Diagnostic Mode Switch/Connector: Description and Operation

Diagnostic Connector (STO/STI):

The Self Test Output (STO) and the Self Test Input (STI) connectors are used while performing the Electronic Engine Control (EEC) Quick Test diagnostic procedure with either a scan tool or analog voltmeter. When the STI connector is jumped to ground, a signal sent to the Electronic Control Assembly (ECA) activates the ECA Self Test mode. The STO connector receives output signals from the ECA which appear as numbered codes on the scan tool or needle sweeps on the analog voltmeter. Codes retrieved from ECA memory represent a malfunction within the EEC system.