Alternative compression test?

[Dean]

Is it possible to compression test an engine electrically using an oscilloscope?

I was talking to a guy who lived in sth africa years ago and used to drive a bug. He said they used to test the compression by connecting the oscilloscope up to the ht leads one at a time and took the electrical reading on each to see how much/little electricity it took to turn the motor.

:?

 

[stagger lee]

mmmm, sounds interesting. i don't/can't see the relationship between the ht voltage and the rate at which the engine is turning, except that cranking current could sap away power form the ignition, so ht voltage could be down?

i s'pose you could measure the battery voltage or current via a shunt and process that. current will be sinusoidalish, increasing as compression increases, until droping once the donk gets past tdc. some clever maths could relate this current to compression with a few known paramters. you'd have to know/fugure the relationship between psi and the starter motor current

so a waveform of periodic pulses would indicate a balanced engine, yeah? the peak of each pulse would correlate to the actual compression. a low comp pot would flag up as a lower magnitude pulse. a flat line would be like turning ti over with no plugs, capisce?

i'm kinda guessing at this though.

based on the above hypothesis. using a pic you could make a realtime compresison tester to monitor stuff up in the cab, but life is too short.

 

[67panel]

crypton testerwill do this by cutting out one cyl at a time and checking rpm drop will show cylinder balance,not foolproof as one cyl may not be pulling due to other faults so needs good operator,still got mine from when had garage good for old school stuff.

 

[Trikky2]

Measuring the current drain on the battery was how the old VW Diagnosis machine worked.

Its more reliable to use a straight forward gauge though.