Shifting and idle still a mystery

[94blue]

Hey all, my stock, 94 Suburban, 4x4 still will intermittantly shift correctly through OD, then while driving, shift back to 4th, then the next day not shift out of 2nd at all. Not in this order necessarialy but very intermittantly. This has been going on for almost a year. I've looked at the grounds, new batteries seemed to help. When it shifts into overdrive sometimes the Check ingine light will come on. Then if it goes off it will shift back out of OD into 4th.
Also at idle, it will go from fast idle to slow idle or just stay in fast idle all day when parked.

I have not checked the codes either.

Any ideas?
Thanks,

[JohnC]

Well, we need to get some terminology straight...

4th is Overdrive.

Due to this confusion, I'm thinking your problem may be the torque converter clutch, not OD. The TCC is the only thing that can be mistaken for a shift up if you're in 4th.

The fact that you also sometimes get stuck in 2nd indicates you may have a problem with power to the shift solenoids. The ignition switch is a common cause. You can put a volt meter on the trans fuse and see if it is steady or not.

And, get the codes...

[94blue]

yup, i wasn't sure about that regarding OD and what gear that is.
Are you talking about the key switch in the column or a relay or harness somewhere?
Thanks,

[94blue]

Also can i check the voltage at idle, or another method.
Thanks,

[94blue]

BTW, where are the transmission solenoids located, etc.
Thanks,

[JohnC]

The ignition switch in the steering column.

Monitor the voltage from the moment you turn the key from "Lock". If at any time it drops below about 9 volts when a solenoid is supposed to activate, you will get a solenoid fault. Worse case it locks you in 2nd gear.

Solenoids are in the trans valve body.

Read the codes. All you need is a paperclip. See the article in the members area of the web site.

[Robyn]

This post goes hand in hand with what John has suggested.

This may be a battery power feed issue. (you mentioned new batteries helped some)

The 94 rig feeds power to the chassis electrical system through a cable slightly larger than a pencil that feeds off of the RH battery and runs down and around and down and up and down and comes up behind the engine and then into the Power junction box on the firewall by the AC unit.

The battery cables on these can look great, but inside the core of the wire can be just crap and not passing current hardly at all.

Now here is another caviat. The alternator is connected to the LH battery and then that battery is fed, then the power crosses the core support via the large cable and feeds the RH battery.

If the LH battery ground is poor (you can't tell be looking at them) things get real hinky.

If the ECM does not see a good constant 12-14 Volts it can get really upset and do all manner of wierd crap.

My suggestion is to replace all the cables with either new factory type stuff or make up a set of new cables using welding cable and solder on the ends.

The side terminal conections will look great, until you cut off the red rubber and prowl deep into the cable.

The original factory ones had a ring of solder around the outer strands of wire, but the core was not soldered. This would allow the core to fill with corrosion and stop passing current.

Run an all new feed line directly from the battery on the RH side to the power stud in the junction box.

If your side terminal batteries are new or nearly so. you can use a piece of all thread and some brass buts and washers to connect some good cables with soldered EYE connectors.

If the batteries are well used, replace with the Optima or similar with top and side terminals and use the top terminals with a stud adapter.

Get the battery grounds, the cross cable (LH TO RH) the feed to the starter and the feed to the power box all in top shape.

Once this is done, my bet is that the tranny will work fine and the rig will start faster too.

With the dual battery setup, one ground cable failure will screw everything all up.

The RH Battery feeds the starter and the chassis and the LH battery gets the charge from the Alternator first.

If the LH battery ground is faulty then the LH battery can't help during start up and if the RH ground is bad the entire system is compromised.

The old manually injected rigs with like a TH400 tranny would limp along with bad cables for years without really anything horrible.

NOT so the computer controlled ones. Voltage to the chassis must be good and grounds must be good.

A poor ground thats causing a back feed in any major circuit can confuse the ECM and drive it nuts, to the point of the SES light coming on, shifting Issues and all other manners of hate and discontent.

Good luck

Missy

[94blue]

I was halfway to doing what you guys recommend. I traced the wire to the distribution box. I even Ohmed it and it looked good, that does'nt mean it will carry current. I have already had the diode on that cable give me fits and the truck was "dead" until that wire got wiggled. Yes, the battery cables have swollen spots in them if you feel around. I will replace all of these cables and report.
Thanks for the responses, You guys are always great help
94Blue