Heater temp lever stuck

Hello all. I am buying a 1974 C20 with AC, so I look forward to using these forums as I learn about the truck. I am picking it up this evening, and the first order of business is the temp control lever for the AC/heater is stuck all the way on hot, which is unfortunate this time of year. Should I first try to spray some WD-40 back there and see if it comes loose, or do I need to take the dash apart and see what is going on back there? I figure this is not an unusual problem so someone on here knows the usual culprit. Thanks for the help.

You can get good access to the plenum and door by removing the glove compartment/door. There are 4 (maybe 5) screws that hold the hinge on, and then just slide the glove compartment out as a unit. I think the screws require a a 1/4" hex socket or driver.

It might be the cable, so lube it and check the operation on top of the plenum. To check the other end you will have to pull out the heater controls and look at where the cable connects.

The more likely culprit is something that fell through the defroster vents into the plenum, and its blocking the door movement. It could be a pencil, pen, bolt or anything else left on top of the dash. You can remove the large black vertical piece on the front of the plenum (two screws on the bottom, one on the top?), and see the door behind it.

Bruce

Wasn't 72 last of the heater controls with cables and 73 first year of square body with vacuum controlled dampers? If so, I would say the problem is in actual lever in dash got something stuck in there since it's sandwiched between upper control for air direction.

My '86 has the cable controlling just the heater blend door. All others are vaccum assist.

My 77 has cable controls and one of mine was stuck. I took pictures of where each arm on the heater box was positioned and then removed the temp control from the dash. I sprayed white lithium lubrication down all the cable sleeves until they slid easily and reinstalled.

On an 73-87 A/C truck, only the heater blend door is cable controlled and the rest of the doors are moved by vacuum motors. Once you remove the glove box and get access to the cable end at the heater plenum, disconnect it and see if you can move it back and forth with the dash lever. If it won't move, lubricate it as CRM suggested, or go online and buy a new cable. If its the door itself that is binding, you need to look inside the plenum.

If your truck is typical, cleaning out the heater plenum is like an archaeology dig. I found a rather large mouse nest (they like that insulation from under the dash), a pencil, a pen, a couple bolts, matchbooks from bars, and a few live rounds of ammunition from a deer rifle. All if it dropped very nicely down the defroster vent when someone set it on the dash.

Also take a close look at the small, hard vacuum tubing that runs under the dash, and see if any of it running to the vacuum doors is broken. Each section of hard line has a short rubber section on the end, or the device it connects to has a rubber nub where the tube plugs in. At the heater controls there is a large molded rubber connector that consolidates all the vacuum lines and connects them to the correct ports on the controls. You can repair small line breaks with a short section of rubber vacuum hose, and most auto stores sell replacement GM plastic hard line if you need a longer run.

Its also fairly common that someone has disconnected the vacuum supply line where it connects to the reservoir/canister under the hood, or has disconnected the line from the reservoir to the intake manifold. In some cases they have taken the entire vacuum reservoir out. Many guys get carried away with "cleaning up" under the hood and they take off the HVAC vacuum lines because they think its something related to emissions controls. Stripping out things under the hood without understanding what they are for can get you in trouble.

Bruce