steve_p

Hi, the a/c in my recently acquired 87 S4 auto worked a bit, although it wasn't that good, so last week during one of the rare hot days we have in the UK I decided to treat it to a re-gas at Kwik-Fit, who have the proper machines.

My car has a plate in the engine bay showing that it was converted in 1999 to R134a, and the correct volume of gas and oil to use. The machine automatically vacuums the system, checks for leaks, and adds the correct charge (I guess you all know this!)

Anyway, the guy doing mine didn't use the low pressure fitting- he was one of those people you can't sway from their way of doing things (...been doing this 15 years etc etc..) and said it's ok to just use the high pressure side.

The machine pulled quite a bit of oil out the system, alomg with 205g of refrigerant, and there were no leaks. System charged ok according to machine.

But after starting engine, pressing the A/C button engaged the compressor pump, but then it disengaged, and now doesn't engage at all. I've checked the 2 fuses marked air con and they're ok.

He let me go without paying, but I'm wondering whether I should have it re-done using both connections, or whether my fault is indicative of something else being faulty. Seems weird that it worked before- could the increased pressure have popped something, or be confusing a sensor now?

Any ideas?

dprantl

I am assuming he charged from the high side with the engine off? Disconnect the A/C clutch wire and apply 12V directly to it with the engine running via the jump post. Do you see the center of the compressor spin? In the car, turn on the A/C and see if it blows cold. If not, shut it off, you don't want the compressor to be running for more than 30 seconds or so when there is not enough refrigerant in the system.

Also, how could there be no leaks if only 205g of refrigerant was found in the system. There is definitely a leak...

Dan
'91 928GT S/C 475hp/460lb.ft

steve_p

Hi Dan, thanks for the speedy response, it's not just the cars that are quick here..!

Yeah, high side with engine off. He said lots of cars now only have a high pressure port, you only need the low side if you want to pull the gas in with the compressor running.

It's midnight here, so forgive me for not trying your test until the morning! Do I find the clutch connection near the compressor, or in the fuse box?

thanks again

dprantl

Clutch wire (usually black) should come up out of the compressor into the main wiring harness that goes across the front of the engine. It is connected via a ~1 inch white rectangular spade connector.

Dan
'91 928GT S/C 475hp/460lb.ft

steve_p

Thanks Dan

by the way, what's the resoning behind this test? Can a high-side recharge leave the low side at too low a pressure, so the clutch isn't allowed to engage? Or am I way off the mark?

dprantl

No. I have no idea why your compressor is not coming on. Doing the above test will rule out all the car's electrical systems and sensors and verify that 1) the compressor is not dead and if it's not dead 2) determine whether there is enough refrigerant in the system to blow cold air out of the vents.

Dan
'91 928GT S/C 475hp/460lb.ft

smyth1

you may have only charged the high pressure side of the system.with the compresser not turning nothing will bypass through it.depending on the position of the expansion valve nothing may have gotten through to the suction side.those machies have to be on both ports to fill up both sides equally.jump both pressure swithches and check the sight glass.

SQLGuy

May be a coincidence. These cars are notorious for burning up the clutch relay (because it's too small for the load). The relay is embedded in the A/C head unit.

Search this forum for AC clutch relay, and you'll find some posts with pictures, testing and replacement procedure, etc.

A quick check would be to run the car and the A/C and jumper the clutch as Steve and Dan suggested. If you start feeling cool air after a minute or so, you've probably got a bad relay... or maybe a disconnected wire or a bad pressure switch or a bad freeze switch. Regardless, you probably shouldn't run the system jumpered like this for much more than a couple of minutes.

dprantl

? Charging liquid through the high side is not supposed to have the refrigerant going through the compressor right away, otherwise the compressor would get very unhappy. I am assuming the machine will be pushing liquid in, and the expansion valve cannot be fully closed so the right amount will go in eventually (which is what the machine is also supposed to be checking, assuming it is charging by weight).

Dan
'91 928GT S/C 475hp/460lb.ft

steve_p

ok, thanks for the advice everyone. The clutch seemed to operate ok before the re-gas, at least I'm assuming it did from the momentary change in the idle speed when the button was pressed. But I guess the increased load with the full charge could have been the final straw for the relay- I'll check it tomorrow (I didn't see a relay for a/c along with the other relays so assumed there wasn't one)

Dan, I just noticed you said this- "Also, how could there be no leaks if only 205g of refrigerant was found in the system. There is definitely a leak..."... I've read somewhere that the a/c can lose up to 10% of its refrigerant per year due to the gas permeating hoses etc.

Is this 10% per year figure generally agreed with?

dprantl

It's true that R12 hose leaks R134a... 10% seems a little high though. Are the hoses going to the compressor oily or dry?

Dan
'91 928GT S/C 475hp/460lb.ft

Hilton

Here's something I posted recently in another thread. Your symptoms sound like the relay - but you'll need to test voltage along the circuit.

JHowell37

With the compressor off, you can charge liquid through the high or low side, you just need to allow a few minutes for both sides to equalize before turning on the a/c.

A 25 year old compressor will allow refrigerant to pass through whether it's running or not. They may be tight, but they're not that tight.

Your problem is related to the recent service. Check all of the wire connections.

WallyP

"Your problem is related to the recent service."

Possibly but not certainly.

steve_p

A/C works fine with 12v to the clutch (it draws 3.3A) so must be electrical I guess.

I've got no voltage either side of what I believe is the freeze swItch either, so I guess it's time to use the service manual CD to locate the a/c control head...