Hey everyone. First post on the forum, so let me say hello to you all.
I definitely found a lot of useful information on the site so far, however, every issue close to mine also seems to be a little different. So I wanted to get some feedback.

So we're looking at a 1999 gmc jimmy SLT 4x4 automatic (Np236).

I run this truck in British Columbia lower mainland. Not by any oceans, but we do get cold snow and rainy seasons.

Bought it 7 years back. About 3 years ago (during winter) it had its first issue where automatic shifting was gone. I had to use [123] to get around. It happened on and off for a couple weeks. Did a tranny fluid change and it was good.
About 2 years later (on a long road trip) it happened again intermittently, but then returned to normal.
The service 4wd light would make an occasional appearance, but never for long, if ever more than a few seconds.

Now it's doing limp mode to a degree where it won't go away. There hasn't been a service 4wd light, but it's got a few added symptoms.

-temp guage immediately spins around to the point it get high centered and stuck.
-I'm doing 160kph in park or any other gear (moving or not)
-oil pressure is at halfway mark when off
-4x4 switch lights are all lit up

Another thing that I'm seeing as a potential symptom is the security light flashing.
There would be times the alarm would go off if I opened the door without pressing the unlock fob. I thought this was normal but I've gotten the idea from other posts that it may be an indicator of a bad ignition switch.

My truck also holds into the key in the ignition when I try to remove it. I have always had a shim just to hit the release button behind and under the wheel.
It's done this from day one.

I got an OBD-ll scanner from a buddy and pulled the codes
B2722 - transmission preference switch circuit low

B2600 - Daytime running lamp control #1 (LH or system) circuitmalfunction

My battery is very bad and has died on me a few times in the last couple months leading up to this.
Going to grab a new one tomorrow and hope it remedies some of these issues.

The truck runs phenomenally when this issue isn't in effect. Never slips, shifts smooth, engine runs great, horn works. So I can't say I feel this issue is mechanically related.

I did a basic 'look and replug' of the TCCM. Cleaned the main grounds. Didn't see anything strikingly obvious, but I can definitely look closer.
Worse I seen was the sub dash bulb had melted some of the surrounding dash shroud. Fuses seem okay as far as not being popped. I have a Multimeter, but I'm not sure what to look at in this case and what readings to expect.
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Thanks for your time and any help would be greatly appreciated!

 

Try hooking up the scanner when the truck is acting up and see if you can read and clear codes and look at live data.

George

 

Looks like it's actually shifting through gears 1 and 2 immediately, then doing 3 and 4 later. Also it won't downshift from 4 until I get to a near stop.

Bypassing it by manually shifting for now.

After replacing battery I have no codes, no errors.
Seems to be Faulty communication.
Possibly BCM/TCCM.
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My diagnostic steps were to see if the class 2 serial data bus is corrupt before trynostics and the parts cannon is invoked. I yield this thread, good luck.


George

 

Update.

The new battery solved the dash issues as the last battery had a bad cell.

Since I could not get a code to reappear after the new battery, I saw this is a communication issue between either BCM or TCCM.
BCM functions all seemed good. This leaves TCCM as the main culprit.

The new TCCM arrived earlier today and solved the issue entirely. 4wd switches work flawlessly and shifts great once again.

My initial theory of it being the ignition switch wasn't accurate. It turned out to be a combination of two issues.
​​​​Two bad parts found causing issues and two bad parts replaced with 100% successful results.

I found out a couple neat things from this while I was part searching.
A couple compatibility tips when looking for replacement parts that may have had part number changes over the years is that you can look in your glove box (in my case) at a sticker with a series of 3 digit RPO codes that match up with the replacement part, or you can call the dealership and give them the last 8 digits of your VIN and they can basically tell you anything about your vehicle.

Hopefully this helps any anyone who happens to have similar issues.

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