On 2014-02-01, 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.
Good -- the brushes are not worn out yet, then.
The truck, as far as I can see, has no fuses.
Anywhere? There should be a number of them *somewhere* --
including fuses for the power feed connector to the trailer.
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
First -- a question:
Does it perhaps run -- only when set to high speed?
I don't know the semi tractor, but I have some experience with a
similar problem in a '96 Mazda Navajo (really a Ford Explorer), and Ford
may do things pretty much the same way over a period of time.
The one in question accomplished the speed control by low valued
resistance elements in the airflow -- an assembly installed into the fan
housing. The problem cause: Squirrels or maybe mice building a nest in
there and partially blocking the airflow. This deprived the resistance
elements of the cooling airflow, and they started sending smoke into the
cab. If switched to high speed at the first sign of smoke, the added
airflow (and bypassing all the resistance elements) allowed it to
continue working. This first happened when my wife was driving it, and
when I looked at it, I discovered the work-around of running the motor
full speed. If it is this -- look for a multi-pin connector snapped
onto the outside of the fan housing. (Is the fan housing under the hood,
or hidden under the dash?
It was a real pain to get to the resistance element and remove
it. Two long screws with very little access, even with the connector
unplugged. The actual replacement element was quite inexpensive.
Now -- the multi-speed assembly probably works the same way in
your older (and much larger) semi, but the resistance elements may not
be in the airflow path to the cab. They may be just somewhere under the
hood. The trick is finding them.
Wiring is sort of like this:
+12---------o----o-- (OFF)
o---WWW--+ (Low)
|
+------+
|
o-+-WWW--+ (Medium)
|
+------+
|
o-+-WWW--+ (Sorta High)
|
|
| (Full Speed)
o--------+------(Fan Motor) ---ground
and the resistance elements are simply coils of bare wire, with the
highest speed before full being a rather short coil of heavy wire, and
each lower speed adding another longer coil of smaller wire, but not too
many turns in any of them.
Look for the resistor assembly. If you can find it, it is
likely to be the problem point, especially if the fan run from the raw
12V sounds faster than you remember from before.
Enjoy,
DoN.
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