Just picked up a 92 mighty max 3.0 4x4 with a low 54k miles with no a/c from the factory. Everything works great except the blower motor.
The trucks wiring is unmolested save a trailer brake/light module that taps into a wire leading to a switch on the clutch pedal and a wire coming of the positive battery terminal, but otherwise seems to be self contained. The ABS light is illuminated, which I'm guessing is related to this.
The ecu was replaced by a mechanic at some point.
When I got the truck the radio fuse was blown, after replacing there's been no issue.
I've checked the battery, fuses, and fusible links, all test good. The rest of the fuse block gets power, but nothing at the "heater" fuse.
Welcome to the site. Nice find on the 92MM! With only 54K that's quite the score.
Going by wiring diagrams for a '91 (and assuming they're the same for your '92), power to fuse #9 (heater) and #20 (rear anti-lock brake system) is from the ignition switch via a blue wire with a red stripe. Test for power to fuse #20. If no power to 9 and 20, test the ignition switch for power to the blue/red wire with the key in the "ON" position. If no power, check the switch. If power, disconnect the ignition switch connector and check continuity of the blue/red wire between the connector and fuses #9 and 20. If there's no continuity to either or both, check the fuse box connections of the blue/red wire, then look for a fault in the wire.
Thanks FMS. There is indeed power to the #9 and #20 fuses (I'm not great with electrons). One of the ground wires on the blower motor was installed incorrectly when it was pulled to replace the ECU. Blower motor works perfect now.
The ABS light is still lit up, need to look into that more. Next step is to remove/clean up the trailer brake module and wiring, while still maintaining the aftermarket trailer light pig tail.
Glad to see the blower was a simple and easy fix. Hopefully the ABS will be the same. I have no experience with the ABS on these trucks, but here are the error codes and check procedure from the '91 FSM. Hope you can make sense of the procedural steps given all the copy editing errors.
Thanks FMS. I have the Haines and Clymer manuals, the Clymer has a significantly more in depth section on the ABS system than the Haines, like pages covering every component of the system is broken down with what it does, how to test, etc. Interesting note, ABS is only for the rear wheels on pickups, the SUV's have ABS on all wheels and a have a dedicated module.
I removed the 2 (two) separate trailer connections, one 7 pin and one 4 pin. My hope was since they tied into the brake pedal and tail light wiring there was a bad ground or something causing a fault, but no such luck. I also hooked an oscilloscope up to the ABS diagnostic connector, and receive the 'normal' wave pattern.
My next step is to individually test each component of the system, of which replacement parts are, to put it mildly, hard to come by. I found a 'brake modulator' which modulates the braking force some how for over $1000 on a Mopar site.
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