Dead 4WD switch in GMT400 Suburban

[rationull]

I’ve got a 1999 (final year of the GMT400) K2500 Suburban where 4WD hasn’t been able to engage for a while. I’ve not bothered looking into it so far because there hasn’t really been a reason, but I want to do something about it now, at least if it’s easy enough. My understanding of the system and its failure points is as follows:

- Front diff actuator: thermal in pre-1996 and electric motor in at most post 1996 trucks. If this fails then you should theoretically be able to engage low vs high gear in the transfer case (i.e. the engagement buttons should still “work”) but get no power to the front wheels.
- Transfer case electronics and actuator: Controls shifting from high to low gear and routes power to the front diff actuator to engage 4WD. Seems to me that if the actuator fails then you should still be able to shift into 4HI, but if the electronics fail then all bets are off.
- Dash switch: Based on a wiring diagram I found, it looks like the circuit that powers the lights in the switch actually goes through the transfer case electronics (i.e. the transfer case maintains state about what mode the system is in, not the switch, which makes sense). I’m not really sure how the TC electronics work though or whether the circuits are actually controlled by the position of the shift mechanisms.
- There’s also a speed sensor to allow or disallow shift-on-the-fly.
- And there is a fuse.


In my case, the lights on the switch are all off (even the 2HI one) and the buttons don’t do anything apparent. Before this, they’d been flakey for a while in that the 2HI light would be on sometimes and off sometimes.

What I’ve done:

1) Checked the relevant fuses (2 of them) and found no failures.
2) Checked the resistance across the relevant pins on the switch and they more or less match what they should based on a diagram I found. One of the settings (4LO I think) read a bit below spec but I’m not sure if that’s a problem.


I was hoping it’d be the switch but based on my test I’m afraid it’s probably the transfer case electronics module.

Is there anything easy I can do to get a little more confidence on what the problem might be? Is there any way the switch could still be the culprit even though the resistance when the buttons are pressed seem to be OK?

 

[Olas]

It's cases like these that reassure me of my appreciation for Land Rover Defender and it's wonderful simplicity.

No button, no switches, just big heavy levers sticking through the floor pan - you pull the lever and FEEL the diff lock up. You push the other lever and FEEL the low range clunk into place.

Levers beat buttons every time (simple and effective and cheap and easy to diagnose)

 

[eljefino]

Thanks for the preachy and unhelpful lecture, Olas.

OP, is there a way you can jumper-power the front axle to see if that locks it? I remember when automatic hubs were the most fragile part of a 4wd system. I would "prefer" to have my axle hardware in good shape and fix/replace a "black box".

If the switch uses resistance ("resistive multiplexing") I can't imagine all modes dying at once. Something little-used like 4-lo should still be there. If you have a "scratchy potentiometer" it should clean itself up with a bunch of switch flicks.

 

[DirkDiggler]

OP, go to the Tahoe Yukon forum. great advice over there. it sounds like the switch is bad. I had to replace mine when the lights when out and the system kept saying "service 4wd drive system". got all my repair advice on Tahoe Yukon forum

 

[kc8adu]

sounds like you have the infamous lucas switch assy.
pull it out and resolder EVERYTHING!
i have fixed many this way.none needed replacement.
takes longer to pull it than to resolder if you are good at soldering.
lucas "prince of darkness" is still at it!

 

[rationull]

Originally Posted By: Olas
It's cases like these that reassure me of my appreciation for Land Rover Defender and it's wonderful simplicity.

No button, no switches, just big heavy levers sticking through the floor pan - you pull the lever and FEEL the diff lock up. You push the other lever and FEEL the low range clunk into place.

Levers beat buttons every time (simple and effective and cheap and easy to diagnose)


Believe me I'm with you on this, although packaging for the wiring and switches is way better than for a lever.

 

[rationull]

Originally Posted By: eljefino

OP, is there a way you can jumper-power the front axle to see if that locks it? I remember when automatic hubs were the most fragile part of a 4wd system. I would "prefer" to have my axle hardware in good shape and fix/replace a "black box".

If the switch uses resistance ("resistive multiplexing") I can't imagine all modes dying at once. Something little-used like 4-lo should still be there. If you have a "scratchy potentiometer" it should clean itself up with a bunch of switch flicks.


AFAICT without the front axle I should still be able to switch between high and low, and the lights on the switch should still function. I could probably jumper it but it doesn't seem like a likely cause. I should probably find somewhere where I can roll slowly with the engine off and try to engage 4HI just to make 100% sure there are no sounds coming from underneath, though.


Originally Posted By: DirkDiggler
OP, go to the Tahoe Yukon forum. great advice over there. it sounds like the switch is bad. I had to replace mine when the lights when out and the system kept saying "service 4wd drive system". got all my repair advice on Tahoe Yukon forum


I've done reading on various sites but not sure if I've seen that one yet. I will check it out. I'm not sure if my truck even has a "Service 4wd system" light -- it doesn't have general message screen -- but there are definitely a lot of posts around where that's one of the symptoms, and it probably is a similar problem.


Originally Posted By: kc8adu
sounds like you have the infamous lucas switch assy.
pull it out and resolder EVERYTHING!
i have fixed many this way.none needed replacement.
takes longer to pull it than to resolder if you are good at soldering.
lucas "prince of darkness" is still at it!


LOL! Yes, when I took the switch out and saw "Lucas" on the back, I immediately thought "hmmm" and the "prince of darkness" came to mind. I was disappointed when the resistive circuit in the switch checked out but it's still possible that there's a broken connection in the other circuits in there.

What kinds of failure modes have you seen that lead to resoldering the switch? Have there been obvious/visible problems with the connections when taking them apart or you just redo everything regardless?

 

[kc8adu]

electronics are cheaper to make than a bunch of handles and linkages.
but are nowhere near as durable.
Originally Posted By: rationull
Originally Posted By: Olas
It's cases like these that reassure me of my appreciation for Land Rover Defender and it's wonderful simplicity.

No button, no switches, just big heavy levers sticking through the floor pan - you pull the lever and FEEL the diff lock up. You push the other lever and FEEL the low range clunk into place.

Levers beat buttons every time (simple and effective and cheap and easy to diagnose)


Believe me I'm with you on this, although packaging for the wiring and switches is way better than for a lever.

 

[rationull]

I took the cover off the switch and nothing looks obviously failed in there. Some of the solder connections on what I think are the neutral light circuit look a little worn but they do appear to have continuity when tested.

I just don't think the switch is the problem, but it is possible that there are trace breaks inside the board and I don't know how to test it much more thoroughly. I would feel more stupid digging in deeper and coming back to find the switch WAS the problem than replacing it to find it's not so I may still order one.

I will go do some searching on the Tahoe Yukon forums.

 

[clinebarger]

Switch testing....
1. Check for 12 volts on the Pink wire with the key on.

2. If no voltage is present, check the Gauges fuse for being blown.

3. If voltage is good, check for 12 volts on pin B Dark Green/White when the 2HI button is pressed.

4. Try the same thing on pin C Gray/Black when the 4HI button is pressed.

5. If no voltage appears when pressing the buttons, then the dash switch is bad.

The Transfer Case Control Module....
1. Go to the Transfer Case Control Module (TCCM) under the dash by the steering column.

2. Verify 12 volt power to pins C6, C8, D14 and D15 with they key on.

3. Check grounds at pins C10, D10, D12, and D13.

4. Try jumping pin D3 to ground with the TCCM plugged in and see if it flashes the lights on the dash switch then to give a trouble code.

5. If power and grounds are OK and jumping D3 to ground does nothing, then replace the TCCM.

TCCM pinouts....




Factory procedure to enter the TCCM into diagnostic mode....They instruct to Ground pin 13 at the Data Link Connector, Above I am instructing to ground pin D3 at the TCCM, These both accomplish the same thing, I find it easier to ground the diagnostic terminal at the TCCM because I am already circuit testing anyway with my Power Probe.

1.The diagnostic trouble codes are displayed on the three transfer case shift select buttons. The shift select buttons are located on the instrument panel when the connector pin 13 on the data link cable is grounded, and the ignition switch has been OFF for at least five seconds prior to positioning the ignition switch to run the shift select buttons will blink various times together in order to indicate a diagnostic trouble code from 1 to 4.

1.1. Position the ignition switch to OFF. Ensure the ignition switch is positioned to OFF for at least 6 seconds.

1.2. Connect pin 13 on the data link cable to a vehicle ground source. The data link connector is located in the cab under the instrument panel on the drivers side.

1.3. Position the ignition switch to RUN.

1.4. Note the shift select buttons for blinking codes.

2.If the shift select buttons all blink one time and stop, and do not continue to blink, no fault codes are stored in the TCCM.

3.The transfer case shift select buttons will blink in order to identify any stored DTC. If only one code is stored in the TCCM memory, that code will blink repeatedly with a three second delay between blinking sequences. If more than one code is stored, the first code will blink once, then after a three seconds delay, the next code will blink. This sequence will continue until pin 13 is no longer grounded.

4.When reading the diagnostic trouble codes, the number of shift select buttons blinks will indicate the code number.

 

[rationull]

Wow, thanks for all the detail. That may yet come in handy.

For now I've, uhh, "fixed" the problem just by putting the switch back in after having it apart. Sort of. On a test drive, all three drive modes worked multiple times, and I got the switch lights to come on sometimes but the lights are still flaky.

I'm ordering a new switch.

 

[rationull]

Update: I put the new switch in today, took it for a drive and everything seems to be working. Unlikely to be a fluke given how quickly the old switch turned flakey again after I put it back in last weekend. $30 well spent.

 

[eljefino]

There ya go!

The solder they use now doesn't have much lead in it for environmental reasons and it cracks after a few years of vibration.

I brought a volvo fuel injection relay back to life by touching the pins where they go through the circuit board with a soldering iron, remelting their blobs, but on your switch they don't seem obviously accessible.

 

[rationull]

Yeah, when I took it apart I thought about resoldering as kc8adu suggested, but all the joints that I could see looked "OK" (i.e. not perfect but decent) and tested fine. Who knows what happens during vibration when driving though.

Only downside is that replacing the switch is so easy that I don't get credit for a real repair on this one