The one thing that has always worked flawlessly in my 95 has been the air conditioning. Today, something strange happened. The car had been sitting parked out in 90+ degree heat. I got in and turned on the ac and cool air came out as usual. As I was driving the cool air suddenly turned warm and nothing I could do would get it cool. I turned off the climate system and waited a bit then turned it on again and cranked the temp way up to force hot air out and then adjusted the temp downwards to see if it would get cooler. Sure enough the system started acting normally again with cool air coming out when the temps were set low.

Seems like a hiccup in some system that went away. Does anyone know what might have happened?

Thanks,
Doug

 

No air conditioning experts out there?? Anyway, I noticed a pattern with this problem. After the car sits for a while (overnight in the garage or all day in the garage at work), the air coniditioning will not immediately come on after starting the car. If I hit the "auto" button, I can hear the door moving behind the dash and the blower kicks in strong but the compressor does not engage. I try alternating between the "off" button and "auto" button but the compressor never kicks in. After about 5 to 10 minutes of driving I hit the auto button and the compressor will finally engage and I get great a/c without problems.

I never get any a/c diagnostic trouble codes, and I also thought that if refrigerant was low, the system would cycle on and off repeatedly. Nevertheless, I do not believe I have ever gotten my a/c serviced in the 10+ years that I have owned the car and is it possible that low refrigerant can cause this behavior?

Anyone?

Doug

 

There is a signal called A/C request that comes from the A/C Programmer. (Little box located to the right of the gas pedal mounted on the firewall and has some vacuum hoses going to it).

When you turn on the A/C, the A/C Programmer grounds one end of the A/C pressure cycling switch which should be closed if there is enough R134a in the system.

(The A/C pressure cycling switch opens when the pressure falls to 23 psi and closes when the pressure rises to 46 psi).

So if the A/C pressure switch is closed, the ground signal passes thru the switch and to the PCM which will engage the A/C clutch if no faults exist.

A tell tale sign that the A/C request signal is received by the PCM is if the engine is at idle speed, the PCM will increase the engine idle speed before it engages the A/C clutch. If no increase in engine idle speed, the request signal is not getting to the PCM.

If you have a Tech 1 or scanner software you could monitor this signal. If you have neither of the above you need to monitor the signal with a volt meter.

With A/C on you should measure 0 volts on the Dark Green/White wire at the A/C pressure cycling switch.

If not, unplug the connector and with an ohm meter, you should measure zero ohms across the terminals of the A/C cycling switch which would indicate it's closed. If the switch is open, the switch is bad. If that tests good, the A/C Programmer is probably bad or bad solder joint on the circuit board.

The problem could also be a bad connection so you could reseat the electrical connectors at the A/C Programmer, A/C Control Head and the Black connector at the PCM.

If you do measure 0 volts on the Dark Green/White wire, and the clutch doesn't engage, the PCM could be bad.

I doubt the problem is the PCM or the A/C clutch circuit since no codes are being set.

 

That is very good information. Is it relatively easy to locate the "black box" you are talking about? I have always noticed that the idle speeds up immediately upon hitting the "auto" switch but, come to think of it, I'm not sure that has been the case since this behavior has started. So this could be as simple as a loose connection or a bad switch and not necessarily refrigerant level? This could also explain why the problem disappears after the car runs for a while (electrical contacts may "seat" better with heat?).

Doug

 

A quick item to check is the ground located on the frame by the #6 ( I believe, possibly closer to #4) plug wire. This caused intermittent compressor operation in my 93 until I finally located it on the wiring diagram. You can't miss it, you can just look straight down at the frame rail by the right bank of the motor and the bolt/eyelet is plainly visable. Good Luck!

 

The PCM is the computer for the engine.
Located above the battery. Has four connectors.
Red, Black, Gray/Clear and Blue.
Disconnect the negative battery cable and reseat the connectors.

The A/C Programmer.
Located to the right of the gas pedal mounted on the firewall has some vacuum hoses going to it. If possible just reseat the electrical connector. I'm not sure if you need to unbolt it from the firewall before you can reseat the electrical connector. Disconnect the negative battery cable before you reseat the electrical connector. (Awkward to access and can give you a pain in the back).

Note: When every you disconnect and then reconnect the battery, the C68 A/C Heater system will go through a calibration period of about 1 minute. You'll hear the blend door move and the blower fan will come on when the calibration is done. That is normal.

A/C pressure cycling switch. Look under the hood passenger side at the evaporator housing. There is a metal tube at the top of the housing with the A/C pressure cycling switch mounted to it. The connector has two wires.
A Dark Blue wire and the other wire is Dark Green/White. Reseat that connector.

The A/C Control Head is the unit that is mounted above the radio with the buttons to turn on the A/C, heater, defroster etc... To reseat that connector you need to remove the bezel that covers it. Requires removing the shift console panel and pulling it back to gain access to some mounting screws before you can remove the center bezel. I don't think the A/C Control Head is the problem so try the other three items listed above and go from there.

As far as the system having low refrigerant, you'd need to hook up some gages to read the pressure.

You say the system cools fine when it does work so I'd say the refrigerant level is ok. The A/C pressure cycling switch or its connector could still be bad or intermittent. So when the problem occurs with the A/C on, move the connector around and see if the AC clutch engages. If the Cycling switch is intermittant it can be replaced without discharging the system. It screws on to a schrader valve.

 

One quick question--at all the steps where you suggest "disconnect negative battery cable", is this really necessary or just advisable?

 

Some electronic devices even though turned off still have power going to them. That's why it's recommended power be completely removed before working on them.

I didn't mean you have to disconnect and reconnect the battery before reseating the connector for each device. I just meant if you were going to reseat the connector make sure the battery is disconnected.

You don't have to remove power to reseat the A/C cycling switch connector.

No reason to take a chance in damaging something when it can easily be avoided.

 

It was easy checking the connectors on the a/c lines near the evap housing (there are actually two connectors--one on each of the lines). The a/c programmer is another story. First I misunderstood the description of the location (right of the gas pedal on the firewall). I thought this meant inside the engine compartment in the vicinity of the pcm and I kept looking but found nothing. Then I finally found what you were talking about mounted to the inside surface of the firewall inside the car to left of the brake pedal and just right of the hood release latch (and, most unfortunately, covered by the left lower instrument panel trim piece). I did not have the patience or flexibility to contort myself to even remove the trim piece!

Anyway, I went to my local Vette guy. As it turns out the a/c was working great at the time (after initially not running when I first started the car). He quickly concluded that the freon was okay due how cold the air was coming out. When I described the problem (pushing the auto button does not do anything upon initially starting the car), he immediately suggested it was the a/c programmer and that this was a quite common problem for these cars. He noted that this typically happens when cars are not driven much and/or a/c is not used for a long time. My car is a daily driver but I had not used the a/c for pretty much the last year. He also suggested the problem may lessen by using the a/c more often.

We'll see.