99 OBS Tahoe 4WD system problems.

[ElectricIsBoring]

Long story short I bought this Tahoe with the transfer case already blown to smitherines. I picked up a used one, rebuilt it, and installed it (along with replacing many other mechanical wear items). Then i ended up finding that the front actuator, T-case motor, and selector switch were bad. Needless to say something was up so i checked the wiring and plug connections under the vehicle, all was good *I thought*, so i got replacements for each. Turns out I didn't check thoroughly enough because when i tried it after installing the replacements, the #2 interior fuse blew loudly after the shift motor was whirring away without stopping. The fuse-relay center direct harness had gotten knocked loose and sandwiched between the driver's side manifold and firewall, it proceeded to melt and short out some of the wires.

Since then I properly fixed the wires, but I have problems still and don't want to throw any more blind money at it. I also went over the wiring again, even behind the dash with a multimeter to an extent. I have checked/replaced all fuses and relays, and the guts of the fuse-relay center. Also I let the electrical system drain, and did a battery re-learn (3 touch offs before connecting battery) to reset everything.

What is currently happening upon either key-on or startup is as follows, no service 4wd light on dash; Switch thinks it's in 4low (i have the shift motor off the T-case currently); When i press any other mode (2wd, 4hi, 4auto) it blinks for a minute but then goes out (The shift motor doesn't show any signs of life); If i try it many times there's still no Service 4wd light (from my research that light should come on if there are repeated failed attempts. it was on when I got it, so I know the light works).

I have done a crap load of research but i've hit a brick wall. The OBD2 port doesn't have a #13 pin present, nor did i find any trace of the wire/pin that should go there, thus i can't do the diagnostics code readout from the TCCM to the switch lights (to my knowledge). The reason i'm trying to do that is to figure out if the TCCM is partially fried/bad or if it's the shift motor.

I'm desperate for answers so any help would be greatly appreciated. I've never made a forum post before so hopefully it's structured ok and I included helpful info. Other potentially useful info: NP246 automatic t-case, It refuses to go into neutral if i hold both 4hi and 2wd at the same time, and i installed a new Passlock module due to random false security lockouts (shouldn't matter though since i did the key and battery relearn after). Lastly I would send the TCCM off to get checked or whatever but I really want this done as soon as possible, and would hate to waste all that time if that has no issues.

Thanks, let me know if you want pics of anything like the OBD port.

 

[OR VietVet]

Welcome to the forum.

I have a 2005 Z71 but my girl has a 2002 TrailBlazer and they are known for TCCM problems. Not saying yours is bad but with all the other electrical problems you have had, it is a possibility. Good luck. and post pics of the new to you rig when you can. Bonus points if has barn doors.

 

[east302]

I wonder if it (VCM) sees your transmission as not being in neutral or moving? I think it will not shift to or from 4L if moving over 3-mph.

Copies of the GM service manuals can be downloaded here:

88-98 Service Manuals

Over the years I have collected a bunch of GM service literature. Links to PDF Scans of service and STG manuals and a links to the Parts manuals. You'll need the FREE Adobe Acrobat reader to open these files. Get it directly from Adobe here {http://get.adobe.com/reader/} If you're not...

www.gmt400.com

I didn’t see mention of what year you had, but the above link only goes up to 1998. The attached images are for that year.

I wasn’t sure if the diagnostic procedure for the Selectable transfer case was applicable to the Auto systems. It didn’t really say one way or the other or I just missed it completely.

A preliminary function test is used by GM and is based on a Tech2 scan tool. That table is below and I suspect that yours would fail it. My 98s do not have your transfer case (two are the non-auto and the other is manual) but I do recall getting DTCs from the Selectable ones using a Tech2 without having to do anything with the OBD connector. But, I don’t think that mine have Service 4WD lights so take that with a grain of salt.

I did see reference to that DLC pin 13 procedure in the manual but it left me scratching my head as to how its ability to report only four single-digit codes differs from the multiple four digit DTCs that can be retrieved with a scan tool. The section referencing “DLC Pin Assignment” is in Volume 1 of the manual.








GM offers the following procedure for a failed function test on a Selectable (I forget which NP### case that is) but doesn’t show a similar one for the NP246. Perhaps they are meant to be the same?





You might take a look at that link and see if you can spot something as I just did a quick scan through it. The above is in Volume 2 of the set.

But, I’d really suggest getting an alldatadiy.com subscription (think it’s around $60/yr) which will have the exact same information as the GM manual specific to your year. Too, it’s a bit easier to navigate as a lot of the references in the tables are hyperlinked.

Good luck, I hope this helps.

 

[Hoesgottaeat2]

@east302, you questioned the year in your post. I do see in the OP’s heading that his Tahoe is a ‘99 old body style.

A lot, if not all, of what you shared would likely still be applicable. You agree?

 

[east302]

Ahh, yes, thanks I see that now. I’d think that it would be pretty much the same as 1998. But, my wife says that I’m wrong all the time so…

If he can manage to pull a code with a scan tool then it would at least give him something to start with.

 

[drakon543]

alot of reading here and i skimmed over a bit soo excuse me if im repeating something. so some issues i had on my 97 were the front actuator was bad and popping fuses(thermal version). 4wd button was faulty so trying to get it to shift or acknowledge anything i wanted it to do was touchy at best. lastly the electronic neutral/shift indicator on the transmission may or may not have a seperate connection that also feeds the tccm. was the last thing i had to fix on mine to get the 4wd to work. had no issues shifting the transmission or any tccm codes i could pull anyway as there isnt a code if it thinks its just operator error. ofcourse i also found the wires going to the neutral safety switch bugered up too.

 

[ElectricIsBoring]

Thank you all, especially East302. I'll get back in a day or two and give an update on how it's going. I don't have access to a non basic scan tool, so for now i'll go through those functional tests and manuals. If i absolutely need it scanned i can probably figure something out, but definitely not my first choice.

Yes it's an early '99, from everything i can tell it follows the '98 year almost perfectly, only thing i've found different is the t-case that was in it used the newer double flat style shift motor mating shaft (instead of the crown gear thing). It got real confusing at auto stores so i just started telling them i have a '98 lol (although i know i have to be careful with that).

Thanks again for all the input. I'll upload pics sometime in the near future of the Tahoe overall. It's not clean or cool yet, but it's rust free and that's what matters to me being in Michigan (I can't stand rust repair). At least this way anything that needed/needs fixing or replacing was super easy instead of soaking every bolt or nut in penetrating oil and torching it to oblivion.

 

[jaeross57]

Long story short I bought this Tahoe with the transfer case already blown to smitherines. I picked up a used one, rebuilt it, and installed it (along with replacing many other mechanical wear items). Then i ended up finding that the front actuator, T-case motor, and selector switch were bad. Needless to say something was up so i checked the wiring and plug connections under the vehicle, all was good *I thought*, so i got replacements for each. Turns out I didn't check thoroughly enough because when i tried it after installing the replacements, the #2 interior fuse blew loudly after the shift motor was whirring away without stopping. The fuse-relay center direct harness had gotten knocked loose and sandwiched between the driver's side manifold and firewall, it proceeded to melt and short out some of the wires.

Since then I properly fixed the wires, but I have problems still and don't want to throw any more blind money at it. I also went over the wiring again, even behind the dash with a multimeter to an extent. I have checked/replaced all fuses and relays, and the guts of the fuse-relay center. Also I let the electrical system drain, and did a battery re-learn (3 touch offs before connecting battery) to reset everything.

What is currently happening upon either key-on or startup is as follows, no service 4wd light on dash; Switch thinks it's in 4low (i have the shift motor off the T-case currently); When i press any other mode (2wd, 4hi, 4auto) it blinks for a minute but then goes out (The shift motor doesn't show any signs of life); If i try it many times there's still no Service 4wd light (from my research that light should come on if there are repeated failed attempts. it was on when I got it, so I know the light works).

I have done a crap load of research but i've hit a brick wall. The OBD2 port doesn't have a #13 pin present, nor did i find any trace of the wire/pin that should go there, thus i can't do the diagnostics code readout from the TCCM to the switch lights (to my knowledge). The reason i'm trying to do that is to figure out if the TCCM is partially fried/bad or if it's the shift motor.

I'm desperate for answers so any help would be greatly appreciated. I've never made a forum post before so hopefully it's structured ok and I included helpful info. Other potentially useful info: NP246 automatic t-case, It refuses to go into neutral if i hold both 4hi and 2wd at the same time, and i installed a new Passlock module due to random false security lockouts (shouldn't matter though since i did the key and battery relearn after). Lastly I would send the TCCM off to get checked or whatever but I really want this done as soon as possible, and would hate to waste all that time if that has no issues.

Thanks, let me know if you want pics of anything like the OBD port.

I kinda jumped ahead a little bit, but the first thing that came to mind is the actuator on the front axle. I always check that first because everything ties in with that and what you describe with the switch is exactly what it does.