96 tahoe transmission issues

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.

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.

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.

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

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?

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!

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

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..........

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.

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

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.

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

...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.

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

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!

My money is on the ignition switch and here's why.

If I jimmy the switch around and get the gear indicator light to come on full-strength the thing runs like a top. But try to start/run it when the indicator is weak or gone and the thing will either slam gears or just go into 2nd gear "safe mode" right off the bat. Sometimes if the switch is in the first position for long enough the light will slowly flicker to life, and if you drive it for a little while and stop, it does not want to come to life again for quite a while.
It is back in the shop as of tonight and should have a new switch and, hopefully, be up and running by Thursday night.

Thanks for the thoughts!

[quote=reverend jim]My money is on the ignition switch and here's why.

how many miles on this vehicle ? most likely the pigtail that shorted out at transmission damaged ignition switch contacts .

Reverend jim
Ign switch is a good thing to check and also wiring and also wiring as J Cat metioned.
There should be a gear selector switch on the side of transmission.
Check it and the wiring to it.

Also some 96 transmissions had a computer wiring/ground problem.
On some they came out with a wiring harness kit to correct the problem there. I can not find the info on it but.
For more details on it go to the link I posted in the PM I sent.
Good luck MT

OK!.... here's the last post (hopefully), it's a Big-Un' !

The problem was directly AND indirectly attached to the ignition switch...not the tumblers (or "key switch") but the electric switch at the base of the steering column. Apparently there is a GM TSB which discusses this which was eventually found by my diligent tranny guy after hours of beating his head against the Tahoe. I will post the TSB # when I get it.

Basically, the '96 was the object of a change in the ignition switch system which replaced the two-piece setup of a switch and a relay to a one-piece unit which encompases both. Apparently, up to this point, the answer was to replace one or the other but it was discovered that just replacing the switch and not the relay, or vice-versa, would burn out the older of the units in just a few months... so GM replaced it with a one-piece unit and put out this TSB to inform everyone, that cared to search for hours and hours to find it, that this was the fix.

The best part is that this repair had already been done at some point in the Tahoe's life (by who-knows what mechanic) but VERY poorly. Additionally, the truck was also equipped (presumably by the same person judging by the electrical terminations and zip-ties) with one of those used-car-lot key-card locks under the dash.

Apparently, the shoddy work and addition of this key card had slowly-but-surely shorted out and severely heated up the main harness plug on the steering column and made it so brittle that, upon removal for inspection, it crumbled like an old cookie...

...still with me?...

...SO... a trip to the salvage (junk) yard produced a nice clean harness plug. The key-card setup was removed and any and all extra wiring and crimp connectors were deleted. And, finally, a new GM switch-relay combo was installed. All this was done with nice solder joints, clean looms, and care not to be a bull-in-a-china-shop.

The grand result is that I now have 600 miles on the Tahoe with absolutely no problems. It has been on long day trips, stop and go stuff, fun in the snow, and all without a hiccup.

So thanks to all of you who threw possibilities at this quandry. All of them were helpful in one way or another. And thanks, especially, to Spath Transmission in Chico, CA who was instrumental in me NOT pushing the Tahoe off lookout point.

This info would probably be helpful here as an Ignition System post also, but I'm not sure of the best way to do so. Thanks - Jim

i've replaced many ignition switches for this problem. the switch isn't putting enough voltage to ignition 2(white wire at switch usually), can also be
checked by putting test light on trans fuse at ipc& turing on ignition any
commanding transmission solenoids with scan tool. also a remote start can cause trans to default with it isn't powering this circuit.

Reverend jim
You are welcome And
Thanks for posting back how it went and letting us know how it went.
Glad all is well and you have got it fixed.
MT

i've replaced many ignition switches for this problem. the switch isn't putting enough voltage to ignition 2(white wire at switch usually), can also be
checked by putting test light on trans fuse at ipc& turing on ignition any
commanding transmission solenoids with scan tool. also a remote start can cause trans to default with it isn't powering this circuit.


On the 96 the switch messes up the trans fuse circuit, remote starters that are improperly installed can bring the same problem.

That tranny guy sure did some homework, the last I knew that TSB was for 97 and newer pickups.