The Saga of a Freaking Out Torque Converter

This actually happened on my ‘97 4Runner (3.4/Auto/4x4), but since it has the same engine and trans as a 1st gen Tacoma, I figured I’d post it here. For about 4 months my torque converter has been randomly locking and unlocking, back and forth for about 5 seconds at a time, then it’ll act normal until it happens again… maybe 15 minutes later, maybe the next day, very random. It started the day after I did a bunch of maintenance - New plugs, wires, coil boots, PCV valve, grommet and hose, cleaned throttle body and IAC w/ new gaskets, cleaned the MAF, and a new fuel filter (all OEM parts).

Naturally, I figured the issue must have something to do with all the work I just did. I mean I’ve owned this truck for 22 years, and it started freaking out the very next day. I thought maybe it could be a misfire, and maybe the loss of power was what was making the TC go haywire, signaling it to shift when it shouldn’t or something? What else did I do that day that could’ve affected the shifting? I checked the kick down cable and I adjusted it right were it was before (I marked it even). My trans has been well-maintained with regular fluid changes every 30K or so and I changed the fluid about a year ago. I always use Dex 3 and didn’t have any problems for almost a year since. Not one engine code either, BTW.

Anyway, I tried putting all the old parts back in to try and maybe “reset” this whole issue and get back to the state it was in before. So old plugs, wires and everything back in, I double check and triple check all the wires, vacuum lines, etc… Even cleaned the MAF (with MAF cleaner) again. TC still freaking out, so I swapped all the new parts back in again since I ruled out any defective new parts.

About 3 months go by and the issue is still happening almost daily, and I still haven’t gotten a single check engine light, no pending codes, nothing. I reluctantly changed the fuel filter again to another new OEM one. I did this because it was the only thing left from the day of the tune up, that I hadn’t re-done again. I knew it was a long shot (maybe the 1st filter was bad and starving the engine of fuel, causing the trans to malfunction??). Nope, still shifting funky of course. I have a spare coil pack in my garage, so I tried swapping that into each spot because I was starting to grasp at straws by now, but of course that didn’t solve anything either. It still seemed to me like it had to be related to something I did that day, otherwise what are the chances it was running perfectly for 2 decades up to the minute before I did the work, but it doesn’t have anything to do with it? The weirdest thing was everything I did that day was engine related, nothing to do with the trans.

I thought about what controls the torque converter… I know the trans figures out when to shift and lock/unlock the TC using data from the speedo, tach, and the throttle position sensor (among other things I’m sure). It got me wondering if maybe the TPS got damaged somehow when I removed the throttle body to clean it. So I went to take a look under the hood and I noticed a coolant pipe (a metal “T” coolant pipe) was pressing up against the plastic TPS. Hmmm… could this pipe carrying 200* coolant, pressing up against this plastic sensor be causing it to malfunction? Why wouldn’t it throw a code after all these months though? Maybe because it’s from last century? I dunno. So I removed the throttle body again and sure enough, there was a little mark on the TPS from the pipe starting to melt into it.

Feeling like I was finally getting somewhere now, I started researching bad TPS problems on 3.4s and I found a thread where a guy said his TC was locking and unlocking randomly like mine for weeks before he figured out it was a bad TPS. His never threw a code either, but he figured it out by watching the TPS on his Scangauge and he noticed a glitch in the 20-25% range, where it would drop by ~10% momentarily for no reason and cause the TC to lock/unlock. I tested my TPS per the manual with a volt meter, and it seemed to read fine thru its range, but again this problem is intermittent - 99% of the time it works right. So I figured between that dude’s experience and the burn mark on mine, screw it, I’ll just buy a new TPS and roll the dice. I hate shooting the “parts cannon”, but I also hate bringing an intermittent problem to a shop, because never in my life has a vehicle of mine ever done “the thing” when a mechanic is watching. Murphy’s Law. I’m an okay DIY mechanic and at this point it had become personal, so I was DETERMINED to figure this out, and without paying a mechanic $100/hr.+.

So I put the new TPS on, which was about $65 at Toyota. I also cut a little piece of coolant hose and zip tied it around that metal coolant pipe so it won’t fry the new TPS too. Well, guess what? Problem solved. The trans shifts perfectly again, every time. I was really starting to question my skills on this one, but I feel like I redeemed myself. This was a tricky one, without any real hints, and very inconsistent and random. The TPS was actually on my list from the start, it just wasn’t on the top of my list because I didn’t specifically mess with it the day of the tune up (which seemed to kick this whole saga off). I still don’t know if the coolant pipe has always been touching the TPS (and heating it up)?.. For 26 years tho? Doubtful, then again that pipe doesn’t really have anywhere else to go either, so I dunno. Did I possibly bang the TPS around and damage it as it was attached to the throttle body when I was cleaning it?? Was the TPS going bad just a total coincidence? Who knows? I’m just glad it’s finally running like a proper Toyota again.

Between the new TPS and the 2nd new fuel filter, I’m only out about $100 bucks (and several hours of head scratching) over this ordeal. So at least it wasn’t a huge hit to my wallet. I’m sure I saved a lot of $$ in diagnostic work too. I have my doubts most shops would’ve figured it out much easier than I did. Anyway, thanks for reading my novel. Hopefully this will help someone else out someday, and save you from going on the same wild goose chase I just got back from!