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Old 12-10-2010, 08:50 AM
wirecutter
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Finally got it all fixed. Replaced the blower motor, which apparently was the root cause of my troubles. Measured the resistance of the good-vs-bad blower motor, and the bad one had about half the resistance of the good one. (Maybe a shorted winding or something, I don't know.)

So I now have two repaired blower amplifier modules, one of which is in the car now. (Yea, heat!) One of the amp modules required a new MOSFET ($3.17, Digikey IRFP1405PBF-ND), and both required new thermal fuses ($0.98, Digikey 317-1134-ND)

(When I replaced the MOSFET the first time, I hadn't replaced the blower yet, so when I tried it out, it blew the thermal fuse. Fortunately, the beefier MOSFET was protected by the fuse.)

Important note: The Digikey thermal fuse is not the original part, but the temp rating is the same. The problem is, unlike the original thermal fuse, the Digikey thermal fuse has one of its terminals electrically connected to the metal case. (I selected the fuse based on specs and on the fact that it was stocked. I checked a couple other distributors, and this was the only compatible part I found.)

If the thermal fuse is not electrically isolated from the case of the amplifier module, the fan will run at low speed whenever the car is on. If you're lucky, as I was, you won't blow out the circuit that drives the amplifier, but don't count on it.

My solution to this problem was to wrap the fuse in a single layer of polymide tape. This is the gold-tinted cellophane-like stuff used in PCB manufacturing. It insulates, but it's thin, can conduct heat fairly well, and can handle temps up to around 500F. (Digikey 81271-ND, non-stock) It's also, unfortunately, quite expensive. I had some around from a previous project. The important part is to insulate the body of the fuse from the module case electrically but not thermally.

Of course, if you're really brave, you can replace the thermal fuse with a wire, a normal fuse, or some other thing. I wouldn't, though.

So now I have a few extra MOSFETs and fuses. Maybe I should offer Maxima heater blower amplifier module repair as a service?

-M
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