View Single Post
Old 08-21-2007, 03:36 AM
b3lha's Avatar
b3lha b3lha is offline
Phil & Belha
Join Date: Aug 2001
Location: Alcyone Limited, Buckinghamshire UK
Posts: 2,671
Originally Posted by JIMSVX
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 !!!
Hi Jim,

Nice to have advice from an expert!

I did wonder if it was ignition or alternator noise, or even Trevor's often quoted discovery of an AC leakage from faulty alternators. However, my 92 has the problem even when the engine is not running, which excludes those ideas.

I understand that for TTL, 0 volts to 0.8 volts is counted as logic 0 and 2 volts to 5 volts is logic 1. So presumably 0.8 volts to 2 volts is "undefined" and could be randomly interpreted as a one or a zero by the MAX232 chip? If so, then less than 1 volt of noise on the line could be enough to cause this problem?

The thing is, the ECU always, without fail, understands the commands from the laptop. It's only the data coming from the ECU to the laptop that gets corrupted.

As we don't have a scope, we need to solve this by experimentation. Shooting in the dark somewhat. Do you think that it would be worthwhile to try and drop the signal from the ECU by a volt or two? How would I go about this? My first thought is a resistor, but what size? Can a resistor pass high-frequency signals without distorting them? My electronics knowledge is very limited.

Subaru ECU and TCU Website
1992 Alcyone SVX Version L
1992 Alcyone SVX Version L
1994 Alcyone SVX S40-II
2004 Subaru Legacy 2.5 SE Sports Tourer
1996 Subaru Legacy 2.2 GX Wagon
1988 Subaru Justy J12 SL-II
Reply With Quote