911 automatic heater control problem

[Fairview]

Kathryn loves the strong and glorious heat in the 84 Carrera that I recently resurrected from a prior owner’s 20 year long storage. I haven’t had the heart to tell her it quit completely the other evening arriving home from work.

Can someone kindly guide me through the testing procedure for the auto heat system? I think I have studied all the threads on the forum but still need help.

Here’s what I have so far:

A) this has the 9 position controller.
B) it was working well, then completely and abruptly quit. There is no fan blowing, or heat rod adjusting at any setting.
C) I checked the fuses in both the front trunk and engine bay.
D) I verified power and ground as described in the Bentley manual
E) I checked the potentiometer set per guidance in this thread and Bentley and the ohms change as I would expect as the switch is rotated from the lowest to highest setting.
F) I checked the cabin and heat exchanger sensors, both about 2.5 in a 55 degree garage
G) I checked that power is getting to the circuit board, and it seems that each connection along the base of the board is getting varying amounts of voltage which I guess is right?
H) The heater blower relay in the engine bay is original, dated 1983, and I checked that there is power there on a couple wires with the key on, and on a couple of the terminals, when I touched my test light probe, I could feel a clicking in the relay as if things are working inside, and
I) when touched with a test light probe, thereby grounding a double wire connection on part of the controller, the footwell fans started blowing. In fact that is the only sign of life to the heater system now and they only blow with me grounding with the test probe, the rotary switch doesn’t turn anything on.

One other thing that might be relevant, when the heater quit the footwell blowers were running and at the last moment seemed noisy like something was dragging.

Would you be able to direct me from here? I’m in way over my pay grade. I’d love to get the heat going again.

Thanks, Jeff

[rokemester]

God bless you brother! All I can do is sympathize. I have an 87 and I wasn’t smart enough to diagnose so I systematically replaced the heat sensing resistor in the driver side heat valve, checked and replaced the resistor in the cabin heat sensor and found a replacement motor drive that you have pictured. Hopefully someone smarter than me will chime in with better advice. Stay safe. Thank God we have this hobby to keep us busy.

Sheltering in place in Northeast Ohio

[Fairview]

Don’t bother answering my inquiry above. I did more research this morning and found a whole series of threads on the engine bay relay 911.618.153.00. That may be the culprit so I will check that first.

Thanks. Jeff

[Fairview]

And another update for the record: After more diagnostics, confirmed that the engine compartment heater blower motor is dead. So I pulled it out and disassembled to find the brushes are shot.

I hope this is the real culprit. Is it possible that if the heater blower doesn’t spool up that the rest of the system is disabled as well?

Thanks

[pkabush]

Pelican member 16V did an excellent write up on the auto heat. I personally think it’s a overly complicated and unnecessary system so, I deleted mine. My car is a 81 930 and has the manual pull up levers as well. I’m not sure if the later cars have both or not.