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Digijet ECU Modification, fuel cut off
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Wildthings
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PostPosted: Sun Jun 11, 2017 5:57 pm    Post subject: Digijet ECU Modification, fuel cut off Reply with quote

Was wondering if anyone knows enough about the operation of the 1.9L Digijet ECU to know how to raise the fuel cut off rpm? With a worn throttle valve and/or a bad retard can these engines tend to idle above the fuel cut out rpm and thus will surge as the fuel is cut off and back on. Would like an easy way to eliminate the surging.
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hdenter
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PostPosted: Sun Jun 11, 2017 7:58 pm    Post subject: Re: Digijet ECU Modification, fuel cut off Reply with quote

Not trying to be smart or obnoxious, but wouldn't it be easier to repair/replace the throttle and/or vacuum can? Would you want to risk frying your ECU when there is a more direct solution to the problem?

Hans
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Wildthings
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PostPosted: Sun Jun 11, 2017 9:01 pm    Post subject: Re: Digijet ECU Modification, fuel cut off Reply with quote

hdenter wrote:
Not trying to be smart or obnoxious, but wouldn't it be easier to repair/replace the throttle and/or vacuum can? Would you want to risk frying your ECU when there is a more direct solution to the problem?

Hans


Even when everything is right you can still get some bucking at times. VW raised the set points significantly when they went to the Digifant system to get rid of the bucking and improve driveability. I have certainly considered swapping over to a Digifant system for this very reason.
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ohhorob
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PostPosted: Sun Jun 11, 2017 10:04 pm    Post subject: Re: Digijet ECU Modification, fuel cut off Reply with quote

I was able to get a copy of the EPROM that holds the code for a DigiJet. I desoldered the EPROM and used a modern microcontroller to drive the address bus and capture the data contents.

None of the widely available & popular software decompilers are still targeting the old P80A49H processor (intel 8048 series). So it's a very manual process to take the decompiler output and trace the program control flow. Interrupts and timers are fun! At least there aren't too many registers or memory locations to keep track of.

If you knew the exact RPM value for the cutoff, and we could find it in the firmware, modify that value and re-program the EPROM.
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Wildthings
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PostPosted: Mon Jun 12, 2017 12:24 am    Post subject: Re: Digijet ECU Modification, fuel cut off Reply with quote

ohhorob wrote:

If you knew the exact RPM value for the cutoff, and we could find it in the firmware, modify that value and re-program the EPROM.


The Bentley gives a value of 1500 rpms. I should put a tach on it and see what rpm it is actually cutting the fuel on and off at.
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