No heat, but AC works fine

[otto huber]

This used to be an intermittent problem, but is now more consistent. I get cold air out of all the vents when the temp wheel is set to max heat. I've never been able to get moderate heat from the wheel consistently. Last winter when I unplugged the monovalve, I got more than enough heat. I tried unplugging the monovalve this evening and still no heat. Any suggestions?

[shertex]

Perhaps CCU is bad?

[otto huber]

Originally Posted by shertex

Perhaps CCU is bad?

Yeah, I figure that the CCU is on the way out since I can't get moderate temps with the wheel, but unplugging the CCU from the monovalve should give me heat since the CCU is no longer part of the equation. Can a monovalve get stuck in the closed position? That's all I can think of right now.

[tangofox007]

Originally Posted by otto huber

Can a monovalve get stuck in the closed position? That's all I can think of right now.

Sure they can. Heater cores can become completely blocked as well.

I think I would disassemble the monovalve. Then think about backflushing the heater core.

[otto huber]

I replaced the monovalve and housing with used ones I pulled from a junkyard last year. All temps work now, very easy and satisfying repair. The old monovalve had a lot of gunk on the mesh but the diaphragm looked good. Is there a link to back flushing the heater core on a 126? There's obviously some cleaning that needs to be done to this system.

[tangofox007]

Originally Posted by otto huber

Is there a link to back flushing the heater core on a 126?

I just disconnect the inlet and outlet hoses for the heater core, then insert a garden hose nozzle in one (and then the other) of the outlet hose ends. You will probably need to plug the unused outlet hose. Start with low pressure and see what sort of flow you get.