Calgary Ole

Bob, I followed your advice from numerous posts. New oil with the compreesor removed(measured), new dryer and expansion valve and flushed all the lines and replaced o-rings.( did not flush condensor or evaporator as I read it takes some specialized equipment to do it right ) Vacuumed as per your advice for several hours.System is working great with a new setting motor from Colin.
The only worry now is I saw in your earlier post about temp. It was a very cool day( 59 f ) but there was very little humidity.

Landseer

Why is it impractical to apply a bit of heat to the system during vac step?

blown 87

Not a problem if you have a paint booth that you can run up to a high temp, for normal folks like me, we just use a big two stage high CFM pump.

You just can not get enough heat to all the places you would need to heat with a heat gun IMHO.

dr bob

What Greg said... You can add heat to a few of the exposed components like the compressor, condenser and drier. But with the system under vacuum, none of that heat is going to migrate to where it's needed most, the evaporator. The evaporator seems to be the part most subject to corrosion damage, but that may be only because it's the hardest part to change.

With your good pump attached, the unencumbered moisture is going to boil off at the pressures and temperatures shown in the Robinaire table. Less obvious is tha fact that it takes a lot of heat to get the water to boil, so as the water flashes it sucks heat out of the surrounding metal or rubber. That heat gets restored/replensihed from the heat in the surrounding air. That's perhaps the biggest reason to leave the pump on for a while-- you'll have local cold spots wherever the moisture flashes, and until that section warms up again it will be tougher to get all the moisture out.

Ole, I wouldn't get real excited about charging on a 60º day if you left the vacuum pump on for a while. RH in the outside air is only important for the time you had the system open to atmosphere, and is critical only if the system metals temp was close to the dew point at the time. You replaced the receiver/dryer at the same time, meaning the dessicant will have a chance to capture remaining moisture. If the system performs adequately for your conditions, I'd call it good and bank your worrying for some other problem ye to be identified.

WallyP

When charging a system in cold weather, I run the engine and the heater long enough to get the entire A/C system hot for at least thirty minutes.

AO

I had a nice long drive yesterday (from Athens, OH to Detroit, MI) and it was about 85*F outside. I paid close attention to how the AC behaved durin gthis drive.

Starting out it took a good 5-10 minutes for the AC to feel cold on Recirc. I think this is about normal.

I would put the fan on "4" and the air blew at a pretty good pace. Previously I thought the AC was getting warmer, but I think that was a misnomer. The airflow was slowly decreasing. If I switched over to outside air (no AC) the airflow would slowly increase.

I beleive the Evaperator was becoming a big block of ice, right? This is what restricted the airflow.

I assume this would indicate a fualty or mis-adjsuted freeze switch?

I'll try pulling it out today to investigate, but any tips would be helpful.

dr bob

Andrew-

Get a handy little dial thermometer and clip it into the center vent. That way you can see the actual temps and make much more informed judgements about system performance as you drive. Instruments are much less subjective, don't include 'chill factor' ingredients including humidity and how hot you are already as you enter the car.

I'm not sure what you mean by 'recirc'. Mine selects recirculate or fressh-air flap open based on outside temp, inside temp, and position of the slider. All via the setting motor, BTW. I hear the fresh-air flap go closed (to recirc mode) when I have the bottom slider at the [] mode one click from the full-left position and I push the AC button. Is yours different?

Data point: My vent temps start to drop within seconds, not minutes, of engaging the AC button. If yours really does take minutes you have some more work to do.

On perceived airflow decreasing, make sure the comb flap is fully retracted and no air is being routed to the floor or dash defrost vents with slider in the [] position. From that point you should be looking at temperature from the center vent with fan on one or two position using the thermometer. If that temp drops below about 35º in first-ten-minutes operation you likely have a stuck freeze switch. If the temp stays warmer than that, and you have the heater valve tied closed with no significant leak-by, your problem is not a block of ice where the evaporator normally lives.

------

The freeze switch is adjustable within a limited range. On the end with the electrical terminals there's a small phillips screw head that looks like it's an adjustment screw; it actually holds the little microswitch to the end of the housing and adjusts nothing. At the other end, there's a flat brown plastic cover with a couple holes next to each other and near the top. The hole on the left (looking at it from the end with the mounting stud pointed up) allows you to push a probe in, and actuate the switch towards the "warmer" position. The hole just to the right allows access to a small phillips-head screw that adjusts pre-load on a tensioning spring. Full-clockwise on that screw is the coldest setting, that's applying the most spring pressure to the little thermo bellows. Turn that screw to the left and the spring is unloaded some, allowing the switch to reset at a warmer temperature. There's a usable range of about six turns from full-clockwise, beyond which there's a risk that the screw will detach from the adjusting sleeve and the spring inside, so don't get too frisky wth the adjustment.

To get the adjustment correct, you'll want to remove the switch from the car. Carefully unclip the capillary tube from the car body and extract the end from the evaporator. Undo the nut on the bracket, disconnect the two wires. Bring the switch and your ohm meter with you to the kitchen or workbench. First, attach the leads of the ohm meter to the switch terminals and confirm that you have continuity through the switch. Push the internal mechanism through the left hole as described above, and note that with a hard push you can hear the switch click as it opens, and that there is then no continuity through the switch when it clicks.

Prepare a large glass of ice water, lots of ice, and drop your thermometer in it along with the end of that capillary tube from the freeze switch. Meter is still attached to the switch terminals. With the ice water and the cap tube at just above freezing (35-38º or so) the switch should open in less than 30 seconds. Adjust the screw in the right of the two holes, counterclockwise slowly and in small steps, until the switch just opens. Confirm your new setting by lifting the cap tube out of the water and watching/listening for the change in switch status. You can fine-tune the setting a little, but basically that's the setting procedure. The switch should open in the 35-38º range to protect the evaporator from icing. The close temperature is not adjustable directly, but should happen within 5-8º of temp rise out of the ice bath.

AO

Drbob-

Thank you for your long post. I'll give that a whirl tomorrow.

BTW, I picked up a 25 micron vacuum pump today. HF had it on sale. I also used a 20% coupon. Pretty sure I got the 20% off $149, so it was a good deal.

andy-gts

Wow you guys are all so good at trouble shooting.....how much would it have cost to get a dealer to work on the system and get it right (if they got it right)....I am just thinking down the line. I have never had any ac work done but if need be I would be going to someone to do the work..

any thought on cost.?

andy

AO

It might have strarted in '90 but the '91 and later GTS, do not have an "off" selection like the earlier cars. Instead, the far-left position of the slider gives you re-circ air.

blown 87

Not saying it will work on a 928, but on the old R-12 Volvo 240's which had AC units that were marginal at best, we used to put about a turn on the allen screw on the expansion valve, right or wrong it did make them cooler and made me a lot of money for years.

I am a firm believer in using R12 as long as we can get it, R-134A just does not work well in Georgia from what I have seen.

About the micron gauges and pumps, I know of no professional shop that uses anything like that, we just put a honking big pump on them after we have flushed the system and pull it down.

That being said, I think i do not know as much about ac work as I should, even though I have improved and repaired countless units over the years.

I have ordered a new thermistor unit from robinair and will be a bit more picky in the future.

I want to add one thing, please do not put any sealer in your system, it screws up our machines and they are not cheap.

dr bob

Greg--

Not sure where that Allen screw might be on a 928 but I'm all for finding it.

The R-134a vs. R-12 debate has been hashed out pretty well. Turns out that the later S4+ 928 has enough extra condenser and fan capacity to work with R-134a OK, so long as the fans ALWAYS work. Net performance is comparable up until somewhere north of 95ºF ambient at engine idle. From that point the condensing capacity is critical, since the condensing pressure goes up faster from there than it does with R-12. Most "criticism" comes from the belief that since R-134a has a lower molecular weight, it won't cool as well. True in theory assuming that your expansion valve passes the same mass of refrigerant. Put the correct expansion valve in the system and it lets the correct amount (by mass) of R-134a in and suddenly you can make ice cubes. The rest of the conversion issues are housekeeping items, like oil selection and compatibility of hose and seals, with good evacuation of air and moisture during the recharge process. I have the option of using either refrigerant, and have a 25# sealed cylinder of R-12 on standby in the garage if I change my mind. So far the system still freezes my fingers, so the R-12 is still sealed.

As far as system performance in Georgia, your car systems rowk harder in that humidity than mine does here in the coastal desert that is the Los Angeles area. Most "hot" days here happen when the wind comes from inland and blows towards the ocean. We just don't have "hot" and "humid" at the same time generally, so system capacity is spent on coolng and not on condensing moisture. For driver comfort the difference is actually a lot smaller than one might think; dehumidifying feels like 15-25º of cooling on a humid day when all else is the same. It just makes performance a little tougher to analyze and evaluate when a lot of capaicty is tied up removing moisture from the airflow.

Now, back to work here...

blown 87

It is shocking how much water they will condense.
The allen screw is inside the threads of one of the connectors, it is what determines spring pressure inside the expansion valve, which can lead to higher or lower high side pressures and flows.

IIRC we were turning them out about 1/2 a turn or so for more flow into the evap core.

Again, I would think this would be location dependent as to this being a good idea or not.

EDIT: I just went and looked at the front and rear valves on a S4, they are expansion blocks, not valves, so no screw in the connector, but the brass plug may turn (Or be removed, that is where the spring would have to be from the way the blocks are built) to increase/decrease pressure, I may try that in the future.

dr bob

The expansion valves have thermo belllows on them, so they are 'smarter' than fixed orifice tube systems. I just didn't remember seeing anything to adjust. Kinda like heater control valves, maybe I got an extra-good one. Mine came from DR way back when, along with the o-rings and seals needed for the conversion. I don't remember for sure if it's a genuine Behr, but I wouldn't be suprised. At the time I cross-ref'd the part number and saw that it's the same valve used in many large BMW sedans, plus many other cars.

I think I still have the original part someplace. Kept it for the next owner, along with every other part that's been replaced along the way. I suppose it would be OK to dissect it and see what's going on inside.

SQLGuy

The original parts on my S4 were Rein, not Behr (or, at least, the date codes on the parts showed them to be made in 1988, so I assume they were original).

The replacements I installed were also Rein, and looked pretty much identical to the originals.

The evaporator itself was marked Behr, as are a number of other parts in the car. Maybe Porsche multi-sourced expansion valves, or maybe Behr sub-contracted them to Rein?