sfierle

This past weekend, I finally replaced my heater valve I have had for about 6 months in the garage. Not a bad job, a little over an hour for me. It would have been shorter if I didn't have to clean out a previous rodent home under the air cleaner housing. Anyway, I replaced the heater valve, but still no change: blows heat all the time when the climate control system is on. I assume it is a vacuum problem somewhere. The system has been finicky since I bought the car, but the a/c did work at the beginning of last summer. Where should I go next? I don't want to replace more parts and hope it fixes the problem.

AO

Steve-
It sounds like you're not getting vacuum to the valve to close it (it defaults to open). For a temp fix, you can zip-tie it closed, but you need to find out why she's not getting vacuum.

Giovanni

Pull off the vacuum hose off the valve and see if it pull vacuum when the car is idling with the a/c is On. If it is like my 1988, it is not pulling vacuum to close the valve although it pulls vacuum when idling with the ac Off. I have been told the relay? under the center dash console needs replacement. Anyone care to add?

Kevin in Atlanta

There is a terrific A/C write up in the tips section of 928 Specialists by Wally. I would start there.

chaadster

Also, sfierle, make sure the hot water valve is oriented correctly, because if it is backwards, it will allow hot coolant to seep past the valve, allowing it to flow into your heater core. IIRC, the actuator arm should be facing the engine, not the firewall; the valve is also marked (in German) with in/out, where out is to heater core.

Anyway, a quick forum search can confirm proper orientation.

Good luck. It would be nice if it isn't a vac issue.

IcemanG17

It does sound like a no vacuum issue.......just plug a mity-vac into the line with the car running and adjust the temp from full cold to full hot......if you get 0 vacuum on full cold...then you have a leak.....you should get at least 10+ inches......as high as 20+ if your system is tight...I got 22 on mine....

Or you could try the zip-tie fix to lock the heater valve closed (no heat)......since its almost summer this works well......but its easiest to spend a bit more time and figure it out.....even if you have a heater valve that holds vacuum and is positioned the right way...it still might be broken internally like this:

Ralph Newman

If you wire tie the valve closed and you still don't get cold air my money is on the underated relay. Great write up on the fix ( replacing it with a $4.00 one from Radio Shack ). Good Luck

bd0nalds0n

If the car does hold vacuum, it could be the mixing, or "setting," motor. That's where the logic function exists that compares outside temperature and cabin temperature, reconciles it with where the temp slider is positioned, and the determination is made whether to open or close the heater valve by energizing the vacuum solenoid behind the stereo.

One failure mode is that the mixing motor never sends power to the heater control valve vacuum solenoid, so the heater valve never shuts, even though the system holds vacuum.

sfierle

Thanks for the tips. I will have to look to see if it gets vacuum. My a/c did work once, so I put the valve on the same way the old one came off, so it should be oriented correctly.

Not an option to zip tie closed, my wife (like many others) is always cold and wouldn't drive in a car with no heat. When it is warm out, I shut off the climate system (no hot or cool air coming in) and open the window and sunroof. It just would be nice to have a/c, especially since it worked a short time ago.

Mongo

I have the same issue. Where does the vacuum line run that connects to the heater valve?

StratfordShark

White line through firewall to vacuum solenoids, but you need to trace the vacuum right back to the brake servo check valve, and each individual HVAC vacuum line to find where the leak(s) is since a leak anywhere will affect entire system. Check whether or not you have main vacuum to the hvac solenoids (black line through firewall). If yes then investigate individual hvac lines, if no then check back to 4 -way connector, check valve, vac reservoir and flappy actuator to see where vacuum leaks.

Wally's write-up will take you logically through the whole system. You just need a Mity-Vac and persistence!

SeanR

http://members.rennlist.com/pirtle/svc_act.html

dr bob

Gio--

The relay in the console only affects the operation of the AC compressor clutch. It has no effect on the heater control valve operation.

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Most Likely Problem is one of more vacuum leaks in the HVAC system. If the heater valve operated OK before it was replaced, there's a reasonable likelyhood that the vacuum hose to the diaphragm is not attached securely. The old rubber elbows get hard and don't grip as well as they once did, so they should be replaced if possible, r at least secured with a small zip-tie to keep the hoses connected. Use the zip-tie like a mini hose clamp.

The write-up at John Pirtle's tips page is pretty good for diagnosing vacuum problems. There are solenoids to control the heater valve, the defrost valve, center vent comb flap, fresh air/recirc flap, and the footwell flap. A leak in any one of those flap or valve control diaphragms will cause the others to operate poorly if at all. Fortunaley you can diagnose with a minimum number of console covers removed. Almost all repairs take a bit of console, pod, or passenger side footwell disassembly for access. I spent several hours with the Miti-Vac and the tools replacing a couple leaky actuators (comb flap and footwell). I got off easy, since I didn't need to pull the pod or the dash.

j.kenzie@sbcglobal.net

Hi Steve,
The heater valve and vacuum systems are suspect on these cars. They control mixing of heat and cold and air distribution. You still have to have a source of cold for the a/c to work. You should have cold air for a few minutes no matter how your heater valve works, with a cold engine before the water heats up. Also you should be able to find cold refrigerant line by the expansion valve near the firewall. I am suspicious that you may not have refrigerant function. You may still have vacuum or valve problems but I would confirm cooling as well. If you don't have cooling, check Wally's article to go through the troubleshooting.
Good luck,

AO

There is a connector just above the accelerator pedal. I know from personal experience on my old 82, that if this is off, you only get hot air (maybe it was outside air... no I'm pretty sure it was hot). Look under the dash and make sure that connector is seated fully.