A really quick way to test if diesel glow plugs are working.

This is a quick test to elimate glow plugs as a cause of difficult starting of diesels in freezing conditions. All that is required is a decent multimeter that has a zero to 20v scale.

All that is required is to observe the drop in battery voltage while the engine is being "glowed" prior to cranking.

Assuming the battery is in decent condition and a reasonable state of charge even in freezing conditions after being parked overnight we can expect to see around 12.4 to 12.5 volts When the glow plugs are operated and fuel pumps priming this will drop considerably. Even on a two electric pump system the fuel pumps will only draw about 10 amps total but the glow plugs will draw about 20 amps per cylinder. For a 4 cylinder this means a load of over 100 amps, this is enough to pull the battery voltage down below 11v possibly nearer 10v. When the glow plugs are turned off the battery voltage should recover fairly quickly to just below the voltage before the start of the test

I like your thinking. I'll be sure to test it on the next 7.3L Powerstroke I get in this winter with starting problems. Each glow plug draws around 18A when new, for a battery draw of 140-150A. This'll be a perfect direction test before putting my amp clamp on the glow plug relay.