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PlayingWithTBI

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I guess one fix for both engines was easier to implement in changes to the harness and the addition of a common low cost module made more sense than to program and stock multiple ECU variations.
Yeah, FWIU the 2 second timing for the fuel pump is in the hardware not the firmware of the ECM so, it's not just a "tune" in the PROM.
 

Eveready

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Actually I went the route of using an Atmel micro controller (ATTINY85) to derive the time delay and a 20A relay designed by Panasonic to make the contact. The CPU allowed me to alter the timing on the fly without any component changes and the relay was small enough to fit. I didn't want a trim POT since they do tend to change value over time.

That sounds far more elegant than my old school approach. I don't think the trim pot would matter much if C was carrying most of the load and R was a wide enough value that minor fluctuation would not affect it much and easily corrected but I definitely think your way would be better.
 

92Landyacht

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The concern I had was, based on the length of the delay being 20 seconds, the values of R & C would become rather large. I thought of using a simple 555 timer but it has similar issues with long time delays and the timer wasn't really designed to be a one shot.

GM went the route of using a 14 stage 4060 ripple counter to allow for a faster clock pulse but then had the issue of having to stop the counter once the final count was reached. This added morel circuitry. They were seriously limited to components available in the late 80's. They also had cost constraints factored in as well, so I'm sure they used the best approach they had available.

Today we have a much larger selection of components and costs are much lower as well. Especially at higher volumes. I build circuit boards in batches of 3 from a prototype vendor (Osh Park). The board for this project came in at $21.50 for a batch of 3 or approximately $7.00 each. The micro controller was around $1.20 each. Actually cheaper then the various capacitors, resistors and transistors that GM used in their original design. This allowed me to "splurge" and build several different designs and test them out. The final design I ended up with is basically a ATTiny85 CPU, a voltage regulator, a single transistor driver and a relay to make the connection. All the other parts were for voltage spike suppression, and visual verification of relay function. The headers at the top left are there for programming the CPU while in circuit on boards I don't put a socket on. Since these are still prototypes I'm socketing the CPU to be able to program it by itself. Final design will have the CPU soldered directly to the board and probably wont have the programmer header pins installed since it's doubtful that any end user will need to modify the program.
 
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