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By Polecat702
#51682
Usually it's the other way around. The cold outside air into the car won't close off, and no heat. Got a new heater vacuum valve and short hose in my rollaway, but I'm thinking it's something else.

What say the brain trust? don't wanna go back to RL, if I don't have too.
Thnx all
joe
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By worf
#51716
First questions:
- have any mods been done to the HVAC system to change default behavior?
- is the behavior independent of engine load? Or, does it work at idle but not under load when driving?

I ask because at least a few folks living in hot climates have mod'd the system to close the heater valve when there's no manifold vacuum. So, we need to know if the default behavior should or should not apply.

And, the VCB kit is a variable since the notion of 'manifold vacuum' isn't stock anymore.

If the behavior is load dependent - works at idle but not under load, then we start at the blue/black vacuum check valve at the brake booster.

If the answers are no and no, then troubleshooting probably starts at the heater valve.

Off the top of my head... I'd first check vacuum behavior at the valve.

- Engine off, disconnect vacuum hose to valve while carefully listening. You should not hear any sign of a 'hiss'.
- Ignition on, slider at full hot; Use hand pump to verify that hose does NOT hold vacuum with engine off.
- Assuming hand pump has gauge, connect pump, start engine with slider at full hot: gauge should show no vacuum.
- Slide gauge to full cold. Gauge should show manifold vacuum.

If vacuum behavior is correct then heater valve is unable to open.
If vacuum behavior is incorrect then we have to go deeper.

Obviously you'll have to remove the intake charge tube for the VCB kit to get to the valve's hose. If it won't idle w/o the tube might want to add an extension to the line to make it easier to do the vacuum tests.
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By SeanR
#51827
I've been finding more and more HVAC Rheostats inside the head unit failing leading to exactly this condition. I've been trying to get one of our own 928 owners to get them reproduced in his factories (in china) and he's had obvious trouble getting anything done. I currently have two customers waiting on me to get these so we can start replacing them.
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By Stepson
#52233
Joe,
When the heater in my GTS stopped working, I found no vacuum at the heater valve. I moved to the inside of the car and checked the vacuum manifold in the console. Sure enough, the vacuum actuator (the small electromechanical vacuum switch that controls the vacuum to the heater valve) was faulty. I bought a whole vacuum manifold's worth of vacuum actuators, so I have some left. If you find that this is your problem, let me know and I will ship you however many you need.
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By Polecat702
#52473
Stepson wrote: Mon Jan 18, 2021 5:12 pm Joe,
When the heater in my GTS stopped working, I found no vacuum at the heater valve. I moved to the inside of the car and checked the vacuum manifold in the console. Sure enough, the vacuum actuator (the small electromechanical vacuum switch that controls the vacuum to the heater valve) was faulty. I bought a whole vacuum manifold's worth of vacuum actuators, so I have some left. If you find that this is your problem, let me know and I will ship you however many you need.
Thanks John, not to be ungrateful, all my other friends. I'm working in Los Angeles, so I won't be able to look at my car till this Thursday. I'll report back ASAP.
Thanks again
joe
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By Stepson
#52823
No worries Joe. The offer stands for whenever you get around to diagnosing it. If you need us, we are there for you. :)
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By Polecat702
#53783
was the heater valve, replaced with the one I had in my tool chest With the short hose. I'm going to have to replace the diaphragms too. I'll be calling Roger next week. Thanks guys!
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