Electronic Shift Vacuum Schematic

Fig 1: Electronic Shift Vacuum Schematic Diagram

Courtesy of FORD MOTOR CO.

Feature inputs 

• MSS 
• Vehicle speed (hardwired to 4X4 control module via ABS)
• Encoder position inputs A, B, C or D
• Transmission NEUTRAL position (automatic transmission)
• Clutch apply signal (manual transmission)

Feature outputs 

• Transfer case dual-cone synchronization clutch
• Transfer case shift motor and Hall-effect encoder assembly
• Transfer case shift motor relays
• PVH  solenoid assembly
• PVH  vacuum pump (certain diesel applications only)
• Cluster indicators are controlled by the IC  which interprets outputs from the 4X4 control module

During 4WD initialization at startup: 

• The 4X4 control module will verify the MSS  position
• If the transfer case is not in the desired mode, the 4X4 control module will attempt to carry out a range shift. If input conditions for 4X4 Low are not present, then the module will either shift to or remain in 4X4 High
• Once the transfer case is engaged, or is already in the correct mode, the PVH  lock/unlock and/or lock solenoids will be energized
• If the MSS  position is 2WD  , the PVH  lock/unlock solenoid will be commanded to energize ONLY at speeds below 8 km/h (5 mph). Once this speed threshold is met, the solenoid will be de-energized. This procedure is to prevent ratcheting induced by an attempt to unlock an already unlocked hub

For a shift attempt from 2WD to 4X4 High or 4X4 Low, the following sequence occurs: 

• MSS  is switched from 2WD  to 4X4 High or 4X4 Low
• For shift attempts into 4X4 Low:
  • transmission must be in NEUTRAL (or clutch pedal applied).
  • vehicle speed must be below 8 km/h (5 mph).
  • if the above 2 conditions are not met, the 4X4 indicator will flash and shifting will not occur.
• Shift motor moves range fork in transfer case, which engages dual-cone synchronizers
• If a blocked shift condition occurs, the shift motor will perform 10 shift attempts (2WD  to 4X4 High or 4X4 High to 4X4 Low) totaling 5 clockwise movements and 5 counterclockwise movements with an approximate one-second delay between each motor movement attempt
• If, after 10 attempts, a shift is not completed, then the module will command a shift motor rest time of 40 seconds, before it attempts any further shifts
• After the 40 seconds have expired, the motor will make 10 more attempts to complete the shift. The motor will continue to repeat this sequence a total of 6 times (60 shift attempts) before ceasing operation. No further shift attempts will be made, nor will any further MSS  commands be recognized. The vehicle must be restarted for 4WD  function to be restored. If this occurs, DTC C1728 (for 2WD  to 4X4 High attempts) or DTC C1729 (for 4X4 High to 4X4 Low shift attempts) will be stored in memory
• If the vehicle is driven above 8 km/h (5 mph) during a 40-second rest period, the module will reattempt the shift strategy outlined previously. Generally, the transfer case will engage as the probability of gear teeth blockage is greatly reduced while rolling
• If no gear teeth blockage is present, the transfer case synchronizers bring the front driveshaft to same speed as rear driveshaft
• Shift fork completes movement and the 4X4 High and (if 4X4 Low) "low range" lamp will illuminate. This ONLY occurs when the driveshafts are synchronized. If they are not, then the 4X4 High or 4X4 Low lamp will flash
• Once the 4X4 High or 4X4 Low lamp is illuminated, the PVH  lock and lock/unlock solenoids are energized for 45 seconds, with approximately 62 kPa (9 psi) of vacuum or more at the hubs. The PVH  lock sequence cannot be overridden at any time (a mode shift change back to 2WD  )
• After 45 seconds have elapsed, the solenoids undergo a 6-second vent period, whereby no commands to either the lock or lock/unlock solenoids are allowed, regardless of MSS  position
• If the vehicle is driven above 8 km/h (5 mph) the PVH  lock and lock/unlock solenoids are re-energized in the same manner, regardless of whether the hubs are engaged or the original 45-second engagement timer has expired

For a shift attempt from 4X4 Low or 4X4 High to 2WD  , the following sequence occurs: 

• MSS  is placed from 4X4 High or 4X4 Low to 2WD 
• For shift attempts out of 4X4 Low:
  • transmission must be in NEUTRAL (or clutch pedal applied).
  • vehicle speed must be below 8 km/h (5 mph).
  • if the above 2 conditions are not met, the 4X4 indicator will flash and shifting will not occur.
• Shift motor moves range fork in transfer case, which disengages dual-cone synchronizers
• If a blocked shift condition occurs, the shift motor will carry out 10 shift attempts from 4X4 High or 4X4 Low to 2WD  , totaling 5 clockwise movements and 5 counterclockwise movements with an approximate one-second delay between each motor movement attempt
• If, after 10 attempts, a shift is not completed, then the module will command a shift motor rest time of 40 seconds, before it attempts any further shifts
• After the 40 seconds have expired, the motor will make 10 more attempts to complete the shift. The motor will continue to repeat this sequence a total of 6 times (60 shift attempts) before ceasing operation. No further shift attempts will be made, nor will any further MSS  commands be recognized. The vehicle must be restarted for 4X4 function to be restored. If this occurs, DTC C1728 (for attempts between 2WD  and 4X4 High) or DTC C1729 (for attempts between 4X4 High and 4X4 Low) will be stored in memory
• If the vehicle is driven above 3 mph (5 km/h) during a 40-second rest period, the module will reattempt a shift (4X4 High to 2WD  only). Generally, the transfer case will disengage as the probability of torque trapping is lower while rolling
• If no torque trapping is present, the transfer case synchronizers decouple the front driveshaft
• Shift fork completes movement and de-energizes 4X4 High, and if 4X4 Low, "low range" lamps. Once the 4X4 lamp is de-energized, the PVH  lock/unlock solenoid is energized for 15 seconds, pulling approximately 41 kPa (6 psi) of vacuum at the hubs
• The PVH  unlock sequence can be overridden at any time such as a mode shift change back to 4X4 High or 4X4 Low, relocking the hubs
• After 15 seconds, the solenoids undergo a 6-second vent period; the vent period can be overridden at any time by a shift attempt to 4X4 High or 4X4 Low