Hello, I have been troubleshooting my 4wd for a while now. Hopefully somebody can help. I have a 1996 K2500 with push button 4wd (RPO NP1) and 4L80E - no auto just 2Hi, 4Hi, and 4Lo. The 1996 is a weird year since the TCCM housing doesn't seem to match the 97+ models. It does seem to match the S10 TCCM but the S10 computer has a blue connector and not white. I'm not sure if either are compatible and the 1996 version seems extremely rare. If anyone knows please let me know.
Here are the symptoms:
1. Does not shift into 4Hi (light blinks)
2. Does shift into 4Lo but does not engage front axle (so basically 2Lo)
3. No TCCM codes present
4. All of this seemed to happen all of a sudden one day. I remember shutting the truck off, going into a store, and coming out and the light started flashing and not engaging. I also remember smelling something faintly like burning electrical but that was during a long road trip earlier that week.
Here is what I have done:
1. Diagnosed front axle actuator wiring. Power probed and found no power on any of the pins. Fuse has power. Ground appears good. My harness might be the Dorman 600-600 model because it only has 3 wires and not 5. It was also spliced into the transfer case switch.
2. Jumpered transfer case switch pins and power traveled up to actuator on both brown and blue wire. Wiring should be good going from transfer case switch to actuator. Transfer case switch had power on brown wire if I remember correctly.
3. Replaced transfer case switch.
4. Replaced actuator.
5. Diagnosed TCCM using GM diagnostic table for (4WD no mode shift). Pins C10, D10, D12, and D13 all were less than 2 ohms for the ground (0.8, 1.0, 0.5, 0.5 Ohms respectively) implying that the grounds are good.
6. Pins C6, C8, D2, D14, D15 all had 12 V. Pins C11, C12, and C14 had 10.35 V. Pin D8 had reference 5V.
7. Dash switch is not shorted to voltage on C4 & C5 but when depressed the switch is properly sending power to TCCM.
8. Diagnostic table said to replace TCCM. Replaced with used TCCM and has same symptoms still.
9. No power on CKT 1695 which is Pin C1 on the TCCM for the Front Axle Switch. No power when 4WD button is pressed or when driven back and forward. Also does not appear to be shorted to ground. Does this just act as an end switch to show that the front axle is engaged to the TCCM or does it actually supply power to the circuit from the TCCM?
Does not having power on C1 mean that the used TCCM is also bad? It was a used version from eBay. Or is it more likely that I have a broken wire somewhere? I'm hoping that I don't have a broken wire because the harness goes right behind the engine by the distributor and passenger side exhaust. I remember seeing another post of someone that had burnt wiring by the exhaust manifold. Mine didn't appear to be but I wouldn't be surprised if I missed it.
The other part of the harness goes over the transmission to the transfer case. The harness coming out of the TCCM (C100) also goes right under the brake booster so it's a PITA to get to. Connector C120 appears to be inaccessible with my transmission and model. It looks like it is on the top of the bell housing according to the diagrams.
I attached the wiring diagram that I have been using to diagnose. If anyone can provide a solution I would be super grateful!
Thanks,
Tad
Here are the symptoms:
1. Does not shift into 4Hi (light blinks)
2. Does shift into 4Lo but does not engage front axle (so basically 2Lo)
3. No TCCM codes present
4. All of this seemed to happen all of a sudden one day. I remember shutting the truck off, going into a store, and coming out and the light started flashing and not engaging. I also remember smelling something faintly like burning electrical but that was during a long road trip earlier that week.
Here is what I have done:
1. Diagnosed front axle actuator wiring. Power probed and found no power on any of the pins. Fuse has power. Ground appears good. My harness might be the Dorman 600-600 model because it only has 3 wires and not 5. It was also spliced into the transfer case switch.
2. Jumpered transfer case switch pins and power traveled up to actuator on both brown and blue wire. Wiring should be good going from transfer case switch to actuator. Transfer case switch had power on brown wire if I remember correctly.
3. Replaced transfer case switch.
4. Replaced actuator.
5. Diagnosed TCCM using GM diagnostic table for (4WD no mode shift). Pins C10, D10, D12, and D13 all were less than 2 ohms for the ground (0.8, 1.0, 0.5, 0.5 Ohms respectively) implying that the grounds are good.
6. Pins C6, C8, D2, D14, D15 all had 12 V. Pins C11, C12, and C14 had 10.35 V. Pin D8 had reference 5V.
7. Dash switch is not shorted to voltage on C4 & C5 but when depressed the switch is properly sending power to TCCM.
8. Diagnostic table said to replace TCCM. Replaced with used TCCM and has same symptoms still.
9. No power on CKT 1695 which is Pin C1 on the TCCM for the Front Axle Switch. No power when 4WD button is pressed or when driven back and forward. Also does not appear to be shorted to ground. Does this just act as an end switch to show that the front axle is engaged to the TCCM or does it actually supply power to the circuit from the TCCM?
Does not having power on C1 mean that the used TCCM is also bad? It was a used version from eBay. Or is it more likely that I have a broken wire somewhere? I'm hoping that I don't have a broken wire because the harness goes right behind the engine by the distributor and passenger side exhaust. I remember seeing another post of someone that had burnt wiring by the exhaust manifold. Mine didn't appear to be but I wouldn't be surprised if I missed it.
The other part of the harness goes over the transmission to the transfer case. The harness coming out of the TCCM (C100) also goes right under the brake booster so it's a PITA to get to. Connector C120 appears to be inaccessible with my transmission and model. It looks like it is on the top of the bell housing according to the diagrams.
I attached the wiring diagram that I have been using to diagnose. If anyone can provide a solution I would be super grateful!
Thanks,
Tad
