GM C3 qns..

[John T Stein]

> Date:          Tue, 25 Jul 1995 09:57:38 +1000 (EST)
> From:          Craig Pugsley <c.pugsley at trl.oz.au>
> Subject:       GM C3 qns..
> To:            diy_efi at coulomb.eng.ohio-state.edu
> Reply-to:      diy_efi at coulomb.eng.ohio-state.edu


> Also, should the 'check engine' light flash fault codes at engine startup
> or only when you bridge the ALDL connector? (This is my main question
> actually)
> 
> Cheers,
> Craig. 

Craig,

The "check engine" lamp only flashes codes when you have bridged the 
ALDL connector with the engine not running.  Bridging the ALDL when 
the engine is running will cause the ECM to enter what GM refers to 
as "field service" mode in which the lamp flashes rapidly (about 
twice/second) when the ECM is operating open-loop, and about once per 
second when in closed loop.  In field service mode the lamp duty 
cycle gives SOME indication of fuel mixture, with the lamp spending more 
time "on" as the mixture gets richer.

John