Principles of Operation — Electronic Shift on the Fly (ESOF) System
The electronic shift on the fly (ESOF) system is an electronic shift system that allows the operator to choose between 2 different 4x4 modes as well as two
wheel drive (2H). The operator can switch between 2H and 4H mode at speeds up to 88 km/h (55 mph). To engage or disengage 4L, the 4X4 control module
requires that the vehicle speed be less than 5 km/h (3 mph), the brake pedal pressed and the transmission in NEUTRAL (or clutch pedal applied on manual
transmission vehicles). This system contains pulse vacuum hublocks (PVH). The selector on the hublock, when turned fully counterclockwise to the AUTO
position, engages/disengages the hub depending on the driver-selected 4x4 mode of operation. The hublocks also contain a manual override. When the
selector is rotated fully clockwise to the LOCK position, the 4WD vacuum and electronic system used for coupling the front axle to the wheels is bypassed.
The transfer case is equipped with a dual-cone synchronous clutch. This clutch is used to synchronize the front driveline to the rear. All other internals
operate in the same manner as the manual shift system. When the mode select switch (MSS) on the instrument panel is turned, the 4X4 control module
powers the transfer case shift motor, which activates a shift fork and engages the clutch. When the shift motor reaches the desired position, as determined by
the encoder position inputs to the 4X4 control module, power to the shift motor is removed. When the transfer case front and rear output shafts are
synchronized, the lockup collar mechanically engages the mainshaft hub to the drive sprocket. At this point, the front axle PVHs are engaged. Instrument
panel "4x4" and "Low Range" indicator operation is controlled via hardwired outputs from the 4X4 control module.

Electronic Shift Transfer Case
The New Venture Gear NV 273 electronic shift transfer case is a 3-piece aluminium design. The unit transfers engine power from the transmission to the
front and rear axles. Under normal driving conditions the unit is in two wheel drive high (2H), but when desired, the operator may shift into four wheel drive
high (4H) or 4-wheel drive low (4L). The transfer case is shifted electronically. The unit is lubricated by a positive displacement fluid pump that channels
fluid flow through holes in the mainshaft.
The encoder assembly used for mode indication is of a Hall-effect type. The system uses a total of 4 independent transistors for mode recognition. Each
transistor is associated with a specific range of motor movement. When the circuit to each transistor is closed, a ground path to any of the A, B, C or D
encoder pins is accomplished. When the circuit to each transistor is open, the connection to any of the A, B, C or D encoder pins is pulled to voltage. Mode
indication is "decoded" by the 4X4 control module based on the varying combinations of signals coming from the transistors.

Pulse Vacuum Hublocks
The electronic shift-on-the-fly (ESOF) system has a feature which allows the driver to override the vacuum-operated hublocks. When the front hublock dials
are manually turned to the LOCK position, the hublocks are locked through the dial at all times, regardless of the position of the instrument panel mode
select switch (MSS).
When the front hublock dials are manually turned to the AUTO position, the hublocks use an internal spring-locking mechanism that is vacuum-operated by
the ESOF system, and are locked with the MSS in the 4H or 4L position and unlocked with the MSS in 2H position. Refer to Hublock Operation in this
section for additional AUTO hublock system operation.

Hublock Operation
With the hublocks in the AUTO position, the 4WD ESOF system uses timed vacuum sequences to lock and unlock the wheel ends when switching the
instrument panel MSS between 2WD and 4x4 modes. A high vacuum level (222 mm [8.75 in] Hg and greater) is applied to the hublocks to lock the internalspring mechanism that engages (locks) the hublocks for 4x4 mode, and a lower vacuum level (114 to 184 mm [5.9 to 7.1 in] Hg) is applied to unlock the
spring mechanism that disengages (unlocks) the hublocks for 2WD mode. The vacuum signals are supplied to the hublocks by system components, including
the 4X4 control module, wiring harness, solenoid, vacuum harness and vacuum seals. As a first step in service, eliminate obvious items such as loose wiring
connections, loose vacuum connections or damaged vacuum lines.

Hublock Engagement/Disengagement Time
With the hublock dials in the AUTO position, switching the instrument panel MSS to 4x4 HIGH or 4x4 LOW results in the high vacuum level being applied
to the hublocks, which lasts for at least 51 seconds (including a venting cycle). The hublocks should engage during this time through the spring mechanism.
Switching the MSS to 2WD less than 51 seconds after the MSS has been switched to 4x4 HIGH or 4x4 LOW will not interrupt the high vacuum level;
instead, the lower vacuum level will be applied after the high vacuum level cycle is completed.
With the hublock dials in the AUTO position, switching the MSS to 2WD results in the lower vacuum level being applied to the hublocks, which lasts for
about 15 seconds. However, the actual time required for the hublocks to disengage by spring force can vary considerably due to the effects of driveline windup.
Road bumps, vehicle speed, acceleration cycles or momentary reversal of direction can assist this process. Switching the MSS to 4x4 HIGH or 4x4 LOW
always overrides the lower vacuum level cycle and results in an immediate high vacuum level and in locking of the hublocks.

Manual Override
The AUTO hublocks have a manual override selector dial, which, when turned to the LOCK position, will keep the hublocks locked (through the dial) at all
times, regardless of the position of the instrument panel MSS. (Always set both hublock dials to the same position.) If the hublock dials are manually turned
from the AUTO to the LOCK position, and the high vacuum level is applied to the hublocks (by switching the MSS from 2WD to 4x4 HIGH or 4x4 LOW),
the hublocks will be locked through the dial as well as through the internal spring mechanism. In this case, turning the hublock dials back to the AUTO
position will leave the hublocks in (spring) locked mode until the lower vacuum level is applied to the hublocks (by switching the MSS from 4x4 HIGH or
4x4 LOW to the 2WD). (This is not an issue if the hublock dials have been in the AUTO position and the MSS has been switched from 4x4 HIGH or 4x4
LOW to 2WD at least once.) Alternatively, the hublock mechanism can be reset to free mode at any time by turning the hublock dial from AUTO to LOCK to AUTO at least 2 times.

I have a PDF file that has more information if you are interested just PM me.

-Art

 

Art, please send it to me, I want to add it to this thread.

Thanks

Stewart

 

OEM 4X4 ESOF Operation, Diagnosis, and Testing .pdf attached.

Thanks, Art!

Stewart

 

Sticky?

 

Sticky what?

Individual tech threads don't get their own sticky. They do get added to the tech folder though...which was why I cut that first post from Apocs thread and put it here as its own stand alone thread.

Stewart