4x4 Malfunction Warning

[DukeCanBuildit]

Rhetorical Question - Why in the hell would they make a relay "Not Serviceable" ???

It IS a relay! Relays fail!

The idiocy of these engineering decisions continue to amaze and stun me.

I guess it right up there with not having tail light bulbs / LEDs that are replaceable. Why spend $5-$15 when you can charge $800 when the tail light goes dark.

But, but, but, they last for 50,000 hours……..until they don’t.

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[Dr. Zaius]

Rhetorical Question - Why in the hell would they make a relay "Not Serviceable" ???

It IS a relay! Relays fail!

The idiocy of these engineering decisions continue to amaze and stun me.

I guess it right up there with not having tail light bulbs / LEDs that are replaceable. Why spend $5-$15 when you can charge $800 when the tail light goes dark.

Because it saves .003 cents on the assembly line.

 

[airline tech]

For a reference on the circuit and general operation

Circuit is (HIGHLIGHTED) as engine start up and 4-WD switch has not been touched.

There is no real description of how the circuit works, so I will do my best to highlight operation.

Switch is powered from DC/DC convertor and powers other sources via Fuse #23
such as IPMA-CCM-BLIS-Rear Cam
This is not an actual position switch - it sends pulse inputs via (HS-CAN 2) data bus with
Vech Configuration Data
Modules that are on the HS-2 are:
ABS-ATCM (4WD Switch)-IPMA-OCSM-PSCM-RCM-SCCM & the GWM

The Gateway Module - Uses this data from the HS-2 and ties in (HS-1) data bus and the (HS-3) data bus.

So, when the truck is powered, it looks for (Switch Position) - data sent via HS-2 to the GWM
GWM reads this (config data) and sends it to the PCM (via HS-1) and the IPC (via HS-3)

If the PCM and the switch position - do not match - then a fault is generated.

So, this is where it can be numerous things for the cause.
1. Bad (Com) data from HS-2 or HS-1
2. 4WD - Switch Power (Issue) aka ATCM
3. VREF - Circuit issue at the (Transfer Case) Control Motor & not transmitting (signal) position feedback. (HS-1 to HS-2)
4. 4WD - Relay issue, sticking - which I believe is the root cause - as at start up and no switch movement, the feedback sense of the Transfer Case is looking for a Ground on BOTH the CW & CCW relays, confirming (motor) position (Locked)
If any of the 4WD relays stick- then the (2-Sided) ground (Locked) will not be seen by the (Transfer Case) Motor

Another main module in Control is the ABS module, this provides (Wheel Speed) inputs to the control (mainly for switching 4-HI to 4-Low, limitations. so, any communication data loss here could also affect the 4-Wheel drive circuit.

The AWD Service MSG - Comes from the HS-1 Can Bus and transmits it to the HS-3 Can bus to display on the IPC and the originator is the PCM - sending that message

Hope this brings a (somewhat understanding) of the circuit

Note: The BJB has 3 Relays
4WD Relay (Main) - Not Serviceable and - The 4WD relay is not even visible and is incorporated into the PCB board (they draw it like a regular relay) for wiring diagram understanding
2 Ea Relays CW & CCW Relays (Serviceable)
So, if the (Main) relay is bad - the whole BJB needs replaced.

For the member that had the truck shift into (4-Low) uncommand there was a short somewhere in this circuit that completely by-passed the (ABS-AWD) Lockout (safety feature) to prevent that from occurring (be it circuit short) internal of the BJB or relay stuck

Note: I am not sure (off-hand) without pulling the (T-Case) motor which direction CW or CCW moves the Motor from (2-Hi to 4-Hi)

My guess is - CW- Moves 2-HI - 4-HI - 4-Low
CCW - 4-Low - 4-Hi - 2-Hi
To coincide with the Switch (Movement)

So when you switch from 2-Hi to 4-High or 4-Low, it is a data transmission sent via HS-2 to HS-1 (PCM) the PCM sends a signal on the VREF (Feedback) signal wire (I request-Movement) and then the PCM applies to Ground to the CW Relay, when it reaches commanded position the Ground is removed thus locking both relays in (Both Grounded) AWD Locked position

Same operation for the CCW - Relay

 

[TJC]

Note: The BJB has 3 Relays
4WD Relay (Main) - Not Serviceable and - The 4WD relay is not even visible and is incorporated into the PCB board (they draw it like a regular relay) for wiring diagram understanding
2 Ea Relays CW & CCW Relays (Serviceable)
So, if the (Main) relay is bad - the whole BJB needs replaced.

Thanks for the info.

Is the 4WD relay on the circuit board a mechanical relay, or solid state device? Mechanical implies it can be replaced with a soldering iron. Solid state implies surface mount with perhaps a heat sink.

Based upon you stating that it is not visible I fear it is a solid state device.

- T

 

[airline tech]

I have not viewed one apart (to see the actual) PCB.
But there is not a physical (Relay) on the BJB like the CW & CCW Relays (In View). One would need to pull the BJB apart to see it.

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