EEC-IV PCM Repair

– EEC-IV PCM Repair — Troubleshooting Guide

Important: Use this guide if you reinstalled your repaired EEC-IV PCM and the problem is still happening.
Important: Please do not open/disassemble the PCM or attempt to probe the pins with tools that can spread terminals.

• Step 1: Verify the correct PCM was installed
  • Confirm the service number/catch code/part number on the PCM matches the vehicle requirements.
  • If a different PCM was installed, confirm it’s the correct calibration for engine/trans/emissions.
• Step 2: Check battery voltage and main power feeds
  • Low voltage can cause no-start, stalling, erratic shifting, and false sensor codes.
  • Verify the battery connections are clean/tight and the engine/body grounds are secure.
• Step 3: Check PCM power fuses and main relays
  • Check PCM/EEC/ECM fuses and the EEC (PCM power) relay.
  • If a fuse blows again, stop and have the circuit checked for a short before replacing more fuses.
• Step 4: Inspect the PCM connectors and pin fitment
  • Unplug/replug the PCM connector(s) until fully seated and latched.
  • Look for bent pins, pushed-back terminals, corrosion/green buildup, moisture, or damaged wiring near the connector.
• Step 5: Confirm clean power/ground at the PCM (vehicle-side)
  • A PCM can test good but still act “dead” if the vehicle has a weak ground, bad relay feed, or voltage drop.
  • Pay extra attention if the issue started after battery work, wiring repairs, or engine work—recheck what was touched.
• Step 6: Check for signs of water intrusion
  • Many EEC-IV locations are vulnerable to leaks; moisture can corrode terminals and cause intermittent no-start/stalls.
  • Inspect the PCM area and harness for water tracks, damp carpet, or corrosion residue.
• Step 7: If you have a “no communication” or intermittent communication complaint
  • Confirm all PCM powers/grounds first (most comm issues are power/ground/relay related on these).
  • Inspect the harness for rub-through near the firewall, battery tray, and engine grounds.
• Step 8: If the issue is drivability-related (misfires, rich/lean, stalling)
  • Verify basics that commonly mimic a PCM fault: vacuum leaks, fuel pressure/regulator issues, ignition components, and sensor connector condition.
  • If multiple unrelated codes appear at once, that often points to power/ground or harness issues.
• Step 9: If the issue involves transmission behavior
  • Confirm the transmission connector and harness condition, especially near exhaust heat and sharp brackets.
  • Low system voltage or bad grounds can cause shifting complaints even with a good PCM.
• Step 10: If symptoms are unchanged after all checks
  • Have a qualified shop perform voltage-drop testing on PCM power/grounds and verify relay outputs under load.
  • Intermittent faults usually trace back to wiring, grounds, relay sockets, or corrosion rather than the PCM itself.