Topic: A/C Vacuum Mode Control Valve Failure.

[1967FEGT]

Are leaking AC Mode control valve failures common?  What are the symptoms?

[TLea]

Is this a Mustang or a Shelby?

[67gta289]

I don't have enough data to know whether or not the term common is appropriate or not.  I think though that if you have a problem, that is a moot point.

Everything can fail, of course.  Given the parts and movement involved, I would think that the most common failure would be that the rubber diaphragm leaks by, resulting in a vacuum leak and "none to partial" stroke.  You might hear the sound of a vacuum leak.

What symptoms are you experiencing?

Sometimes the problem is in the control switch - you can divide and conquer using a vacuum gage.

[67gta289]

I suspect that the vacuum porting switch in the controller (aka control head) is bad.  Part # C7ZA-19B919-C, picture attached, and not serviced separately.

[67gta289]

I should note that I have these pictures because I have a friend (me lol) that installed this, and followed up with a functional test using a vacuum pump.  I had a similar failure - a few actuators would move, but not all.  Then out is came and I bench tested the head and found it faulty (vacuum not applied to the correct ports as noted in the shop manual).  I have a couple extra units - too pitted up front to go on a car, but as is often the case the party out back was good, so I was able to move the ugly duckling head unit vacuum porting switch over to the nice one.  It bench tested ok (fool me once...) and went in the car, where a fully functional test proved everything out.

[jwc66k]

Sometimes the problem is in the control switch - you can divide and conquer using a vacuum gage.

Along with a vacuum pump. (Which also works very well for bleeding brakes, and checking out distributor diaphragms.)
Jim

[67gtasanjose]

The vacuum switch disassembles and usually is repairable in my experience.
I also found "just the switch" new before and have one for backup if ever needed.it

[J_Speegle]

The vacuum switch disassembles and usually is repairable in my experience.
I also found "just the switch" new before and have one for backup if ever needed.it

Parts available for the rebuild?  Think my 71 has a leak it its control valve. Not looking forward to getting to it but could be worst

[Bob Gaines]

I should note that I have these pictures because I have a friend (me lol) that installed this, and followed up with a functional test using a vacuum pump.  I had a similar failure - a few actuators would move, but not all.  Then out is came and I bench tested the head and found it faulty (vacuum not applied to the correct ports as noted in the shop manual).  I have a couple extra units - too pitted up front to go on a car, but as is often the case the party out back was good, so I was able to move the ugly duckling head unit vacuum porting switch over to the nice one.  It bench tested ok (fool me once...) and went in the car, where a fully functional test proved everything out.

This reads eerily similar to my experience. I am glad I am not alone.

[67gtasanjose]

Parts available for the rebuild?  Think my 71 has a leak it its control valve. Not looking forward to getting to it but could be worst

I cannot speak of any parts, I took my 67 valve apart, cleaned the mating surfaces, lubricated, reassembled and it quit leaking...soon afterwards, a NOS service part showed up and I bought it too. The repaired one was still not leaking this fall when I put the car together, it had sat 3-4 years boxed and waiting...I wondered if it might leak because no parts were changed out (just cleaned/lubricated) so was half expecting trouble but no such drama.

[1967FEGT]

I used my hand pump and pulled down enough vacuum in the reservoir to show 20 in on the gauge on the Mightyvac.  I couldn't get any of the vacuum motors to work with the switch. They all work fine with the MightyVac.

[ruppstang]

If you need a different switch I have one. PM me

[67gtasanjose]

I used my hand pump and pulled down enough vacuum in the reservoir to show 20 in on the gauge on the Mightyvac.  I couldn't get any of the vacuum motors to work with the switch. They all work fine with the MightyVac.

Try taking the valve off the controller assembly, un-do the spiral spring carefully to take it apart. Clean, lubricate. Check for any blockage or internal corrosion and reassemble.
Be sure you have the hose routing correct and it should work or if unrepairable problems are discovered, take Marty up on his offer.