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.

You just haven't found them yet. Ford used fusible links on some
things for years, so look for those, too.


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

I never worked on semis, but the car and truck HVAC switches were
sometimes routed through both vacuum -and- mechanical controls. Your
best bet is to buy a service manual for your exact truck. Libraries
sometimes have them. Or see if your usual truck service manager will
allow you to check the schematics at the shop.

I "grew up" with Mitchell manuals at work and Chilton manuals at home.
Mitchell was $$$$, about $400 a pop, but they had good, solid info.
I want to cry: http://tinyurl.com/kkekhyg but odds it has your big rig
in it are slim to none.

Also check your manuals site. You may have one hiding there. vbg

Oh, here you go: http://tinyurl.com/lpmmnfc

--
I have the consolation of having added nothing to my private fortune during
my public service, and of retiring with hands clean as they are empty.
-- Thomas Jefferson, letter to Count Diodati, 1807

Too bad -none- of the current CONgresscritters are willing to do that. -LJ