Tom in Austin

This summer both my DD and my wife's car had weak A/C. Since 928ers are very knowledgeable on all things automotive, wondering your thoughts on what we heard about our cars.

In both cases an evacuate and recharge was done. The dealer who did my car (under warranty) told me "A/C's working good, we couldn't find anything wrong". The independent shop who did my wife's car gave us a printed disclosure saying they had evac'd and recharged the system for diagnostic purposes and to come back for additional troubleshooting if we notice further difficulties.

Is there a gov't regulation now saying a shop cannot add refrigerant (R134a in both cases) to a car if it's low, so that to stay legal they have to claim to be doing a 'diagnostic service'? Obviously, evac and recharge will result in a full charge of refrigerant ...

FredR

Tom,

One of the problems with loss of refrigerant is that the shop does not know how much oil is left in the system. Too much oil can kill the performance, too little the compressor will go kaboom.

The safer bet is to evacuate the system and effectively start again adding the specified known weight of gas and a measure of oil. Whether or not they can get all the old oil out remains to be seen. Best way to do that is to perform a system flush then fit a new filter/drier.

A franchise dealer will do this for fear of losing their franchise if they do not. Independent dealers will probably take a more pragmatic approach.

I recently had my 928 gas way down. I calculated how much might be needed to top up the system and it came to the same amount as was in the top up can I purchased. I figured it probably had too much oil in to start with as the local shops tend to throw the stuff in wily nilly. After adding the can my system now works better than ever- probably sheer luck but...?

Not sure if that addresses your query but best I can think of!

dr bob

Tom --

Good thing it never gets seriously hot in Austin.

Meanwhile...

Guessing that your DD's are 'murican iron. Since the 1990's, American cars (and trucks -- same as a "car" in Texas) have used a system that relies on a fixed "orifice tube" instead of the more sophisticated (and expensive) expansion valve system used in the 928. It's a little plastic tube with an array of tiny holes in it, used to meter the amount of refrigerant liquid passed into the evaporator to cool the car. The orifice tubes end up being the first filter for crud passing through the system, as there's no receiver-dryer between it and the compressor and condenser. Any serious service involves removing and cleaning or replacing that orifice tube. They are really cheap pieces, so for me it's inspect the old one for any blockage or damage, then put in a new one.

The system is somewhat self-protecting for either, as compressor operation is regulated by a switch in the suction side that drops out typically around 40 PSI or so, corresponding to an evaporator temp somewhere around 40-45ºF. On either orifice tube blockage or damage, the system doesn't carry as much heat away from the cabin air, and actual discharge air temps go up.

Diagnosis on the system is a little more fun, as there's typically no "sight glass" available to verify liquid flow to the tube and evaporator. Everything depends on "perfect" conditions. Manufacturers share guidance on what pressures you might expect, but the windows of acceptable pressures are really wide. Variables include ambient and cabin temps, cold air discharge temps, and relative humidity. The acceptable windows are too big to be really useful, so what looks "acceptable" on a chart may not actually make the air cold enough.

Consider taking the car(s) to a real AC expert at a non-dealer shop, as Fred R also recommends above. You'll get the local wisdom and experience that the dealer-trained technician has probably not accumulated. For instance, in my long-gone 90's Ford Explorer, the recommendation was for a change in orifice tube size (larger) and a different (lower) pressure switch setting. Vent temps dropped about 12º with those relatively easy changes, somewhat valuable in a black car in Los Angeles heat but with low humidity. Austin's generally higher RH may require something different because of the significantly higher moisture-condensing duty and a bigger concern for evaporator icing there.

928 Content: There's some variability in the expansion valve function that seriously affects actual system heat-carrying capacity. The expansion valves in the later 928's are shared with several other German manufacturers like BMW, so the new Behr actual valves sold today are much better than the ones they supplied to Porsche for the cars originally. For sure you want the newer valve for R134a-conversion cars as the R12 have pressure-temperature curves are specific to the refrigerant used. Having an actual dynamic flow control with the expansion valve offers better capacity in any off-design condition than an orifice-tube system, at least in my limited personal experience.

Tom in Austin

Good info, thanks Dr Bob and FredR.

My understanding of car technology is a bit narrowly-focused on 928s, so sometimes wonder about things people say about other cars, like our Honda and Mercedes.

dr bob

FWIW, my 928 vent temps are significantly colder than the 2013 Honda Pilot DD vent temps. The Honda "rear air" in the middle section that works, plus vents in the rear seat area too sharing that rear evap capacity. Like the 928, it's set the temp and forget it. The Honda "adds" fan speeds and diverter-damper controls while in auto. Good thing it isn't 928 black.

We don't get the heat or anything close to the humidity you enjoy in Texas, such that both cars have more than adequate cooling capacity for where we are now. Plenty when we were in Los Angeles too. K complains about the sub-freezing center vent temps in the 928; I need to replace the anti-freeze switch one of these days. First one failed closed, second one failed open. There's a jumper in there now.