GM Fuel Pump control scheme

[Ludis Langens]

WEG1192 at aol.com wrote:
> 
> To me its obvious that Steve Ravet was right in that the OP switch is used to
> keep the pump running during CPU failure, otherwise limp home wouldn't work
> period. I can also say for certain that the ECM keeps the pump running as
> long it recieves ignition pulses from the distributor. [...]
> The only plausible resaon left is to keep the pump running during
> limp home mode.

One problem with your theory - the fuel pump relay is controlled 100% of
the time by the limp home circuit.  The CPU (and PROM) need to do
nothing to operate the fuel pump relay.  In fact, they can't control the relay.

When initially powered up, the limp home circuit turns on the relay
briefly, and again whenever there is a distributor reference pulse.

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/


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