Test car battery with oscilloscope during cranking

[TaylorD93]

I have a 2016 Toyota Auris, and the workshop manual actually quotes a minimum permissable voltage during cranking. Certainly with Toyota UK you can pay a few £ to access the entire workshop manuals online for a few minutes.

Another thing to be aware of when looking at the cranking voltage however is the "relative compression test". It is a test where you use an oscilloscope to measure the battery voltage during cranking (usually done with the injectors disabled), as the engine rises on the compression stroke, the starter motor is put under more load and therefore the battery voltage will sag at each compression stroke. so if you have a cylinder which is lower compression relative to the other cylinders, you would see a smaller drop in battery voltage on that compression stroke.

If the 2nd channel of the scope is set to monitor a coil pack pulse (often can be held on the back of the coil pack, if coil-on-plug setup) you can then find which cylinder is actually low.

My point being, engine mechanical health will impact your readings.