On Fri, 31 Jan 2014 22:38:43 -0600, 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.

The truck, as far as I can see, has no fuses.

From our motor-heads, I wanted to find out what would be the most
typical issue with those fans. Maybe it can help shorten my search for
the trouble. Thanks
The LNT WILL have fuses somewhere.
should be on passenger side dash, there are two screws near the
windshield and then the cover flips open, fuses in there.