Heater vent controls not working

[Deezelcaddy]

My heater vent selector vacuum switch isn't working. I replaced the control switch with a junkyard one and still no help. I had a broken vacuum line and fixed it. I tested my vacuum under the hood and it was within limits. I'm guessing this is a check every vacuum line and trace down the broken one situation but I just wanted to make sure there was no common fix to my problem. Thanx to all in advance for your help.

 

[OLDBULL8]

When you have a vacuum problem, the heater reverts to the defroster air flow. You should have at least 20 inches of vacuum from the pump. A real good pump will be around 27 inches of vacuum.

 

[bike-maker]

What year truck?
80-86 have a blend door with plastic hinges that commonly breaks.

 

[Deezelcaddy]

Sorry 92 and when I tested the vacuum under the hood I was at 20-21+

 

[chris142]

look for a broken line near the pass hood hinge.

 

[The Warden]

Is the air coming out of the defroster ducts, or where exactly is the air coming out of?

 

[Deezelcaddy]

Defroster only

 

[The Warden]

Defroster only

As the others said, that's a classic case of the system not having vacuum. You said you tested vacuum at the pump; do you have a Mityvac or similar, or are you using a standalone gauge?

Granted, I'm looking at vacuum problems from the Mercedes-Benz world (where almost everything is vacuum-operated, including the door locks and the engine-shutoff ), but a Mityvac really makes life easier when tracing vacuum leakage

My guess is the main vacuum feed line to the climate control is broken somewhere...if you have a Mityvac, disconnect the main feed line on both ends, plug one end, and put the Mityvac on the other and pull a vacuum. If it doesn't hold vacuum, the problem's in that line somewhere. If it does hold vacuum, the problem's either in the control switch assembly and you happened to get a second bad one, or it's elsewhere in the system...test each circuit individually. I have diagrams for the systems on slant-nose ('80-'86) and brick-nose ('87-'91) trucks; I'm not sure how different the system is on OBS ('92-'97) trucks.

The plastic vacuum lines can get brittle with age...I had the main feed line to the climate control break right at the firewall. But, again, be thankful you're not troubleshooting the vacuum system on an M-B diesel!

 

[Deezelcaddy]

Thanks for the advice. I don't have a mighty vac although I have coveted one in the past. I just have a vacuum gauge I use to tune Holley carbs. I will troubleshoot the system from the beginning. Before I got the truck the braided ground strap got extremely hot and melted some of the vacuum lines under the hood. I thought I fixed them all. I'm guessing it was starter related as it had a newer starter. This is as definite blind spot for me as a mechanic. I will keep checking and if I don't find i guess I'll get it smoked. X2 on the benzs my brother had one. Between vacuum lines and the hydraulic system on the all wheel drive all we did was replace lines, and the $25/qt proprietary hydraulic fluid.