A/C but no heat

1981 C10 A/C works but heater does not. Climate control unit is new. Where do I start?

I assume the blower is working when it's set to heat since you said the AC works? I would first check the coolant level. If coolant is full and temp on your temp gauge is good, possibly a plugged heater core.

are heater hoses hot, temperature difference between the 2? Temp split around 20-25 degrees indicates a plugged core, no or low flow. Check after warm and heater has been operating. When coolant was added did you bleed air out?

Because he says he has a new heater control, I'd be checking blend door operation

3/4 of a gallon low on coolant. See if that helps…

So I’m checking the routing of the hoses. And I think they are on backwards the top hose coming out of the firewall from the heater core should go to the radiator and the bottom hose coming out of the firewall should go to the engine, correct? Mine are the opposite.

Jaybird81 said:

So I’m checking the routing of the hoses. And I think they are on backwards the top hose coming out of the firewall from the heater core should go to the radiator and the bottom hose coming out of the firewall should go to the engine, correct? Mine are the opposite.

Click to expand...

The heater core isn't directional, so which hose is on the top and which is on the bottom shouldn't make any difference as long as there's coolant flowing through it. (Which it sounds like there might not be.)

Make sure your HVAC ball is hooked up to a vacuum source. It should be on the passenger side next to the A/C box. Ignore anything anyone says about a check valve for it, ricko for example, it has one built in.

It's what comes through the firewall, to the vacuum splitter (Controlled by the settings lever on A/C controls) and moves the doors. it's not electronic like the new ones.

Laqota said:

Make sure your HVAC ball is hooked up to a vacuum source. It should be on the passenger side next to the A/C box. Ignore anything anyone says about a check valve for it, ricko for example, it has one built in.

It's what comes through the firewall, to the vacuum splitter (Controlled by the settings lever on A/C controls) and moves the doors. it's not electronic like the new ones.

Click to expand...

Laqota is 100% correct.

I was googling and searching, trying to figure out the OEM configuration for reconnecting my 1983 HVAC vacuum controls to the engine (a PO never reconnected it). I was thinking what good would a vacuum reservoir be if there was no check valve between it and the engine to keep the HVAC reservoir under vacuum... like there is with the cruise control reservoir.

After seeing Laqota's reply, I verified this was true for my truck's HVAC ball. I took the reservoir out, put a vacuum gage on the small nipple (HVAC side) and applied a vacuum to the large nipple (engine side). I verified a vacuum on the gauge and then removed the vacuum source by completely disconnecting the larger line. The reservoir held a vacuum without leaking, verifying the small ball does have an internal check valve on the engine side. Once I disconnected the vacuum gauge from the small nipple, there was an obvious hiss of air as it rushed into the reservior.