TECH QUICKIE – Understanding the Crankshaft Oscilloscope

One of the most important diagnostic tools at your disposal with the DTASwin software is the crankshaft oscilloscope. If the ECU cannot “synchronise” – i.e. confirm that the tooth and missing tooth count set in general engine settings matches what is being measured from the crank/cam sensor(s) – the engine won’t run.

The crank oscilloscope can show you what the ECU is reading from the crank/cam sensor(s) and suggest which “flywheel mode” you need to set. It can even give you an indication of where TDC is in relation to the crank sensor.

How does it work?

How it does this is by measuring the time between teeth passing over the crank/cam sensor(s). If you tell the ECU that the engine is running a 36-1 crank trigger wheel, it will look for 35 small gaps and 1 big gap. If it sees something different, it won’t synchronise.

Below are a couple of examples of what a good, strong crank signal should look like. Each vertical line represents the time taken between each tooth on the crank/cam wheel(s) passing the sensor. The taller the line, the longer it took and as the engine will slow during cranking as it approaches TDC, the tallest line is generally around TDC.

The numbers above the lines are the tooth number. If you divide 360 by the total number of teeth on the trigger wheel, including the missing, you get the degrees of crank rotation per tooth, you can then use the tooth values for roughly calculating crank/cam sensor position in relation to TDC.

Note on the 6-cylinder image however, there is a slow down in crank rotation speed before TDC, this happens to be the longest time between the short gaps so the ECU has incorrectly assumed that this is TDC – this can happen with certain engine configurations, which is why we always recommend checking sensor position in relation to TDC with a timing light.

What can I diagnose with it?

Interference (bad shielding)
If you want to test for crank/cam sensor interference, do a coil output test and click gather on the crank oscilloscope page. If all is well, the page should sit there wating for you to crank the engine. If it starts gathering lots of data, you know there’s a wiring issue somewhere.

Backwards crank sensor wiring
The tallest line (biggest gap measured) should always be representing the missing tooth (green, blue or red for crank sensors). If this is not the case and the tallest lines are yellow, try swapping the wires around in the sensor connector.

TECH QUICKIE – Understanding the Crankshaft Oscilloscope