'89 Speedster A/C temp working intermittantly

[Knucklehead]

Hi.... I'm new here, looks like a great forum. I've owned a few 911s, and my most recent Porsche is a 1989 911 Speedster. It's been retrofitted with a C2 A/C system, which has really helped keep the car cool here in the Arizona heat.
The A/C has worked perfectly for almost 9 years, blows real cool.

I took the car out today, and as it was hot I turned on the A/C. It worked okay for about 30 minutes, and then started blowing warm air. I turned the A/C off and on a few times, and it worked for a couple minutes blowing cool air, then it turned warm again. I turned off the A/C and headed home. I've since tried the A/C a couple times, and it seems to be working okay again. I'm hesitant to head out again and have an A/C problem since it's still summer weather here.

Any advice would be appreciated, hopefully it's something simple. I have an appointment for service next week. Thanks very much, Robert

[ruf-porsche]

Your evaporator could be icing up so the air is no longer going through the evaporator coil, but around it. Buy a pocket thermometer and check the vent temperature. If it is coming out less than 32degree, your evaporator coil is icing up. The compressor is not cycling off so the capillary tube could be the problem.

[mthomas58]

Originally Posted by ruf-porsche

Your evaporator could be icing up so the air is no longer going through the evaporator coil, but around it. Buy a pocket thermometer and check the vent temperature. If it is coming out less than 32degree, your evaporator coil is icing up. The compressor is not cycling off so the capillary tube could be the problem.

A tell tale symptom to look for in evap freeze up is a noticable reduction in airflow as the vent temps rise. You may have a leak in the evap itself or it could be that you have the temp setting set so cold that the compressor is never cycling off. Extended compressor use can contribute to freeze ups - particularly if your thermostat is not functioning properly or calibrated to cycle off at sub freezing temps. The compressor needs to cycle on and off to keep ice from forming on the coil and blocking air flow. Try turning up your thermostat setting up and see what happens.

[Knucklehead]

Thanks for the reply. I had a pocket thermomter in the vent when it happened. It started out going as low as 55F, then within a minute it climbed up to the 70s, it peaked at around 80F.
When I turned it off and back on after a minute or so it was blowing cool again, but then did the same thing. I'm going to check it out today and try backing off the temp knob as recommended.
Normally I run the A/C when it's well over 105F here, even higher, Yesterday when this malfunction occured it was 91F.

Appreciate the advice.

[ruf-porsche]

Originally Posted by Knucklehead

Thanks for the reply. I had a pocket thermomter in the vent when it happened. It started out going as low as 55F, then within a minute it climbed up to the 70s, it peaked at around 80F.
When I turned it off and back on after a minute or so it was blowing cool again, but then did the same thing. I'm going to check it out today and try backing off the temp knob as recommended.
Normally I run the A/C when it's well over 105F here, even higher, Yesterday when this malfunction occured it was 91F.

Appreciate the advice.

If you have a set of manifold gauge check the pressure on the low and high side. If the vent temp was only 55 degrees I doubt that the evaporator coil is icing up. You could be low on refrigerant or have a blockage in the system such as at the expansion valve.

Is the ac system still R12 or has it been converted to R13a?

[john walker's workshop]

see if it works better with the fan speed turned down.

[Knucklehead]

Originally Posted by ruf-porsche

If you have a set of manifold gauge check the pressure on the low and high side. If the vent temp was only 55 degrees I doubt that the evaporator coil is icing up. You could be low on refrigerant or have a blockage in the system such as at the expansion valve.

Is the ac system still R12 or has it been converted to R13a?

The system is R12, and I have not had it topped up since the A/C was retro-fit 9 years ago. Thanks for the advice on checking for blockages such as at the expansion valve.

I'm going to also see if it works without problem with the fan speed turned down.
But I want to find out why this problem occured.

Appreciate all the good suggestions and help!!

I'm planning to take the vehicle in for routine stuff this week, so hopefully I'll have some info from the shop on the A/C issues if I can't get it strasightened out before then.