Solutions to EFI interference Problems on Saab 99

[David Sackett]

Hi, I have recently written to this list regarding my Saab 99 fuel injection
computer based on the ATMEL AVR8535 micro. Thanks for all the suggestions
regarding my idling problems.

I have been having trouble with accurate measurement of RPM and I believe it
is due to interference from the ignition system. When the engine is running
RPM jumps around all over the place. If the engine is running at 1000rpm
occasional values of 3000, 5500 & 15000 will be calculated by the computer.
I thought that the input and power supply filtering I had designed was
inadequate so I constructed a new circuit board based on the megasquirt's
power supply, coil based external interrupt trigger and TPS input (thanks
for the design Bowling and Grippo). I still get the same interference!

I have mounted the circuit board in a metal box and have wires running from
the back of a D25 connector to the circuit board. The only things I can
think of is that the filters should be closer to the back of the D25
connector to avoid having unfiltered wires floating around in the box and
also I only have a single sided circuit board and don't have any large
copper areas on it (unlike the Megasquirt design).

Are these two things critical to filtering the noise? The computer works
fine with a signal generator, is there anything else I can try?

Thanks

David

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