2002 SD 7.3 shifted from 2H to 4L on its own?
 

Had a new gremlin show up tonight.

Driving on a straight, smooth, flat stretch of road about 60 mph, I feel a little jerk, and then hear gears grinding. I saw the 4x4 and Low Range (or whatever it says) lamps on. I kicked it into neutral, and as I slowed on down it finally slammed into 4L.

I rigged up a 12V jumper wire and tested the motor by unplugging the 2-wire connector on the pass side of the ESOF actuator. It would try to turn but I never got the t-case out of 4L. I took the back roads on home.

I understand that I can pull that module and shift it back to 2H. actuator

What would cause this, though? I know of another truck that did the same thing, but it was built from a donor cab / bed swapped onto a rolling chassis. I chalked that up to a wiring gremlin. Should I start chasing electrical stuff, or is it a common thing for the ESOF actuator to do this?

Is there a manual shift lever retrofit?

Any guidance appreciated.

Thanks

I found the two relays - I think, in a small mount next to the master cylinder, had an orange and yellow heavy conductor from them which appears to go to the actuator motor. Anyhow, when I pulled them they were warm to the touch. I have 12V across both coils - (85 and 86) - a schematic I found indicated that the 86 terminal was full-time energized and the 85 terminal has ground applied to suck in the relay. That is, term 86 to chassis reads 12V (as expected), term 85 to hot reads 12V - I wouldn't expect this.

Anywho, I pulled the actuator motor so I could shift to 2H. I applied 12V to the 2 terminals in both polarities, and got no movement. I removed the plastic motor cover and the motor smells burnt, and the varnish has cooked out, also indicating this.

I assume the 4x4 control module behind the glove box supervises the operating of these relays and actuator. So, is this thing failing a common occurrence, or should I start tracing the wire harness and look for unintentional grounds?

The schematic I found is here: https://goo.gl/images/QS2MNu

TIA!