Use your cell phone's video function

1991 300-SEL M103 - Pierburg pump

12V test light - connect alligator clip to probe end of test light.
Connect alligator clip to fuel pump electrical connection that black wire attaches to. Connect red lead of test light to the fuel pump electrical connection that brown wire connects to.

Set cell phone to video record and point at business end of test light that contains bulb.

Turn key in ign. switch to first position; then crank. The cranking was not necessary, but test light continued to illuminate while cranking. Cranked 3X in video. All the times it lit up. Pump is getting voltage, but that does not prove it's functioning.

Will next test fuel tank for flow to pump and pump for flow to fuel filter; then fuel filter flow to front. Suspect pump. If it's fails test, will replace it and fuel filter.

Video is upside down - you still see the results. First crank about 30 secs. into video.

M103 Single Fuel Pump Test_zpsuvqydbbf.mp4 Video by lesterl | Photobucket