Might be labeled as a waste of time, but the ac in these cars aren’t the best. After souring the forums, I decided to jump in and install a factory style heater valve. Since my car was originally an LG4 I didn’t have the metal lines, but a few of the molded hoses that fit the TBI, and a 3/4 tee allowed me to route in a similar matter. To give it a factory look I grabbed some spring clamps from the junkyard. Cleaning up the wiring is next on the list!

 

FYI, Current(?) G-vans use a two port heater block valve - in and out. I used one on my 87 and it works well. Out of the pump, into the valve, out to the heater core, no problem.

 

the heater valve appears to be doing the job, however, I needed a solution to trigger the valve. From the factory, I believe it received vacuum via the max air setting. However, I opted to go a different route and trigger the valve to bypass anytime the AC compressor is running. What I did was repurpose a GMT400 vacuum actuator and tied it in the circuit with the throttle kicker solenoid. Typically, I’d run a relay with a negative output as a ground trigger, but my understanding is that this solenoid will pulls very little amperage.

GM#NV236. Shares a vacuum source with the HVAC. The factory reservoir/check valve prevents a drop in vacuum when accelerating.

Here’s the vacuum reading with the AC engaged. Not a bad vacuum signal for a mild 350!

 

I would advise STRONGLY against that.

Reason being, the compressor runs when in Defrost. Meaning, in your scheme, you'll have no heat to the windshield EVER in Defrost.

Better plan would be, get the little vacuum valve disc thing out of a car - almost ANY GM car from the late 80s / early 90s - and run another of those thin plastic vac lines through the harness into the engine compartment to the heater valve. You should be able to just push one through there. Doesn't have to be very large. Worked GREAT on my 83 car, same overall situation as yours.

 

That's an... Interesting... Way of handling it. Iirc when I was doing mine, vacuum was applied when the temp setting is on cold or the first inch or so of travel. That way you could have cool, non ac, outside air if you wanted. This would make the failure mode was coolant flowing through the heater core at all times, which is preferable to no heat.

 

get the little vacuum valve disc thing out of a car

More info please. I have an 84 TA and want to do this too.