TA 167 - Starting / Running current measurements

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Steve Smith
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Re: TA 167 - Starting / Running current measurements

Post by Steve Smith » Mon Feb 16, 2015 2:30 pm

Hello Spencer and thank you to for these posts as they highlight the features we take for granted such as multi channels and high resolution.

Regarding the custom probe settings, yes you have the option on how you wish to scale your probe and display units, colours and filtering etc., which is ideal given the temperature probe options you mention. These options can be explored using the free download PicoScope 6 Automotive software (in demo mode) and I have included a screen shot of the “scaling” stage during set-up below
Using a maths channel as mentioned above you can obtain instantaneous RPM values for the whole capture. The maths channel technique is robust and reliable so long as the correct information is entered during the “creation of the maths channel” and the adequate number of samples are selected pre capture (1MS as a typical bench mark-using PicoScope)

The maths channel waveform returned by the scope present a visible RPM trace that will track the acceleration and deceleration of the engine, right down to events pre and post combustion (opening up misfire detection opportunities)

Regarding compromising the RPM signal via crankshaft sensors, this may be as a result of connecting across a “floating crankshaft” sensor signal when other channel inputs are connected to other sensors/ ground references. (Inputs sharing a common ground) This is not an issue with the new 4425 and 4225 scopes.

The auto zero issue in your application does sound like a nuisance from a monitoring point of view. Would you not be able to zero the clamp away from the negative cable before commencing your test? The offset (post capture) could work assuming the current draw of the coolant circulation pump is a known fixed value? (The zero off set feature is also available pre capture under the channel options button)

Moving onto cranking voltages:

Below I have attached a typical cranking voltage waveform obtained from a V8 petrol engine that indicates a fall to 6.33 V and where the OCV was 11.70 V. On this vehicle the crank and start time had no effect on the startability of the engine yet 6.33 V does give cause for battery concern for sure.
The PCM does have various operational modes that allow for the application of various fuelling and ignition maps during cranking. “Starting mode” and “Voltage correction Mode” are typical examples of the PCM’s ability to function even under adverse voltage supply conditions. Key inputs here would be the “battery voltage” value and/or “crank signal” where the relevant “map" would be applied.

Without researching the “Capacitive Buffer” theory (and one I do like) I cannot answer the question and any input here would be most valuable on PCM activity under cranking. (And other conditions for that matter)

It looks like you have now resolved your question and proved the theory and it has been interesting to follow your posts and sharing your research

Take care……Steve

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