dprantl

You are correct about the oil. Putting PAG oil in with mineral oil is a very bad idea. Flushing and filling with ester oil is the way to go for an R134A conversion, which is what you did.

Yes, the condenser should show gradual heat loss from top-pass side to bottom-driver's side.

If you have only one bad/blocked expansion valve, the other evaporator will function properly.

About your blockage scenarios, it's difficult to tell exactly how they would manifest themselves in terms of pressures when you have two valves and two evaporators in there.

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

worf928

I'll check this on Monday.

Agreed. Here are the scenarios I think apply:

- If blockage is between the low side port and the "Y" in the hard line to the expansion valves you would expect lower-than spec pressure.

- If the blockage is between the low side port and the compressor then you would expect higher-than spec pressure.

- If the blockage is in one of the branches between the "Y" and a specific expansion valve then all bets are off.

worf928

It's too cold now to do any meaningful diagnosis.

On Monday I attempted to get some condenser temperature gradient data. After allowing the car to idle (normal and higher than normal idle) for the better part of an hour and cycling the compressor periodically I never saw condenser temperatures above 100 F and gradients across the condenser were less than 5 degrees. It seems to me that the ambient temperatures (mid-50s in my garage) are simply not high enough to do any diagnosis.

If anyone has any good ideas about how to do A/C diagnosis when it's cold I'm all ears. And the WSM's pressure charts are off the edge of the world below 60F...

worf928

New compressor and hoses. Same behavior. Low-side pressure high, high-side pressure in-spec. Vent temperatures crappy. Expansion valves not at all cool.

I heated up the garage to 65-70 degrees so as to have good ambient temperatures.

I've spend a couple hours on the phone with someone who's done a lot of A/C work on 928s. Neither of us can think of a failure mode that hasn't been addressed.

It's as if there's no expansion valves in the system or as if the new compressor isn't doing its job. I've examined two new expansion valves likely from the same manufacturing batch as the new valves in the car and compared them with the old valves. Nothing obvious wrong with the new ones - like the piston or ball missing.

I'm outta ideas.

Anything?

worf928

More: new condenser, new r/d. (System vacuumed for hours and hours, leak checked, etc.)

worf928

New Info: When I turn off the rear A/C and turn the temperature **** all the way down, the low-side pressure drops by 10 PSI, into the high-side of spec, and the front center vent cools down to about 50 degrees.

On the other hand, now that the car's sat for several hours, over all, the pressures and vent temperatures seem better. (?)

dprantl

Ok so now you have a new compressor, condenser, receiver/drier, compressor hoses and both expansion valves. Am I reading that right?

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

worf928

Yup.

dprantl

Have you measured the temps of the lines going into and coming out of the evaporator after the system has been running for a while?

Since you now basically have a completely new system with the exception of a few lines and the evaporators, the list of suspects is narrowing. Could you also give the exact step by step description of your charging process?

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

ZEUS+

I have done many conversions on various makes and models including my shark. I converted mine in june of '06. I replaced drier and o-rings at drier and re-sealed compressor. Prior to conversion I flushed with 134 as I have a recovery station. I charged system with 2.2lbs 134 and 8oz ester. No other seal or componant was replaced. My pressures are 225 high and 45-50 low. I dont have the cooling like r12, but it cools well as long as I dont use the rear ac. I believe with a new compressor it would be better.
With any restriction, you will see it in the pressures. Usually a bad expansion valve has extreme high side pressure and the low side can even suck vacuum. 134 operates at higher pressures. If your pressures are 175HS and 23LS with 37oz, the compressor is weak.

worf928

What follows are annotated r-134a charts from the WSM for the 10PA20 Compressor (10 cylinder N-D used starting MY'90.) As I noted in my OP, with another car with the 6E171 compressor these charts were dead-on.

worf928

Not yet. But, I will tomorrow.

Car idled to close-to-operating temp.
Cycling power to A/C clutch with jumper from pressure switch connector to jump post so that I don't have to hop in and out of the passenger compartment.
Gauges attached. A/C button on so electric fans operate. R-134a attached through middle gauge hose, charging gas through low-side port.

Power clutch, open valve on gauge, raise engine rpms to ~1800, allow about 3oz of r-134 to trickle in, close valve, keep clutch powered for 20-30 seconds. Disengage clutch. Let car idle for ~30 seconds. Engage A/C clutch for ~10 seconds until low-side pressure drops.

Lather rinse repeat. Occasional longer clutch operation - minutes - with fill valve closed while watching pressures and vent temperatures when r-134 charging starting at about 24oz until 36oz. I probably spent 2 hours slowly charging the system.

Anything weird there?

worf928

The car now has a Kuehl compressor. So, I'm not taking the charts as Gospel. The high-side tables that came with the compressor indicate operating pressure that are at the low end of Porsche's range. No low-side tables are given but the manual indicates low-side pressure should be in the 20 - 40 PSI range. With rear A/C activated the low-side pressure struggles to get much below 40. With the rear A/C off the low-side pressure heads towards 25.

I'm going to let the car think about it overnight and gather more data tomorrow.

ZEUS+

The hoses should be purged prior to charging. After purging let the vacuum suck in the refrigerant, starting the engine when needed for low side suction to complete the charge. The compressor should never be engaged on an empty system holding vacuum.

worf928

I forgot to mention above that I let the first bit of r-134 into the low-side before starting the engine. I let about 10 PSI build on the low-side.