After a year of procrastination I was finally motivated by the cool
weather to fix my Jeep heater. Like many others I was having the
problem of the "low" and "medium" settings not working. I figured I
would post this and hopefully save someone else the few hours it took
me to track this down.

This information is based on my 95' Jeep Wrangler YJ and while I
understand there are variations in the heating systems throughout the
years, the concept should be the same. I suppose this problem could be
caused by the switch, however the usual culprit seems to be the
resistor pack. While the switch can quickly be ruled out by checking
it with a multimeter, it might be better to check out the resistor pack
first.

The resistor pack is located on the passenger side just above the vent
on the blower housing. Upon removal of the wiring harness that is
connected to it you will notice that it has 3 prongs and is held in
place by 2 (1/4 inch?) bolts. Prior to removing the pack, ensure that
the ignition is off and the heat is in the off position, lest you shock
or burn the hell out of yourself if the pack is actually functional
(The resistors get RED HOT).

When the pack is out you will see that it consists of 2 resistors and a
thermal protection fuse. The resistors in this case are merely coils
of wire and should be obviously deformed in some manner if they are the
problem. The thermal protection fuse is a more subtle failure, but can
be easily checked with a multimeter. You'll either have continuity or
an open. In my case, the fuse read open.

If you have no soldering knowledge or no will to learn, I suggest a
trip to your local Jeep dealership where they will promptly break one
off in you and sell you a new pack for approximately $17. If you can
solder, the fix is relatively easy. The fuse I found appears to be the
exact same one in the resistor pack and costs a paltry $1.49. Head to
your local Radio Shack, turn down the barrage of cell phone offers, and
ask for part number 270-1321, which should yield a Thermal Fuse rated
at 444F (229C).

Unless you posess super-human strength and patience, simply clip off
the old fuse, rather than attempting to pull it out of the blade it is
mounted in. Form the leads of the new fuse in the shape of the old to
position it in the center of the pack and prevent it from contacting
the resistors, then solder it in place.

A special note on the soldering. Remember that this is a thermal
protection fuse and as such, may be damaged by the soldering. Use a
heat sink of some sort. You may call these hemostats . . . I call them
roach clips. Also, if available, use high temp solder rather than
typical 60/40. I'm not a mathematician, but the difference between
60/40's melting point of roughly 370F and the fuse's limit of 444F
concerns me. I imagine the worst that will happen if you do use 60/40
is that the fuse will fall off, causing your heater to once again only
work in high.

I hope this helps anyone who may have been in the same situation as me.