Helpful Hints in Scope Pattern Interpretation Anyone who knows Ohms Law understands that the effect of resistance is in direct proportion to current flow. When we have a poor connection at the battery post of as little as 0.01 ohm with the headlights turned on, it creates only 0.1 voltage drop — not even noticeable in brightness.
However, during cranking, that same resistance at 300 amps cranking load means 3 volts less at the starter. This certainly will slow down the cranking speed. This also holds true for air flow or fuel flow. A kink in a garden hose does not make much difference if you want a cup of water, but it becomes a problem when sprinkling the lawn. A fuel filter restricted at 80 percent will not make any difference at idle, but the car will not have the horsepower to make it uphill.
The point is simply that testing under load makes all the difference to complete performance analysis. Because you have the scope hooked up, it might take less than a minute in the bay or up to 20 minutes on a test drive. There are different options or methods of testing under load, but they all have two things in common. First, it needs to be done with someone in the driver seat. Second, the leads need to be long enough to reach the equipment inside the vehicle.
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