been chasing down an AC gremlin, i've replaced the AC compressor, charged it, and checked the sensors on the back of the compressor manifold, and the one closest to the firewall.................. and after all that the compressor will cycle on and off, and if i screw with the top slider i can get it to stay on for awhile, also the lamp on the defogger **** will not come on.......... does it sound like i'm in need of a new climate control unit?

thanks in advance

 

are you sure you have enough freon in it. Usually when you don't have enough your compressor cycles on and off

 

yes, it is full of freon

 

i have been chasing some AC problems on my 1995 Impala SS. i dont know if this will help, but this link says what i have been going through.

http://impalassforum.com/cgi-bin/ult...=003806#000012

 

The only way to tell for sure is to get some gages on it and see what pressure the compressor is cycling at.

Could be many things.

 

Yes, get gages on it.
It's really good practice to replace the dryer when replacing the compressor. Also the expansion valve.
Moisture in the system (especially if the dryer wasn't replaced) or a partially plugged up expansion valve will produce your symptoms. And too low suction pressure under operation. Actually the moisture will freeze in the expansion valve and act like dirt, making the expansion valve seem too small.
The last dryer I bought (a year ago) was about $40 and the expansion valve was under $10. Most of the cost was in the Freon recovery, pump down and recharge.
Good luck

 

Check your high pressure switch, too. I've seen them cause the compressor to cycle on and off erratically when they go ****-up.

 

I am having the same problem with my 84 corvette too. I just replaced the compressor oriface tube and accumulator... and my compressor will cycle on and off...

 

You need to obtain and post your pressures. Low side switch is going to open at 25 psi and cut power to the a/c clutch. Hide side will open at 400 psi and do the same thing. Specs are 8 cycles or less per minute, but on a 70 degree of better day, a fully charged and clean system shouldn't see these pressures with the a/c on max. A good rebuild will be pushing a tad above it, maybe 28 on the low and somewhere around 200 on the high - assuming the main fan is running (and on the '84, it should be). Once we know what the pressures are, we'll be able to help you figure it out.


SSLT1KID - Your Impala is a different animal with a/c clutch engagement controlled by the PCM and if it uses a variable displacement compressor, it doesn't have a low pressure switch. Even if it does, the PCM monitors overall charge via a thermistor or pressure switch. That switch is mounted on the high side and gets a 5 volt reference from the PCM. The monitored parameter is variable voltage which rises with system pressure and each returned volt is about 100 psi. In order for compressor engagement to occur, that return usually has to be between .8 and 4 volts - less or more and the system will shutdown (and may set a troublecode which you can retrieve with a scanner). PCM also looks for a voltage increase (pressure rise) whenever the clutch engages. If it doesn't increase, that will also shut it down. Easiest way to quickly diagnose it is with a scanner. That will show you the signal and pressure. Any a/c trouble codes will have to be cleared (disconnect and reconnect the battery) after any repairs (including a recharge) - otherwise nothing is going to work because the PCM still thinks it's broken.

 

ok, i think i have finaly found out how this thing is wired......... when idling the compressor will cycle on and off, but when driving it will stay on, and work as designed. i got to thinking about this, and about 10 years ago i rented an oldsmobile achevia, and it did the same thing. so i figured this is how GM probally set it up to conserve fuel while idling

 

Not at all - It's set up that way so that when the refrigerant reaches a temperature that will make ice, the compressor shuts off. Refrigerant temperature (pressure) is dependent on the outside air temp and the efficiency of the design (which includes how fast the compressor is spinning. GM specs are 8 cycles or less per minute at a 70 degree ambient. Realistically, a clean and fully charged system shouldn't cycle at this temperature once the coolant temp reaches the thermostat. Cylcing is hard on the compressor - you should check operating pressures and vent temps to make sure it's right. If you've charged without knowing them; ie, without a manifold gage set, there's a high probability that it isn't right.

 

I have the same problem with my 94. The compressor keeps switching on/off, although I just replaced the freon.

But noticed that whenever I play with the A/C buttons the compressor turns on.

 

The '94 monitors how often it cycles and will shut it down if there's too much cycling as that is indicative of a low charge or a plugged up system. You need to see what the pressures are to fix it.