Pfc. Parts

On my car, which only has about 68K on it, I wrestled with the 134a conversion question until I found, as Dr. Bob mentions, that a conversion isn't nearly as simple as some would have you believe. In the end I decided to get the EPA Technicians Cert ($20, a couple of hours study & test time), then buy R-12 on eBay. I did rebuild my compressor after finding it wouldn't hold a vacuum; there are a few good YouTube videos on the subject put out by Century Auto Air detailing the procedures for replacing all the seals on these compressors for about $35. Another ~$100 will buy you a decent DIY vacuum pump and manifold gauge set.

Typically, you will still need to put temporary R-134a adapters on your car before re-charging it with R-12 because just about all the manifold sets sold these days have 134a fittings on them. Make sure you find good quality adapters if you try this. You'll need the ones that have the pin through them that presses down the existing schrader valves and they are, as someone notes, flaky and unreliable by design. On the upside, you use them and remove them. No reason to keep them on the car. They also aren't all that expensive. I had problems getting them to securely mate to the 134a connections on my manifold. Trial and error with vendors is the best I can recommend, I haven't yet found a supplier that makes a truly good adapter.

I've been very happy staying with R-12.

For reference, the trails and tribulations of my A/C refresh can be found here:

https://rennlist.com/forums/928-foru...ggestions.html

CraigL

Griffiths: Would one of your PF condensers increase the cooling capacity of the standard (R-12) system enough to allow the rear A/C unit to function decently? (Decently = cold)

SMTCapeCod

Here, this might be better than the little more/little less frame of reference- TSBs for refrigerant capacity various models:

http://the944.com/ac/default.htm

Pfc. Parts

An F-4 with no A/C? Who would have guessed?

redpathtribe

Interesting discussion, and timely as I've been considering whether to convert to R134a or stay with R12. I have a slow leak in the high pressure hose that I still need to fix. NOT fun in Phoenix right now! Consistently see 122 deg. on the outside temp sensor after the car sits out in the sun for a few hours.

FWIW... In doing some checking on PET, IIRC and as far as I can tell, condenser and compressor part no's. are the same for my GT (R12) and later GTS's (R134a). Compressor part no's. show the same for my GT and GTS's in the WSM as well.

Rob Edwards

Yes, the '90 to '95 compressor is the Nippondenso 10PA20c. There's no difference in either the compressor or the high-side / low side hoses for those MYs.

griffiths

Nice shark you got there Craig.

Good question; whether with R12 or R134a. I don't have a shark to play with to confirm the concept.

But, I've owned a shark with rear air, and as you know when you turn on the solenoid to open the circuit for the back evaporator the vent temps rise as much as 10F. I've seen quite a few rear air shark's demolish compressors because the oil accumulates back there; this happens when you have the rear air off and the solenoid is partially open, trickling refrigerant while holding back much of the oil which reduces the oil/refrigerant ratio to compressor and compressor overheats, locks up or grenades.

Frankly, the rear air is useless in design in my opinion, you can't even keep a six pack cold on it. Most of our driving is done without rear passengers and the interior is not as large as an Suburban, bus, or mini van. So the need for rear air is minimal. Most SUV's, for example, simply run a vent from the front condenser HVAC system to the rear.

Dictys

I upgraded the condenser on my 90 S4 (had a custom one built locally with slightly more capacity), fitted a new Nippondenso 10PA20c and it runs R134a. All other fittings as per factory.

It kept the cabin ice cool in Dubai despite the outside temp being 110-120f in the height of summer.

CraigL

>
> Nice shark you got there Craig.
>
Thanks. And it has one of your dashboard evaporators in it! (I'm still trying to forget the gory details of that job).

>
> Frankly, the rear air is useless in design in my opinion ...
>
Thanks to you and Greg Brown for discussing the inadequacies of the rear AC.

I did an extensive R&R on my system two years ago ... new compressor, evaporator, o-rings, accumulator/drier, expansion valves, flushed all of the lines and charged by weight with R12. The dash vents are ice cold but when the rear system is used, the system (front & rear) has a hard time keeping up with the Missouri heat. As a consequence, I tend not to use the rear system at all. Now I know why!

And, given your observation about the leaky rear solenoid and the compressor oil, I might cap the lines at that solenoid and pull the fuse the next time I have to evacuate the system.

I anyone has other suggestions about how to make the rear AC unit functional, I would appreciate hear them.

Thanks!

Alan

The rear solenoid valve is never partly open. It's always either fully open or fully closed & it cycles as needed based on the cooling level control & the rear evaporator temp sensor. It is controlled only by a simple relay in the control unit. It only ever opens when the rear fan is in active positions.

Maybe there is a similar accumulated effect when it opens having been closed for a long while (as is typical).

I do agree the rear air is next to useless - in really high heat it is quite counter productive from a front comfort perspective. Since it has no integration at all to the front system controls they can end up working against each other... that seems totally nuts to me...

Alan

dr bob

Since the system capacity seems to be limited by the condenser, it's expected that the rear evaporator will steal capacity that would otherwise be available at the front. I don't think the rear unit was ever intended to increase the total cooling capacity, only to provide local cooling for rear-seat passengers sitting in the greenhouse back there. That said, it's probably a Good Idea to run the rear system once in a while to keep the o-rings and solenoid properly lubricated.

The solenoid sits on the pressure side of the rear system, so I don't readily see how it could collect oil and then suddenly dump it to the compressor suction when opened. Perhaps a more likely cause is refrigerant condensing in the low points in cold weather, then flashing and leaving the oil when weather warms up. Start the rear system and the oil accumulated in the low-point sections between the bulkhead fittings under the seat and the main pipes at the firewall. Just a theory.

DKWalser

If the car is heat soaked, I agree the rear air is useless. When temps are milder -- say below 95 degrees -- I really like the rear air. I don't like the feel of a fan blowing in my face, so I prefer to have the rear air help with an indirect flow of cool air. When things are hot, I focus the front vents on my face and turn off the rear air.

Alan

David you may not have encountered this yet - (it is a bit of an oddball) but in mild temps with rear AC on high and front AC on but at mid-temp ranges with front fan on low (close to what you described...). The front system can switch to quite aggressive heating when its trying to warm the cabin back up to its climate set point (and making little progress).

If you have the front on footwell and vents closed this is most noticeable - and quite strange.

Alan

OTR18WHEELER

what was the first year for the R134a in the 928?

Rob Edwards

'93, in the US.