Carpokes

DR Bob really helpful explanation as have a working heater vacuum valve but only ever hot air on our 944.

Used to be able to get cold by setting the control knob to fully hot and then back to cold but now that does not work.

Off to do some fault finding.

The heater control valve is vacuum-to-close, so any leaks in the vacuum system will manifest as heat in the cabin full time. In the 928, there are limit switches at end-of-travel on the temperature slider in the control head. Assuming that the vacuum system is intact but there's a problem in a temperature sensor, pushing the temp slider all the way to the left feeds current to the heater control valve solenoid to apply vacuum and close the valve. Slider to the hot end of the temperature scale isolates the solenoid, disconnects vacuum, and allows the valve to open.

I'll confess that I know nothing about the temperature control system in the 944, other than following the thought that Porsche (and Behr) work together on these systems. There's a lot of learnings from each car that gets passed on to subsequent models and from other cars of the period. 924 and its progeny inherited a huge amount of VW/Audi technology in their development cycles too. One big but not always happy family if you dig deep into their histories, but that didn't keep borrowed technology from wandering among the various brands as design talent moved project to project.

Mercedes and Porsche seemed to share a lot of engineering talent in the 1970's and '80s. Behr worked closely with both manufacturers on HVAC design and supply along the way.
-- For the S+ 928 for instance, major AC system components like expansion valves are shared with/from BMW systems, from the 944 series in the receiver/drier.
-- All but one of the too-many HVAC vacuum actuator diaphragms on the 928 are common to period M-B models, so silicone-rubber pieces now available for those cars will upgrade the 928 systems.
-- The 928 heater control valve is common to many period and contemporary Mercedes models. In the case of the HCV, Porsche still directly supplies the original version from way back when, while M-B has made several version updates along the way. It makes a whole lot of sense to search out a more contemporary M-B replacement, with better sealing flap and vacuum-actuator diaphragm materials. Cheaper too.
--On the 928, the short coolant hose that connects the HCV to the fitting on the cylinder head is a known weak spot, to the point where a then-prominent 928 shop owner referred to them as a cooling system overpressure protection device.

Bottom line is that there are significantly better other-suppliers component pieces available now vs. when the cars were built.

Regardless though, performance of the system really does rely on everything working perfectly together. I spent weeks chasing what looked like a defective front expansion valve, only to find that a seeping HCV was bleeding hot water into the heater core. In SoCal, I'd tie the HCV closed for all but December and January, when there were some days where A/C wasn't needed. Moving north to Oregon and real winter, it took a bit of head-scratching to figure out why AC system pressures were so unbalanced. Just a little bit of hot coolant flowing next to the evaporator was enough to do it. In a very rare and fortuitous instance of something, having it solved by the most inexpensive and easiest to access and replace component in the system is pretty amazing. I did install the M-B version and a real modern connecting hose and fitting, just to be sure. I also have a couple new Behr expansion valves in spares inventory.

I think of all this stuff as minor features in the older-Porsche ownership experience. Changing that first 928 timing belt was certainly one of several right-of-passage milestones in that process. Diagnosing interesting symptoms is part of the fun in the giant scheme of things.

2Types wrote: Mon Nov 13, 2023 6:55 am DR Bob really helpful explanation as have a working heater vacuum valve but only ever hot air on our 944.

Used to be able to get cold by setting the control knob to fully hot and then back to cold but now that does not work.

Off to do some fault finding.

If this is a 944, be sure to check the heater clip! See video here: viewtopic.php?t=371