Dougs951S

Im trying to figure out why my heat is stuck on full blast, again. The first time about 8 months ago it was the plastic clips that broke. After replacing them it was fine until recently when it started getting hot and tried to turn the heat off in the cabin, which was when I realized it was stuck on. I double checked and the clips are intact, so I'm assuming TCU failure? If its possible, will converting to the old manual style valve from an early car allow me to turn the heat on and off from under the hood? its 65+ 10 months out of the year where I live so I can totally live with that. The car is pretty devoid of creature comforts but having hell wind blowing on my face when its 110 out is going to suck on a whole 'nother level. Barring all that, any way I can fool the system by putting vacuum to the valve another way to force it closed? Could it just be the servo motor is dead? is there a fuse for it??

theedge

Are you sure the valve isnt broken or there isnt an unplugged vacuum line? The valve is supposed to fail in the open position from what I remember, so it could be something simple.

Dougs951S

Ill look into that. Off topic but Brian, do you happen to have an old working msq from your old megasquirt build? Trying to troubleshoot. Or perhaps your input in my thread concerning my maps? I just know you are one of the few successful megasquirt users here.

ibkevin

Thanks for the reminder, my new clip is screwing with me so I have to check it out.

Grandpa#3

Check the vacuum operated heater control valve next to the fire wall in engine compartment. The diaphragm could be broken or the vacuum hose is loose / broken. An inop HCV will default to the full heat position.

Cheers,
Larry

sketchers356

I have the same problem. I tapped the line after the blue/white check valve and had that lead to the heater control valve. When the heater control valve is under vacuum it stays closed. The line after the check valve is always under vacuum.

turboS_swede

Yep, the vacuum diaphragm on my 88 951S failed, causing the heat to run at full blast. The canister held vacuum though, but the lever didn't move when vacuum was applied. Replaced it while I had the intake off and all was well again.

Dougs951S

But this required the diaphram to be intact, yes? I'm pretty sure mine is dead. I live in south texas where its 90+ 9 months out of the year so I'm really considering just bypassing the heater core, at least for now.

turboS_swede

Might as well fix it right and not deal with half-assed stuff when it does finally get cold.

Dougs951S

Not really half assed...I've owned my car for 3 years and have actually had real need for the heat MAYBE 5 days out of those 1000+. I try to not drive my car when its crappy out.

turboS_swede

ok, but its a $5 part and it takes a couple of hours to replace.

Whatever, its your car do what you like.

Dougs951S

Yep, but bypassing the heater core takes 5 minutes and I can replace the valve next time I have the intake off.

sketchers356

You dont need to take the intake off the car to replace the valve.

You do need to have the valve working for my method to work.