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A/C problems!

2K views 19 replies 7 participants last post by  ohnotagain 
#1 ·
Hey guys, I have some issues with my AC compressor not staying on, I have replaced the entire AC system, new everything, and I have the low side pressure switch jumped, new AC relay and fan relay. But the compressor will not say on, it will kick on for about 5 seconds then kick back off. And keep doing that. Nothing is plugged up, the pressure equalizes when I shut the car off so i know there is nothing plugged. The more I put into the AC system the less time the compressor will run. BUT THE PRESSURE SWITCH IS JUMPED???? So it should stay on no matter what. Once I go past 10 psi on the low side the compressor starts kicking off. Does anyone know if the ECU controls anything on that relay? I am at a loss ??? it doesn't make any sense
 
#3 ·
Oh sorry it's an 89 Shelby Daytona.... the car's compressors are suppose to cycle 4-6 times a minute?? That is weird, I've never heard of a car doing that when the A/C is functioning properly. Every other car I've ever owned the compressor stays on as long as the A/C is fully charged and the high side and low side have the correct pressure. The AC is cold, but the compressor just won't stay on
 
#6 ·
How much R134 is in the system? A rule of thumb is that you fill an R12 system with 80% R134. Most our cars take 38oz of R12. If I remember correctly.

At 80* low should be 16-30 and high should be 180-235
At 70* low should be 10-24 and high should be 140-210

I think that your low side pressure is low as is your high side pressure. But I will have to go look at the FSM to confirm.

How much is in the system please
 
#7 ·
System on R-12 should be 38 oz, that would equal 30 oz. of R134.

If you do not know how much refrigerant is in the system you will need to recover the system and start over.
1)Evacuate @ 28" HG for 15 - 30 minutes.
2)Fill system to 30 oz. R134A
3)Pressures...
a)High Side - minimum of 100 PSI over ambient temp with cooling fan cycled on
b)Low Side - compressor should cycle off at 20-25 PSI.

Once you know you have the correct amount of refrigerant in the system post back.
 
#8 ·
Your car also has an electronic cycling switch inside the AC casing, it is in between the control head and the underhood damped pressure switch (which you have jumpered). In answer to your other question, from there it goes to pin #45 on the SMEC 60 way connector. The SMEC turns on the engine fan and the AC compressor relays (AC compressor relay coil is fed from the light green fan wire).

If the cycling switch sees a coil temperature below 32° (may be higher or lower) it will open the control circuit, this may be what the issue is.
 
#10 ·
There is a temp probe that is actually stuck into the evaporator inside the HVAC box under the dash. Found mine when I had to replace the heater core on my Daytona. But the actual "Switch" you asked about I believe is inside the SMEC.
 
#11 ·
The cycling switch is the probe in the evaporator core (inside the AC casing) The wire from pin #45 on the SMEC goes through the damped pressure switch, then through the firewall on pin #37 of the bulkhead connector, then to the Electronic Cycling Clutch Switch then to the control panel where it is grounded with the AC button depressed and the fan on. If you ground the brown wire at the Damped Pressure Switch (on the valve block at the firewall) the compressor should run continuously. If it still will not, then you have either a SMEC problem or other electrical problem.

As soon as you ground that wire with the engine running the fan and compressor relays should close, fan will come on and compressor clutch will pull in.

Pin #56 is AC clutch command (grounds relay coil), pin #57 is fan command (also grounds relay coil).
 
#13 ·
Without that probe the compressor will constantly stay engaged and will not cycle and then pressure will get to low and the evaporator will freeze (In A/C temperature and pressure are directly related) and you will not have A/C.

My suggestion, diagnose and repair the actual problem.
 
#14 ·
Soooo is this possible..... it is too cold outside (51 degrees today) for the outside air coming past the evaporator, thereby making the evaporator get cold really fast and inturn the probe getting cold really fast, and causing the probe to kick off the compressor really fast. Could I be on the right page here or am I sounding like an idiot?
 
#18 ·
Soooo is this possible..... it is too cold outside (51 degrees today) for the outside air coming past the evaporator, thereby making the evaporator get cold really fast and in turn the probe getting cold really fast, and causing the probe to kick off the compressor really fast. Could I be on the right page here or am I sounding like an idiot?
My A/C cycled normally with the defrost on during the winter month's when it was 40 degrees outside but yes, the colder it is outside the quicker the compressor will cycle because...
As I stated, temperature and pressure are directly related, if it is already cold outside/in the car then there is not as much heat to absorb (hot moves to cold to equalize)so the line pressure is already lower to start.
(Did I confuse you even more???)

The BIG question is...
Do you actually know how much refrigerant is in the system?
R134A is not like R-12 was.
R-12 you could keep adding and it would keep getting colder, R134A will not do that, too little or too much = warm air and a poor functioning A/C system.

What are your gauge readings?

The high side will change more than the low side with Ambient Temp and Humidity Levels but should be at least 100 PSI over ambient temp.
The low side depending on ambient temp will be in the 25-35 PSI range and will cycle off at approx. 20-25 PSI.

If the pressures are changing quickly and the compressor is cycling quickly then there is an issue and you have to start with the basics and that would be is the system under or over charged?
You may have to evacuate and recharge to be able to accurately diagnose the issue.

NAJ, did they do away with the expansion valve when the cycling switch was added? I know on Ford and GM vehicles with the CCOT system it will freeze up big time if the switch is bypassed. The reason I am asking is I am using a 1992 Imperial EATC system and it does not have the cycling switch.
The Chrysler manual system still used an expansion valve, at least they did up until 2008 when I was still at Jeep.
Funny, never had an issue with mine (knock on wood) but the Jeeps used to stick closed and the customer complaint was A/C blowing warm air.
Hook up the gauges and the low side was in a vacuum.
 
#15 ·
NAJ, did they do away with the expansion valve when the cycling switch was added? I know on Ford and GM vehicles with the CCOT system it will freeze up big time if the switch is bypassed. The reason I am asking is I am using a 1992 Imperial EATC system and it does not have the cycling switch.
 
#17 ·
If the ambient temp is cool, then it will cycle more. The air is too cold to transfer much heat to the evaporator so there really isn't much load on the A/C system. R134A runs at lower pressure than R12 on the low side so that will exaggerate the problem if it's a retrofit and the low side switch is for R12.
 
#19 ·
Yeah, our 2005 T&C had one, actually had a sight glass on the receiver/drier. I never did get my 1985 konvertible with the turbo II radiator to run with R134, compressor would fry the belt in traffic. I am going to add a condensor pusher fan on the 1986.
 
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