I have 4 old glow plugs from a 300D which may work perfectly. (one is broken) I've seen on youtube a way to test to see if they glow by attaching the positive lead from a battery charger to the threaded shaft, and the ground to the glow plug body.

I've done that will all my plugs, and none even get hot. Does this mean they don't work, or my battery charger is busted? The charger shows 12 Volts.

I have continuity in all the plugs, so is there a reason to replace them considering that they will probably cost me $30-40 each at a dealership?

THanks

Mark