Transmission Switch????

gw1500se · November 6, 2015

I purchased a used 2000 GMC Sierra 1500 4X4 long bed (4.8L) and found that occasionally the shifter indicator was off by a notch (e.g. when in D the indicator showed N). Also the backup lights, when in reverse, would not come on but I could see them flash occasionally when moving the shifter to P. I thought this might mean a problem with the reverse switch (if there is one) on the tranny. I ignored it for a while and mention it now because it might lead to a diagnosis of my sudden serious problem. Also, on occasion when riding a speed, the doors would unlock, which again seems to imply a transmission indicator issue. Then the seemingly unrelated repair after which things went crazy. I turned the brake rotors and replaced the pads. Now the ABS light comes on, the brake warning light frequently flashes but not when actually braking and it will not shift out of 2nd. The fact that this followed the brake work is probably just coincidental since there were symptoms prior. Hopefully you have a some idea what this might be (control solinoid?). TIA.

gw1500se · November 7, 2015

After some research, it appears that when it runs in 2nd only, it is called "limp mode." That means replacing the 2 tcc solenoids. However, I did not find any indication that these also signal the computer to turn on backup lights or unlock the doors. Nor have anything to do with ABS. Am I on the right track with all symptoms or am I looking at multiple switch problems? I only want to pull the tranny cover once. TIA.

jimlem · November 8, 2015

It's no big deal. I had the same exact symptoms after a AT replacement. Tech shattered gear selector box during re-install. Easy fix.

On the D's side of the AT, there's a sort of flat black plastic box. The trans linkage hooks up to it and there's 2 (IIRC) wiring plugs hooked up to it. This is the controller for your NSS and and all gear positions P thru D1. One big clue is the door locks locking and unlocking as you drive down the road. Another is the gear reading being off by a gear or so. Unless of course you have a violent poltergeist haunting your truck. If so, don't trust any jack stands.

The new part is about $40 max I believe (I did end up with my 1st new one being defective) so keep it in mind just in case. R&R the old for new. There's 2 bolts in 2 slots holding it in place. One is a crescent shaped slot to adjust it to match up with the physical gear locations so don't tighten either bolt 100% right now. Have a helper sit in D's seat turn key far enough to allow AT lever/linkage to move and gear selector to show. Park is at one end of the curved slot and 1st at opposite end. You can get a feel for that by watching linkage, on the old one. when in park and again in 1st.

Get a helper to assist in moving gear lever while you watch linkage. Confirm by manually moving linkage a notch at a time and have helper call out gear reading. Avoid temptation to use a 6 year old son as your helper. By trial and error you will fine tune each linkage/gear position, both at the linkage end and the gear lever position readout. When satisfied, carefully tighten two bolts on the electronic box on AT so as not to slip out of alignment. Drive and watch for gremlins that occasionally pop up and go back under to fine tune them. I had to do this twice just to be sure my OCD self was satisfied.

Depending on how well you and your helper communicate, it can easily be a 30 min. job. For the rest of us it's a 60 to 90 min. job. easy to do w/o jacking up truck plenty of room to work under there. Be sure to firmly set P-brake and chock a wheel or two as you be in neutral many times.

Good luck, be patient and it's just a walk in the park. Let us know if any of this was helpful.

Jim

gw1500se · December 6, 2015

Just to close this out for someone that has a similar problem, Jim nailed it. When removing the old switch we found that the rear bolt was missing and the switch was loose. This was likely the cause of all the problems. We replaced the switch anyway including the missing bolt and that solved all the problems. A big thanks to Jim for his daignosis.

However a word of warning to anyone that tries to do this. Because it is a 4X4 there is very limited space to access the switch due to the driveshaft location. The most difficult part was unplugging the connectors from the old swtich without breaking them. There is really no good way to grab them and, at least mine, were extremely tight. We had to actually destroy the shroud on the old switch body to get them apart. The rear bolt was also therapy work because of the difficulty accessing it. We didn't have to remove the old one because it was missing but I can see why whoever worked on it before did such a lousy job that it worked its way out. Be sure to use a thread locker when putting it back in. Finally replugging the connectors was not easy either. Partly because they were hard to grab and because it takes so much pressure to get them locked in. This is not a particularly complicated job but a very frustrating one.

gw1500se · November 8, 2015

Thanks for all that. However, will this cause "limp mode?" Am I still looking at also replacing the tcc solenoids or does your suggestion fix everything? As for your NSS controller, I have no idea what that is. Nothing shows up on parts stores web site for that. What else might it be called?

gw1500se · November 6, 2015

Something I forgot to mention. While all this is going on the speedometer bounces around and then goes to zero. At some point it will start working again.

jimlem · December 8, 2015

Sorry I missed the 2nd question and causing confusion, I was away for awhile. NSS was meant to be neutral safety switch. They probably call it something else now. Glad it worked and I was able to help. BTW, did you end up having to replace tcc solenoids? Or did the repair cure it all?