2004 Tacoma - Weird transmission issue

Also try it with your headlights off and on

 

I went cheap on this truck, no cruise control installed.

--Dan

 

Same with or without headlights.

--Dan

 

I can feel the truck hesitating acceleration (surge), it's not just the needle movement. I have a voltmeter and will do some testing when I get back from work today. I will report back what I find.

Thanks for all help so far.
--Dan

 

Also check and see if it does it if you turn on your hazard lights.

 

If it had a trailer wiring harness, cut it off and see if it still does the problem. I'll put any amount of money on it that its that, or one of your bulbs has a filament out.

 

I am back from work, so now I will get busy testing out everything suggested. My truck does have a trailer hitch and harness that was dealer installed since they did not have 4X4 on the lot with the towing package from Toyota.

I will eliminate things one by one, and report back later tonight once I am done with the testing. Thanks!

--Dan

 

I'll put any amount of money on it that its the wiring harness that was installed, my dealer one took a crap. But let us know what you find out it drove me nuts for about 6 months before I finally started looking.

 

Sir I owe you a million. You were dead on, as soon as I disconnected the crappy wiring harness the truck was working like a champ.

You know what is worse? This was all installed at the dealer (Freeman Toyota in DFW area) and they freaking installed a Uhaul trailer light thingamabob. I cannot believe that they put a Uhaul wiring harness in there when I paid the dealer for a Toyota one, but I signed so many things who knows if I gave them the right to install this crappy stuff.

I bough this truck in 2003 when I was 19 and it was my first brand new truck, so I was excited and probably did not look at the paperwork much.

Anywho bellassaiw10 you saved me a ton of money, who knows if the Dealer would have figured this out and how much $$$ I would have had to pay.

--Dan

 

That's cool. Glad you got it fixed.

 

Probably not the dealers fault the little controller usually goes bad I had a Uhaul installed one. Cut it out and installed a universal one from the local auto parts store

 

OP, thanx a ton for posting up the solution. I never would have thought of this.

I still can't understand why an un-related electrical system would affect the tranny shifting in and out of OD. Are the trailer-related wires in close proximity to the tranny harness, and shorting?

 

I think it has something to do with the eay toyota used grounds for their electrical

 

I have a 96 tacoma with v6 auto tranny 35 inch tires, had 3:91 gears just regeared to 4:88 now I have a simalar problem but I drive almost all the time on two lane country roads so not much lane changing, since the gear swap around 60 mph my tranny goes back and forth from over drive to passing gear, what could be the problem, when i turn my over drive off it's actually is a lower gear than the tranny is jumping back into from overdrive?

 

I usually ride my motorcycle, but since it rained this morning I drove the Taco. Still getting the stuttering with the turn signal off. I will disassemble the tow harness this weekend and report back. Thanks OP and TWorlders for this informative and intuitive thread.

 

Reviving an ancient thread. I'm having the exact issue described by the OP. Did anyone figure out how the turn signal wiring is related to the transmission wiring?

 

I have a ton of issues with my trailer lights and my cruise control doesn't work, and my truck has an aftermarket light connector spliced into the tail lights for the trailer wiring.. after reading this, I'm getting the feeling that the cruise and trailer light issues are connected...

 

I may have a lead here. This is from the Wiring Diagrams in the FSM regarding the 4th gear shift (aka lockup, aka overdrive). I've highlighted the sections that I think are key:



This is my current theory: The turn signal wiring is not grounded properly or shorting. The stop switch circuit has a residual voltage. This voltage is causing the transmission to be "tricked" into thinking the brake is being applied, and thus takes the transmission out of overdrive, causing the fluctuating with every turn signal flash. This short could be explained by faulty trailer harness wiring or shorting, as the trailer harness typically taps off the rear light housings.

I think with this info, I will be checking the grounds, stop switch, and perhaps poke around with a multimeter to see if I can find the stray voltage. I've added some relevant wiring diagrams if anyone cares to follow:

 

See my post above. If the truck is "tricked into thinking" the brakes are being applied, it would cancel cruise control.

 

Definitely did not update like I said I would. Here's what happened:

With the truck in the garage, I was able to see that when my turn signal or hazards were on, a faint illumination of the brake lights could be seen which synced with the flashing of the turn/hazards. This apparently was enough to signal the transmission to unlock the torque converter. The results of this being a "pulsing" of RPMs on the highway that are synced with the turn signal flash. This would also cancel cruise control as the truck is interpreting the brake lights on as "brake applied". Once I cut out the shoddy wiring job of the trailer harness the issue disappeared, no more transmission issues.