4WD Won't engage, '96 K1500 Suburban

['72 Chevy Suburban]

My truck is in 2HI and when I push the button for the 4WD Hi or Lo, the light flashes, but nothing happens. Here is what I have done:

Checked the 4x4, trans and TC fuses and all are ok
Per the FSM, checked to make sure all lights come on for 1 second during startup

I followed the FSM through the troubleshooting up to Table E until I got to DLCs and DTCs and that is where they lost me.

There is a normal sounding clicking noise coming from under the dash and I assume that the TCCM is operating. By process of elimination, does this mean that it is probably the Actuator(?) that is located on the transfer case?

Is there more troubleshooting that I can do in order to further narrow this down?

Thanks,

[sandking]

I had the same problem with my '96, light flashing and not engaging, it ended up being the actuator. Replaced it and fixed the problem.

['72 Chevy Suburban]

Thanks Sandking. I am pretty sure that it is the part that is bad, but was hoping to better troubleshoot it before dropping the $120. I have been known to throw parts at vehicles and am trying to get better in my old age...

Thanks,

[LIVE RUST]

Same problem 3 times in 4 years on my Junk out of the box, more wheel bearings & ball joints than I can count, 2 fuel pumps in five years, 99 ZR2. One time it turned out to be a pin hole in the vaccum line to the front diff acutator, then the other two times it was the actuator on the transfer case. If its the T-Case one, try taking the actuator off and follow the wires into it (under the plastic cover), They usually corrode where they're attached to the motor and somtimes are repairable. What the hell was wrong with front leaf springs and a shifter to put it in 4wd?

[72 CustomCamper]

Originally Posted by LIVE RUST

What the hell was wrong with front leaf springs and a shifter to put it in 4wd?

Too many people wanted a truck that rides like a car...


Chuck

['72 Chevy Suburban]

I replaced the transfer case motor and now I am hearing it actuate when I hit the switch. Now, I don't have 4Hi, but I have 4Lo. The light just keeps on flashing. Any ideas?

Thanks,

['72 Chevy Suburban]

bump... Help!

[LIVE RUST]

Are you sure the fronts are spinning in low?

[89 crew]

Originally Posted by LIVE RUST

What the hell was wrong with front leaf springs and a shifter to put it in 4wd?

Simple! They didn't get to see any parts for those! Anyone can make a piece of steel linkage! They wanted parts you would have to buy from them! If you want a sad thought think about the new trucks being built! What percentage will be rebuildable when they are 40 years old? I'd be willing to bet there will be more 67-72's around then than there are 99-06's!

['72 Chevy Suburban]

Originally Posted by LIVE RUST

Are you sure the fronts are spinning in low?

I am pretty sure. I felt a clunk and heard the extra whine that is usually associated with the fronts turning and I didn't take it above 30MPH because of the whining. It had the same feel as 4LO. The only thing that I don't remember was whether or not the light stayed on in 4LO. I googled it at lunchtime and came up with a pretty interesting link. I am going to check this stuff out this weekend: http://autorepair.about.com/library/a/1/bl378.htm

Do you think that this system and troubleshooting would apply to my truck? It sounds like a similar setup and it goes back to your earlier comment about a leaking vacuum hose.

Thanks,

[DirtyLarry]

These electric shift junkers have 4 main components and 10 miles of wiring to go wrong.

1) Control head (button assembly)
2) Tranfercase control module (TCCM) - under the dash
3) Encoder motor - located on the tranfercase
4) Front axle disconnect – located on the front axle

If you actually hear and feel a clunk and the tranfercase will actually go into low range but you have no front wheel action, then you have a problem with the front axle disconnect. Unlike the link you posted regarding the S-10 Blazer, the full size rigs did not use vacuum to engage the front axle. 1996 was the first year for a fully electronic front axle disconnect from the previous thermal axle disconnect. It sounds like that or the wiring to it could be your culprit.

Click this link for the ultimate fix for front axle disconnect issues. http://www.4x4posi-lok.com/app_chevy-k.html

['72 Chevy Suburban]

Thanks for the info, Larry...

Is there any way to test the front axle disconnect to verify that it isn't working? Or, would that have thrown a code?

Thanks for the help,

[DirtyLarry]

You might want to try running through this diagram and verify the power and grounds. Outside of that I could not a find a formal procedure to test it.

['72 Chevy Suburban]

Excellent info! Thans a bunch, Larry. My buddy happened to have his scanner with him today at work and we pulled the following code:

P1860
TCC PWM Solenoid Ckt

I'll have to google it to see if this means that the solenoid is bad or maybe a voltage, as you mention.

Thanks,

['72 Chevy Suburban]

It appears to be a Torque Converter Solenoid related code. I also found this alternate description which leads more toward a 4wd problem:

P1860 TCC PWM Solenoid Circuit Electrical/ Transmission Transfer Case Contact Plate 'D' Short Circuit To Battery

Any idea of how to troubleshoot that?

Thanks,

[DirtyLarry]

See if this helps chase after the P1860 code. Seems like a stretch to be related to the lack of front axle engagement or 4wd in general. I think you have two issues going on.

DTC P1860 TCC PWM Solenoid Circuit Electrical

Circuit Description
The Torque Converter Clutch Pulse Width Modulation Solenoid Valve (TCC PWM Sol. Valve) controls the fluid acting on the converter clutch valve. The converter clutch valve controls the TCC application and release. The solenoid attaches to the control valve body within the transmission. The solenoid receives ignition voltage through circuit 1020. The Vehicle Control Module (VCM) controls the solenoid by providing a ground path on circuit 418. Current flows through the solenoid coil according to the duty cycle (percentage of ON and OFF time). The TCC PWM Sol. Valve provides a smooth engagement of the TCC by operating during a duty cycle percent of ON time.

When the VCM detects a continuous open or short to ground in the TCC PWM Sol. Valve circuit or the TCC PWM Sol. Valve, then DTC P1860 sets. DTC P1860 is a type A DTC.

Conditions for Setting the DTC
• The system voltage is 10-16 volts.

• The engine speed is greater than 450 RPM for 5 seconds.

• Not in fuel cutoff.

• The VCM commands first gear.

• The TCC duty cycle is less than 10% or greater than 90%.

• All of the above conditions are met for 5 seconds and either of the following fail conditions occurs for 5 seconds:

- The VCM commands the solenoid ON (90%) and the voltage input remains high (B+).

- The VCM commands the solenoid OFF (0%) and the voltage input remains low (0 volt).

Action Taken When the DTC Sets
• The VCM illuminates the Malfunction Indicator Lamp (MIL).

• The VCM inhibits TCC engagement.

• The VCM inhibits 4th gear if the transmission is in hot mode.

• The VCM freezes shift adapts from being updated.

• DTC P1860 stores in VCM history.

Conditions for Clearing the MIL/DTC
The VCM turns OFF the MIL after three consecutive trips without a failure reported.
A scan tool can clear the DTC from the VCM history. The VCM clears the DTC from the VCM history if the vehicle completes 40 warm-up cycles without a failure reported.
The VCM cancels the DTC default actions when the fault no longer exists and the ignition is OFF long enough in order to power down the VCM.
Diagnostic Aids
• Inspect the wiring at the VCM, the transmission connector and all other circuit connecting points for the following conditions:

- A bent terminal

- A backed out terminal

- A damaged terminal

- Poor terminal tension

- A chafed wire

- A broken wire inside the insulation

- Moisture intrusion

- Corrosion

• When diagnosing for an intermittent short or open condition, massage the wiring harness while watching the test equipment for a change.

Test Description
The numbers below refer to the step numbers on the diagnostic table.

This step tests for voltage to the solenoid.

This step tests the ability of the VCM and wiring to control the ground circuit.

This step tests the resistance of the TCC PWM Sol. Valve and the Automatic Transmission Wiring Harness Assembly (A/T Wiring Harness Assembly).



DTC P1860 Torque Converter Clutch Pulse Width Modulation Solenoid Valve Circuit - Electrical Step
Action
Value(s)
Yes
No

1
Was the Powertrain On-Board Diagnostic (OBD) System Check performed?
--
Go to Step 2
Go to Powertrain On Board Diagnostic (OBD) System Check (4.3L) or Powertrain On Board Diagnostic (OBD) System Check (5.0L, 5.7L) in Engine Controls

2
Install the Scan Tool .
With the engine OFF, turn the ignition switch to the RUN position.
Important: Before clearing the DTC(s), use the Scan Tool in order to record the Freeze Frame and Failure Records for reference. Using the Clear Info function will erase the stored Freeze Frame and Failure Records from the VCM.


Record the DTC Freeze Frame and Failure Records, then clear the DTC(s).
Are DTCs P0740, P0753, P0758 and P0785 also set?
--
Go to Step 3
Go to Step 4

3
Inspect the TRANS fuse.
If the fuse is open, inspect circuit 1020 (PNK), the solenoids and the A/T Wiring Harness Assembly for a short to ground. Repair the circuit, the solenoids and the harness if necessary.
Refer to General Electrical Diagnosis Procedures in Wiring Systems.

Did you find a short to ground condition?
--
Go to Step 15
Go to Step 4

4
Turn the ignition OFF.
Disconnect the transmission 20-way connector (additional DTCs may set).
Install J 39775 Jumper Harness on the engine side of the 20-way connector.
With the engine OFF, turn the ignition switch to the RUN position.
Connect a lest lamp from J 39775 Jumper Harness cavity E to ground. Refer to Automatic Transmission Inline Harness Connector End View .
Is the test lamp on?
--
Go to Step 6
Go to Step 5

5
Repair the open or short to ground in ignition feed circuit 1020 (PNK) to the TCC PWM Sol. Valve.

Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?
--
Go to Step 15
--

6
Install the test lamp from cavity E to cavity U of the J 39775 Jumper Harness.
Using the transmission output control function on the Scan Tool , command the TCC PWM Sol. Valve ON and OFF three times.
Does the test lamp illuminate when the TCC PWM Sol. Valve is commanded ON, and turn OFF when commanded OFF?
--
Go to Step 8
Go to Step 7

7
Inspect circuit 418 (BRN) of the engine wiring harness for an open, short to ground or short to power condition. Refer to General Electrical Diagnosis Procedures in Wiring Systems.
Repair the circuit if necessary. Refer to Wiring Repairs in Wiring Systems.
Did you find an open, short to ground or short to power condition?
--
Go to Step 15
Go to Step 9

8
Install the J 39775 Jumper Harness on the transmission side of the 20-way connector.
Using the J 39200 Digital Multimeter (DMM) and the J 35616-A Connector Test Adapter Kit, measure the resistance between terminal E and terminal U. Refer to Automatic Transmission Inline Harness Connector End View .
Is the resistance within the specified range?
10-15 ohms
Go to Step 11
Go to Step 10

9
Replace the VCM.

Refer to VCM Replacement/Programming (4.3L) or VCM Replacement/Programming (5.0L, 5.7L) in Engine Controls.

Is the replacement complete?
--
Go to Step 15
--

10
Disconnect the A/T Wiring Harness Assembly at the TCC PWM Sol. Valve.
Measure the resistance of the TCC PWM Sol. Valve.
Is the resistance within the specified range?
10-15 ohms
Go to Step 13
Go to Step 14

11
Measure the resistance between terminal E and ground, and between terminal U and ground.

Are both readings greater than the specified value?
250 K ohms
Go to Diagnostic Aids
Go to Step 12

12
Disconnect the A/T Wiring Harness Assembly at the TCC PWM Sol. Valve.
Measure the resistance between each of the component terminals and ground.
Are both readings greater than the specified value?
250 K ohms
Go to Step 13
Go to Step 14

13
Replace the A/T Wiring Harness Assembly.

Refer to Torque Converter Clutch Pulse Width Modulation (TCC PWM) Solenoid, TCC Solenoid, and Wiring Harness .

Is the replacement complete?
--
Go to Step 15
--

14
Replace the TCC PWM Sol. Valve.

Refer to Torque Converter Clutch Pulse Width Modulation (TCC PWM) Solenoid, TCC Solenoid, and Wiring Harness .

Is the replacement complete?
--
Go to Step 15
--

15
In order to verify your repair, perform the following procedure:

Select DTC.
Select Clear Info.
Drive the vehicle in D4 with the TCC On. Ensure that the following conditions are met:
- The VCM commands the TCC PWM Sol. Valve ON, and the voltage input drops to zero.

- The VCM commands the TCC PWM Sol. Valve OFF, and the voltage input increases to B+.

- All conditions are met for 5 seconds.

Select Specific DTC.
Enter DTC P1860.
Has the test run and passed?
--
System OK
Go to Step 1

[70rs/ss]

Man I'll never complain about my 205 fighting me into 4X4!! You could do the front solid axles conversion?! I know that's no help but an extreme option!

['72 Chevy Suburban]

Originally Posted by DirtyLarry

See if this helps chase after the P1860 code. Seems like a stretch to be related to the lack of front axle engagement or 4wd in general. I think you have two issues going on.

Lots of good stuff there Larry! Thanks again...

['72 Chevy Suburban]

Originally Posted by 70rs/ss

Man I'll never complain about my 205 fighting me into 4X4!! You could do the front solid axles conversion?! I know that's no help but an extreme option!

Yeah. Say a little prayer for me, 70RS...

['72 Chevy Suburban]

Transmission Transfer Case Contact Plate 'D' Short Circuit To Battery

Does anyone have any ideas as to what this means?

Thanks,