Saturday, August 2, 2008

Oscilloscope and engine RPMs

I finally got the new handheld oscilloscope in to try and figure out what I was doing wrong in creating the RPM signal for my gauge and to enable the electronic power steering to work. The scope is a Velleman HPS10 and I picked it up new on ebay for $140 shipped. I didn't know what to expect never using a scope before but looking at the features I was pretty sure it would do what I needed. I'm really impressed with this scope for the money after using it. It showed me that my timer circuit I had built wasn't behaving as I thought it was :)

So ultimately what I'm trying to create here is the signal the RPM gauge wants to see. However, this signal comes from the ECM which it creates based off the crankshaft position sensor and they are not the same pulse. After trying a few different things my circuit still wasn't pulsing what the gauge wanted to see so I decided to simplify things and reconnect the ECM to the engine and crank it over while watching the signal the ECM generates for the gauges on the scope. Instantly I realized my problem. Unlike all the other sensors so far this pulse is NOT 5 volts, it's 12 volts. I was never creating strong enough pulses to be measured.
After building another quick circuit and increasing the frequency a bit I was able to get the gauge to move as I wished. At the same time I heard a relay click and sure enough I had power steering! I tested it a few times trying to turn the wheel without the assist and then turing it on to make sure everything was in working order. I did notice the EPS light is remaining on even after the "engine is started". In other words the RPMs are not zero and I would think the light should turn off now. I'll have to look into that next.

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