I have a 1991 F150, 5.0 with warm AC. It appears to still be R12... threaded schrader valves and no conversion stickers. All tests were done with the AC and blower set on high and an ambient temp of 92 degrees (with exception of static pressure).


The clutch engages and runs continuously without cycling. There are no loud noises from the compressor or belt slippage. Warm air blows from the correct vents. The temperature does increase to heat if moved to the hot selection (So I assume the venting is functional). The condenser line just after the orifice tube does feel cold but the other end exiting the firewall is ambient temperature.


I assume the compressor is shot but wanted a second opinion before I spend the money to replace it and convert the system. You think that is a fair assessment? If not what else should I do to further narrow down the problem? Thanks in advance!


Static pressure: ~100psi (Both sides)
Idle: Low side ~80psi, High side ~120psi
2000 RPM: Low side ~60psi, High side ~130psi

 

Those really are crappy readings. In light of the high temperatures and most likely humidity the readings can be a bit skewed. I would lean towards poor compressor performance or another possible cause is a restriction in the suction line. The latter may be caused by the compressor letting loose then sending debris downstream which can plug the orifice tube.

Open up the system then pull the orifice tube for a visual inspection. That will tell you what is going on.

 

How long since the AC system has been charged?

 

I just purchased it. The previous owner of one year advised it never worked and didn't know.

I suppose I will inspect the orifice tube. If it has only sludge clogging it I may replace it and attempt to use a type 2 conversion kit. I just hate to blow $50 on the kit if the compressor is defective. Would be nice to have a recovery unit for this type of repair . I was under the impression if the orifice tube was clogged both the low and high side would be low mimicking a low charge. Obviously if I see any metal particles in the orifice tube I can rest assured the compressor is either already shot or will be very soon (true?).

Does this sound like a good plan of action or am I most likely wasting my money on the conversion kit? By the way, YES, I am a cheapskate .

PS @rla2005 That 92 degrees was in Florida, so high humidity...feels like it just rained 24/7 here lately .

 

A clogged orifice tube will not always show itself with low readings on both sides. I am really leaning towards a bad compressor with the resultant restriction of the orifice tube. As noted it could be a low charge, but I would expect a lower "at rest" pressure on the low and high side.

Without knowing exactly what has been done to the system I always recommend cracking the system open for an inspection. Then decide from there what to do. High heat/humidity can mask what I typically see for pressures/vent temperatures in the midwest.

I also highly recommend to check the fan clutch as well as verify the seals between the radiator support and radiator/condenser are still in place as well as the fan shroud being intact. All of these will contribute to poor A/C performance.

Personally I am not a big fan of conversions to R-134a. R-12 systems work better with what they were designed for. There are alternative replacements available worth considering.

 

The condenser line just after the orifice tube does feel cold but the other end exiting the firewall is ambient temperature.

Check the suction line which is the large line.... you need a charge accoring to those readings. You can't get R-12, you need to convert it.

 

R-12 is still out there in both large cylinders and small cans. To buy it on the open retail market you will need an EPA609 certificate (easy online course for about $25 USD). Now that the initial craze has blown over the price is not really that bad.