ecu voltage problem

[Tom Cloud]

>I am an FSAE student and we built an EFI computer for the competition.
>There seems to be a problem running it off the car's power supply, ie the
>battery.  When it is hooked up to the battery the computer holds pulse
>widths at around 1 ms per revoltution regardless of throttle position.
>When we hook it up to the standard voltage regulator used for testing in
>the lab the car works perfectly with full rpm and throttle range, but
>can't move because it is attached to an extension cord.  We're using a
>68hc11 processor.  If anyone has had any weird voltage problems like this
>any help would be appreciated.  Thank you,

What kind of regulation circuitry are you using to drop
the voltage from the automotive 12 volts to your circuit.
Keep in mind that your regulator needs to be able to deal
with voltages as low as 8 or 9 volts (cranking) to over
15 volts, with voltage spikes maybe as high as several
hundred.  A resistor/zener or varistor input could squelch
the spikes (lo-R so you don't lose voltage from current
drain of circuit).  Put this before your regulator.
In-line inductors (chokes) work also, but they can
get big.  Also consider you may have some sort of
ground loop going on.

Tom Cloud <cloud at peaches.ph.utexas.edu>