rickjaffe

I'm still working through my air conditioning issue. to recap:

I've replaced the head unit (twice)

lost most of the cooling after two weeks.
pressure appeared to be good (like 200 to 225 high side maybe 45 on the low)

put some dye in the system and looked like the compressor was leaking, and weak.

so I got a rebuilt compressor.

it still only cools to 60 degrees (of course it's in the mid 90's and humid here in houston);

my a/c guy (general mechanic, good on a/c issues but not a porsche mechanic) found that when the door is slightly opened there is positive pressure which indictates that it's taking outside air rather than recirculating air. He's thinking that maybe there's something wrong with the switch between the two.
how do you check that?

so after replacing the head unit and putting in a rebuilt compressor, what else could be wrong?
thanks

Kevin in Atlanta

First, have you read Wally's AC write up? If not, that is a place to start. See the 928 Specialists's website under the tips section.

rickjaffe

thanks, I think I need more specific information:

anyone one have handy a picture of the plenum box, where the switch between fresh air and reciculating air should be found?

bd0nalds0n

Check to make sure the car holds vacuum. If it doesn't, your recirc won't work, and the heater valve might not stay shut.

Also, check the mixing motor to make sure it's working properly.

bd0nalds0n

BTW, I would think that a 35 degree or so temperature differential is pretty good..?

ckabee1

Crank your car and let it idle. Turn the AC on. Sit in the passenger seat and feel for the plastic grate behind the parcel tray (if you have one) back near/above the CE panel. In the middle of the grate is a small access hole. If you feel the flap through that hole, then your recirc flap is inoperative (it defaults in the closed position). Meaning, your car is trying to ac outside air rather than recirculate. Cuts down on AC efficiency bigtime.

Go to this site for other trouble shooting the ac vacuum actuators:

http://members.rennlist.com/pirtle/svc.html

SeanR

Thanks for this bit of advice. I don't know how I missed it. After doing an entire vac system rebuild, for some reason checking this never crossed my mind. I checked that today and now I have some troubleshooting to do. I do remember that the blue line held vac, but I can't recall ever hearing the flap actually move up/down.

Thanks.

inactiveuser1228

Also, if you have rear air and it is colder than the air comming out up front, your recirc flap is the problem.

rickjaffe

doesn't the cruise control also work off the vacuum system? MIne works, so if so then I don't think I have a vacuum problem

IcemanG17

35 degree differential is good for most cars, but not for a properly running R12 928 system.....I've seen better than 50-60 degrees at times...

the recirc flap runs off the vacuum supply, which also controls the heater valve....chances are you have a vacuum leak which causes the heater valve to always be open, allowing heat and the recirc flap will be stuck in fresh air only...so your a/c has to overcome the ambient outside air...then the heat from the heatercore.....before anything gets cool....

As said above the easiest test for the heater core is the temp differential between the front and rear a/c....if the rear a/c is much colder.....thats an easy one....Tony has a great write up on how to manually zip tie the heater valve as a temporary fix

IcemanG17

yes the pre 1987 cruise control runs off vacuum...but the post 88 are electric

rickjaffe

not familiar with that write up. could you direct me to it?
also I don't have rear air so no differential to test.

prior post said pre 87 is vaccum and 88 is electric. how about 87 vaccum or electric. if vaccum and my cruise control works good, seems like the vaccum is ok

Earl Gillstrom

http://www.millionaires-media.com/acprobs/

http://members.rennlist.com/v1uhoh/index.htm

A small vacuum leak in the hvac will cause the heater valve and reciculate to fail and may not effect your cruise control. Also, even if the heater valve is closing it could be leaking coolant. This failure mode is very common. Also be sure that the valve is not installed backwards. Even new ones leak when installed backwards. You can test for leaks after removal by blowing or sucking by mouth.

docmirror

From post 6, start there, and then lay upside down in the pax seat and actually watch the recirc flap. It's behind the glove box, above the CE panel. Take a long probe with you and a flashlight. When the AC is running, the flap should be tilted up. If it is not, another fault is the flap sticking to the edge sealant. Use the screwdriver on the rear side(hinged at front) to gently but firmly push it up. If it stays, it was stuck to the sealer, if it falls back, it's the recirc vacuum actuator. Contact ROG100 on this board for parts and the fix. You'll need to take out the blower motor, but it's not too bad a job.

Heater valves all seem to leak. I replaced mine two years ago, I don't drive too much and I know mine istarting to leak again. Look for an aftermarket replacement that has a metal flap. The OEM unit is pure crap. Vac leaks in the actuators aren't big enough to affect your cruise control. But they have a devastating affect on AC operation.

IcemanG17

Rick
All 87's still had vac cruise control (well maybe some really late ones don't)...."if" your cruise control works fine.....chances are your vac is okay..BUT there are many other factors too.... Buy a mityvac and start testing the vac system..... I start at the brake booster and work in from there....

the blue-black check valve is probably leaking (it should only flow one way towards blue, but when failing flow both ways)..pretty much every single 928 I've ever checked had some degree of failing blue-black check valve..

the 4 way rubber "T" is probably old and dried out (leaky)....order a new one from just about any auto parts store (3mm vac line)...the after market ones are bigger (longer) so they fit easier & are cheap (the original porsche one is steep)

There are 4 lines into the "T"....the blue black line (source from brake booster), the black line that goes into the firewall (HVAC line) the short line that goes to another "T" thats the vac tank and cruise control in the fender, then the white (or light grey) line that goes along the fuel rail to the flappy actuator....sometimes the flappy is hooked up backwards which causes a leak...or the flappy itself if failing (PITA) which causes midrange performance problems....its easy to check once you find the solenoid....if the line that goes under the intake holds vacuum...& the flappy spins...its fine

Refer to the link to Tony's page and Pirtles for more precise info with pictures.....everything I learned about the 928 vacuum system I got from either of those sites....& my experience fixing my own hvac problems (vac leaks, heater valve, recirc flap, backwards hooked up flappy, etc)

the all metal heater valves are a good idea too...I use one now (after one broke internally)