951 A/C question, which is high side, which is low?

AaronM · 06-26-2005, 03:53 PM

'86 951, will be doing some R&R on the A/C. Pressue is down below the low-pressure cutoff, but both fittings have pressure behind them so it looks like an instance of "normal" leakdown over the past several years (PO never used the A/C) rather than a catastrophic pressure loss somewhere.

I have the R-12 and the taps and such, but can't figure out which is high side and which is low-side. I had hoped that the fittings would be in the same place as on my 944, but they aren't. They are both right next to each other near the driver's side shock tower and I can't figure out which is which since R-12 fittings are identical for high and low side. One of the fittings is on a much fatter pipe and my instinct says that the fatter pipe should be the low-side since the R-12 will have expanded in the evaporator but I want to make sure.

Thanks much,

Aaron

carsonmx114 · 06-26-2005, 05:06 PM

the large line is low presure the small is high presure. both hoses will have the same amount of presure when the system is not working.

AaronM · 06-26-2005, 05:47 PM

Quote:

Originally posted by carsonmx114
the large line is low presure the small is high presure. both hoses will have the same amount of presure when the system is not working.

Thinking more, you're right. For some reason I was thinking there would be a separation of the sides. But the fact that there's still minimal pressure means that, unlike my 924S, the condensor isn't swiss cheese and the leak isn't complete and catastrophic.

Thanks much for the confirmation on the fittings.

Aaron

bearone2 · 06-26-2005, 09:02 PM

you'll need a manifold set/gages.

how much pressure?

take one of the caps off and put the red line/hp on 1 of the 2 lines, close both valves, start the car, turn the a/c on.

if it's the low side you could see 25-30, the hi side more than 60.

if you put the red on the low side, shut the car off, let the pressure drop, remove the line and put it on the other fitting and add thru the blue hose with the can on the yellow line.

if you have an r12 source, pm me please.

if pressure is too low the lo pressure will keep the clutch from engaging and you won't see any pressure.

Zero10 · 06-26-2005, 10:05 PM

The bigger line closer to the passenger side of the car is the low pressure line.

If you would like to nail it down for certain, and are working on another car, or something like that, remember how the A/C system works.

Compressor compresses the refrigerant into a liquid, it then passes through the condensor, to the receiver/dryer, then to the expansion valve, and evaporator. So, if you follow the lines, one will go from the compressor to the firewall, which will be hte low pressure line, the other will go from the compressor to the condensor, which must be the high pressure line, follow it.

Long story short, low pressure is usually the bigger pipe

AaronM · 06-27-2005, 04:40 AM

Quote:

Originally posted by bearone2
you'll need a manifold set/gages.

how much pressure?

take one of the caps off and put the red line/hp on 1 of the 2 lines, close both valves, start the car, turn the a/c on.

if it's the low side you could see 25-30, the hi side more than 60.

if you put the red on the low side, shut the car off, let the pressure drop, remove the line and put it on the other fitting and add thru the blue hose with the can on the yellow line.

if you have an r12 source, pm me please.

if pressure is too low the lo pressure will keep the clutch from engaging and you won't see any pressure.

Pressure is too low to allow the compressor to come on, but there's residual pressure in the system when I measure it (a couple psi, nothing big, but just enough to indicate that there's no gaping hole anywhere).

If the compressor could run, I'd have just done as you suggested, but it hasn't been run in several years (definitely hasn't been refilled since 2000, probably not touched since the mid or late 1990's) and it looks like it's all just normal leakdown that's gone un-noticed.

No "source" for R-12, I just keep an eye out for individuals selling it. Picked up my last bit from someone cleaning out his barn/workshop.

bearone2 · 06-27-2005, 08:53 AM

i used to bridge/bypass the low pressuire switch to get r12 in the system to leak/ops check to see if the compressor/clutch were working but can't remember how to do it.

good luck

AaronM · 06-27-2005, 12:08 PM

Quote:

Originally posted by bearone2
i used to bridge/bypass the low pressuire switch to get r12 in the system to leak/ops check to see if the compressor/clutch were working but can't remember how to do it.

good luck

I tend to dislike that method since it chances running the compressor without enough oil. I generally add a can of oil charge to the system to bring the pressure up before running it if it's that low. Haven't had a problem with doing it that way in the past.

bearone2 · 06-27-2005, 02:11 PM

i guess there's enuff pressure in the can to get into the system without the engine running.
hi to lo.

AaronM · 06-27-2005, 02:48 PM

Quote:

Originally posted by bearone2
i guess there's enuff pressure in the can to get into the system without the engine running.
hi to lo.

Yup, more than enough pressure in the can to charge the system enough to get the cut-out switch to back off and allow the compressor to run. Usually one can will do it.

Zero10 · 06-27-2005, 08:58 PM

A 1lb can will be more than enough to get the low-pressure cutoff switch to re-engage the compressor. If you have 0psi in the A/C system (no vacuum though), a 1lb can will almost completely fill the system, or at least that's been my experience, I got proper high and low side pressures from just a 1lb can when I was trying to find leaks in mine.

To bypass the low pressure switch, pull the 2 spade connectors on the line beside the receiver/dryer back a little bit, and insert screwdriver

I had to do that to get a friend's system to charge. I attached the can, opened the valve, and the pressure shot up to about 55psi (can pressure), and wouldn't come down, and the compressor wouldn't kick on. I jumpered the switch, and it immediately fell to 15psi, and continued filling, until it hit about 40psi, which was correct for the ambient temperature. Sometimes you have to jumper the low pressure switch if you're not super-patient for the pressure to fully equalize, and the diaphram in the low pressure switch to snap back. Can take quite a while sometimes...