merphie

I have a weird problem that has occurred since I bought the car a few years ago. Recently, I replaced the compressor, orifice tube, pressure sensor, and accumulator. I pulled a vacuum and charged up the A/C with a relay bypass. It worked fine.

However, today the compressor won't come on unless the HVAC switch set to MAX a/c when I start the motor. If I turn off the controls and turn it back on the compressor won't come back on until I turn the engine off.

I can change the temp, fan speed, and location without a problem except the clutch won't reengage.

Any suggestions?

hrcslam

Check your pressure sensor.

merphie

I've replaced the sensor already. Maybe the connector has a short? How likely would it to be the HVAC control?

hrcslam

That's a good point on the HVAC control. Maybe you need a schematic.

I had a weird issue with my AC (caused idle issues too). Turns out I had a bad ground on the AC clutch circuit to the AC relay. I ran a new ground and no more issues. The weirdest part was the ground was good when I tested it for continuity, but would lose continuity once power was sent through it. It started off doing it intermittently then became constant. Could be something to look for.

merphie

It appears to be the PCM not sending a signal. I put in a switch powered from the accessories to the clutch since I am not ready to buy a new computer.

hrcslam

This is a bad idea. The PCM will turn the compressor off in certain scenarios; low pressure, high pressure, PE mode, etc. You need to find out why the pcm is not telling the clutch to engage and fix that. I doubt it's the pcm, it's likely the pcm isn't getting the right signal from a sensor or ground.

merphie

I agree it's not a good solution but I've replaced everything and retested. the problem is the pcm isn't sending the command to the cluch reliably.

I'm open to other suggestions.

hrcslam

I'd check the pressure sensor wiring and all grounds. You may have to find the chart for what the sensor should be reading. Also check the grounds and connections to the HVAC control switches too.

merphie

I replaced the sensor and checked the voltage. i also checked the controls. Everything is what it should be.

The only problem is voltage to the relay from the pcm during the senario above

hrcslam

Have you checked to make sure the wire from the pcm to the relay is not broken?

merphie

Wouldn't the fact that it initially works prove the wire is good?

hrcslam

Not if there's a break/corrosion in it or any other wire used within that system.

RockinWs6

When you turn the HVAC system to AC the HVAC control sends a request to the PCM to turn the compressor on. I would check that request circuit 1st, If its good to the PCM then the next thing to do is diagnose why the PCM isn't turning the compressor on. It can be as simple as a poor connection at the pressure transducer.

merphie

I checked the control switch (and ground) all the way to the compressor. I tested the voltage coming off the pressure switch too.

1. Turn the A/C control on
2. Start the motor
3. Compressor comes on and car starts to cool
4. turn off the A/C control (engine running)
5. Turn the A/C control on (engine running)
6. Compressor doesn't come back on
7. Turn off Engine.
8. Go to Step 1

I checked the control, pressure switch, relay, and clutch between each step. The only point of failure was from the PCM to the relay. At step 6, the PCM wasn't sending voltage to the relay.

hrcslam

What year is you car? LT1? LS1? V6? I want to pull up the schematics.

I know on my car (1995 Z28 LT1) the PCM interrupts the ground signal from the relay (on the coil side) and the power side comes from the fuse block shared with the clutch. The clutch ground is not interrupted. The PCM will determine when to kill the ground signal to the A/C relay.

If your schematic is wired the same as mine, it's not a PCM related issue and the fix you did is legit if you only re-wired the positive side. But I'd have to confirm this with a schematic.

There can be other things that can cause the AC to not engage from the PCM, not just the pressure sensor. Do you have any other engine codes (not all codes light the SES so you'd have to scan it)? If you have a MAP high voltage code, it can cause you A/C to not work. If your MAP is reading higher than it's supposed to that can cause it (you'd need to data log for this). Same goes for TPS.

merphie

2001 Pontiac Firebird Transam

hrcslam

Your pcm interrupts the ground signal side of the coil in the relay, just like mine. Your AC clutch is fed positive through the relay directly from the fuse (both positives for the AC relay are from the same fuse and ate both direct from that fuse to the relay), the clutch ground is direct to ground from the clutch assembly. The fuse is hot only with the key on.

The PCM will activate the ground signal only if all the the sensors are showing within the limits identified in the tune. These usually are high and low freon pressures, PE mode (usually based on MAP and TPS values), and HVAC control switch positions.

You'll need to make sure all those values are correct. It sounds like your pcm is seeing all the values as acceptable to engage the ground with the engine off, but with it on and already running it's getting feed back from one (or more) of those sensors telling it not to engage the AC. That leads me to an engine sensor (MAP maybe?), a data log can confirm or eliminate this though. Or or could be a load issue feeding the HVAC control switches once the engine is running and drawing electrical power.

When you ran a new wire, how did you do it? Did you run a new power signal directly to the clutch? If so, you need to monitor the AC pressure to prevent it from going to high (this will destroy a compressor quickly) and make sure you turn it off before going high throttle (passing or racing).

With the engine on and running, do you have 12+VDC at both positive terminals of the relay block?

merphie

There's no codes in the system. I would think if it was just a bad sensor then I should see other issues with the car. I also had a mechanic look at it and confirm it's the PCM signal.

Yes

hrcslam

You wont always get a code. That's why I said datalog.

When you said you wired in something did you wire in a new positive to the clutch? Or did you re-wire the ground signal?