Air conditioning woes, appeal to experts

I've been on a journey trying to get the a/c working in my 1988 coupe. Effort has been spent undoing curious decisions made by previous owners. (Previous thread).

The compressor shuts off before I can fully charge the system.

During the charging process everything is working as it should and the a/c in the car is blowing beautifully cold.

I have a set of gauges while I'm doing this and everything reads as it should, but low.

After leaving the car a while, the a/c will work again, but then switch off after 10 minutes or so, whether stationary or driving.

The current situation is: fixed leaks, new receiver dryer, new low pressure switch, new oil (correct type and amount). The condenser fan is working as it should, I pulled a vacuum for hours before attempting to charge.

Any ideas? Any help much appreciated.

 

Link out the Lp Switch on the drier until you get the full charge into it. Keep an eye on the gauges in case the expansion valve closes in due to low evap temperature- you can run fan on max speed with windows open

 

There is a line temperature sensor switch on the condensor that may have failed. Part number 61047900.

Here is an old thread where I dealt with the issue. Easy to diagnose and replace. The sensor is now being reproduced at a reasonable price.

https://www.ferrarichat.com/forum/threads/ac-condensor-fan-questions.554436/

 

yep, my thermo switch (also called a bi-metal switch) was bad, same symptoms, I just bypassed it and the fan runs all the time when the AC is on, not a problem here in FLA and I think the fan is rated for continuous operation, not sure about the wiring. IIRC, when I did the fuse box maintenance a few months ago, I found a burnt connector on the white connector far left, I fixed it, but didn't bother to look it up, but it may be the AC condenser fan circuit.
Alden

 

Ben could your evaporator temperature sender be faulty and switch off the compressor at too high a temperature?
Could your high pressure switch be faulty and cut the compressor at too low a pressure?

 

Thanks for all the replies guys.

Interesting, I appreciate the link, moysiuan. It sounds like that could be my problem. That's a pretty cheap part, even the Ferrari price is reasonable. How accessible is that?

Oh gosh, I'll have to figure out how to test that, that's buried pretty deep into the dash?

The drier switch and sensor at the receiver drier end of the system are all brand new. Are you talking about something else?

 

Can you post the gauge readings that you have?

 

(with the compressor running)

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Does the sight gauge (between the valves) still show bubbles? BTW depending on the outside temperature when you shot this......the pressures are okay. A bit higher on the high side than I would expect and a little lower on the low side for a R134 system.

Nothing that should be tripping the high or low pressure switches. As mentioned earlier....they are easy to by pass just to confirm what the issue is. I don't have a schematic for a 1988 Mondial. If you have one please post it.

 

There is an ecu that controls the heat and ac, it is under the passenger floor panel. That gets temp readings from the three sensors, in the outlet air and the evap area and the cabin temp. This ecu actuates one of the three underdash relays which turns the compressor on and off. Perhaps you have a failing relay. The ac relay underdash is the same as one other of the three relays, you can swap to see if the relay is a problem. And you can use an ohm meter to check the three ecu sensors at the harness connector to the ecu.

 

These are the relays I am referring to...

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Will it keep running on those pressures ?
Do you get a period where the suction pressure drops and it then cuts out ?

 

Ben you mentioned
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but did you also renew the high pressure switch? I think on some cars there are 2 separate ones, on mine they are combined into one.

You can easily check the evaporator sensor by measuring the resistance on the ECU plug pins 1 & 3 of the HVAC wiring and then comparing the value to the chart:

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This would help exclude some more failure points.

 

Thanks again for the replies everyone.

Hmm. I don't recall seeing any bubbles.

Great, thanks! Good to know.

No, it doesn't keep running. Pressure doesn't drop, the compressor just stops. (And the readings go up).

Awesome, thanks Urs. That's a lot of info. Much appreciated.

 

When the compressor cuts out, you will see the suction pressure rise and the discharge will drop until they are equal. The fact that the suction does not drop seems to rule out the lp switch, but to prove a point you can take 1 wire off the switch and see if the compressor will run( i dont think it will run) then short the 2 wires and see if it cuts out after running.

 

Some questions:

-How exactly are you charging the system (you mentioned running, so assuming you are going in through the low-side as a gas?) and what are you doing to ensure the correct amount of refrigerant is being charged? Are you using a scale or just trying to eyeball pressures? With R134, you also may need to adjust the amount of refrigerant as opposed to the original R12 spec.

-You can charge the system at rest on a ambient-temp car by heating the refrigerant tank. This will raise the tank pressure to overcome the system pressure of the cooled car. Also invert the tank and charge through the high side as liquid refrigerant and will go right in. Do not do this on the low side with the A/C running or you will slug the compressor..

-If the the pressure sensors, etc test out, you may have a weak coil in the compressor. Might be going open due to heat. Or the gap distance is larger than desired with the clutch. Check for 12v at the compressor at all times when the system is on. If you have 12v at the compressor and the compressor is not engaging, then it's the compressor. I would do this first before chasing anything else.

That 20psi on the low gauge seems low. Normally you want to be 30psi - 40psi depending on Ambient temps and humidity.

 

This is an awesome feed. Very educational. A project I am getting ready to do on my 86!

 

Not sure this will help. Just a reminder, my factory repair manual for my 88 3.2 says to get the specified pressures with the engine running at 2500 RPM, not at idle, Yours may be different, but probably not.
Alden

 

Hi all. Thanks again for the replies. I spent some time today trying to get a handle on things.
When I first started the car today, the a/c was running great, I was able to introduce more refrigerant into the system, was not quite the full amount when it shut off again.

Good idea. I checked it with a multimeter. When the compressor shuts off, there's no power going to it.

I tried bridging the low pressure switch for fun, that didn't make any difference.

Oh gosh, really? I can't imagine that's the problem I'm having but maybe someone can chime in if they think it is.

I'm still scratching my head but I have a couple of other things mentioned in this thread still to try. If anyone has any thoughts, feel free to share, thanks.

 

Best to have a look at the desired temperature switch in the central consol, if this is playing up it will disconnect the compressor clutch if it thinks the car is at the required temperature . If you rig a multi meter up on it and twist the dial to see if the ohms value changes.
In ref to the wires on the lp switch , if the compressor clutch was engaged then by removing 1 wire or shorting the 2 together should have made the clutch disconnect