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OK, so I purchased a car that the seller informed me the AC had been removed for performance...almost a deal breaker but I bought it anyway. He lives lives in Canada and didn't need AC. I live in VA and definitely need AC. When I picked it up, his mechanic showed me that all the major AC components were in the car, compressor, condenser etc. So I went into the deal knowing I would have to fix that. However, when I was driving it back from Canada, it got a little chilly in upstate NY. I texted him and asked about the heater (it was warm when I test drove it). He said, it works, the vacuum lines under the air box are plugged...
So, after pulling air box off and doing some research, I'm sort of confident I know the problem but need a little help. The four way splitter coming off the brake booster has a plug. One line coming off the four way splits again and goes into fender well. So that should be cruise control and vacuum reservoir. Second line goes to check valve then to brake booster. Third line is red (been partially replaced, then turns to black line and runs to fire wall, so that should be the line to HVAC vacuum console. So, the plugged one, should be running to intake flap actuator correct? I have an idea what that looks like but not sure where it is located
to re-connect.
This is for an '81 manual tranny.
Also, why would anyone plug this line? Seems to be somewhat common the vacuum lines get plugged. I'm not to smart on vacuum stuff. Only slightly familiar after working on my '86 944.
So the heater was working on test drive, but not on drive home? It could just be rod to mixing flap has become separated from mixing motor - you need to get heD in footwell and take a look.
Do you have a vacuum tester? This is good time to check all actuators and solenoids hold vacuum. If any fail to do that, it will undermine performance of whole system.
So the heater was working on test drive, but not on drive home? It could just be rod to mixing flap has become separated from mixing motor - you need to get heD in footwell and take a look.
Do you have a vacuum tester? This is good time to check all actuators and solenoids hold vacuum. If any fail to do that, it will undermine performance of whole system.
Oh, sorry. I wasn't clear; the weather was warm during test drive. Never had to worry about heater not working before, so I didn't check it. When I asked the previous owner about it, he said the heater works, just that vacuum line was plugged under air box...just need to figure out where plugged line is supposed to go.
So the heater was working on test drive, but not on drive home? It could just be rod to mixing flap has become separated from mixing motor - you need to get heD in footwell and take a look.
Do you have a vacuum tester? This is good time to check all actuators and solenoids hold vacuum. If any fail to do that, it will undermine performance of whole system.
Sorry, I wasn’t clear; the weather was warm when I test drove it. I have never had to worry about a heater not working before, I didn’t think to check it. I do have a vacuum pump. I’ll look into testing. Thanks.
So the port that is plugged is supposed to be plugged? I’ll call the previous owner and see if he knows what exactly they did to it. I will check out the link, thanks.
OK, so some new info...since I was by myself, I took a video of the heater control valve to see if it was working. I started the car, then got out and started the video, then got back into the car and started playing with all the HVAC controls: fan speed, temp switch and moved setting switch from "off", to "econ", to "auto" and finally the defroster. I think I moved the temp switch from cold to hot and back at some point, and well as the setting switch. I got out, grabbed my phone and watched the video. About ten seconds into the video, the lever on the valve actuated, then about 35 seconds later it moved back. So, something made it move. So I got back in the car and did the same thing again, but only one change at a time, and I got out after everything movement of the interior controls. Except nothing actuated the lever again. I tried everything 3-4 times and nothing! So I pulled the vacuum line off the valve and the lever immediately moved. I got in the car, and sure enough, I had heat. I could feel the vacuum on the tube. So, question #1, is there any reason it would work initially but not after a few minutes? It obviously doesn't need the vacuum to produce heat (similar to my 944, defaults to full heat when something breaks), so there seems to be a disconnect between operating a switch to add vacuum to eliminate heat. Question #2, which switch should be removing the vacuum? And any ideas what would cause it not to work? If I remember what I read, there is some kind of solenoid actuated when the HVAC controls are operated, not sure if these are common failures. The good news is that I have heat, if I leave it disconnected, and reconnect in the spring. I will try to post the video but it was to big to email. I'll try to upload directly from my phone.
It sounds like one or more of the vacuum pods is leaking get your tester and check them
My ignorance is starting to show. I've done my research on the system, but what are the vacuum pods? What does a vacuum actuator look like vs a vacuum solenoid?
Short Version - a vacuum from the engine runs into the interior, to a set of "switches" called the vacuum solenoids. The HVAC computer uses electricity to switch these solenoids, which directs vacuum to different end points - the vacuum actuators. Each actuator moves something - one of various air diversion flaps, a "comb" flap, and the heater valve you have shown in your second photograph. In 928S4s it also moves a flap on the intact system that changes the volume of the intake manifold. Failure of a solenoid means that the vacuum actuator it controls will not move when sent vacuum, bc the solenoid will not "open" to allow vacuum to pull at the respective actuator. If a actuator leaks, when it is sent vacuum it can release much/some/all of the vacuum in the system, causing temporary mayhem. The vacuum actuators are little rubber diaphragms that can crack with age.
The heater valve's default (No vacuum supplied position) is to allow hot water to the heater core - the logic seems to be if the system fails in winter you will have heat.
To test the system's vacuum actuators, you find the vacuum solenoid setup, which is in the inner side of the left footwell (or think of it is the left side of the center console). Mr Merlin linked to a comprehensive discussion of the system, and Dwight's writeup is pretty excellent : https://rennlist.com/forums/928-foru...re-w-pics.html
You will get some clues testing in the way you've tried, but as you've experienced the sheer number of combinations and interactions make it almost impossible to make definite diagnoses (though at least your heater valve vacuum actuator sounds as if it's working).
That's why it's a little more work to get to the vacuum manifold in centre console, but ultimately the fastest and surest way to fault find the HVAC using either of the writer-ups you've been pointed to. Once you have access to the vacuum manifold you can isolate each actuator and each solenoid, and you'll know very quickly which are sound. You're then in a position to decide whether you want to fix any leaking components (some actuators are harder to get at than others) or temporarily just plug leaking lines so that other parts of system are not compromised.
A good vacuum tester will be less than $20, and is a handy tool for lots of other jobs/tests on the car.
Thanks for the replies. I think I have a pretty good idea how it’s supposed to work now. I’ve never had to work on something like this before. I have to drive up to NY to pick up my cousin from her first semester at West Point tomorrow and bring her back to VA for Thanksgiving. I at least figured out how to have heat for the trip. I’ll keep trouble shooting and will follow up with any results.
11-16-2018, 07:31 PM
HVAC Vacuum help
