GM Early 90s ECU/ECM

GM Early 90s ECU/ECM Post-Repair Troubleshooting Guide

Important: Use this guide if you reinstalled your repaired GM Early 90s ECU/ECM and the problem is still happening.

Important: Please do not open/disassemble the module. This can damage the unit and may void warranty coverage.

Step 1: Confirm correct ECU and installation

• Verify the ECU part number matches the original
• Confirm connectors are fully seated/latched (no “half-seated” plugs)
• Inspect for bent pins, pushed-out terminals, corrosion, moisture, or oil in the connectors

Step 2: Battery voltage + basics

• Battery fully charged (weak battery can cause crank/no-start or no-comms)
• Clean/tighten battery terminals
• Verify engine grounds and chassis grounds are clean and tight

Step 3: Check ECU fuses and power feeds (do not rely on visual checks)

• Check all ECU/ECM/IGN fuses with a test light or meter
• Verify ECM/IGN relay operation (swap with a known-good relay if possible)
• If the vehicle has fusible links, verify they are intact and not high-resistance

Step 4: Confirm ECU has power and ground at the harness

• Key ON: verify ignition feed to ECU is present
• Verify constant battery feed to ECU is present (if equipped)
• Verify ECU ground circuits have low resistance to battery negative
• If power/ground is missing, the issue is vehicle-side (wiring, fuse, relay, ignition switch)

Step 5: No communication with ECU/ECM

• Confirm scan tool powers up at the OBD/ALDL connector
• See if other modules communicate (if applicable)
• If nothing communicates, suspect OBD/ALDL power/ground or a blown fuse
• If other modules communicate but ECU does not, re-check ECU power/ground and connector pin fitment

Step 6: Cranks but won’t start (common checks)

• Confirm fuel pump runs/prime occurs (listen/key-on prime)
• Verify fuel pressure meets spec for the vehicle
• Check for injector power feed (often ignition-fed through a fuse/relay)
• Verify spark is present; if no spark, check ignition module/coil/distributor inputs and related fuses

Step 7: Fuel pump issues

• Check fuel pump fuse/relay and relay control circuits
• Confirm power and ground at the pump during crank
• Inspect common ground points near frame/engine for corrosion/looseness

Step 8: No pulse to injectors

• Verify injector fuse is good and injectors have power with key ON (one side)
• Check crank/cam sensor signals (many systems won’t pulse injectors without reference signal)
• Inspect distributor/ignition module connections (common on older GM systems)
• If spark is present but no injector pulse, inspect wiring to injectors and ECU grounds

Step 9: If the issue started after recent work

• Re-check anything recently touched: grounds, fuses, relays, distributor connections, fuel pump wiring
• Look for pinched harnesses, swapped connectors, or blown fuses caused during the repair

If the issue is still present

• Have ready: VIN, ECU part number, symptom (no-comms / crank-no-start / fuel pump / no injector pulse), and what you’ve verified (fuses, relays, fuel pressure, spark).
• If the problem happens immediately after install and you need fast help, call or email UpFix for quicker support.