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Old 08-20-2007, 10:55 PM
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JIMSVX JIMSVX is offline
.... what a sweet ride !!
Join Date: Apr 2005
Location: St. Louis, MO
Posts: 179
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Originally Posted by Nomake Wan
But as soon as I start driving, the data gets more and more corrupt until it's not even readable. If I then turn the car off and back on again, the ECU is back to normal and sending proper data... until the next time I drive it (or let it idle for a LONG time, as was the case with the time in the Fry's parking lot).
Could it be ignition or alternator noise riding on the TTL pulses due to the engine running. TTL is a high speed logic that could be detecting the noise as data pulses. As a result your TTL converter could be generating weird data when the engine is running.

The amount or shape of the ignition noise could be dependent on the engine load or speed. If it's alternator noise, then it could depend on the bus load that changes as the battery charges up, or with the change in engine speed.

You might need a low-pass filter in front of the TTL converter to strip those pulses off the real data. Or the noise could be coming from your laptop and being sent on the USB power to the converter. Is the laptop powered by the car's 12V or internal battery?

As Trevor said before - a scope (picture) would be worth a thousand words, in this case.

I spent many years chasing Electromagnetic Interference (EMI) problems like this. Lots of luck !!!
- Jim

'02 Outback - Wife's car
'96 Legacy Wagon - Son's car
'95 Legacy Sedan - #5 Daughter's car
'93 Impreza Sedan - #4 Daughter's car
'92 Claret LS-L - My car ____________
TOTAL = 595,000 Subi miles
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