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Tony, et al, thanks for the reply. Scope use for troubleshooting our cars is extremely useful IMO. For example, looking at the pulses from the reference and speed sensors. You can get an idea from a DVM that's there's something coming in to the DME but the DVM is not fast enough and also tends to RMS (root mean square) values, not peak envelope like a scope. So, scopes are basically Y axis voltage and X axis time, which is settable in milliVolts or Volts per division (Y) or micro, milli or Seconds per horizontal division. So for the reference sensor, which is supposed to give off about 2 volts, once cycle per revolution, you could set the Y (vert) to 0.5 V/div and the horizontal timebase to about 100 mS/div (1 Sec over the screen width, for 10 divisions) you should see about 5 pulses and they should be at least 2 Volts for the ref sensor (Bentley). Not sure if that's 2V peak-to-peak or RMS but should be a healthy signal. If you look at the Speed sensor, you should see a lot more pulses (6 X) at a slightly higher voltage (2.5 V).
The scope has a high-impedance input, meaning it shouldn't add very much load to the circuits on the DME. The better the quality scope and probes, the higher the input impedance. That DSO 138 cheapy scope has 1 Meg ohm (10^6) so it's not the best but seems to work for me on my DME. The scopes you mention are probably much better than the DSO 138.
As for where to put the leads, for the ref and speed sensor, attach the scope ground lead to the chassis and with the DME disconnected, at the plug, you can probe either of the two outputs wires from the sensors but the scope should be set to AC coupling. Do not put the ground of the scope on one wire and the probe on the other. The output from the sensors is a balanced 2 wire with a shield, so neither of the 2 wires is grounded. Each wire should read about the same except they will be out of phase with eachother.
Tippy, you had asked about the coil output. As far as I understand from reading this forum, the output is high (12V through the primary of the coil) which is dragged to ground for firing the coil. It's that rapid change from 12V to ground that is really an AC pulse that is transferred to the secondary and multiplied many times because of the prim to sec turns ratio. I just scoped the pin 1 output on the DME and also on the primary of the coil, looking for those negative going pulses. need to set the vertical sensitivity to 2 V/div but seems to work quite well. That Velleman scope you mention for $120, seems like a very good deal also. Would be much more flexible than this DSO jobby.
Triggering a scope to give you a nice waveform from our cars is difficult but with a storage scope like this DSO you can capture then look at a later.
I'm pretty new to this, so would appreciate any inputs from anyone with much more experience in this area than me.
Mike
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