Beatin' on the ol' AC

Troubleshooting the air conditioning in my 98 Lumina. Using Torque Pro and a custom PID set, I finally managed to pull AC telemetry from the PCM:

  • AC Request responds to cabin controls (okay good, that rules out quite a lot of wiring)
  • AC High Side Pressure is 0.22V ~= 6.96PSI (Ouch. If that’s not relative to ambient, the sensor or wiring harness is DEFINITELY toast)
  • AC Pressure Out of Range is on (no shit?)
  • AC Relay is off (again, duh)

That narrows it down to three possibilities:

  • wiring fault between PCM, AC pressure sensor, and/or ground
  • faulty sensor
  • or simply no charge in the system.

(Well, it could be a PCM fault, but the chances of that are pretty damn low.) My gut says the sensor is blown open, that reading seems absurdly low for it to simply have leaked away to nothing. Either way, I think I should verify the wiring harness first.

Right now, I definitely will:

  1. With key in RUN (so OBD port is active), engine off, AC Request OFF (fan speed in position 0 should do it), jumper AC high pressure sensor connector, 5V to sense wire. If reading doesn’t change to ~450PSI, wiring problem.
  2. Under same conditions, measure voltage between 5V and ground on the same connector. If not at 5V +/- 0.2V ish, wiring problem.

Either of those, I’ll definitely fix myself. It’s electrical, it’s what I know.

Otherwise… I have an idea, stop me if this is stupid.

I have half a mind to give it just enough charge to run, so the leaks weep, and go at it with UV dye, spot fix from there. But, pretty much every recharge kit on the rack mixes in a “stop leak” clogging agent. Circulating that kind of material through the entire system strikes me as a catastrophically bad idea, and from what I’ve seen, most mechanics agree. So, is it possible to get a can of premixed 134a + oil that does NOT include that garbage? Or should I not be adding oil and just use a refrigerant only can? I spotted a few of those, I’d have to get the gauge/hookups separately but that shouldn’t be a problem.

And to charge it, I’d need a pressure gauge anyway, so I might as well first see if the gauge reading matches the sensor. I’m not trying that swap myself though. I know it’d have to be evacuated and I’d never be able to get the equipment for that. That’ll be a shop visit.

I wouldn’t use dye.

I’d use my refrigerant leak sniffer before starting any work on the system.

You have to locate the leaks first before you fix anything.

And a dye won’t tell you if there’s a leak the evaporator right away. Where a leak sniffer will.


Leak sniffer, didn’t know that was a thing. Those are… pricey. Is it a likely AutoZone rent?

I’m 99.9% certain it’s either properly charged and the sensor/wiring is faulty, or there’s nothing left. AC hasn’t worked at all since I got it half a decade ago, I can’t imagine there’d be any pressure difference vs. ambient at all at this point. So I think I need to put something in it before the leaks will, well, leak.

First thing is to get a set of gauges on it and compare the sensor readings to actual pressure. Don’t bother with the electronic side of it until you know it’s mechanically sound.

Went against my hunch and reached for the refrigerant. Glad I did, saved me a little work. Hooked the recharge nozzle up to it, gauge read dead zero. Start pumping! It was only after pouring refrigerant into it that I realized I didn’t purge the air out of the hose. Whoops… Oh well, it’s probably got a bunch of air in it anyway.

I decided to leave the AC controls off while I added a little bit first, check the high side pressure sensor. It tracked the reading on the gauge no problem. Okay, ruled that one out.

Once it hit ~35PSI, pressure out of range flag turned off. Allright, let’s try it. Set fan control to 4 and vent control to AC MAX. AC Request flag turned on, relay flag turned on! And the clutch did, indeed, engage! System operating! The charge hose gauge dropped to ~25 pretty quick.

I spent the next ten minutes dumping refrigerant into the system, using my phone to watch the ECM’s high pressure reading while using the gauge on the hose to see the low. Low kept settling down to ~25 while the high side gradually climbed higher. I got it to about 230PSI and called it good.

Around this time I noticed that there was a significant amount of some bright green substance splattered across the passenger half of the engine bay, that wasn’t there before. Hm. Closer inspection revealed that it was coming out of the AC clutch. This mystified me at the time. Some kind of lubricant? (I figured it out later.)

Welp, let it run for half an hour, watching the high side pressure. It was definitely working, I was getting very cold air. High pressure wobbled back and forth, from about 170PSI to 230ish and back again. That’s interesting. Seemed to get more stable as it ran. Just fresh pressure settling?

Turned the AC off to note the idle pressure. 94ish.

So, killed the engine and went to disconnect the hose. At this point I heard a faint hissing coming from the passenger side. Where exactly? The AC compressor. It was at this point I put two and two together.

For the sake of argument, took it up on the loop anyway. High side pressure dropped as I got up to speed, settling around 127PSI. Some kind of variable flow trick? Air stayed plenty cold. My phone died about 3/4 of the way around the loop. Damn.

Got it back home, managed to get my phone to turn on, and got another idle reading. 74.7. Ouch. That’s a bad leak. (The damned phone died again not minutes later. I really need a new phone…)


  1. Everything electrical about the AC system is 100%. High side sensor is fine.
  2. I’m not the first one to take a crack at it. Someone dumped leak dye in it at some point. Great, saves me a few bucks and a lot of time.
  3. The root problem is, in fact, a leak, somewhere on the face of the compressor. Most likely (excuse me for not knowing the proper term) the compressor axle gasket.

I wanted my leak location so I could try my hand at a spot fix; well, I’ve got it. Now, do I have a prayer of rejuvenating or patching that gasket from the outside? A new compressor may cost more than the car’s worth, but I’ll call my favorite shop and get a quote anyway.

-E- For the record, I used AutoZone house brand refrigerant only cans. I made a point of verifying they didn’t say “stop leak” anywhere on them. They were the only cans the shop had that DIDN’T have stop leak. Scares me slightly.

I’ve put together a plan.

Tomorrow I’m picking up a new shaft seal. Friday I’m taking it to a local bay rental place to have their guy vac it out. Then I’m gonna rip the clutch off and swap the seal, and have him charge it back up. And then I’m gonna put a thousand miles or so on the car over a weekend trip. If it holds, done! And based on the car’s history, I have good reason to believe it’s more likely to hold than not. If not, whatever, I still save $100 over having a shop do it, and learn a lot in the process.

If there’s another leak somewhere else, well, that’s something I’d have to separately address anyway, even if I have a pro do the compressor.