IcemanG17

This is typical...I go from a no vacuum, heat on all the time to a constant vacuum, no heat all the time!!

Back in June I had a myriad of vacuum leaks so I zip-tied the heater valve closed (Tony-fix) to help increase the A/C. It worked great, car ran slightly hotter but the A/C was 20-30 degrees cooler! A week after I zip tied the heater valve I tracked down the vacuum leaks. Recirc & center comb flap were leaking, so I capped them both. That solved all the non-op problems with the other flaps.

Today I decided to cut the zip tie off to set the system back to normal. I cut the zip tie off and start the car. The heater valve is closed. Change the temp to full heat & set the airflow to the footwell....heater valve is still closed? Put everything back together and go for a drive....try every combo possible...all the different vents work like they should, but nothing but cool air?

Get back to the house and take it apart again. Heater valve is still closed? Disconnect the vacuum line...heater valve opens no problem....hook the line up to the miti vac and start the car....sure enough it pulls a steady vacuum of 14 inches not matter what I do with the control head. Now I get heat & only heat! Damm this is frustrating!

I'm thinking the control head is partially broken. It appears the top (temp) selector is permantly stuck on full cool (65), the rest of the control head works like it should? The temp selector moves fine and doesn't feel disconnected or anything like that?

Does anyone rebuild the control heads or do you just need to buy a new one? Any other ideas....fuse or relay maybe?

the flyin' scotsman

Brian...........I believe the control head is doing a little more that just switching the vacuum solenoids. Its taking where you have the heat set, the outside temp and inside car temp into consideration and making adjustments accordingly to the recirc, heater valve solenoids..........hence the autoterminology.

There's a few pages and tables in the WSMs on the topic but its still not clear. Perhaps Wally or someone can post a table that shows what solenoids would be open or closed at given temp settings and outside/inside temps.

BrianG

There is no doubt that Wally is the HVAC wizard amongst us. Look here https://www.928gt.com/default1.htm for some help.

Select - "Tips/Links - Wally's World - The HVAC System"

IcemanG17

I did find some good info on Nichols site that said occasionally the vacuum solenoids fail individually and the failure mode is to apply vacuum? It sounds like its easy to switch a broken solenoid for a working one (hopefully one of my leaking actuators). Another thought is the problem with the temp sensors sending false info to the control head? Maybe it wasn't cold enough outside to engage the heater (80 degrees ambient?) I'll have to try it on a colder day?

BIMMERMIKE

Did you sort out your problem? I'm stuck on heat setting!

Mrmerlin

Hi Brian why dont you investigate the outside temp sensor search for fix your hvac, I posted a quick writup on this sensor and how to fix it Also use the mytivac to test all of the vacuum pots on the left side of the console

IcemanG17

it appears I never posted the solution to this problem and since another 928er experiencing the same problem here goes

The problem was the solenoid behind the stereo...it died and defaults to vacuum all the time.... I think it was about a $25 used part from intl.... Not too hard to change either if you have removed the solenoid strip before and got that annoying center screw out.....

the flyin' scotsman

So it was the solenoid feeding vacuum to the heater valve?

IcemanG17

yes.....not that hard to fix