Why draw testing with a test light doesn't work

I caught myself trying to explain exactly why drain testing with a test light doesn't work in a recent Repair thread, annnnnnd I wasn't happy with my answer. :blush: Obviously, it isn't a valid test. I just struggled to explain why.

Hey, that's what personal cars are for! I started with a normal drain test, with the meter in series with the negative battery cable:



After about 18 minutes, we're well below our 50ma rule of thumb. I repeated this several times. Then, I put the test light in line with the meter AND the cable:



Tried to get the flickering light to show up. :silly: I left it there for over half an hour, just to give it every chance to go out. Never did. That's a false positive in my book. Imagine the rabbit holes you could go down looking for a draw that never existed...

The real key was checking voltage drop across the test light. With current flowing through the meter and test light, I connected between the battery and the test light:



Dropping seven volts across the light leaves the car about five to work with. :lol: Modules don't like that!

I agree. Using a test-light, by it's self, for a draw test is not advised. However, a test-light in conjunction with a volt meter can be very useful, as you demonstrated with the voltage.

The key is to use a "knife switch". Connect the test-light to each side of the switch.With the knife switch closed, modules can go to sleep, as they should. Then open the switch. Now, take a voltage drop across the test light and apply Ohm's Law. (A 1 Ohm resistor, instead of the test-light makes the math MUCH easier. 1mv drop = 1ma current on a 12v system.)

Thank you! Everyone I know tries to use the test light!

My response is always the same when someone pushes back at the correct procedure: "this is not a 1970 Monte Carlo"