The first thing to check with no O/D is the coolant temp sensor. On my 94 there is an additional temp sensor for the transmission on the coolant bypass block (along with the other sensors). Other years may be equipped differently. That sensor, among other things, keeps the A340H from shifting to O/D with a cold engine. So if it fails ....

For fourthyota, 12.8volts is too low for engine running. http://personal.utulsa.edu/~nathan-b.../2onvehicl.pdf (but an after-market auto voltmeter never comes with a calibration certificate, so before you start chasing that I'd get a regular voltmeter (even a cheap HF one) and compare readings.) While you might have a weak alternator, I think you've got a wiring problem somewhere. The transmission should shift even with a dead alternator, with just battery (about 12.4-12.8v), so I'm guessing you're getting a big voltage drop at the transmission ECU after your voltmeter.

 

• UPDATE!! UPDATE!

So here's the deal...with Winter on the way up here in the Panhandle, I figured starting at the alternator (original one) and working outward couldn't hurt. I had to replace belts anyway (Squeeeeeeeeeel) so I started there, and with 277K on the clock, I figured a new alt for the Winter couldn't hurt. Unfortunately while the new belts made my 'rnr considerably quieter, the new alt didn't remedy my shifting problem.

So I started checking connections as recommended by Scope103 (and others) and found a couple of loose nuts at the negative battery terminal. Tighten, tighten, and OOOOPS...



Like a good yota owner, I have a *few* spare parts in the shop, and actually had a new terminal. So cleaned the battery post and slapped the new terminal in, and BAM, got 14.4 volts with just the engine running, and so then I cranked up the headlights, put the heater on high, and turned up the stereo. NO SHIFTING ISSUES. Shifts like buttah again!!

Thanks to all who helped. Hope this thread helps others. Was thinking about an upgrade to a 2nd gen, but I guess I'm keeping her now.

 

Thanks for the update, for sure is gonna help somebody.