how many volts should there be at glow plugs when ignition on ,?

If you are testing the voltage with the supply wire connected to the glow plugs, the voltage will be less than 12 volt, because they draw a lot of current, but if you disconnect the wire & test the voltage, it will be nearer to 12 volt.The best way to test them is to remove them & put them across the battery, they glow red hot for about 1/2 inch on the tip, if they are working properly

jim
June 2010

it will be 12 volts (60amp fuse supply) if testing glow plugs disconnect strip attached to plugs an put wire on live (battery) and touch each tip of glowplugs if they ark (spark ) then glow plug should be working if no spark then change that plug

peesian
June 2010