So I came home from being away for a year and my truck was stuck in 4L. Ok, motor looked swelled so I replaced that because at the very least it appeared on it's last legs. Well, new motor wouldn't switch gears. I finally discovered that the truck is stuck in 4L. I can manually power the motor with a Power Probe to switch it into 2H. No indicator lights on the dash operate whatsoever. As soon as I hop back in the cab with the tcase hooked up it goes to 4L. Gem module sees the signal from the switch, so I figured no output from gem, and they're usually a culprit at that point. Swapped that out. Still doing the same thing. It's definitely 4L, with hubs locked. I hate firing up the parts cannon and I feel I started that with replacing the gem module. So I'm looking for suggestions on where to look next. I'm aware of a relay pack in the dash behind the radio.... Pretty sure I check that but unsure as it's been some time now. Any chance a factory service manual is online for free download anywhere?

 

Not exactly factory, but it's free and has everything.

https://drive.google.com/file/d/1-4d...ew?usp=sharing

 

You should take time to understand how the system works first.
Then observe what you have and relate to the system operation before going into parts replacement.
Here is what you need to know.
The dash switch 'request' the Gem to perform the first operation 4H by closing a relay to run the case motor to the 'first position, then stops.
The fire wall solenoid closes engine vacuum to the front actuator. This actuator slides a splined collar to lock together the passenger side CV axle.
There are no operating Hubs on these trucks.
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Here is your catch: 4L cannot normally be entered unless the shifter is in Neutral, your foot on the brake and truck is not moving.
If your sure it's a 4L issue, the case motor would have to run to it’s 'second' position for that to happen. In this position, the case engages the lower gear train ratio and changes the motor shift sked. so the engine does not RPM in order to shift the transmission from first to second etc in low range.
If your sure the drive line is in 4L, the case motor would have to be in it's second position as explained above.
If that is so, you have a power issue that has operated the dash 'relay 'to make it run to the second position out of normal control by the GEM..
Be sure what condition you have before replacing any parts.
Find out why, then replace as verified.
I would investigate for a leaking windshield that has dripped onto the GEM module.
This situation can cause a lot of different issues from circuit board corrosion.
There is more to this than meets the eye so consider how it works first.
Good luck.


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Thanks @Bluegrass So I've replaced the GEM because I did see it communicating with the switch. I had replaced the motor because of how rusty and swelled the motor case was. So really I'm only out the gem module. Believe I had tested continuity to the relay, but since I have the p1820 I would start to suspect the relay. Now I also have the P1838, and that leads to a motor. Wonder if the relay is bad and caused the motor to burn out. I'm gonna retrace my wiring and check for shorts, but a short to power is usually rare. Just wish there was a difinitive way to check that relay module for operation.... (as I can't find a good breakdown)

 

The code 1838 is the result of code 1820
1820 is the relay/circuit open or shorted to ground.
`1838 is the case motor did not move. It can'r operate if there is no power to it.
An open can't supply power. A ground could blow the supply fuse.
Doubt it has damaged the motor but would blow the fuse first.
The circuit has to be fused so it cannot cause a fire, by original design.
You know pretty well what's going on, now.
Good luck.