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Old 12-08-2010, 10:59 AM
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Default 2003 Maxima heater blower, again

Greetings, all, from another poor slob whose Maxima ac/heater blower quit because of the amplifier failing. What I've learned so far:

2003 nissan maxima

So now I have 2 of these bad amplifiers. I decided to do some real troubleshooting on them. Later, I'll post schematic of the module.

You can do a basic test to see if the module is smoked, and if so, maybe how. Take the module out of the car. I've "numbered" the pins of the module as follows. This would be viewing the module's male pins:

<sp>--- 2
1 --- --- 3

If you short pins 1 & 2 together, you should measure a very high resistance (over 1 megaohm) across pins 2 (negative) and 3 (positive). Now, if you apply 5-12 volts DC across pin 1 (positive) and pin 2 (negative), you should be able to measure a very low resistance across pin 2 (negative) and pin 3 (positive). (less than 1 ohm, but you probably have an ohm or two in your test leads. The "low" reading should be close to what you get when you short the meter leads to each other.) Remove the voltage across pins 1-2 and short pins 1-2 together, and again you should see a high resistance across pin 2 (negative) and pin 3 (positive).
In my case, I never read the low resistance - it was always high. So, I applied the voltage to the other side of the thermal fuse. Voila! Very low resistance. So the problem is that the thermal fuse opened up.

When you pop the cover off of the amplifier module, you'll see a little circuit board. You'll also see solder joints on each end of the board for parts mounted on the other side. The 3 joints on one end are for the MOSFET, the 2 on the other end are for the thermal fuse. You can test whether the fuse is good by reading the resistance across it.

-Mark
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Old 12-08-2010, 11:08 AM
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Thank you for the information.
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Old 12-08-2010, 11:09 AM
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Default Schematic

Now let's see if I can post the schematic. Yes, this is a photo of my dry-erase board.

-M
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Old 12-08-2010, 11:13 AM
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Oh, and for what it's worth, both of my modules have blown thermal fuses. The bearings (or something else in the motor) cause the motor to draw excessive current. (My new motor is being delivered today.) Excessive current causes excessive heat, causing the thermal fuse to open. The replacement fuse costs about a dollar. I've also got replacement MOSFETs on hand, just in case. (about $4 each)

I'll post the results of my adventure when I once again have heat in my car.

-Mark

Last edited by wirecutter; 12-09-2010 at 08:57 AM. Reason: Need to verify fuse part before posting
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Old 12-10-2010, 08:50 AM
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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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