1970 3 Port heater control valve issue

[Bob H.]

I traced vacuum leak to the 3 port valve in the center console. Since the center port is blocked then I can only assume that rotating the heat side valve either passes vacuum or cuts it off to the rear ventilation. Well, that all works fine.

Blocking the vac in line at the valve keeps the system vacuum from escaping. Testing the line from the valve to the rear, it holds vacuum perfectly while closing the rear vents. So air must escape from the valve after sitting a few hours. I tried a second valve, same thing. I decided to take it apart expecting to see some hind of seal but there is none, just the two sides sliding back and forth and I certainly could see that easily leak. What am I missing? This vacuum loss causes my wiper door to open upon startup and then close. No leaks elsewhere!

[Terry M.]

Re: 1970 3 Port heater control valve issue

A former Texas Chapter member who owned a 1971 faced this problem. I used past tense because he has passed. He found a Ford solenoid vacuum valve that is normally closed and opens when power is applied. Unfortunately, for obvious reasons, I can no longer ask him for the part number. He wired it to his vacuum heater supply and had his 1971 vacuum system hold vacuum until the next day. He was good friend with Bob Demmel so if Bob reads this perhaps Rolf shared the details with him.

I have never tried this, but if the disks in the HVAC control were really smooth (as in sanded smooth and flat) would they seal better? I don't know, but if one had a sacrificial set of HVAC controls it might be worth trying. Perhaps someone has already tried that and can share their results. Or someone can tell me why I am on the wrong track?

[Mark E.]

Re: 1970 3 Port heater control valve issue

"This vacuum loss causes my wiper door to open upon startup and then close."

How did you test this? When the HVAC vacuum source is pinched (the tee for the source is in the engine compartment between the master cylinder and wiper door vacuum relay), does the wiper door behave?

[Bob H.]

Re: 1970 3 Port heater control valve issue

It behaves fine when you pinch off on the split T under the console going to the valve. The 9 port valve holds vacuum

[Bob D.]

Re: 1970 3 Port heater control valve issue

Terry

Unfortunately Rolf did not share his solution with me, but there is another person who he may have shared it with, Dave Anderson. I will see Dave at the regional this week and will ask him.

Bob

[Mark E.]

Re: 1970 3 Port heater control valve issue

It behaves fine when you pinch off on the split T under the console going to the valve. The 9 port valve holds vacuum

It sounds like you isolated the problem part. You say a second valve had the same issue. I'm wondering if the diagnosis is correct but you had the misfortune of getting a second defective part.

Did you bench test the two valves to confirm they leak? If you're confident they are the problem, try a third valve, preferably NOS, but bench test before installing.