For you guys who want to know how to test your Glow plugs

[wideopen]

Useful information to help you guys out with glow plug questions

To check your Glow Plugs simply unplug the Glow plug harness going into the valve cover and check the resistance on the terminals to see if the glow plugs are good! your residence should be under 2 ohms the lower the better! Anything over 2 ohms should be looked at being replaced..

 

[Max Power]

I always assumed that a glow plug would either be around 2 ohms or a completely open circuit. Is that not the case? You can actually have a glow plug that shows say 20 ohms resistance?

 

[Ford_Forgotton]

Some details to the above:

1. The truck must be COLD! If the glowplugs have been used, or the truck has been run, you must WAIT for them to cool. As the glowplugs get warm, their resistance increases.

2. "new" glowplugs should read between .1 and .6 ohms. Anything over that and the GP's are getting weak.

A little math for you:

Ohms law is a calculation of Resistance, Current, and Voltage. If you know any two, you can solve the third. Volts = Resistance x Current.

12v = .1 ohm times unknown current. 12 divided by .1 = 120 amps!
12v = .5 ohms times unknown current. 12 divided by .5 = 24 amps.
12v = 1 ohm times unknown current. 12 divided by 1 = 12 amps.

As you can see from the above math, a difference of just .4 ohms results in a loss of almost 100 amps!!!

You really want to be around .2 ohms. No more, no less.