code 1780 giving me fits

Hello friends, I'm having fits trying to figure out a wire issue in my 2001 Tacoma, 4 door, extended cab with a 3.4 v6 motor..The problem started after I replaced the windsheild and gas tank....the truck sat all last winter with the battery unhooked. This spring I installed a brand new gas tank, charged the battery hooked it up and turned the key and like always the old gal fired right up. A few minutes of running i noticed the alternator pulley wasn't spinning so I shut her down. A shot of wd40 and stout turn with a pipe wrench the alternator freed up and spun and was putting out power. All seemed good for a couple of days until I was backing out of my driveway and the CEL popped on. My nephew stopped by and put his code reader on it and found that it was PO1780, and at the time I noticed the back up lights weren't working. A search on the internet listed the cause as a neutral safety switch malfunction and the no back up lights was a likely cause. I pulled the taillights apart cleaned the connectors, bulb sockets , checked the wiring and ended up ordering and replacing the whole taillight unit on the passenger side. I also removed an old trailer wire harness that was tucked up onto the spare tire. Everything works as it should, reset the code and a day later the CEL returns. I took her to the local mechanic and he said the Park neutral safety switch was bad and needed to be replaced. They replaced the PNSS and the transmission cooling lines they broke trying to get the safety switch off. I paid the bill and as I was leaving the mechanic says "by the way that code in on again". Ive checked out and cleaned the shift interlock devices adjusted the park neutral safety switch and have been looking for defective wires. I have it down to the code only trips when the shifter is put in REVERSE. Im pretty sure its an open circuit in the reverse wire....Pulling the wire harness looks to be a bear of a job. Wondering if anybody else has ever had this problem and what the right fix is. Thank you in advance for any helpful suggestions.

 

Better to understand the circuit before changing parts, I have attached the 96 wiring manual, hopefully its close enough. I would say that you should verify switch operation at whatever module it ends up on, probably the body control module? I assume you have a multimeter or rig up a test light.

It must also go to the ECU to cause a check engine.

You might also look around/under the truck for critter nests, a truck sitting that long almost always gets one although usually seems to be in the engine area and I suppose you or the mech would have noticed by now.

I had a coworker who knew a guy that had a auto electric shop. He charged by the hour and most of his business came from dealerships. A general mech can't spend a lot of time debugging these issues although yours should not have done the work if he wasn't sure of the fix. Unfortunately this is common.

 

Mr alexh, Thank you kindly for the wiring maps, they are awesome and Im sure they will be very helpful. I do own a multimeter, a test light, testing leads, and now a code reader. So far I checked the fuses and tested the relays. I've also pulled apart most of the connectors and checked for corrosion. I will start a more concentrated search using the wire diagrams you shared..Thanks again and best regards

 

Hello Friends, After an exhaustive hunt I finally have the PO1780 code resolved. Thank you again Mr "alexh" for sharing your information. Regards