Pesky P1780 (1 Viewer)

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Joined
Jun 21, 2020
Threads
2
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Location
Vancouver, WA
Hello all,

I have been chasing issues associated with the P1780 code being thrown out. Exhausted all searches here on mud and completed the tasks they presented with. Hoping to get a direction to go from those who may have been there before me.

Here's what's going on: P1780 will come at random while driving or sometimes immediately when I put it into 2 or L. Zero cruise control and leading up to the codes being thrown I had intermittent issues with it. When in Low gear it will intermittently kick in and out of that gear. No issues starting and except for the burnt out D light all gear selector lights are on as they should be.

Zero corrosion in the connector housing to safety switch or connectors near firewall. Safety switch itself is 100% good to go too. I'm currently trying to work through the FSM troubleshooting, but I am a new to working with a multimeter so it's slow going.

Thoughts on this? Thanks.
 
I just had mine come on and it had to do with the alarm system. Someone mentioned on another thread about the relay in the alarm system potentially being the issue. I pulled my alarm out and the code went away and cruise control is back.
 
I just posted this exact issue with my '96. @LuckyLong1720 did you ever get this issue resolved?
Faulty ground with the kill switch connected to my ignition. Lucky find actually, but I narrowed it down to wiring in steering column/dash area. I was driving around with the dash apart, pulling on wires with the truck in the then defunked low gear. I happened to accidently bumped the kill switch which kicked the low gear on.
Months of stress gone in a second.
Start checking for shorts to ground. Especially if you have an aftermarket alarm connected into the ignition.
Good luck. Let me know how it goes.
 

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