96 Suburban 4WD electronic shifting problem

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danp - 29 Nov 2006 00:05 GMT

My 96 K1500 will not go into 4W High. The 4WD High light keeps blinking.
Originally my problem was it would not shift into 4-wheel drive at all. I
replaced the 25-amp fuse, front drive actuator and transfer case switch and
now it does shift into 4W Lo (light on and front axle engaged), but not 4W
Hi. The front axle actuator was upgraded to the new solenoid style
(pn26060073) and appropriate wiring harness.

When I select 4W High (from 2W High), I hear a noise from the transfer case
encoder motor but the front axle actuator never gets powered to engage. I
know the transfer case switch is good as it was replaced and activates
during 4W LO selection. Also, I verified the 4WD High selection switch on
the dash is good (measured at the TCCM). I checked for error codes and there
are no DTC codes stored in the TCCM. I am now focusing my troubleshooting on
the transfer case encoder. I have wiring schematics and but they don't
indicate what signals levels or "states" I should have out of the transfer
case encoder for a 4W High selection. My diagrams show Encoder Channels A,
B, C and P.

Can someone tell me what happens inside the transfer case during a 4W High
selection and what the transfer case encoder signals should be at that time?
Is the encoder assembly servicable without removing the transfer case? Any
ideas would be appreciated.

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SnoMan - 29 Nov 2006 01:06 GMT

>Can someone tell me what happens inside the transfer case during a 4W High
>selection and what the transfer case encoder signals should be at that time?
>Is the encoder assembly servicable without removing the transfer case? Any
>ideas would be appreciated.

Here is a take on a manual shift GM T case , your NV243 works the same
except it has a encoder motor to shift it

Shifting into the 4HI range causes the following conditions to occur.

1. The front axle indicator lamp does not come on until the front axle
engages.

2. The torque flows from the input gear to the mainshaft the same as
in the two-wheel position.

3. The shift linkage moves the mode synchronizer sleeve into
engagement with the clutch teeth of the drive sprocket. This locks the
drive sprocket to the mainshaft through the synchronizer sleeve. The
torque is transmitted through the drive sprocket and the drive chain
to the driven sprocket and the output shaft. The torque then flows
through the front propeller shaft to the front axle.

4. The shift mechanism in the transfer case closes the transfer case
switch.

5. The current is then applied to the front axle thermal actuator and
the front axle switch. In the K30 vehicles, the power is also supplied
by the transfer case relay to the transfer case synchronizer. The
thermal actuator contains a heating element, a gas charge, and a
piston.

6. When the current is applied, the heating element heats the gas. The
gas expands, pushing the piston out after a delay of a few seconds.
The piston actuates the shift fork in the front axle. This connects
the right axle output shaft to the front axle differential. The torque
is then available at the front wheels. The front axle shift mechanism,
when fully engaged, closes a switch, causing the front axle indicator
lamp to come ON.

7. If the shift lever is moved back to the 2 WHEEL position, the
operations in the preceding steps are reversed. The current to the
thermal actuator is turned OFF. The gas cools and the piston retracts
allowing the shift fork in the front axle in order to return to the
two-wheel drive position.

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TheSnoMan.com

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