A/C Compressor Control Problem

1998 Chevy S-10 Blazer LT

4-door, 4x4

145,000 miles

Issue: The A/C compressor will not turn on.

I recently had the intake manifold seal replaced on my truck. In the process, the A/C dryer sensor was hit and broken which vented all of the freon. I took the truck from the mechanic’s shop to an A/C shop where the dryer was repaired, the switch replaced, and the system was given a clean bill of health. Unfortunately, now the compressor will not come on when triggered to do so by either the A/C control in the dash (Computer Climate Control box) OR when the sensor on the dryer is shorted. Note that it was working fine before the engine repair work occurred.

Now, it IS possible to get the A/C system to work. The A/C guy hooked up a wire from my firewall to the engine fuse box and I can jam this wire into where the A/C fuse plugs in and the compressor will kick on and give me nice, cold air. So, the compressor, condenser, hoses, etc are all fine. It’s just getting the compressor to come on without Frankenstein-ing my truck!

Both the mechanic and the A/C guy said the cause was likely that my ‘Computer Climate Control’ box had coincidentally died during the engine repair and would need to be replaced. So, I replaced it. And… it didn’t fix the problem!

What is there left to check? I can only guess that there MUST be a wiring problem - that maybe something else was damaged or didn’t get reconnected properly after the manifold seal repair. I asked the repair guys to check everything one more time and they claim that everything is in order. However, I don’t know where to look or how this stuff even connects up. And, I can’t afford any more ‘exploratory’ work.

Any useful suggestions would be greatly appreciated. The heat index is now in the triple digits here. I’m literally baking in my truck.

I’m afraid that I’m going to be zero helpful here without a schematic but I tend to agree with you that this problem likely has something to do with a fault caused by the original intake work.

Just my opinion, but the shop that broke the system should be the ones paying for this problem and how can the A/C shop give it a clean bill of health when the compressor was not energizing?
Or were they doing the wire trick simply to show the basic system did cool fine?

I doubt that I have a schematic for this particular vehicle but I’ll take a look tomorrow at some similar Chevy diagrams and see if I can weed something out.

You might also consider getting someone (AutoZone, Checkers, etc.) to scan the car for codes. There are codes that have to do with the A/C system and maybe something will crop up there. They will do this scan for free.

The mechanic shop took no responsibility for the blown freon seal. They acknowledged that they ‘probably’ did it, but claimed that ‘it happens’ when you’re doing this kind of work. I have to say that fluorescent green dye all over the engine compartment is a sure giveaway, but the guys there would only shrug it off and say there’s no way to say for sure that they actually did it.

The A/C shop repaired the dryer - took it out, actually - and replaced all the seals and the sensor, pulled a vacuum on the system and fully recharged it. When they were done doing the work, that’s when it was discovered that the compressor wouldn’t turn on when activated by the dash control unit. The repair tech is the one that showed me the wire-in-the-fuse-box trick to get it to work - and to prove that his repair of the other components was successful. He figured that the mechanic shop simply failed to reconnect something and suggested that I take it back to them to check everything out. I did. The mechanics looked at it and said everything was fine and that it must be that the computer climate control died.

I took the truck back to the A/C guy for a second opinion and he concurred that it was possible (although strangely coincidental) that the unit died at the same time that the sensor switch was broken - maybe the act of breaking the sensor is what killed the A/C control?

Anyway, since both shops seemed to think that getting a new dash controller would fix the problem, I bought one. And, as we now know, it still doesn’t work. Meaning we’re back to the wiring.

We have an AutoZone here. I’m not aware of them doing code checks, but I’ll run by and see.

If you have a schematic for anything, that would be great. FWIW - every other function on the A/C controller appears to work fine. It’s just turning on the compressor that is the problem.

Anyone else have any ideas?

You did check the fuse for the compressor clutch?

I rummaged through my books and could not find anything close to a 98 Trailblazer; just around 95ish certain GM models and these would likely be quite different.

The AutoZone website provides some wiring diagrams but I haven’t mustered up enough fortitude to even check that yet. Since they redid their site a couple of years ago it has become pretty unwieldy and for me at least, has caused a lot of grief by either taking forever to pull up or either locking my PC down. The latter causes me to have to manually shut down and reboot.

I was thinking along the lines of Tardis about checking fuses so I agree with him; check all of them as it’s likely the A/C system works off of more than one. One of the diagrams on one of the 95 GM cars I looked at showed power provided from more than one source so it’s quite likely a 98 is not going to do with less.

The climate control module sends a signel to the Vehicle Control Module through the high pressure switch. Thus there is that circuit to check.

The compressor clutch is powered by through a 10 amp A/C fuse and an A/C compressor clutch relay contacts. The relay is energized using voltage through a 10 amp Ignition E fuse and grounded by an open collector drive in the VCM again. Thus, check that circuit.

Hopefully that will lead you in the direction of the culprit.

THANK YOU everyone for your assistance. We have to be getting close. There’s not much left to check.

I think the A/C guy checked all of the fuses and such. He was in both of the fuse boxes during the repair. I also systematically pulled and checked every fuse in the in-car fuse box about two weeks ago hoping it was something that simple. And, I pulled and checked every fuse in the engine compartment yesterday. Of course, there was the A/C relay (and all the other relays) which I couldn’t figure out a way to test. So, that was one possible source of the problem.

I drove over to AutoZone this afternoon and the guy that ‘helped’ me wasn’t a very motivated individual. He wasn’t interested in scanning the truck for error codes and would only sell me a new, $15 relay - which didn’t do any good.

Researcher: Thanks for the information! I’m embarrassed to have to admit it, but I think I only understood about half of what you said. I knew that there were two fuses (only because they were marked) for the A/C system - one in the engine box and the other inside the vehicle. I wasn’t aware of the Ignition E fuse relationship. I double checked all of these fuses today and they are still good.

The climate control module sends a signel to the Vehicle Control Module through
the high pressure switch. Thus there is that circuit to check.

The high pressure switch is the one that’s connected to the dryer (silver cylinder)? This is the one that, if the wires are disconnected and jumpered with a paperclip, should trigger the compressor to come on? This switch (sensor) is what was broken during the intake manifold repair job. It was replaced with a new unit and should be fine. However, shorting the wires as mentioned previously will not make the compressor come on. The end of the circuit should be the ‘Computer Climate Control’ unit in the dash. I have replaced that, too, without remedy.

The compressor clutch is powered by through a 10 amp A/C fuse and an A/C
compressor clutch relay contacts. The relay is energized using voltage through
a 10 amp Ignition E fuse and grounded by an open collector drive in the VCM
again. Thus, check that circuit.

I should correct an earlier mis-statement on my part. I said ‘fuse’ where I should have said ‘relay’. The compressor WILL turn on and give me cold air if a wire that is attached to my firewall is stuck directly into one of the holes (upper left) for the relay and the relay is plugged back in.

I have confirmed that the fuses are good and replaced the relay with a new one. This is all I know how to do at this point for troubleshooting. Is there anything else I need to do to test this circuit?

For the time being, the system is working again. And, it didn’t require installing a toggle switch in the cab.

I took the truck back to the A/C repair shop and asked them to go over everything again. I tried the mechanic, but he was busy this week and could only figure that installing the toggle switch would work.

There was definitely one issue - maybe two - that together caused the system to not work. Here’s what the A/C guy told me when he handed me the bill.

Problem #1:
The high pressure sensor that the A/C company installed was bad. ‘It happens,’ they told me , ‘and there’s no refund or exchange for electrical parts once they’ve been installed.’ I’m not sure I appreciate a policy that makes the consumer pay for poorly manufactured and/or dead parts.

Obviously, a bad sensor would cause the compressor to not come on. However, I would have thought that pulling the wires and manually jumpering them would have revealed that the part was bad.

Problem #2:
The A/C relay in the engine compartment fuse box was also bad. They said they replaced it, too, and the system worked. Of course, I bought a replacement relay yesterday that didn’t make the system function, so I asked them to show me how it was now fixed with their relay. During the explanation, the compressor stopped working again. We jiggled the relay, and it came back on. It seemed that the real problem was loose connectors for the relay. They offered to repair/replace the fuse box. Instead, I pulled out the new relay I bought and plugged it in and the system restarted. No amount of jiggling, shaking, or fist pounding on the relay or fuse box would make it lose the connection. I did not pay for another new relay. I think this points back at the bad high pressure sensor they installed.

In the end, I’m still not sure exactly what was the real problem. I guess I’m just happy that it’s working again. Financially, it could have been much worse. Of course, that doesn’t mean I’m pleased, either. In case someone else experiences a similar situation, here’s a rundown of my expenses.

$140 initial A/C repair
$60 for replacement A/C control head (from junkyard)
$20 for wire, connectors, toggle switch, and box (back-up plan)
$15 for new relay
$70 for trouble shooting / replace high pressure sensor (again)

This took three months of back-and-forth trips between the garage and A/C guys to accomplish.

I would now head to small claims court with the shop that broke it in the first place. “That happens when you work on things” is a pitiful excuse and the green dye all over the place when you picked up the car is conclusive.