Ford Differential Electronic Module

Ford Differential Electronic Module (DEM) Post-Repair Troubleshooting Guide

Important: Use this guide if you reinstalled your repaired Ford Differential Electronic Module (DEM) and the problem is still happening.
Important: Please do not open/disassemble the module. Opening the module can damage it and may void warranty coverage.

Applies to:
Ford Five Hundred (2004–2007)
Ford Freestyle (2005–2007)
Mercury Montego (2005–2007)

Common symptoms this guide helps with:

• All-Wheel Drive (AWD) not working
• Communication issues with the DEM
• Water/moisture-related faults (intermittent AWD or warnings)

Step 1: Confirm the symptom and warning messages

• Is AWD completely inoperative, intermittent, or only fails under load/slip?
• Note any messages/warnings (ex: AWD disabled, traction control warnings, etc.).
• If the issue happens right after other work (battery, alternator, wiring repairs), recheck what was touched first.

Step 2: Battery voltage / charging system check

• Weak voltage can cause AWD modules to drop out or throw communication faults.
• If the battery is old/weak or voltage dips during crank, fix that first.

Step 3: Check all AWD/ABS-related fuses

• Use the vehicle fuse legend and inspect fuses tied to:
  • AWD / DEM
  • ABS / stability / traction control
• If a fuse is blown, don’t keep replacing it—inspect the harness for a short.

Step 4: Reseat and inspect the DEM connector

• Key OFF.
• Unplug/reseat the DEM connector until the lock fully clicks.
• Inspect for:
  • Corrosion/green tint on pins
  • Moisture/water marks
  • Bent or pushed-back pins
• If you see corrosion, the vehicle-side connector/harness needs to be addressed or the problem can return.

Step 5: Inspect DEM mounting area for water intrusion

• Because “water damage” is a known issue on these, verify the area around the module is dry.
• Look for signs of past moisture:
  • Damp carpet/trim nearby (if applicable)
  • Water trails, staining, or corrosion on brackets/fasteners
• Fix the leak source (sunroof drains, cowl leaks, rear hatch leaks, etc.) or the issue can repeat.

Step 6: Scan the vehicle for codes (don’t guess)

• Perform a full vehicle scan and record codes from:
  • AWD/DEM
  • ABS module
  • PCM
• Communication codes across multiple modules usually point to vehicle-side power/ground/network issues, not the DEM alone.

Step 7: Clear codes and do a controlled retest

• Clear codes.
• Key cycle OFF → ON.
• Drive-test in a safe area:
  • Straight-line acceleration
  • Light turns
  • If possible, low-traction test (wet/loose surface) to see if AWD engages
• Re-scan and note what returns immediately (hard faults) vs later (intermittent).

Step 8: If AWD is not working but DEM communication is OK

• This commonly points to vehicle-side inputs/outputs that the DEM relies on:
  • ABS/wheel speed sensor faults (AWD may disable if wheel speed data is invalid)
  • Wiring to the rear differential/Haldex-style unit (if equipped)
  • Power/ground integrity under load
• Address any ABS/wheel speed related codes first.

Step 9: If the DEM will NOT communicate

• Focus on basics before blaming the module:
  • Confirm fuses are good
  • Confirm connector pins are clean/tight
  • Check for moisture/corrosion in the connector
  • Inspect harness routing for chafing/rubs
• If communication is still not possible, the issue is often wiring/power/ground/network on the vehicle.

Step 10: If the problem is intermittent (works sometimes, fails sometimes)

• Intermittent AWD/comm issues are commonly caused by:
  • Moisture intrusion returning
  • Connector pin fitment issues
  • Harness movement (broken wire inside insulation)
  • Low voltage events
• Recheck Step 3–5 carefully and look for conditions that trigger the fault (rain, car wash, cold mornings, etc.).

Step 11: What to send UpFix if issues continue

• VIN
• Exact codes (photos/screenshots preferred)
• Whether the DEM communicates on a scan tool
• Whether the fault is constant or intermittent
• Any signs of corrosion/water at the connector or mounting area