89 GM truck with check engine light but scanner won't communicate

Symptoms:

This GM truck was brought to a mechanic friend's shop from out of town (yesterday...here in Calgary) and when it runs, the check engine light stays on and flickered.The idle is also a bit erratic or rough. The mechanic changed the ecm (computer) but it has no effect. We then cut the SES light wire terminal at the ecm but the check engine light bulb remained on. We suspect, the bulb is getting a back feed due to a shorted wire or connection.

Basic Strategy:

As soon as you start this vehicle, the check engine light should come on a few seconds.After starting, the light should go away unless there is faulty (shown by codes) sensors or components in the electrical system.

Recommended fix:

Access the ecm wire terminals and check the readings of all ground connections. Any ground reading higher than 0.250 volt is a suspect circuit. Check also the reference voltage which is usually 5 volts. If reading is different from this, undo all the load connections like IAC motor, injectors, alternator or other solenoid until the reference voltage returns to normal. The last one you touched is the culprit load circuit.

Note:

Looking at the engine compartment (see picture), I would change the complete wiring harness first. It is too costly to do the tests (above) which can still re-occur in other circuits.Also,is the customer willing to pay for ALL the wiring tests? A complete wiring harness is usually available from most salvage yards.

For proper ecm test and wiring diagram, please contact ATS.