Heater Fan Resistor(s)

jemdandyJanuary 4, 2010

Wouldn't you know it. Coldest day of the year and the speed control resistor (for the heater fan) on my '98 Jeep Cherokee decides to burn out! At least, I still have one speed: Full blast.

The resistor is located below and behind the glove box and there is a kick shield covering the area. Replacing the resistor assembly requires bends and joints in my old body that only a contortionist has, and the eyesight of a young man. And then, there is a trick electrical connector to pull apart. My information says this connector has locking wedge that has to be pulled up before the connector can be released.

Although the repair manual does not mention it, apparently the job will be easier if I take out the glove box.

Due to the extreme cold, this job might best be accomplished by taking it to a repair shop with a heated garage and lift. We are into frost bite conditions at my locaction.

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I don't memorize the location of each of those resistor assemblies. But I can tell you, 51 year old eyes, even with reading glasses combined with what it takes to get my 200lb frame under a dash can make replacing these not a lot of fun even in a heated shop. VBG..

I normally use a 1/4" air ratchet and 1/4" flex sockets to assist me with the fasteners. Sometimes they just won't fit and there is no choice but to do the repair by hand. Watch for the wiring harness pigtail to have been overheating and possibly have damaged connectors.

One step often overlooked is measure the current that the motor draws. Figure 60% of the fuse rating for the circuit. If the motor at high speed draws more than that, then its starting to fail and could be the reason that the resistor block failed. (I.E. 20 amp fuse, no more than 12 amps current on high speed.)

    Bookmark   January 4, 2010 at 8:42AM
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The temperature rose to 20 F today, so I donned my snow suit and tackled the job. Removing the glove box was easy and a good move. I had warmed up the cabin before I began (ran a coule of chores) and left the engine idling with the heater on while I took off the auxillary stuff such as the glove box and kick shield. This let me work with bare hands. I shut down the engine and took off the battery ground cable before I began removing the resistor assembly and breaking open electrical connections under the dash as a precautionary measure. It was advised to break the battery circuit and allow two minutes for the air bag capacitors to discharge to prevent accidental deploying an airbag. There is a passenger side airbag on this vehicle located above the glove box.

I replaced the resistor assembly without fanfare. Most of the time required was getting the kick shield back on, but at least by then, I could run the engine and get heat when needed.

The connector for the resistor was in good shape. The design seems to have considered reducing heat flow to the connector stabs.

The failure was typical of "notching" aggravated by corrosion. The high current resistor had parted just above where it was fastened to its connector. Otherwise, all the resistors were as expected. The unit had moderate to severe corrosion whose appearance was typical from air borne salt spray. Obviously, it had been wetted.

['Notching' is a description of a failure mode common to lamp filaments and hot resistor elements. To learn more, look up notching failure.]

Ain't winter fun?

    Bookmark   January 5, 2010 at 1:45AM
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