P1780 issue

[MillerPKA]

1997 3.4 auto

This code came up last week and when I plugged the obdii scanner in the light went away about 30 minutes later while driving. i just figured to forget it for now. well i'm leaving for work yesterday and my roommate calls and said when i was leaving my reverse lights didn't come off, sure enough i pull into work and check and they wouldn't come off. drove up to advance again to make sure it was the same code, she plugged it in, confirmed it was the same code, walked to the back and said the reverse lights weren't on anymore. i walked back to the driver seat and the check engine light was still on. i said thanks and drove away, less than a mile after leaving the cel went off.

so now there's no issue but i feel its going to come up again, i've done some research but didn't see anything too conclusive as to what i should do. i read up and it has to do with the neutral/reverse switch. also, for some reason when i reverse i have a tendency to pop it into neutral after i have enough momentum to roll back, do you think this could be a cause? any reason i should stop doing this? thanks a lot guys

[Good Times]

The P1780 code is related to a malfunction with the park neutral position switch. Based on what you're experiencing with the reverse having a tendency to pop into neutral I'd say it's definitely the switch that's causing the problem.

I believe the switch is located on the tranny behind the shift selector lever. If you can find that switch, try removing it and cleaning it as it could be dirty. If that doesn't fix it try replacing the switch.

Good luck

[MillerPKA]

thanks for the insight Lance. I crawled up under there and there are 2 sensor plugs behind the lever and 2 more behind the side. I don't feel like driving it so I can't get the exploded view from the dealership right now, but according to the guy on the phone its towards the front of the axle where it bolts to the engine?

my guess is if the check engine and reverse lights are coming on and off it must just be a dirty or loose connection.

[corax]

It could also be a misadjusted shifter cable

[Good Times]

Didn't think about that simple check. Check the cable first! :) Hell that might be an easier check rather than digging up there to clean up the switch.

[MillerPKA]

I checked the cable, seems to be alright. Located the switch, took it off and took it apart. Don't really know what a working one is supposed to look like but this one looked alright, cleaned the copper contacts, housing and connectors and reinstalled. RPMS at shift are higher than normal still but reverse lights were off. Went to advance and cleared the code, pretty sure the reverse lights were on when I was leaving. Just tried starting it up and no code and no reverse lights, but I have a feeling it will be back soon, guess I need a new switch. Really pisses me off a few copper strips and plastic is 250 bucks

[MillerPKA]

Okay update...sort of

Got a new switch and picked it up yesterday at my shop in Atlanta. Installed the new switch and figured out that wasn't the problem, the code popped right back up. Pulled one of the shop owners out and had him check it out....we are both confused but we did come up with something conclusive. We plugged in the obdII scanner to it and cleared the code, we found out that every time I turn on my headlights the code comes back on, and with it the reverse lights. I had severed 2 wires up underneath the dash (light wires, go figure) but when we figured this out we took my temp splices I had rigged in there and did some proper rewiring. The light stays on and cannot for the life of me whats causing this. The only thing that was conclusive is that somehow when I activate the headlights it somehow throws a p1780 and to some extent messes with the switch and corresponding pnp. Sorry if i'm rambling i'm a few down in pre thanksgiving screwdrivers. Hopefully someone has some insight cause I am completely stumped.

Happy Thanksgiving everyone!!!

[YotaFun]

I know this may sound weird, but what about the headlight switch/turn signal stalk?

I know its a different vehicle but on my olds cutlass, the turn signal stalk went and I would sometime loose my brake lights or even my running lights...

Just a thought...

[MillerPKA]

Yeah honestly anything could be the case right now I'm up for any possible causes. Besides the reverse lights none of the problems are sporadic. The head lights come on every time I turn them on and have yet to fail, same with the tail lights, blinkers and brake lights. Just this damn reverse light. Any way I could test the stalk to see if there are any problems?

[Seanz0rz]

ill grab you a test how to tomorrow when i get to my real computer.

[MillerPKA]

ill grab you a test how to tomorrow when i get to my real computer.


let me know when you get this i'll see if i can test it when i get back from work

[Seanz0rz]

unfortunatly the fsm doesnt really have the info i hoped it did on the stalk switch. it does have this however:



The park/neutral position switch go on when the shift lever is in the n or p shift position. when it goes on termal NSW of the ecm is grounded to the body ground via starter relay, thus the NSW voltage becomes 0. when the shift lever is in the d, 2, l, or r pisition, the park/neutral position switch goes off so the voltage of the ecm terminal NSW becomes battery voltage, the voltage of the ecm internal power source. if the shift lever is moved from the N position to the D position, this signal is used for air fuel ratio correction and for idle speed control, etc.



detecting condition: after confirming DTE P!&*) use the toyota handheld tester to confirm the PNP switch signal from "current data"



pm me your email addy and ill email you some scans of the FSM that will help you. i will not post them here, sorry (copyright reasons).

[MillerPKA]

Thanks a lot man, I had found bits and pieces of that online but its good to have in its entirety.

Found another odd problem that should lend another hint as to what the problem is. Noticed it today by chance but every time I shift into reverse the dash clock light dims as if I'm turning my lights on. I'm about to finish the semester so I don't have time yet to really diagnose, if anyone has any opinions shoot em at me!

[L33T35T Tacoma]

I probably way off here, but it sounds like you're battery voltage is low (no charging voltage) when the reverse lights go on so you're losing power everywhere else. Just make sure you have 14ish volts when the truck is running, simple things first.

[Seanz0rz]

Thanks a lot man, I had found bits and pieces of that online but its good to have in its entirety.

Found another odd problem that should lend another hint as to what the problem is. Noticed it today by chance but every time I shift into reverse the dash clock light dims as if I'm turning my lights on. I'm about to finish the semester so I don't have time yet to really diagnose, if anyone has any opinions shoot em at me!



after hearing this, my verdict (with out looking at anything or knowing your vehicle) is you have knackered dash wiring somewhere. do you have an aftermarket (or hell, even a "factory installed" alarm? any other accessories tapped off of headlight wires or backup wires?

[MillerPKA]

There was an aftermarket alarm installed by the p.o. but I had it deactivated when I got it 3 years ago. About a year ago I did cut some wires up underneath the dash in a failed attempt to wire my roof lights to come on with the brights. Ended up patching the wires and haven't had an issue since. In the months leading up to this i haven't done any wheeling or any erratic driving to prompt any of this being a factor, just in case redid the connections properly when i went up to atlanta to put the new switch on which changed absolutely nothing. This reverse thing definitely has me focused in a direction which is good, any wires in particular (hell even fuses) that I should check for this? I'll have the battery tested tomorrow too thanks for the tip

edit
Rechecked the connections on the taillights and under the dash, repaired several on the tails for good measure. put the reverse bulbs in and they're working like they should. dash clock doesn't dim anymore when shifting into reverse and rpms have seemed to calm down. the cel remained so i cleared the code, hasn't popped up again yet we'll see how long this lasts. i'm really gonna kick myself if it was trippy wiring

[Seanz0rz]

goad you got it worked out maybe.

ive seen so many hack jobs on alarm installs, even by "professionals". my moms car has an alarm from circuit city. now the door locks wont work when its cold. if its warmer but not hot, the door locks will work, but not the keyless entry. if its hot, then the keyless entry works.

[Crinale]

circuit city, best buy, installers are not professionals... they are usually high school/college kids that had 1 day of training on wiring... i wouldnt trust them with swapping speakers in my doors, let alone wiring up an entire alarm