jjbyron

I feel a little silly worrying about this in Los Angeles, but...

I'm not a mechanic. Can anyone give me steps to diagnose why my heater wouldn't be working? Its a 79 928. AC works, fans work, I can hear changes in vacuum ducts when I move the switch. it only blasts cold air.

Is there a separate fuse for the heater coil? I don't have a manual so I can't decipher the fuses.

Thanks!

dogleg

heater works on hot engine coolant pumped through a small radiator under the dash board.if you have no hot air when engine is warmed up it will be one of a few things wrong first make sure the coolant level is at the max next follow the cable coming off the hot cold lever it goes to an arm under the dash to the right of the steering column you should be able to move that arm as far as it will go towards the bulkhead.failing that the heater rad itself may be blocked this can be flushed through witth a garden hose from the engine bay,good luck hope this helps.

Mrmerlin

for more info about how the heater works,
go to 928gt.com then on the top right click page 2,
then on top left under tips and links click on Wallys HVAC system tips,
this will give you a brief on how the 928 HVAC system works your car is slightly different because its earlier

StratfordShark

Is the 79 system like my S4, where a rod attached to mixing motor on RHS operates the flaps governing proportion of ambient and heated air?

If so get in footwell with a flashlight and check that arm is connected at the mixing motor end (while you're at it check the motor travels to each extreme as you move temp lever from hot to cold).

My arm is prone to popping off at the mixing motor end. It does this because it rubs slightly against other parts as it moves between hot and cold positions, and because the clip which costs a few pence/cents is missing a chunk so when the arm is stressed rubbing on another part it pulls itself out of the clip, or the clip pops out and I find it rolling around under the carpet!

I really must order a new clip. meantime does anyone have a bright idea for securing the arm to motor?

James Bailey

Odds are someone has tie wrapped the heater valve closed because it was passing hot water ALL the time.....due to vacuum leaks etc. In some cases people put a plug in the hose to block the flow. You will need to remove the top of the airfilter housing to get more room to look at the back of the engine firewall area where the heater valve lives and the hoses are located.

UncleMaz

A little more information: JJ (Jeff) replaced his radiator a few weeks back. I helped him do this. The old radiator had a side tank separation issue.. He replaced it with an all aluminum, better cooling, slightly used one. It is also important to note that the fluid was very dirty and while we did a simple back flush, the system was not flushed in its entirety - probably not for a long time.

Jeff, did you run the heat while/when you bled the system?

jjbyron

Thanks Michael, I don't know why I thought it was unrelated.

I did run the heater when I bled the system, but I don't know if it was blowing hot air. I didn't really notice since I wasn't in the car and I just assumed it was hot.

The radiator seems to be working fine. It was a relatively simple install, with only a few snags.

UncleMaz

You should always run the heater when you are bleeding the cooling system. It opens the heater valve so that the coolant will flow through the heater core. It could very easily be your issue with the heater. Re-bleed per WSM and turn on the heater when you do it.