Vacuum switch on the A/C control

[badmilk]

I have two A/C control units. One has 3 vacuum switches and the other has 2. Other than that they look identical. What’s the difference? The truck had the one with 2 vacuum switches. Ohhh yeah ist for my 72 with factory air.

[lbpd719]

been down this road..

See:

http://67-72chevytrucks.com/vboard/s...d.php?t=225592

And..

http://67-72chevytrucks.com/vboard/s...controls+lines

Goodluck

[wibilly]

it appears the third port goes to the water controal valve.that one was plugged on mine. i'm just now trying to sort this out two

[badmilk]

i capped the port on the left and it seems to be working.

[davepl]

Originally Posted by wibilly

it appears the third port goes to the water controal valve.that one was plugged on mine. i'm just now trying to sort this out two

It's possible, but I don't think so. According to the manuals, the water control valve just T's off the cowl vent vacuum line.

I'm also in the position of having a 3-port repop control. They're numbered 1-2-3 and I've searched a lot on the board... no posts seem to list what's what.

I've seen the shop manual drawing, but it doesn't indicate which port is actually which.

Any help?

[leddzepp]

He is correct. Look at the second diagram in post number 5: http://67-72chevytrucks.com/vboard/s...controls+lines

The third port goes directly to the water control valve. You can also just cap it off and t off of the other line as the two port setup does.

[Andy4639]

It depends on which manual you look at. The 71 manual is this way.





The 69 shows it this way.

[davepl]

Only problem is that I'm using a 3-port replacement control in a '70 where I'm going to use the proper '70 routing (heater valve off a T under the hood).

I just don't know which of the 3 ports correspond to the older 2-port units.

[leddzepp]

Start capping them off one at a time and see which two you need to use to make it function properly.

[davepl]

What's properly in this case? In which switch position should it pass vacuum and which should be closed?

I may be able to find the old one to check it, but just in case...

[leddzepp]

With the A/C on, the inside passenger vent door (recirculating air) should be open (vacuum applied) the cowl vent door (fresh air) should be closed (vacuum applied), and water valve closed (vacuum applied)

You will just have to use a trial and error method as to which switches do what. The above diagrams have all the info needed.

[Andy4639]

Easy man look at the 71 picture with three lines. The bottom line is for the water shut off. Cap this line. Then tee the line out of the center one for the other vacuum operated valves. The top is for the source of vacuum.

[davepl]

Originally Posted by Andy4639

Easy man look at the 71 picture with three lines. The bottom line is for the water shut off. Cap this line. Then tee the line out of the center one for the other vacuum operated valves. The top is for the source of vacuum.

Thanks! In the picture you're referring to which do you consider top and which is bottom? I assume you mean that the "missing" one is the water one. Which is weird since that means the ports 2 and 3 do the same thing - so why they'd add complexity to the switch to remove a plastic vacuum tee I don't know. It even requires more hose and additional run through the firewall. I wonder why they changed it?

[Andy4639]

The reason for I'm not sure of other than if you get a cut in the line for all three vacuum operated valves. With 2 seperate lines at lest you have a vacuum source. Not really sure though.

[davepl]

So the problem I'm now seeing is that the switch really doesn't hold vacuum worth a crap. Nor does my old original. When I connect a vacuum pump (brake bleeder thing with a gauge) I can pump up vacuum but it bleeds off in a jerky fashion about as fast as I can pump it.

If I connect the vacuum pump directly to the vent solenoid, it'll hold vacuum for hours, so its not the pump or the solenoid.

I guess if you're talking about a 400cid engine rather than a hand pump maybe it doesn't matter, but I still find it odd. Seems like a vacuum leak and I don't want it affect the idle and so on.

Am I the first to test the switch or has anyone had a similar (or different) experience with them?

Thanks!
Dave

[Andy4639]

How did you test it? Did you cap each line off other than the one you pulled vacum on? If so did you check the caps to see if they where the leaks. if so I would buy a new one.

[davepl]

Buy a new one what? Both the old and the new do the same thing.

Yes, I capped them all but one at a time.

Hey, I always appreciate your input, but have you considered not making your posts bold red and shortening your manifesto of a signature? It makes your answers hard to read :-)

[davepl]

According to the 70 manual it follows the 71 method of a line for the heater switch all the way to the vacuum switch. Which is weird because my truck is a 70 but came with the two-port.