I got a 97 GMC Sierra 2wd 350 engine. This March will be 2 years Ive had it pretty much trouble free. I hope I can explain this well enough y'all can understand what's going on. Back in October I noticed the torque converter would unlock quicker than normal. the small hills I use to go right up the TC would unlock and lock right back up soon as I was at the top. It done this 3 or 4 weeks and just stopped. It was back normal. I went another few weeks with all normal lock ups and 1 day out of the blue it unlocks going down a flat highway for reason. It done off and on the rest oof that day, the next day it was fine. It got to where once a week or so it would mess up a few times and get back to normal. It has never acted up shifting any gear, never slips, shifts all of the gears like a new one. I also noticed when it is acting up the cruise will not work. Its getting more frequent now and I guess I gotta figure out what is going on. Any ideas? What actually sends the signal to the TC that makes it lock?

Thanks in advance for any ideas

 

I believe the VSS sends the signal to the PCM which tells the TC when to lockup.

I'd start by inspecting/checking your transmission fluid.

 

its fine. looks good, level is good, has no burn smell

 

Honestly I don't know where to start to look here, but unless the actual torque converter is loosing fluid, I would suspect a physical blockage in the hydraulic passages inside the tranny, or an electrical issue that isn't giving the PCM good enough means to keep the torque converter locked.

What I would do, is grab an expensive scan tool and watch the PCM data stream, and look at the TC lockup request. If it's switching on and off, then it's my guess the issue is with the VSS signal. If the TQ lockup request is always on, and the TC still unlocks randomly, then you're still looking at a tranny issue.

Pure speculation though. I'd also call a transmission shop and ask them. Avoid driving it in to ask, because they'll just try to sell you services. See if you can pick someone's brain about it without them having to test drive it.

Sorry I can't help otherwise.

 

My 2001 used to do this. On a flat plane or down a hill it seemed as if it was slipping out of overdive but give it gas and it grabs the gear. I hated it, it did it more when it got hot out. I never figured it out, I had my tranny rebuilt for more power, and it doesnt do it anymore. I read it was the shaft for the lock up gettin worn out, a replacement fixed it. The shaft on the valve body. Ill try to find that article for u.

 

Ive ran it a couple thousand miles or so in the last week. SEVERAL tanks of gas the last 10 days. Friday through Monday it locked up perfectly every time. Even going through the hills with the cruise on. It would go right up the hills and never unlock. I had to go to Memphis yesterday and on the way back it started unlocking again. I'm just wondering could it be computer/electrical issues. Ive had some strange things happen electrical wise on the truck. Back in 2010 a few months after I got the truck. I got off work one afternoon when I started home. The check gauges light was on, The orange indicator light on the gear selector was off and it would only shift into 2nd, no 3rd or torque converter lock up. It drove me crazy for 3 days trying to trace it down. I think that's how I found this board. I was ready to give up and take it somebody smarter than I am. I cranked it, the check gauges light was off and the gear selector light was back on.It shifted every gear perfect. That was almost 40,000 miles ago and its never happened again. I don't have a clue. The windshield wipers don't work sometimes. Ive changed everything and it still does it on occasion. I'm not worried about the wipers but wondering if it is, a electrical issue with the trans could it all be related.

I either dreamed or read somewhere one time that there was a way to straight/hot wire the TC lock up. Anybody ever hear of that? Hell,if I can I will put a toggle switch on it to use on the days it acts up........lol

 

Ok now Im thinkin yours is power related. The ignition switch in the steering column is what powers the trannys electrical. When this build up resistance it will start to fail, then the tranny goes into default mode where only 2nd works. The ignition switch is easy to replace but costs a little money.

 

yea OK, We might be getting somewhere. You talking about the box below the dash, on the steering column where a wiring harness hooks up, not the key cylinder but the part the key cylinder moves?


Is it really really common thing to happen? What I'm getting at, would I have at-least a 50/50 chance of getting a good one from a salvage yard? atleast to try figure out if that is it.. I don't mind buying one. I just don't wanna buy it and have the same problem.

 

Well like all electricals, they build up resistance and then fail, like spark plug wires. So a new one would be the prefered way to go. Do understand Im not saying this will fix your problem, but when I saw your post about only having 2nd gear, this popped up in my mind as a possibilty. And yes the electrical box right below the steering column not the cylinder. I really dont want ya to throw parts at it and become dissapointed. If you could find someone with a bad *** scanner to show the electricals workin while ur driving it would narrow it down.

 

I haven't had a chance to get a ignition switch yet but I do have some changes to report. It locks up pretty much every time now until I turn something on. I can turn the heater fan on, it unlocks and locks back up within 3 or 4 miles. I can turn the wipers on or the headlights and it does the same thing. Does this new symtom make you lean more towards the ignition switch? or less?