1999 Tahoe (purchased this week and need help with trans issue)

[Nelson L tompley]

I'm new to this forum and this is my first post.

I recently purchased a 1999 Tahoe 2 door that was sitting up for 5 years,
I did the usual, change fluids, etc ...
The vehicle runs great but I'm having a shifting issue (Note-this is why the previous owner stopped driving the vehicle)

Issue:
(with the 4x4 control panel in 2WD) Put the trans in Drive and take off,
it shifts to 2nd and as ur accelerating to 3rd gear, the transmission acts like it shifts to a low gear and the rpms jump up high.

This doesn't happen all the time, sometimes it drives perfect but moew often than not is occurs.

Today I replaced the 4x4 control panel but the same thing occurs.

4x4 control panel lights:
Sometimes when you start the vehicle all of the lights on the control panel are lit on and stay on.
Sometimes when you start the vehicle the 2WD light on the control panel is on and stays on.

The sudden downshift issue happens no matter which control panel setting you select (except) if you select Auto 4WD/2WD the sudden downshift issue doesnt occur BUT I don't use this selection because sometimes the trans sounds like its having a hard time deciding which mode you want to use.

Sometimes the 'Service 4WD' light comes on but not all the time ...

Could it possibly be a defective Shift Solenoid?
or a Throttle Position Sensor?

Before I start throwing additional money at this vehicle I've always found it's best to ask fellow forum members.
The wheels been invented, I just need help finding the correct spoke

Thanks for your suggestions ...

 

[east302]

Since the issue doesn’t occur in Auto mode, I’d probably start with the transfer case code, see what it is if you have a scanner. If not, a parts store may be able to pull it for you. GM has diagnostic tables for each code that will give a step-by-step procedure. Post the code and I can pass along the relevant tables.


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[drakon543]

if your friendly with any local garages a more advanced scanner might be needed depending on the scanner your local parts stores have. if your just trying to pull the codes and thats it as quick as it is some garages will do this for free. smaller shops atleast dont expect that kind kf service from a chain shop or a stealership. basic scanners generally dont talk to the abs module or the pcm.

 

[Nelson L tompley]

East302, the sudden downshift doesn't occur in Auto mode but I cant use this setting because the vehicle shutters and the trans sounds like it cant decide which mode it want to run in.
When you say Scanner are you refering to an ODB scanner? If yes, I used a scanner and it's not throwing codes.
Drakon543, Thanks for the tip

 

[east302]

If it tripped the service 4WD then it should have a code stored. Some of the inexpensive readers may not be able to read that. A shop (with a SnapOn, Tech2 or similar) may do it for you for free if you ask really nice or have something else done along the way. I think transfer case codes start with a B or C.


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[Nelson L tompley]

I'll locate a shop with a good scanner.
If the scanner reveals codes I'll update my post.
Thanks for yhe suggestions ...

 

[Nelson L tompley]

Expensive scanner results:
C0315 Motor ground circuit open
I'm having trouple locating this grounding spot.
Suggestions ...

 

[east302]

This is the diagnostic table for C0315 for a 98, maybe it will help. It looks like it points to a bad ground or defective transfer case shift control module.

Ground G200 is on the dash support bar kind of above the parking brake.












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[east302]

A few more to show where the auto transfer case module is located under the driver side dash...






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[Nelson L tompley]

WOW!
I'm overwelmed.
I will try to decipher all this info.
The older I get I wish I had someone tell me Bill do,
1.
2.
3.

Thanks for all of the great information.
It's kinda like 'Be careful what you ask for, you might get it' and then some ...