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post #1 of 20 Old 04-19-2017, 08:12 PM Thread Starter
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A/C problems!

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
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post #2 of 20 Old 04-19-2017, 08:44 PM
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Re: A/C problems!

What are we working on here? Most of the 80's turbo dodge cars are expected to cycle 4-6 times a minute.
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post #3 of 20 Old 04-19-2017, 09:55 PM Thread Starter
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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
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post #4 of 20 Old 04-19-2017, 10:14 PM
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Re: A/C problems!

Are you running R12 or R134?
You say the pressures are right...so what is the low side and the high side reading after the A/C runs for a minute or two?
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post #5 of 20 Old 04-19-2017, 10:46 PM Thread Starter
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I'm running 134, low side at 10 psi high side at 80 or so, once I add more on top of that, my compressor starts cycling more and more on and off every 8-10 seconds or so
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post #6 of 20 Old 04-19-2017, 11:36 PM
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Re: A/C problems!

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
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post #7 of 20 Old 04-20-2017, 05:49 AM
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Re: A/C problems!

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.
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post #8 of 20 Old 04-20-2017, 06:55 AM
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Re: A/C problems!

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.

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post #9 of 20 Old 04-20-2017, 10:36 AM Thread Starter
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Where exactly is this switch? It's inside the compressor? There is only one switch going to any of my AC lines and that's the low side pressure switch (already jumped)
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post #10 of 20 Old 04-20-2017, 12:51 PM
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Re: A/C problems!

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Originally Posted by curt1987 View Post
Where exactly is this switch? It's inside the compressor? There is only one switch going to any of my AC lines and that's the low side pressure switch (already jumped)
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.
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post #11 of 20 Old 04-20-2017, 01:08 PM
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Re: A/C problems!

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).

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post #12 of 20 Old 04-21-2017, 05:25 PM Thread Starter
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Thanks 85lebaront2! So I grounded the brown wire at the pressure switch under the hood and it works! Is there any way to remove that stupid probe from the circuit? I looked under my dash and couldn't see Any sort of wire going into my evaporator box?
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post #13 of 20 Old 04-21-2017, 05:33 PM
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Re: A/C problems!

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.
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post #14 of 20 Old 04-21-2017, 05:44 PM Thread Starter
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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?
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post #15 of 20 Old 04-21-2017, 06:44 PM
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Re: A/C problems!

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.
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