This is the automatic (electronic) 4WD system (4 buttons on the dash)...

Does anyone have schematics or troubleshooting guides for the 4WD Electronics (like the TCCM)??? Moreover, what feeds power to the 4WD system (fuse#’s, relays, etc)?

I have a problem with the 4WD... the TCCM doesn't click (self check) like it should at start-up. So it will not choose a mode (2WD , 4 Auto, 4Hi, or 4Lo) and all lights on the selector switch go blank. I had someone look at it (they replaced the selector switch, but it didn't help). The TCCM seems fine (visual), but I have no idea how to truly test it. After the last battery problem, the TCCM worked a few times and it was in 2WD. I got it to switch to 4 Auto, but no other modes could be selected. After choosing 4 Lo, it tried for a minute then popped the service 4WD light. That light is normally on all the time when the TCCM doesn't pass self check at start-up. The actuator motor in the Transfer case is supposed to be good according to the mechanic and it moves freely.

Any help would be appreciated. Thank you!

 

Here are a couple of wiring schematics for the switch and encoder motor.
As far as diagnostics go, it's pretty hard to figure out without a scan tool.
These pictures I'm sending , you'll probably have to put into a photo editing program so you can blow them up larger to see them properly.I can send you what I have for diagnostics, but they include the use of a scan tool.

If you click on the thumbnail and then click on the image, there is a tool to expand it a little.

[IMG]local://upfiles/2910/EA8FB7874E0A4E21B20E457EC8F11D86.gif[/IMG]

[IMG]local://upfiles/2910/979E82D56FA5443D8287486BA44CDAE8.gif[/IMG]

 

Anything you have is appreciated at this point...

It sounds to me, like the TCCM isn't getting powered correctly at start-up and the Service 4WD light stays on from initial crank. The control switch (dash) blinks all lights once for just a millisecond, but the normal self check clicking and light patterns are not there. I was wondering about some sort of ignition relay that controls power to the 4WD system, since it worked briefly after the battery change.

Thanks for the schematics... I'll look them over.

 

The battery voltage is supplied by the ATC fuse in the underhood fuse block via circuit #1640 which is a 3mm orange wire connected to the black 4-pin connector terminal D at the tccm.

The Ignition voltage is supplied by the 4WD fuse in the instrument panel fuse block via circuit #241 which is a 0.35mm brown wire connected to the big connector terminal #E10 at the tccm.

You realy need a scan tool to check this thing for codes though, because if there is an internal malfunction, it will set a C0550 in the tccm, and turn ON the service 4WD light.

Hope this helps, and keep us posted please.

 

I'm still working on this one... The schematics have been helpful, but the details are too grainy to read. I checked on resetting the 4WD system (like what happened when I changed the battery) by pulling the two fuses RDO BATT and ATC, but I cannot duplicate the reset. I'm interested in the Body Control Module and how it impacts the 4WD system. One mechanic indicated that he didn't think the switch (dash) was getting power like it should, but the LEDs all do a quick flash (fraction of a second all at once) at crank.

 

One other question... does the '99 have all the vacuum actuator equipment that so many of the other model-year's refer too. Furthermore, is there anything that controls 4WD under the batter tray in the '99 (since I had a battery leak acid all over the ground I suspect anything under the tray could be damaged).

 

Yes there is a vacuum actuator under the battery tray.
Data Memory Retention Check
The TCCM checks the stored diagnostic trouble code memory to see if the memory has changed since the ignition switch was last positioned to OFF. This self test checks in order to see if the RAM standby power supply has been interrupted. If the TCCM detects a RAM standby power supply interruption, (the loss of stored diagnostic trouble code) the TCCM stores a DTC of 1 indicating RAM standby power failure.

This condition occurs when the TCCM is disconnected from the wiring harness or battery power is removed. The DTC of 1 automatically clears from the TCCM after cycling the ignition switch ON and OFF five times
Transfer Case Control Module Description
The TCCM performs the following functions:
- Receives input signals

- Processes the signal information

- Develops output signals

- Sends the output signal in order to control the shifting of the transfer case

The TCCM receives input signals from the following sources:
- The transfer case shift select buttons

- The NSBU switch on the vehicles with automatic transmissions

- The clutch position switch on the vehicles with manual transmissions

- The vehicle sped sensor calibrator module

- The encoder/motor range and the mode feedback signals

- The diagnostic enable

- The front axle mechanical status signal

In order to ensure the electronic shift system is operating properly, the transfer case control module (TCCM) continually performs diagnostics checks on itself and other parts of the electronic shift system when the ignition switch is in the run position. The following are different types of system checks that the TCCM continually performs.

Range Shifts
The range shifts are the shifts between the HI and the LO ranges.
- 2HI to 4LO

- 4HI to 4LO

- 4LO to 4HI

- 4LO to 2HI

A range shift can only be made with the automatic transmission in neutral or with the manual transmission clutch fully depressed. The shift occurs when the vehicle speed is below three miles per hour. Whenever a shift into 4LO is initiated, the 4LO shift select button blinks. The 4LO shift select button will continue to blink until the shift is completed mechanically or until 30 seconds elapses. The 4LO shift select button must be on and not blinking prior to shifting the transmission into gear or releasing the clutch pedal.
If a range shift is initiated when the transmission is engaged or the vehicle speed is above 3 mph, the 4LO shift select button will blink for 30 seconds and no range shift actually occurs, the TCCM will default and position the transfer case into 4HI.

 

Have you ever figured out what was the problem? My wifes blazer is doing the exact same thing. The atc fuse was blown so i replaced it. I verified that there is power to the orange wire of the black plug on the TCCM and power to the brown wire of the orange plug. The service 4wd light is also on and the lights on the switch flash for a split second. I have a OBD scanner but it does not read 4wd codes. Thanks.

 

Well unfortunately, I have no answers, but I do have more information...

I cannot find any obvious cause of the problem. However, I got into my blazer yesterday after it sat in the Texas Rain (2-3 inches and the normal temperature dropped by 20 degrees) for 2 days. I started it up, drove to work, and noticed the SERVICE 4WD light was not lit and the system was in 2WD. I thought it would go back to failure after turning off the vehicle. At lunch, the normal under-dash clicks occurred at crank and it started up in 2WD again. While headed down the road I pushed in Auto 4WD and it changed to that mode. I pushed 4HI and it went into that mode. I thought surely it would fail 4Low, but in a parking lot in neutral rolling below 3 mph, it clicked into 4Low. The darn thing works in all modes (when it wants to). It came back out of 4Low at the push of a button and I cycle through the other modes one my way back to 2WD. It continued to work through 5 or 6 more stops and restarts, but this morning (the day after) it comes up with the SERVICE 4WD light again at crank. GRRRRRR!

So everything must work mechanically (transfer case and tranny). The vacuum portion, whatever it is, must work. The dash switch must work and the TCCM can work (although the problem could still be intermittent there). Anyone have any thoughts… Something that could get wet or that changes with colder temperatures? A ground problem on some portion of the system? It seems like it has be electrical/electronic since it functions in all modes (when it wants to).

As usual, any help or thoughts are appreciated.

 

I think I’ve found something that affects the problem...

I have noticed that a shift from D to R seems to influence whether or not the 4WD works. More specifically, at crank the 4WD wasn’t working. I shifted into R and then turned off the vehicle. I turned the key back on and heard the 4WD system do the self-test (like it would work). I kicked it into neutral, cranked the engine, and the 4WD system came on in 2WD as normal. The next time I went to drive the vehicle (later in the day) 4WD didn’t work at crank. I put it in D, then in R, turned off key, moved to neutral, cranked the engine, and the 4WD system came on in 2WD.

I thinking either some kind of interlock switch or connector that gets squeezed or pinched with the change in tranny torque???