Heater stuck on

[CogitoErgoZoom]

Thought I'd tap the collective wisdom of the forum to help troubleshoot a strange issue I've been having with my 74 Targa 3.2 swap. I fairly recently had an engine out service to address some of the typical engine oil leaks and at the same time had the transaxle fully rebuilt along with a new clutch. Nothing else was touched or changed.

Everything has been fine and good but oddly since I got the car back, the cabin is getting hot air like the heater is stuck on wide open. It's not responding to the floor hand control as normal. No matter what setting the floor control is on, it's always a full blast of hot air coming into the cabin.

I'm not very familiar with the way the heater system and controls are implemented on the G series, but it seems like there's a valve somewhere stuck in the open position.

Getting the car up on jackstands this weekend and ready for the trek west from Austin to Monterey for Rennsport Reunion later this month, so any advice on where to start the troubleshooting would be much appreciated.

[Nditiz1]

There is a switch under the lever that activates a ground when up. This completes the circuit and turns the blower on at least on a car equipped with a blower. There is a relay in the engine bay for the blower. Check both of these

[CogitoErgoZoom]

Thanks. I'll check both. I've always assumed wrongly it was a purely mechanical system and that floor control was rotating a valve that allowed the main engine fan to push scavenge air.

[Nditiz1]

So if you have the older style which is what your car would have come with from the factory, then it is mechanical only. The lever next to the handbrake closes 2 valves. If they are rusted closed then you will constantly get heat. Do you have a blower motor or is it backdated with the 3.2?

[CogitoErgoZoom]

I have a blower motor at the front of the car, but I think that's for fresh air controlled through the dash slider controls? I don't ever hear an electric blower motor engage when I lift the lever on the floor. Warm air just starts quietly flowing, so I think I must have the passive/mechanical system.

I have a feeling the cable was somehow left disconnected when the engine was reinstalled and the valve is stuck in the fully open position.

I'll start by checking both ends of the cable connection. I just wished a simple diagram of the system existed for the 74 but haven't found anything.

[Nditiz1]

It's a very simple system
Lever -> cable -> valve
When the lever is in the full down position, the cable pushes against a lever on the valve opening the ports for heat to escape AND closes a flapper valve.
When the lever is pulled up, this pulls the cable which closes the ports on the valve AND opens the flapper to allow heat.
When the engine is on heat is always flowing to the heat valves, but if the flapper is closed then the heat drops out the bottom of the valve via open ports

[buster73]

Could be a loose clamp at the heater valve just above the drive shafts. The clamp also holds the heater cable. Easy to fix if you have a lift. Otherwise you have to jack up the rear of the car to reach the clamp from below the car. Had a similar problem after an engine swap.

[EC900]

“Heater stuck on”
With heat exchangers…”it’s always on”.

There’s a lot info in the Search for proper use and positions for the dash controls, it’s not an air tight system, some heat can enter and most noticeable the hotter it is outside.

With the engine swap, was the engine compartment assist blower reinstalled?
This fan only provides an additional boost for hot air to flow thru the heat exchangers and into the tub and hvac system. That fan can simply be disconnected to reduce air flow, but most heat will still come up from exchangers w/o the fan. Sure, easy just disconnect the hoses from the heat exchangers too and keep interior temp a bit cooler in summer.

It is a simple system, and maybe the dash controls are confusing and need adjustment but your problem could be broken/loose cables either at the two red center levers or at the flapper valves being open, rusted flaps or broken mount studs impeding operation.

See what you have, then post a pic or two.

[CogitoErgoZoom]

Thanks for the recommendations, all. After hitting the "books" last night, I have a much better understanding of how the system is implemented, although there is a lot of variation by MY obviously. These pages were very helpful and had a good overview of the system I was looking for: https://members.rennlist.com/imcarthur/911autoheat.htm
https://members.rennlist.com/imcarthur/heatcomponents.htm#Manual%20Heat

@buster73 noted and thanks, I'll check for that.

@EC900 I don't think it was carried over, but I could be wrong. I'm going to get the car up on stands this weekend and see what I'm working with. I have the system with only one single red lever near the parking brake and, again, I've never heard any electrical fan or motor engage when I've operated it, but I'll confirm all that once I have the car up. I'm fairly certain I just have the "passive" system driven solely by the engine fan.

['78 SC]

I had this happen the last time the engine was installed in my '78. The right side heater was stuck on full blast. I checked under the car and saw the cable for the heat valve (as shown in the photo above) was snagged on something, which closed the valve and forced heat to the cabin. I don't remember specifically what it was caught on, but it was some projection. Check the heat valve cables.

[Gretz]

I had a similar problem on one side as 78SC. The ferrule/seal on the cable had disintegrated and wouldn't allow the valve to open so all the heat was going into cabin regardless of the lever position. Cleaned up the cable end and now the heat lever functions correctly.

[PeteKz]

I had one of the cables/wires from the lever get kinked before it went into the tube in the tunnel. Consequently the valve did not close fully and I was always getting heat. Look under the rear of the car first and make sure those parts are working.