Chalkboss

In my quest to have the system perform properly, I seem to have one remaining issue on my 87, and that is the recirc flap will not open when the slider is in off (the slider for air distribution) position. I have: replaced hot water valve, replaced all the diaphragms, they all hold vacuum, replaced some vacuum solenoids (that probably weren't bad), checked the vacuum connections, check valve, and reservoir under the hood (all good), fixed the cold micro switch that was not making contact in the HVAC control unit, checked proper operation of setting motor, cleaned all the electrical connections in said unit, still can't get it to open to recirc the cabin air in off position.

The recirc line is getting is weak vacuum (5 in. Hg), and that's not enough. It opens at around 7. It does open when the cold is set to max, ac on, and lower lever is in the up/down, and down position. It is closed in every other combination.

I am not very good at electrical testing (learning though) and this system baffles me. It is way over the top complicated to my simple mind. I don't understand why it would see a weak vacuum in one case and (off) strong in the other (AC). Especially as I can't find anything leaking.

One final observation- when the lower slider is to the far left (off), I can now turn on the fan. It seems like that was never the case before... it would only come on when the slider is moved from the "off" position.

MainePorsche

Sounds like a solenoid/actuator fault of the recirc component. This one sits at the end of the chain of them and is right next to the heater valve solenoid/actuator. Your recirc vacuum is 5 on its own but when AC is on and heater valve vac is maximal it then has normal function. Could it be some fault exists and when the neighboring heater valve vac is 'on' you get spillover vacuum to the recirc line (?).
Check solenoid function as per Dwayne's write up. If there is no problem at the solenoid console level, then the fault will be on the hvac head board.

http://dwaynesgarage.norcal928.org/1...0Procedure.htm

Chalkboss

I think this is starting to make sense- when both sliders are in the far left position, car running, here are the voltages I measure at the solenoids:

Recirc 13.3
Heater valve 12.3
Center comb 12.2
Defrost 13.3
Footwell 0

So... too many solenoids getting vacuum when they should be off (i.e. defrost). So does this mean the HVAC head is bad?

The Forgotten On

Open the HVAC head and clean the slider contacts and test it again.

It could be that the traces are shorting due to some contamination on the contacts.

Also check to see if the switches themselves are broken. 1 of mine is and it is giving me all sorts of issues. It only costs ~$15 to fix if it is part of your problem.

Chalkboss

I have already had the head apart and cleaned everything, no change. Apart from a cold microswitch that was not engaging (fixed) the sliders seem to be doing their thing, and making contact on the board.

MainePorsche

Hey there Don,

Not sure where your problem lies, but the top two areas on my differential are (1) at the hvac board rheostat and/or (2) solenoid actuator level. I'm leaning toward rheostat issue as per data you have provided. I attached link to online WSM - pages 87-104 and beyond are what you want for testing of this part of the hvac system. I'm not the savant like Alan is with this material, so I'd hate to lead you astray and can only say these pages will give you the definitive diagnostic algorithms to follow.
Best Wishes
Craig

http://www.ligeti.com/928/Porsche%20...Volume%204.pdf

Chalkboss

Thank you very much Craig. I should change this thread title to I make myself insane. After a lot of help from a very smart friend who kept telling me the voltages were correct, I went back to the first thing I should have done and tested the actuators. Turns out the comb flap and defrost are bad!!! I didn't bother to check the comb flap because I replaced it 16 months ago. Hopefully the line just pulled off the comb flap. The defrost was fine when I checked a month ago.

So after all this it was very simple. This car just keeps giving me lessons in humility...

MainePorsche

Good to hear you got it sorted.