Bill Ball

Yep, in terms of what I described, it is the same. I'm basically trying to point out that the heater valve operation is a little more complex than you might think - at least more complex than I thought originally until I worked on a car that had an AC system that seemed to work but not as well as it should. I got fooled initially when it appeared the heater valve had vacuum and would close fine until I realized it was only closing when the climate control was switched OFF. As soon as I switched the climate control ON in that car, the valve opened regardless of temp setting. Of course, that's not right. Then looking at the wiring diagrams it became clear that the mixing motor control was in the path to the heater valve's control solenoid. Anybody that has any formal training in automotive AC should know this, but I'm just an amateur learning bit by bit as I go along.

9two8

Don,t know about that Bill, you sure sound convincing to me
and thank,s for taking the time, appreciate it.

Ken
80 928 S
5 Speed

UK

SteveG

I just want to confirm that the valve can appear to be closed but have an internal malfunction like a piece of gasket hung up inside keeping the valve from sealing shut. It was only visable when sighting through the valve after removal.

Bill Ball

Also, if the valve is installed backwards it is subject to at least a partial internal leak as the guillotine door in the valve will be unsupported against the coolant flow out of the engine block.

9two8

Well just a small update, I have,nt been able to get the car into the shop yet. i started the car from stone cold, just to see if the compressor kicked in, yes it did, so it should I guess as it,s only three months old, along with the drier.
The site glass was clear I could,nt see any flow.., and the air con was ice cold ( cold engine)
Am I clutching at straws , or does this mean that the heater control valve is failing, as I noticed the last time she was out from cold, the air con was working as normal.

Then after stopping for thirty minutes, on re-starting the cold air gradually warmed up somewhat, and started to get cooler after about three miles.., is this hot water escaping through the dodgy valve, and then it has to be cooled again ?

If it,s the valve I,m going to try a Mercedes one, part No A000 830 5784, I can buy one of these for around the £16 or $12 mark, as opposed to a genuine Porsche one at £60 or $80 to 90 dollars, prices from Porsche for these valves is now criminal,( but not in the states ) especially given the quality.

So has anyone any experience with the Mercedes valve ?

Ken
80 928 S
5 Speed

UK

9two8

....., bump

Ken
80 928 S
5 Speed

UK

Bill Ball

I still use the Porsche valve. No other experience. But, yes, your issue could be a partially leaky valve. As the motor heats up, this interferes with the AC more. In this case, with the AC off, the vent air will become warmer than ambient by 10-20 degrees or more, depending on the leak, as well. If it were a FULL leak, the vent temp would be VERY hot all the time. So, check the vent temp versus outside air with the AC off. I mentioned this could also be due to a problem with the mixing motor control of the heater valve and mixing door as well. As you move the temp slider from one end to the other you should be able to observe the mixing door move inside the center vent area. You can pull off the center vent grill (should come off very easily) to make this easier to see.

9two8

Many thank,s for that info Bill..,

Ken
80 928 S
5 Speed

UK

dimmer

You can test the heater valve in place by pulling off the vacuum line and attaching a spare piece of vacuum tubing;suck on the tubing and see if the diaphragm pulls the valve. You also need to ensure that there is a vacuum on the line when the temp control is moved to the full cold position. There is a switch inside the temp control that causes the vacuum to close the valve.

If you have vacuum and the valve works, take it back to the a/c shop. There are a few things that could be the problem:
1) If you have too much refrigerant, it will not be able to expand adequately. This will be most noticeable on hotter days when the added heat increases the pressure in the system even more. You may also notice that restarting the car with the a/c on is more laborious.
2) If the old compressor failed leaving debris in the system the a/c shop would need to properly flush the system or the expansion valve will clog.
3) you have a leak/too little refrigerant

As far as sourcing a different heater valve, I don't think that I would bother. The diaphragm are rubber and the heat in the engine compartment is harsh on it. If you really wanted to get more life out of it, you would probably be better off trying to relocate it to a cooler location.

Alan

Well at least part of the issue after stopping is that the heater valve defaults to open with the car off. So the valve opens and water circulates through the heater core by convection - if the engine was fully hot the HVAC airbox will warm through substantially. On restart the valve closes - but the heater is still hot - the AC will eventually cool the heater core - but even with the mixing flaps fully closed the air will not be as cold for many minutes. When the car is in full cold mode the heater valve will always stay closed when its running...

If Porsche were based in Phoenix the defualt position would have been designed to be closed.

There have been some threads wrt trying to fix that issue for hot climate cars...

Alan

Tampa 928s

Why don't you try closing the valve by using a wire tie it to test it. If you see it closing while running it is easier to attach the wire tie. If your car stays cold you have found your issue.
Other issues are the recirculation flap and center door that will cause a low cool condition. Just went through this on an 80 now it's 35 degrees

fraggle

If your valve dumps immediately at rest, you may have another vacuum leak somewhere. The whole system, when operating properly, should hold vac for many minutes once the car is shut down due to the one way valve at the bvac source.

9two8

...., thank,s again everyone for the help and advice, and taking the time, much appreciated .., I actually ordered a new HCV, and picked it up today.
After reading an old post on here regarding the valve, in which it said that if the valve is five years or older, it,s a wise move to replace it.., so I bought the Mercedes one ! it,s exactly the same one as the Porsche valve at a fraction of the cost .
Everything seems to check out, thanks to the advice of members on here !, so I,m just trying a quick substitution, with a known new part, to establish the credibility of the current valve, so as I can give it back to the shop in as good working condition as possible.., if I need to ? don,t want them to say to me, your HCV is only sometimes working which is causing your meltdown problem !

I,ll fit the new valve this week-end and try the system out hot/cold etc , and see how it behave,s, if it,s still suspect then I,ll take it back to the shop, and ask them to sort it, after all it cost me almost £500.!

Ken
80 928 S
5 Speed

UK

dr bob

My experience is that the little solenoid for the heater valve relaxes when the key is turned off. This opens the solenoid's valve port to atmosphere, and the heater valve immediately opens.

-----

Testing fate here, but I still have the first replacement heater valve that I bought from one of the Big 2 (at the time) vendors, and it still seals fine when closed at least ten years later. The original one from 1989 didn't leak when I replaced it as a PM task from group advice. Those old ones must be a LOT better than the newer parts.

9two8

Just a small update really, as I have,nt had a chance to run the car on the road ..,(work got in the way)
Last night, I took the HCV off the car, compared it with my new Mercedes one and it,s IDENTICAL in every way, just to re-cap the Mercedes valve was £19, as opposed to the Porsche valve at £62 ( Go figure ) I guess these prices would be US $23, and say $73 respectively !

When I got the valve off and not having a " Mityvac " I attached a piece of vacuum hose to the nipple on the valve body, manually closed the valve and nipped shut the vacuum hose, this held perfectly, so I guess the vac side of things was working.
I then filled the inlet side of the valve with cold water and within 20 seconds it had drained through, so I think from that little test the shut off portion inside is warped ?
All to do now is get a chance, and take her out and try the air-con and see if it,s cured meltdown ?

Ken
80 928 S
5 Speed

UK