I am just going to throw this in. I don't like to use incandescent test lights because when the time comes to use one the bulb is often shot and it is often difficult to find good ground for it near where you are using the light. I prefer a volt meter because it tells me how many volts I am getting. I have found that to be of the most use most of the time.

I admit I had a Car battery fail and it showed 12 Volts but had so little current that it barley would spark if you shorted a cross the Battery Terminals and that is how I found out it was no good.
In that particular case a working test light would have helped.

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84 300D, 82 Volvo 244Gl Diesel