Update. Took the car back to the ac guy. Checked the valve stem on the low pressure/cycling switch and it hissed, so okay there. Found a small leak on a schrader valve he installed on the accumulator, which he tightened up. He discharged and recharged with his ac machine. Still no compressor kick-in. Rechecked continuity from low pressure switch receptor to compressor, which was okay. Again jumped the receptor and the compressor clicked on, but with minimal if any cooling. He concluded the new switch was bad, so I went and bought another. Same story. He was at a loss for a logical explanation and said he will ask his boss when the boss returns from vacation in a week.

Any other suggestions? I didn't see the gauges on his machine, so I don't know what the pressures were. Seems to me it should have cooled well when we jumped the switch receptor and had the compressor running. Maybe his gauges are messed up?

 

This is going to sound harsh but you need a new AC mechanic. He obviously doesn't understand the principals of refrigeration and how to troubleshoot your system. It;s not that hard but does require some knowledge of the refrigeration cycle and electrical circuitry. Both seem to be lacking here.

 

Typically with the system full and compressor running(get the gauge readings) the only reason it would not be cold is the heater /cooler door was not working(stuck on heat). If he indead filled it compressor pressure would create cold air at your accumulator(in the vehicle) and the blower would push it int vents. Wait for the boss, this is a head scratcher to say the least.

 

One other observation from today: there was no continuity across the pins of any of the three low pressure switches we tried on a presumably pressurized system. If this switch is normally closed with adequate pressure, shouldn't I have continuity?

Silver, you may be right but I have several degrees and licenses to prove my intelligence (albeit one stupid beer blunder), I've studied this enough to understand how the refrigeration and electrical work, I have personally replaced the ac systems in three other vehicles (except for discharging, vacuuming, and recharging), and I know this gentleman and have watched him work often enough to feel he knows what he is doing. Nobody on this board or others I've talked to seem to have the answer, either.

xrav22, the blend door is functional. Sliding the temp **** to heat puts out warm air.

My current guess is that his gauges/machine are/is inaccurate/broken, and that the true low pressure is not enough to keep the switch closed. I'm going to go buy my own gauges and see what they read. Any reasons why the low pressure line might be too low (e.g., a leak, a restriction somewhere)?

 

Bought new gauges and jumped the compressor. Readings were 202 high and 41 low at 82 degrees and 30% humidity. Isn't that acceptable?

 

That is acceptable but was it getting cold, is the air blowing in?
There is a reason why the compressor is not kicking on, mabey high switch bad?
I guess try adjusting the switch as it ma be adjusted too low or high.
I don't have an adjustment could you take a pic of it?

 

You should be able to feel the suction line from the evaporator getting cold. If the high switch was bad, the compressor wouldn't run. The high side switch is NC, and should open only if the high side pressure exceeds about 400 psi. Leave the car doors open to increase the "heat load" to make the a/c work harder.

 

Just got home and wanted to test some of your replies. Warmed up the car, the fan turned on at 195, and the car settled in (and usually runs) at 200 degrees. So I assume, Dogs, that my fan continually runs whether the compressor is running or not. Top off, LP switch plugged in, ac on max, and blower on high, vent temps were 96 or so (car has been in a hot garage all day). When I slid the temp to warm, temps rose to 120+ rather quickly, so that shows the cable and blend door are functioning properly. Of course, the compressor was not running.

I then jumped the LP switch, kicking on the compressor. After about ten minutes, the vent temp was about 78 degrees. Some, but not as much drop as I would expect.

xrav22, looking at the wiring diagram, I assume the HP switch would have to be good; otherwise, jumping the LP switch would have no effect on the compressor, right? Also, the adjusting screw for the LP switch is right between the prongs. Unplug the switch and you should see the screw easily. I have not adjusted it on any of the three LP switches I've tried (including the original that worked fine before the rebuild.)

Hot Rod, the suction line did get quite cold when I jumped the compressor. Vent temps just didn't drop enough. As above, I concur on the state of the HP switch.

Silver, if you're still checking this thread, I apologize for getting overly defensive. Let's just say I'm confident my ac guy hasn't screwed anything up. He just can't make sense of this like the rest of us. And, I will consult someone else if this isn't resolved by next week.

 

Could that diode thing mabey causing an issue, I haven't even checked that lately.

 

I'm no electronics wizard, but I think a diode prevents current from running the wrong way. If it were messed up, why would the compressor run normally when jumped?

 

The system is not normal, you stated that you jump the low pressure and it was not cold inside. I don't know about the diode BTW, but it seems that the accumulater may not be functioning(mabey clogged, not cooling enough), something is wrong I did complete clean and retro of 1985 and I want to fiqure this one. If you jump the compressor it should be cooling if the freon is correct. I am stumped.

 

The accumulator is brand new (has to be to get warranty on compressor); it absorbs moisture in the system. I don't think the diode is an issue or the compressor wouldn't keep running. Freon might not be perfect, but that isn't the real issue. Why do I have to jump the LP switch to get the compressor to run? I think it's an electrical issue of some sort, not my strong suit. Maybe a ground?

 

 

With the suction line cold, and no cold air from your ducts, that tells me you've got some restriction in the cold air path. Dirty evap coils, internal insulation clogging the ducts, fan speed too slow, fan clogged, or temp control doors not working properly. After you get proper air flow thru the ducts, you'll need to add refrigerant.

 

Hot Rod and Dogs, I think you guys are on to part of my problem. The evaporator and blower fan are new, so it's doubtful they have anything wrong. BUT, I have had bits and pieces of foam insulation blowing out the vents the past couple years, so I suspect I do have a lot of leakage. When I had the evaporator out, I also had the control unit out so I couldn't test the movement of the blend door (if it is even visible through the hole behind the evaporator). I guess I'm going to have to pull the dash out. I can't remember if I've had the dash out of the Vette, but I know I've done several other cars in the past and it's a PITA. Do you have any sources or recommendations for replacement closed-cell foam?

Dogs, is the heater valve in line on the heater hoses next to the evaporator? If so, I replaced that a few years ago; if not, I don't know where it's located. Also, the LP switch connector has two black wires running into it. I figured out lead and ground by putting a voltmeter on it with the car running. I "jumped the switch" by sticking a piece of wire into each female part of the connector, not the switch itself (although I did do that early on which was my stupid beer blunder). Lastly, I did replace the rotary selector switch inside the control unit as someone else suggested; the old one didn't look broken or burn't, but it was kind of gummed up with old grease so I put the new one in.

 

Mystery solved! The new evaporator had a nipple (the thing with the schrader valve in it) about one millimeter longer than the old one, so the pressure switch did not seat against the schrader valve. Solution was to ream out the threads inside the switch enough to fit over the nipple and contact the valve to pressurize and flip the contacts inside the switch. Cost me $50 and a six pack. Thanks to those of you who tried to help me out.