Okay, I've been reading a lot here and tinkering with the 4x4 system on my 99 TB. Sometimes it works, sometimes it doesn't. Looking for help!

I have done the following:

• replaced vacuum lines under the hood (they were rotten). I also still question if the vac hose connections at the switch on the firewall are correct,
• verified cable movement at the actuator (it seems to move freely),
• checked TCCM connections (they were okay but applied di-electric grease when re-connecting), and
• checked fluid in T-Case (a very slight amount dribbled out).
• I had the dash switch out tonight and it was hit and miss when it worked, but when wiggling the connection on the back I did get the 4 hi light to flash and could hear clicking.

Instead of continuing to wander aimlessly I thought I would have the codes read. I did this recently when I also had a check engine light on (turned out to be P404- the gas cap). I asked if the reader would read transfer case codes and he said yes, but it had no codes stored...

What to do?
Will reading codes diagnose a bad dash switch?
If I have codes read again, what kind of reader should the tech be using?

Thanks!

 

Kept reading and found one answer to a question... I need the tech to use a scanner capable of accessing the BCM. Will try to get that done today. Looks like a switch is about $25 so may roll the dice a buy one of those too.

 

If the service 4wd light is on, you have stored codes in the TCCM. Generic scantools cannot access the TCCM. Transfer case codes will not be stored in the BCM, but if the scantool can access the BCM, then it should be able to access the TCCM as they both are serial bus communication devices.

The vacuum switch is fairly easy to test and you must have the hoses on correctly or the front axle will not disengage properly. I created a vacuum diagram showing the valve and proper routing of the hoses in one of the 4wd related tech articles.

 

Thanks swartlkk.
So I drove her to work this morning and got no service 4x4 light... Once I got to work the auto, 4 hi and 2hi switched back and forth as it should but 4 low would never engage, light just flashed. Will get codes read today, hopefully!

 

If you unplugged the TCCM to check the terminal condition, the codes would have been reset with the loss of power so no surprise there. You will have to wait for the 'Service 4wd' light to come back on to retrieve any stored codes.

The fact that everything works now indicates to me that the code is likely to be a failed shift and/or encoder fault type code. By no means should you start replacing parts based on this information without further diagnosis.


The failed shift to 4LO doesn't surprise me much either. These transfer cases can be very finicky when it comes to successfully getting a range shift from 4LO to 4HI. To increase your chances of a range shift, it helps to be moving slightly forward or backwards with the transmission in neutral when you hit the button. This allows the gears to mesh more easily allowing a possibly weak encoder motor to get through the shift before timing out and returning to the previous mode/range.


You can test the dash switch both at the switch itself as well as at the connection to the TCCM to make sure that you get the appropriate resistance measurement across the switch wires. The NV236 Wiring & Diagnostic Information thread has a blurb near the bottom discussing the switch and the resistances that should be observed for a properly functioning switch. Testing at both the switch and the TCCM harness connector ensures that the proper values are making it out of the switch, through the wiring, and into the TCCM.

 

dhud64: Take the 4wd switch back out of the dash, carefully separate the two halves and reflow all the solder connections, especially the connector pins...add a dab of solder to these....reassemble and try again.

 

To clarify, the service 4x4 light was seen once after disconnecting the TCCM, but haven't seen since (fyi, it only gets driven about 30 miles a day).
Wasn't able to get code read yesterday, but tested the 4x4 this morning and everything worked; auto, 2hi, 4hi and 4 low, the only thing was the 4 hi light never illuminated but it did lock in 4x4. So, this doesn't appear to be anything major as far as causing damage if continuing to drive, so I think I will monitor the situation and maybe try re-soldering the connections on the switch.