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Fluctuating A/C manifold gauge readings for '99 Lumina

The A/C on my hand-me-down '99 Lumina has never been outstanding, but has kept the Florida summers at bay. I recently turned it on again (I only ran it once or twice this winter) and the system was empty. I refilled it and it was empty again in 72 hours. I was hoping the seals had just dried out, but it continues to empty after several other attempts to recharge it, including adding 134a with lubricants. I added 134a with UV dye in it and the only leak I found is around the compressor clutch. I accept that the compressor probably has to go, but am puzzled by the manifold gauge readings taken when I added the last can. I just don’t want to put in a new compressor and find out that wasn’t the only problem.

With the system off, both High and Low sides read just under 70. (Normal Low and High side operating pressures for this car are listed at 42 and 340 respectively.) When the system is turned on, Low drops to 30 and High goes to 80 and there is no air cooling. After 30 seconds the High side slowly climbs to 200 and the Low side slowly drops to 26, with almost no air cooling.

When the High side hits 200, it drops back down to 80 and the Low side goes back to 30. After 30-45 seconds it does it again and keeps repeating. The compressor clutch stays engaged the whole time.

I looked in the Haynes A/C Techbook as well as online and can’t find anything that resembles this description. If this is just symptomatic of a bad compressor, or is there more evil afoot?

Also, while I have the system open are there any parts I should replace at the same time given the age of the system?


Your compressor is shot. You already know that. That is the only moving part in the entire system. It is responsible for the obvious leak you have and the fluctuating readings. It is simply not compressing up to the required pressure. 200 psi max is not enough. Most systems run from 280-360 psi. Also, if the system was completely down, a vacuum is required to pull air and moisture from the system. If this was not done, you have too much air in the system. You must pull a vacuum if the system is ever purged.

BTW, I hope your planning on flushing the lines and replacing the orifice tube and the accumulator/dryer. These are basic tasks when replacing the compressor. Also, don’t forget to pull a vacuum on the lines once the system is cracked open or it will never work.

That’s kind of what I figured, but it’s nice to have it confirmed. I’ve never flushed an A/C system before and had thought about having a shop do the discharge and flushing, then replacing the parts myself. I’m planning on renting/borrowing a vacuum pump to evacuate the system before recharging it.

I had heard it was a good idea to replace the accumulator, but will replace the orifice tube as well. Thanks for the help!

I have never developed the “hand on the steel line” technique to determine a full system charge. Myself I would charge this one by weight (after the orfice tube replacement,with an eye for massive metal shavings as some shavings are going to be there). In short,a weighed charge and then another look. When the low is too low and the high is not high enough I exhaust the possibility of a low charge. I have replace many,many of this type compressor and not because they were wore out,but because they leaked where the casting halfs joined. Just be glad it is not a Lumina APV as this is a miserable compressor replacement job until you get a few under your belt.

The only things I will add is this.
The system is seriously undercharged if it has a static reading of 70 PSI at rest. Normally, you look for around 115-120 PSI on both sides with the system equalized.

I would ignore that 340 figure in the book. That is way high and many manuals, including the factory ones, carry a lot of erroneous information. It will vary based on a number of things but figure on 250-275 on the high and preferably around 35 on the low.

Once it’s assembled and you pull a vacuum on it shut the vacuum pump and gauges off and allow it to sit for half an hour. Recheck the gauge readings and make sure it’s still holding the same vacuum as when you shut everything off. This will tell you if there’s a leak without blowing through refrigerant.

When recharging and before opening the gauge valves, tap and open the can of refrigerant.
Loosen the center hose on the gauge set slightly and then retighten it. This will purge the hose of air. Of course, NEVER open the high side on the gauge set with a tapped and open can of refrigerant and wear safety goggles.

Thanks, guys. I really appreciate the great advice. It should help a lot.