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1987 Supra a/c clutch disengages for hours

Supra on Sat December 03, 2011 8:29 AM User is offlineView users profile

Year: 1987
Make: Toyota
Model: Supra
Engine Size: 7mgte
Refrigerant Type: R134
Ambient Temp: 60
Pressure Low: 22 ish
Pressure High: 155ish

The a/c has been low for a year so I converted to R134A, drained most of the oil (the tsb said draining the old oil isn't required) from the compressor, replaced the dryer and all o-rings. I installed about 7oz of ester oil. When I initially started charging the system the low side went to zero but the compressor kept going but since I was adding freon I kept going. Several minutes later the low side was up to about 25# and the clutch disengaged but did not re-engage until the car sat for several hours. I figured the low pressure switch was bad so I replaced it but it is still doing the same thing. The car has auto climate control with various sensors inside and out as well as an electronic head unit and amplifier. Anyone got an idea?

PS: The compressor clutch will stay engaged a few minutes before disengaging and I have installed about 1.75 cans of R134A

Edited: Sat December 03, 2011 at 9:00 AM by Supra

mk378 on Sat December 03, 2011 10:52 AM User is offline

There are a couple of people here with extensive Supra experience-- much of it not good, especially when R-134a is involved. On any car, a major reason for the clutch refusing to engage when hot is because the gap between the clutch plate and the pulley is too wide. Those cars should have an A/C amplifier which would detect a problem with the clutch and make the A/C light on the button blink rather than stay on steady.

Supra on Wed December 07, 2011 3:04 AM User is offlineView users profile

I haven't noticed the a/c light blinking. How does the amplifier work? Where is it located? You mentioned others have had trouble woth the R134a conversion on a Supra, can you give an example? The compressor clutch has never been worked on in the cars 43k miles. Is it possible the amplifier is bad? What should the clutch gap be?

NickD on Wed December 07, 2011 5:56 AM User is offline

Have an 88 Turbo for over 23 years now with a the five volume set of shop manuals, full parts lists, and original brochures. MVAC system is microcontroller where the circuit diagram is a four page fold out, so where do I start? Sensors all over the place, is an automatic temperature control system with a manual override. In basics, uses a dual function high side switch that should remain closed between the 40 to 430 PSI range. All those temperature sensors, ambient, in-car, solar feed into the processor circuit that determines whether the system calls for heat or cold. Key component is an evaporator sensor that limits compressor operation between 33-39*F.

Yes, all these sensor are tuned for R-12, really no big deal, system only holds around 26 ounces, and you ask why R-134a doesn't work. Well it can if you can reprogram the processor to work with R-134a, bit too much for me to fool with and fooling with it is the precise word. Did know this system quite well, but that was around twenty years ago. Just took my car out of storage last summer and replaced the timing belt. That was one heck of a job, not that it was due, didn't want to risk a 24 year belt with an interference engine.

I gapped my clutch at 15 mils, but can go up to 25-30. As I recall, system has diagnostic codes to add in trouble shooting, but gosh, that was over twenty years ago. Mine still works so didn't dig into it again. If you know anything about P-T charts and this system you will know why R-134a is not ideal.

Nick is my real name, why do you call yourself Supra? Are you a Supra expert? Maybe you could help me if I have problems.

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