A properly functioning glow plug will glow red hot - the plug is placed very close to the injected fuel stream. When the atomized fuel hits the red-hot plug it ignites - even if the compression level is too low because of slow cranking speeds. Glow plugs act as a catalyst in a chemical reaction.

I have not had good luck with champion or autolight glow plugs - I would stick to Bosch or Beru. Also avoid "monarch" plugs, they are made in Germany but I bought 4 of them for my 240D and all failed within 1 month.

The best test of a glow plug is a current draw test - a good plug will draw 9-15 amps in operation.

A healthy engine with good glow plugs at an ambient temp of 32 degrees should start faster than 4-7 seconds. My 240D with a rebuilt engine would start in about a quarter of a second at freezing and on the first compression stroke in summer temps.

Tim