groppo

This coming weekend in Oz, Melbourne, is going to be hot. Forecast is 40C (105 f), so long drives are rather uncomfortable when A/C blows warm air.
I've read a few other threads on the problems that can occur. The system seems very complicated, so here are all my A/C symptoms to hopefuuly make a diagnosis easier. The car is a 1990 S4 with dual A/C (front and rear).

History
- never worked properly since purchasing the car, worked better for awhile after various repairs
- previous work done (2-4 years ago), converted to new gas, replaced compressor thermostat, compressor checked, replaced heater tap, replaced control flap servo, replaced "O" rings, replaced 1 way vacuum valve.

Symptoms
- air from rear vents are slightly colder than front, front air only a little colder than when A/C turned off
- both thermo fans are working when A/C switched on
- air flow is stronger out of vents when A/C is on, compared to when A/C is off
- air is a fair bit cooler (but still no where near cool enough) when outside temperature is cooler

So, if anyone can help, I would be a lot cooler under the collar!

groppo

jeff jackson

when you have the AC switched on...you can "see" the clutch running on the compressor ?? If yes...then the refridgerant charge is suffiicient to allow the clutch to engage. That said...it doesn't mean the charge is OK for decent interior cooling, particularly in these extreme temperatures you are enduring. Sounds to me like you are low on refridgerant...also....R-134 does NOT make as cold of air as the old style R-12 (although its more "environmentally friendly"), it simply doesn't offer the chill factor of good old R-12...also, unless the conversion was done by a competent shop...that knew that the capacities on your system for the different type refridgerants are different between R-12 and R-134 systems...your could have been "charged with the wrong ammount of refridgerant...Just guesses mind you...But NO one I've met...has ever told me that the system after the "upgrade" to R-134 made as cold of "conditioned air" as the old ecologically dangerous R-12 systems did.

groppo

Jeff, thankyou for your reply. The air temp out of the vents is still not cold enough even for R-134. I found a place nearby specialisinbg in A/C's and they can regas the car this afternoon. As you mentioned this may improve things. I'll check the clutch.

Cheers,

groppo

AO

Have you replaced the hot water valve behind the airbox? They are notorious for leaking/failing. On my GT, the PO had the AC serviced at least 4 times by the dealer, always complaining, "AC not cold enough!" The dealer would recharge the system, replace some o-rings/expansion valve and charged the guy $600+ each time. I suspected the hot water valve, so I pulled it, and sure enough, eventhough it appears to be closing, when I blew on it, i could hear & feel air passing through. I replaced it, and now it truly is ICE COLD for $35. Certainly worth looking at. YMMV.

AO

BTW, I used and Audi valve. Hopefully it will last.

Bill Ball

Did it work well after the conversion 3-4 years ago? It should have. R-134 can get the vents as cold as R-12 or close enough not to matter. If it didn't work well, that should have been addressed by the technician then. The system isn't all that complicated. If you have had this technician work on it repeatedly without good result, you need to find another shop.

You say you are going in for a regas. The technician should confirm basic operation of the clutch and compressor. If it does need more gas, then the leak needs to be located. A good AC system should hold a charge indefinitely. If the clutch never works, then it is a matter of checking the electrics. If there is no power at the clutch it could be faulty pressure valve near the condenser (bypass it), faulty freeze valve in the windshield cowl (bypass it) or a bad relay in the controller (if none of the aforementioned are getting 12 volts). This relay is infamous. The controller can be replaced with a rebuilt unit or the relay can be replaced with a substitute (we use one from Radioshack in the US). If the controller is bad, the clutch will never engage. If the clutch engages, the charge and pressures are right, then it is likely the expansion valves (front and rear) are bad. With bad expansion valves, everything else will check out, but the AC will not get cold.

If the heater valve mentioned were the culprit you would have hot air all the time. Still, an apparently good heater valve can have a slight leak that will warm the vents a bit over ambient and compromise cooling somewhat. I used to have air 10F above ambient (hand in front of vent compared to hand out the sunroof) from the vents all the time. The heater control valve was the culprit in that case.

SharkSkin

If it was originally an R-12 system, the hoses that came with it are almost certainly not up to the task of containing the smaller R134 molecules. Were the hoses changed when the conversion was done? Also, look for any oil on the hoses and around the fittings. When freon leaks out, it carries oil with it.

Tails

R 134a gas capacity with rear A/C unit is 1030 grams.
Oil capacity 130 +/- 20 ml (special oil suitable for R 134a refrigernant).
R 134a requires correct quantity of gas, not filled until no bubbles in sight glass.
Tails 1990 928S4 Auto

JET951

hi

at our work in sydney we have converted over60 r12 to r134a 928s, and we would have fixed another 40 or so. when we do AC on a 928 we do everything. we do everything because we dont like the hassles of customers coming back saying "hey you charged my system up and it only lasted a month". well thats because you only told us to gas it and to SPEND the MINIMUM. so for the past few years as i said we do everything.

we replace both the hoses that come off the compressor, we replace the liquid line hose(we have special tools to remove and replace this hose without taking the line off the car). we send the compressor away to get rebuilt, we replace the TX valve, and we replace the low pressure switch, we replace the thermo switch, we replace the reciever dryer and we modify the cooling fans to cut in automatically when the AC is engaged. we dont leave anything. although this is expensive we have never had one come back for a problem. and we see a constant 3-4 degrees at the vents on a 30 degree celcius day.

one more thing we do most of the time is delete the rear AC. it is troublesome and pointless. and we have found that with the rear AC deleted the main system works more efficiently.

AC on 928 is very simple. spend money and do it right the first time and it will save you in the long run. be cheap and it will cost you more!

Sean Buchanan, Buchanan automotive

AO

Just to interject a bit further on this possibility. My AC would feel cool, but never cold. Outside air was, as Bill said, about 10*F warmer than outside air. I first mis-took the warmer air as engine heat. But then found the culprit.

WallyP

If you will send me an email (NOT a private message on this system!), I will send you a tech paper on the HVAC system that will give you more details on the system. techAT928gt.com - change AT to @

IcemanG17

It sounds like you need a recharge.....everything else you mentioned seems to be in working order.....since the rear A/C is only slightly colder then the heater valve must be okay? If the recharge doesn't help then there is other things to address...

A properly working system should drop temp about 50F from ambient....mine will drop up to 60' under the right conditions! I've seen airtemps as low at 21 degrees out of the center vent! But thats with R12 too! (But no recirc flap either)

groppo

Had the system regassed with R-134 (system had been changes over) and now it works fantasically.
It had been regassed a year ago with very little improvement, but now it's blowing 6 C (43 F) out of front vents (with 25 C outside temperature). What a difference. Regassed by a specialist this time. The weekend after I drove the beast on long drive in 42 C (106 F) heat. With outside temp that hot the vent temp went up but still was cold, but not frigid. Technician also said that R-134 doesn't get as cold as R-12. No complaints though. He said compared to a lot of new cars it's colder. But I'll still keep a look out for leaks and other gremlins as mentioned in replies.

Thanks, groppo

Bill Ball

OK, you've established the system works just fine. No need to worry about expansion valves and most of the other hardware we thought might be an issue.

Hopefully the tech evacuated the system and checked for leaks too as part of the recharge. If not, then you will know it leaks if you need a recharge in the next few years. Look for oily residue around the AC line fittings. If you can get under the car, look at the lines that run to the rear AC. In the meantime, enjoy the AC.