no change. Changed AC relay and checked fuses, still no change. It seems that it is related to the car warming up as if I let it cool all the way don it runs again for a few minutes before the cycle (or lack of cycle...) starts all over again. Car uses FAST XFI 1.0 if that matters... Anyone have any ideas?
 
Chuck Leeper said:
What's the rad fan set up?
Fan come on when the AC is turned on?
Click to expand...
Single large SPAL electric fan, it's on at all times, not sure if it has a high/low setting but likely does. Engine idle increases when AC is turned on, stays elevated even when compressor is disengaged. If I disconnect (but not jump) the low pressure switch, engine idle returns to normal.
 
Not sure on the FAST, was told it did not.

Took it to a shop, they said it was low on 134a and charged it. I left the parking lot and it was cooling. I also noted the exhaust gas temp gauge in the console pod was INOP. About 3 miles later, suddenly blows hot now that car is up to temp, i happen to notice EGT gauge is about 700 degrees. Took it back and after an hour they showed me that the "green wire with the yellow strip off of the relay isn't grounding. It's supposed to ground from the ECU. It's gonna get real expensive to chase it down."

He did show me that when he grounded the light green/yellow wire, the AC clutch engaged. Thoughts?
 
So far I've found the issue seems to be the ECM? The earlier mention of the EGT seems unrelated (or I viewed it wrong)

The green/yellow wire is where the ECU grounds the relay to engage the AC. When the car is cold, it works fine but once it's hot, the ground from the ECU goes away. Could this be a bad ECU or would there be something else that might cause this? Seems odd that it only occurs once the car is up to operating temperatures AND DRIVEN. The shop said they left it idling 45 minutes with no cutout. I made it maybe 5 minutes before the AC died.

Are there any other functions of the ECM that would affect this?
 
theman1990 said:
Seems odd that it only occurs once the car is up to operating temperatures AND DRIVEN. The shop said they left it idling 45 minutes with no cutout. I made it maybe 5 minutes before the AC died.
Click to expand...
Driving and the hood is down = engine compartment temps rise, relay gets hot.
Was the "test" they did with the hood up?
Have you replaced the relay?
Quik test: When it quits, put a rag with cold water on the relay. Start working?
 
I replaced the relay already that didn’t make any difference. They did show that if the ground the green and yellow wire clicks the relay over every time. The problem seemed to be in between the green and yellow wire and the relay.
 
I took it out this morning and it blew nice and cold for about 10 or 15 minutes before it cut out. I then noticed that it blew cold for approximately 30 to 45 seconds every four or five minutes. When I got home, I got out my stopwatc and timed it, have screenshots but not sure if I can upload them here. Basically what I found was that it would be disengaged for approximately a minute and a half to two minutes and it was engaged for anywhere from a minute to a minute and 10 seconds. I then put the car in gear and backed it up about 10 feet and the next time it disengaged for five minutes and when it reengaged it was only for 10 or 15 seconds before it disengaged for five more minutes and then I gave up. when I backed the vehicle up, I did not bounce anything or even give it any gas. It definitely seems to be something thermostatically related.
 
Took it out again on the same route I took this AM, AC died within 100 yards of the same spot. Brought it back home and timed it again.. Almost Identical. The top pic is test 2, the other are test 1. AC compressor is engaged on even laps (via the ECM Ground) then the ECM shuts it off... This is CRAZY frustrating... My significant other was not impressed with my seemingly impulse purchase and this is definitely not helping matters.


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Hot wire the relay and see if it stays on.....
Jump the ecm wire. Probably a ground. Check the other side of the relay coil and see if there's 12 v there, to be sure.
Or, unplug the relay and hotwire the load wires together.
 
Chuck Leeper said:
Hot wire the relay and see if it stays on.....
Jump the ecm wire. Probably a ground. Check the other side of the relay coil and see if there's 12 v there, to be sure.
Or, unplug the relay and hotwire the load wires together.
Click to expand...
Yea I already grounded the ECM side and it kicks right on. It's DEFINTELY the ECM shutting off the relay... The question is WHY and why it seems to be temperature related?
 
Then try another ecm.............
Bob Bailey and others may have a workaround.
Someone may have a spare to try.
 
Chuck Leeper said:
Then try another ecm.............
Bob Bailey and others may have a workaround.
Someone may have a spare to try.
Click to expand...
Yah I was thinking that might be the next step. I'm a new member here so I don't know who anyone is :(
 
Welp, looks like the FAST XFI 1.0 totally replaces factory ECM... I'm beginning to suspect someone wired the AC relay to a fan switch? Do we have any FAST gurus here?
 
From what you have described this may be the culprit..... Open up the PDF link.

I'm not a FAST guru but......
Try reading page 131 Section 20.0 and see where your parameters are set too in the software.
Maybe your A/C CTS (degrees Fahrenheit) setting is too low?
This parameter tells the ECU to disable the A/C clutch if the engine coolant gets too hot by the setting in Max CTS(*F) for AC Enable thats entered. If your coolant temps go above the MAX CTS setting it turns the A/C compressor off.
Max %TPS tells it to turn the A/C off by throttle position also so set that high like 90% if it isn't already.

I hope that helps and I'll keep my fingers crossed for you :cool:
 
XracerX said:
From what you have described this may be the culprit..... Open up the PDF link.

I'm not a FAST guru but......
Try reading page 131 Section 20.0 and see where your parameters are set too in the software.
Maybe your A/C CTS (degrees Fahrenheit) setting is too low?
This parameter tells the ECU to disable the A/C clutch if the engine coolant gets too hot by the setting in Max CTS(*F) for AC Enable thats entered. If your coolant temps go above the MAX CTS setting it turns the A/C compressor off.
Max %TPS tells it to turn the A/C off by throttle position also so set that high like 90% if it isn't already.

I hope that helps and I'll keep my fingers crossed for you :cool:
Click to expand...
Once I find a 100 year old laptop capable of connecting to it I'm definitely looking here!!!
 
You should have the FAST program to work on this car with, even if there's no AC issue.
No reason your LT, assuming it's not DOS based, can't load that program.
Before you do that, determine if the ac relay is even wired into the FAST.
From what I gather, the stock ecm is still in use and the FAST is on the engine. Y/N?
 
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