On 2014-02-02, Jon Elson wrote:
Ignoramus16088 wrote:

We have a 1985' Ford LNT9000 semi tractor. Its heater used to work. It
was switched by a four position heater switch. It recently stopped
working. I just started troubleshooting this problem, and found it to
be very painful to do, due to cramped panel and complicated wiring
that is hard to access.

So far, I have been able to locate two wires that supply electricity
to the heater fan. If I apply 12v to them, the fan runs vigorously.

OK, so the problem is the fan doesn't run? Anything 1985 vintage would
not have electronic speed controls, but probably has a resistor
unit that is in the airflow path. So, poke around the fan assembly
looking for a block with some wires attached. Pull it, and you
will likely see some coils of resistor wire, and likely a one-time
thermal fuse. If the fan motor stalls or the airflow is restricted,
the resistor unit burns up, and the thermal fuse often is wired to kill
even the high-speed setting. These resistor units are fan-cooled,
so they have to be in the air path.

This is a good start. I am going to work tomorrow and I will work on
this semi tractor. It is parked inside, thankfully.

i

And, of course, there HAVE to be fuses somewhere.

Jon