A/C Vacuum Control Valve

melloelky

Top Fueler
Jul 1, 2009
3,654
31
48
mass
has anyone had to replace this?it's the vacuum selector switch on the hvac control.my hvac is defaulting to defrost/floor vent.I've got vacuum from the motor to the vac ball,i tried taking the ball outta line and hooking the hoses together but still nothing inside the car.I've been floating around under the dash but the vac hoses look ok and was just wondering if anyone has run into this before.
 

Doober

Moderator
Jun 2, 2003
14,704
1
38
Catalina, AZ
www.cardomain.com
None of my Malibus had their lines hooked up, so I wasn't sure what went to where under the hood. I had one hooked up and heard vacuum from the control switch area... never did get it to move around right but I wasn't attempting a full revamp of the system either... this next time I will, and I'm considering going to an electronic retrofit.
 

Macrisel

Weekend Racer
Nov 11, 2016
52
0
0
Southeast Missouri
I ran into the same thing in my car and worked on it for three nights before I figured it out. I even replaced the HVAC head unit and it still didn't work.

Turns out, my line that supplied vacuum to the unit was stopped up with GUNK from the vac ball all the way to the unit. I tried cleaning it out with wire, but it was no use. I ended up just clipping the plastic OEM lines at the plugs and splicing in some thick rubber line. Worked like a charm!

In fact, I'm pretty sure that line had been stopped up for a loooooong time. When I spliced in the new line and changed the switch over to vent, there was enough debris flying out of the vents to choke a horse!

Good luck with it!
 

melloelky

Top Fueler
Thread starter
Jul 1, 2009
3,654
31
48
mass
thanks,that's certainly food for thought i appreciate it. i'll try something like that.
 

Doober

Moderator
Jun 2, 2003
14,704
1
38
Catalina, AZ
www.cardomain.com
Something I found while looking around... I did know some G-Bodies had electronic hvac control, but it was apparently all a ruse. I'll have to find the link, but someone retrofit one into an El Camino and there is actually an electronically controlled valve that STILL runs vacuum to the ductwork doors :lol:
 

Doober

Moderator
Jun 2, 2003
14,704
1
38
Catalina, AZ
www.cardomain.com
Looks like I'm mistaken on the control donor, but it appears they still used the awful vacuum control into the '90s on some cars.

My plan involves the controls for the late '80s/early '90s GM trucks (GMT400 platform), possibly the late '90s if the controls are still the same and the control head isn't too big.
Early GMT400
41u6hecz7tL._SL500_AC_SS350_.jpg


Late GMT400
image.php


I started doing some looking and discovered they use a motor similar to the NBS (GMT800, '99-'06) and possibly newer trucks.
image.php


The early controls used relays to run power through each blower speed resistor, I'm not sure about the later ones though... it's not real clear but you can see the relay bracket here (behind where the glove box mounts):
464432_DSCF0457_1.jpg

My thought is it should be possible to just tie it in place of the factory G-Body relay module in the engine compartment, matching the speed settings up to the relays for the newer controls, since many/most of the GMT400 trucks had 4 speeds like the G-Bodies (not all, I had an '88 Chevy with manual heat/defrost controls and low/med/high with a mechanical switch).

If there are other ideas related to electronics/retrofit/etc. I started a new thread since this stuff isn't really related to the factory stuff.
 
Last edited by a moderator:

MalibuRacing.com Gear

Stickers & Shirts!!