Trouble diagnosing 1st gen p0705 and p0850

This is my first post in Tacomaworld!

I'm hoping I can find some help. I recently bought a used 2004 automatic extended cab 4x4 tacoma. The check engine light was on and the codes were p0705 and p0850. The truck ran well in all gears and the 4x4 worked. I have been trying to diagnose this code and have learned a lot about it. I have also checked and replaced some things. here is what I have found.

1. Replaced P/N switch. Didn't solve the problem (used)
2. Replaced ECM. Didn't solve the problem (used)
3. Tested wires from switch to ECM for continuity, all good.
4. replaced junction block 3 under driver dash to the right of steering column. Didn't solve the problem (used)
5. Serviced the transmission with new fluid and filter. Didn't solve the problem.

I see that a lot of people have water/corrosion inside the left kick panel junction on the 2nd gens. I don't know if there is anything there to look for on the 1st gens.

Each time I've done something to attempt to fix it, I have cleared the codes. It is a two trip light, and every single time the code has come on immediately when I shift out of park on the second start.

Again, there are no noticeable symptoms. The truck drives, shifts, lights, blinkers, all work.

Any advice or help would be appreciated.

 

sorry if this is redundant information.

 

That is helpful. I've seen the wiring diagram, but not the checklist on troubleshooting. Thank you.

 

in reading the fault that causes these codes... it seems too simple to be giving that many issues. There is a difference between the 3.4 V6 and the 2.7 4 wiring... which motor do you have?

 

It does seem simple, the fact is it isn't causing a single issue I know of, other than the code tripping the light. I can't let it go though. I have to figure this out. I have the 3.4 v6.

 

for mine, I had a relay connected to the reverse light that threw the code and would not allow me to "shift" up when using throttle but would "drift" fine since it was already in 3rd gear

check for an errant wiring harness or trailer wire harness that might be causing back-feed into the circuit

https://www.tacomaworld.com/threads/p0705-error.593307/

 

I have tried unplugging the wiring harness for the trailer which didn't change anything. Do you think it could be further up the wiring? I'll put that on the next place to look list. Thanks

 

always could be, wiring is never fun to track down a bad wire :\

 

So I investigated the trailer wiring connector. I used a multimeter and couldn't detect any current. I went behind the driver's seat and looked at the wiring for the trailer.


There was a black box the large wiring connector plugged into and four separate 5 amp fuses labeled: left turn, right turn, park, reverse...something like that. One of the fuses was burnt out.

I didn't test the box.


I then used the multimeter to check this connector for power to the trailer wiring.

It had power to a couple spots, and "blinking" power to two spots (I had the caution lights blinking and lights turned on). This makes me think that the power to the trailer wiring harness is good. I don't have any 5 amp fuses on hand so I'll grab some of those.

The real discouraging part is that with the wiring completely unplugged I reset the codes. Started and put the truck in each gear. Shut it off and on the 2nd start as soon as I put it in gear the code p0705 came on. This makes me think that the problem with the trailer wiring harness can't be the cause.

 

It is fixed! There were two issues. One I probably caused while trying to fix the other. As part of trying to remove the codes, I replace the p/n safety switch. When I did so, I did not properly align the switch. There's a mark on the transmission and the switch that must be aligned. It was reading correctly in park, but not neutral. This was verified by trying to start it in each position. Before adjustment it would only start in Park, after adjustment it started in neutral as well.

After clearing the code, the p850 was gone, but still had p705. We removed the driver's side dash and disconnected the cruise control. It looked aftermarket, and low and behold the code did not come back. We knew through testing connections in the wiring that there was some voltage feeding into the system. With the cruise removed it worked like a charm.
This cruise control never worked since I bought it.


These are two connectors that plugged into the cruise control wiring that I removed.
These are wires that were left after I removed the cruise control switch from the column. The red connectors plugged into splices to the ignition and something else...

 

You are correct, that is aftermarket cruise control. It was good of you to look at anything aftermarket installed with wires/circuits, or even dealer installed when truck originally sold, highest probability of the issue would be with those things. So good job on the sleuthing hermano

 

Yep when you take your truck in to add a feature like cruise control, etc. They never add it cleanly and usually mess up the reliability Toyota created.