1999 Blazer, Shifting issues

[Hildy]

Alright so this thing came in and had a complaint of low power. I took it for a test drive and felt right away the transmission was not shifting properly. It would go from 2nd to 4th locked up almost by the time i got to 20 kph. The only code in the entire system was a 4wd lo s/w code. I test drove it with the scanner, in graph mode watching tranny data. Whenever the vehicle acted up, i noticed that about 5 PIDs would glitch like crazy, ( TC RPM, OSS, 4wd low s/w, desired gear etc. I did what research i could on a certain program we have at work ( i wont mention the name ) and found that if the VCM sees that the transfer case is in 4wd lo, it commands different shift points, corrects for vehicle speed etc. So i figure this has to be coming from whatever is making the 4wd lo PID go from off, to on, while i am driving. Now this is a touch button selectable transfer case with the auto 4wd button as well. So it has a transfer case control module. So the best i can figure the system is made of the shift(encoder?) motor, wiring, TCCM, dash switch, and VCM/PCM. Now when the PID glitches, the light on the dash does NOT change from the 2wd position, and the Transfer case is NOT going into 4 lo. After numerous test drives to try and catch the conditions it glitched under, i decide to pull it into the shop and do some wiggle tests. Nothing showed up there, because it just kept randomly glitching. So with the battery charger on the vehicle, i put it in KOEO, went through and proceded to unplug all the sensors on the Transfer case watching for an obvious change..... Nothing. i even unplugged the dash swithc and the PID remained randomly, intermittently glitching. So with everything but the TCCM unplugged, i walked away for a 5 minute breather, when i came back i looked at the PID history in graph mode and seen that it glitched out about 40-50 times in 5 minutes, with pretty much the entire system minus the TCCM unplugged, while it was sitting on a hoist KOEO, and the vehicle was not moving. Finally i unplugged the TCCM while watching the PID and it went to off, plugged it back in, and it glitched to on again. So the glitching completely stops when i unplug the TCCM. GREAT! so i have an idea if whats kinda going on now. I know that the "4wd lo switch" PID is fed to the scanner from the VCM, which gets that info from the TCCM. This is the part where i need some help. I cannot figure out where the TCCM gets the info for the "4wd lo switch" PID from. I can only assume that once the motor engages 4lo on the transfer case, that it sends its signal to the TCCM saying "we're engaged", and the TCCM then in turn sends the signal to the VCM for shift points etc... I have a TCCM on order, but i am not a parts swapper, i like to have solid diagnostics, but in this case i just cant get my hands on the info, so there is a long post, but thanks to any of your guys' ideas

[AMX77]

hey buddy, you still fighting this one or did you call time of death on it?

[Jay G.]

1999 Blazer

Below is the wiring diagram of the switch. Also this information can be found in Modis CTM in Transfer Case section.

Operation:
Transfer case shift select switch is a stepped resistance type switch. Shift select switch sends information to tcscm (transfer case shift Control module) to inform module of selected Range.

Tech note on shift motor:

Running motor while disconnected from vehicle may damage motor internal components.

When shifting this transfer case into neutral, the following conditions must be MET. Key on Engine off, automatic transmission must be in neutral or clutch pedal down on standard trans.

Hold brake pedal down while holding down both 2H & 4L buttons at same time for 10 seconds.
Neutral light will glow when neutral is achieved.

To reengage into any Range selection from neutral, first shut off Engine to avoid gear clash. Then, KOEO, select a Range before restarting Engine.

To shift transfer case into or out of 4L, KOER, hold foot on brake. Auto trans in neutral or standard shift with clutch pedal depressed


Best test Location:
At 8-pin & 4-pin connector on transfer case select switch
Component Location:
On left hand side center instrument panel to right of steering wheel
Connect test leads:
Red=select SW signal
Blue=known good ground


Check values (Plus Minus10%)
In switch positions:
3.18v
7.52v
4.81v.
2.39v
3.95V
No buttons pressed 0.79v

Key off, disconnect connector & connect test leads to switch terminals:
RED=tcscm(+8V)
Black=select SW signal

[Hildy]

Alright guys, so here is the update, after installing a new TCCM, all is well, fixed the power loss, erratic speedo, and horrible shifting, thanks for the help guys, talk to you again soon i am sure!

[crackerclicker]

just to try to answer your question i believe the vcm gets the information from the transfer case module over the bus lines. so, with that in mind, you would have still had a good idea what your issue was. glad you got it fixed!