Vacuum issues? Heater, A/C controls not working

[Jehu]

I think I'm missing something on the vacuum line routing. The nipple was broken off of the Charcoal Canister and the line was dangling over the side of the engine. Rather than trying to find a way to fix the canister, I just flinched while paying the $85 for a new one. I ran the line straight from the canister to the throttle body. It seems to run okay. I'm pretty sure that there's no more vacuum leak but there's absolutely no control response in the heater / ac. The electrical works. The fan and A/C come on but there's no air exit from anywhere. I know that the ducting controls are a weakness of the GMT400 but this seems like a little more than just a bad solenoid or two.

Should there be a tee in the vacuum line? Where does the heater control get its vacuum supply?

It's a '93 PU with 5.7 (350) and stock A/C.

 

[Ken K]

The vacuum supply line that disappears inside is PPL plastic line. The intake should have a fitting with live vacuum possibly where the power brake booster plugs in. This is where a rubber nipple to plastic line should be going into a vacuum reservoir on the passenger side. Then to the PPL vacuum supply line for the switch / mode control. Yes, the vacuum switching valve (HVAC) can leak and cause a hissing noise inside the dash. I am not sure about the vacuum reservoir on trucks (GM400) but has an internal check valve to hold vacuum volume that can be lost during WOT operation, like passing or towing. Washer tubing can be used to replace broken hard plastic lines and route them away from anything hot. By law (DOT), with a loss of vacuum should cause the system to default to defrost. This is true with all type of systems except for cable types.
As for the carbon canister, the fittings should be marked with (TANK / CARB). One goes to the gas tank for vapors at the top with a roll-over-valve, the other goes to a thermal vacuum switch that opens when warmed up. In closed loop, the ECM can proper control air/fuel ratio while evacuating raw hydrocarbons from the gas tank, stored in the canister, that your paid for. This is called the "Canister Purge". A vacuum routing sticker should be under the hood on the core support, but with time may be gone. The thermal vacuum switch is in the coolant jacket portion of the intake and have fittings large enough to match the hose coming from the canister. With electronics, these VSV's are solenoids. Service manuals from this period are paper and not electronic. GM started the change over to electronic in 1997 with only engine & transmission only prior to this change over. Hopefully it is still there. Best of luck.

 

[Jehu]

Thanks, Ken.
I'll look for something that resembles a vacuum reservoir on the passenger side. Hopefully, I can figure out where the vacuum line goes through the firewall and find a place to check it.
Jon

 

[someotherguy]

There is no vacuum-related function of the HVAC on your 1993 GMT400 truck. Zero. It is all electronic, regardless of a few factory service manual references to vacuum - cut and paste from squarebody-era trucks (1973-1987.) Just an FYI so you don't go down the wrong path troubleshooting.

Richard

 

[Jehu]

Since reading Ken's post, I've been looking for diagrams, etc. I found a few discussions about how the GMT400 went back and forth between vacuum and electric on the controls a few times during the '90's. It sounds like I have the typical warped duct flaps and faulty solenoid controllers. I would like it if the defroster would at least provide an outlet.

 

[Jehu]

I am now convinced that I need to get into the duct flaps. What's the best way into them? Glovebox? underneath?

edit>>
I do get the typical thump, thump sound after making changes in controls or shutting the engine down.

 

[Jehu]

I solved the mode actuator issue. It turns out that the linkage is missing a screw. I slid it back into place (still no screw) and worked the controls. The mode selection works great. Now if I can just figure a way to put some 12A refrigerant in the system, I'll enjoy the summer a little more. I wish I had this figured out when I needed the defroster.