Black PCM w/ Red Plug 80s-90s PCM

Black PCM w/ Red Plug (80s–90s) — Troubleshooting Guide (SKU Group)

Important: Use this guide if you reinstalled your repaired Black PCM w/ Red Plug (80s–90s) and the problem is still happening.
Important: Please do not open/disassemble the PCM or probe the connector pins in a way that can spread terminals.

• Step 1: Confirm the correct PCM
  • Verify the PCM part number matches the original unit for the vehicle.
  • If a replacement PCM was used, confirm it’s the correct calibration for engine/trans.
• Step 2: Battery + grounds first
  • Confirm battery connections are clean/tight.
  • Inspect/clean engine ground straps and battery-to-chassis grounds (poor grounds can cause random codes/no-start/stall).
• Step 3: Check PCM fuses + ignition feed
  • Check all PCM/ECM/IGN fuses (under-hood + interior).
  • If any fuse is blown, don’t keep replacing—find the short first.
• Step 4: Confirm ASD/Main relay operation
  • Make sure the ASD (Auto Shut Down) / main relay clicks on with key-on and stays on while cranking.
  • A bad relay or relay socket can mimic a bad PCM.
• Step 5: Verify power and ground at the PCM connector
  • Look for loose pins, pushed-back terminals, corrosion, or damaged wiring near the PCM.
  • Pay extra attention to the ground pins and ignition-switched power feed.
• Step 6: Inspect for water intrusion
  • Check for moisture/corrosion around the PCM area and inside the connectors.
  • Any green/white buildup = clean/repair terminals and confirm seals are intact.
• Step 7: If the complaint is “no start / stalls”
  • Confirm crank sensor and its wiring (common cause and can shut down spark/injectors).
  • Check ignition coil/coil connector and distributor pickup (if equipped).
  • Verify fuel pump operation and fuel pressure (don’t assume PCM).
• Step 8: If the complaint is “no communication / scan tool won’t connect”
  • Confirm diagnostic connector power/ground (a dead port looks like a dead PCM).
  • Re-check PCM power/ground and ASD relay output under load.
• Step 9: If the issue happened right after wiring/engine work
  • Re-check anything recently unplugged: grounds, sensor connectors, vacuum lines, and harness routing near exhaust/edges.
• Step 10: If symptoms are unchanged after all checks
  • Have a shop do voltage-drop tests on PCM power and grounds while cranking (this catches hidden resistance that a basic meter check can miss).