96 tahoe transmission issues

[reverend jim]

My 96 tahoe sport started having transmission issues yesterday and I'd like to narrow down the issue if possible.

The truck starts ok and seems fine when cool but, at temperature, it seems to be starting in 3rd and will not go into OD. At slow-down, the truck does not seem to downshift. I pulled codes of p1860 - p1864 - p1886 at a local parts house and then cleared them but, on the way home (1/2 mile into a 1 1/2 mile trip), the tach and then speedo were only working intermittently.

A friend said that it's almost certainly electrical (sensors) and not mechanical but I really have no idea. Any thoughts? Thanks.

[MT-2500]

IT IS ELECTRICAL but
Codes do not say sensors bad they say check sensors and wiring circuit.
1860 is PWM solenoid or wiring circuit problem.
1864 is tcc solenoid or wiring circuit
1885 is 3/2 solenoid circuit or solenoid..
If speedo or speed sensor quiting will cause transmission to go into limp mode
also PWM solenoid or wiring circuit.
Read out and test them speed sensors on scanner.

If all three codes come right back check the fuse and power feeds and wiring and plugin to transmission solenoids.
MT

Recheck codes and see what has come back.

[reverend jim]

I appreciate the info. looks like loose wires, bad connections, fuses and then the speed sensor will be my first go-round. Where is the speed sensor located on this vehicle?
I did a brake job including a new master on this truck within a week or so, any wiring in that area of the engine compartment that strikes you as being a clear possibility?
Same friend that told me it was electrical and not mechanical said he's come across bad battery cables causing problems like this. Sounded odd to me. Thoughts?

Thanks again.

[tempfixit]

Yes check your battery cables. corroded cable ends will create many problems. Take the cable with 2 connectors completely apart, the bolt may be corroded and the cable ends may have a varnish on them. Cheap fix if that is the case.

Good Luck

[MT-2500]

The input speed sensor will be on back of transmission or transfer case.
Front wheels and some rear wheels will have speed sensors for abs brakes.

Has any codes come back?
If so what codes?
Is Transmission still not shifting right?

[reverend jim]

The batery cables were fine and have recently been cleaned and the studs replaced.

As time was critical, I was forced to let a local transmission guy work on it. The 1-2 solenoid was replaced as it was not functioning correctly as was the other on that side as it was cheap and already torn apart.

The truck ran flawlessly for a week and then Sat night I went to drive away from the house and it started it's old tricks. It was at 'running temperature' or just below and the shift indicator on the dash did not light up at all. No 1st and no OD as before. Key off, shut down, restart and it worked just fine again... very frustrating.

Back at the tranny shop this morning and it pulls the 1860 code again. 1860 definition seems to be a bit vague from most sources so he is currently trying to pick through the system and find it.

All thoughts appreciated. Thanks for the previous info!

[MT-2500]

1860 is PWM solenoid or wiring circuit problem.
One of the first codes and problems you had to start with.,

A good transmission man should be able to fix it for you.
Let us know how it goes.
MT

[j cAT]

Originally Posted by MT-2500

1860 is PWM solenoid or wiring circuit problem.
One of the first codes and problems you had to start with.,

A good transmission man should be able to fix it for you.
Let us know how it goes.
MT

my friend owns 96 tahoe about 3years ago he got those error codes did not have 1st gear started out in second, replaced the 1st gear solinoid did not read correct ohms, we found bad solder joint on coil wire. checked all solinoids for resistance before closing up transmission did not repeat. hope this helps..........

[reverend jim]

It is the pigtail for the circuit itself. It was shorting to ground from general age and when it shorted (if only for a split second) it would throw off all the shift sensors and solenoids and put the trany into 'safe mode' in 2nd gear.

Multiple occurrances had fried out the two solenoids that were replaced previously.

A new pigtail is on the way from the dealership and it SHOULD be fixed this evening. I will keep the forum aprised of the outcome and thanks as always for the replies.

[MT-2500]

Reverend Jim
Thanks for posting back how it is going.
Sounds like they have found the main problem which is good.
Good luck and let us know how it all goes.
MT

[reverend jim]

OK... got the truck back tonight after work and here's the lowdown. It has to do with the PWM wiring circuit as stated by MT-2500 and there is actually a GM response to it.

Tech Bulletin #583 came out in Dec '06 and states that the reason for the symptoms I have on a 4L60E tranny is a poor crimp on a termination in the 1020 circuit.

Essentially, there is a known problem with the crimp termination on circuit 1020 (shift sensors etc.) at the bulkhead connector #C100. circuit 1020 is also shown as A2 on the bulkhead connector diagram.

Will this fix it?... I dunno, but I'm game to try and make it fail for a week and the tranny guy is into finding the root problem if this isn't it so I'll give it a go! And yes I might have been able to find it on my own but the loss of work time would have far offset the cost of an honest tranny guy, and his resources and experience far outweighed mine!

Thanks for all the help.

[MT-2500]

You are welcome.
Yes a bad wire or connector can be a problem.
And good luck.
MT

[reverend jim]

...well that didn't do it. but I have some more tid-bits to add. If I put the key in the ignition and turn it to the 1st position, sometimes, the shift indicator light (which shows park) does not appear very strong at all and will even flicker to life from time to time. Switching it off and then back to 1st pos. will usually bring the indicator light to life and, as soon as it does, the truck starts strong and runs, drives, and shifts as if nothing were wrong at all....
... attempt to start it when the indicator is weak or off and it will go back into "safe mode" as stated previously.

I am willing to replace the ignition if that turns out to be the problem but no one has been able to tell me if that would make the indicator light act the way it does and cause those problems. I hate to buy a $250 switch if I don't have to but I need my truck back.

Any thoughts appreciated, especially those on whether I could drive it 1.5 hs to the airport on Sat. a.m. the way it is.

[MT-2500]

I am from the old school.
If it is broke do not drive it untill you fix it.
Good luck

[16th hippy]

in a nutshell, ckt 1020 supplies the 12V feed to the tranny. if for whatever reason there is not enough power goint through ckt 1020, it will cause multiple codes in the tranny. i have personally seen where on one occasion the tranny pigtail was to blame, and another where the ignition switch was at fault. more sitches bad than pigtails to be honest. I am not saying the switch is your cure all, but would lean toward it enough to give it a serious look through. and i couldn't agree more with MT, if it aint fixed, don't drive it. it could strand you somewhere. best of luck!