How do I know if my glow plugs are working?

[Iplayfarmer]

How do I know if my glow plugs are working?

 

[Mike_Lipke]

3 ways:
1.Take a test light, ground one end, put the other end on the wire to each plug, activate the glow plug circuit. The test light should light (shows they are getting power only).

2. Pull them all out, hook up to battery, see if they glow.

3. If the ambient temp is under 40, and the engine starts on the 3rd or 4th revolution, they are working.

 

[HGM]

I'd like to add one obvious warning to #2, DONT TOUCH THEM!!! They get glowing red hot. /forums/images/graemlins/grin.gif

Also , if you have an amp meter, you could hook the inductive clamp over each wire at the plug. Cycle the switch and monitor the amp draw. I'm not sure of the draw of your particular plugs but on a Ford truck they typicaly draw 50A each when cold.

 

[RalphVa]

Good advice on checking them. Note that I've NEVER had a glow plug problem in driving Benzes 195k miles on the first one and coming up on that mileage on the current one.

In my experience, if they're working and the engine compression isn't gone by WAY too many hours or miles, and if you work them the prescribed time (about 5 seconds with the newer ones), the engine starts on the first turn, even at -20 F. This was the case even on my old 1973 Benz. On it, you had to run those old glow plugs for a minute or so at -20 F.

Only diesels with glow plugs I had trouble starting were a couple of rental ones in cold weather (which isn't all that cold) in Sicily. Both the Fiat's and Opel's odometers had gone around the bend a few times.

Ralph

 

[Iplayfarmer]

<font color="blue"> If the ambient temp is under 40, and the engine starts on the 3rd or 4th revolution, they are working. </font>

That's definately not happening. At warmer temperatures there's an obvious difference in starting when I use the glow plugs vs. when I don't. Now that it's cold, though, it's taking a while to get her started.

Do glow plugs usually just go out altogether, or do they get "weak" before they go out?

 

[PineRidge]

With glo-plugs there is no in between. They are either good or (open) bad. Think of a filament in an incandescent light bulb.

 

[cp1969]

You might have one or two that are not working and that will make it considerably harder to start.

Disconnect the wire from each of them and use a continuity tester to see if any are open, i.e., dead. A beeping multimeter is perfect for this, otherwise look for infinite resistance.

 

[LeRoi]

Here's one I have experience with. If you are getting 12 volts to the glow plugs from the wiring harness, take a 12 volt test light, the one with the bulb in the handle and connect the wire to the + of the battery. Touch the end of the glow plug with the probe and if the lamp lights up, the glow plug has resistance and should work. If the lamp doesn't light up the glow plug has no resistance and should be replaced.
If you have a 4 cylinder diesel and one or two plugs are bad, it will make cold starting a real chore.
HTH
Leroy

 

[BobReeves]

</font><font color="blue" class="small">( Here's one I have experience with. If you are getting 12 volts to the glow plugs from the wiring harness, take a 12 volt test light, the one with the bulb in the handle and connect the wire to the + of the battery. Touch the end of the glow plug with the probe and if the lamp lights up, the glow plug has resistance and should work. If the lamp doesn't light up the glow plug has no resistance and should be replaced.
If you have a 4 cylinder diesel and one or two plugs are bad, it will make cold starting a real chore.
HTH
Leroy )</font>

I wouldn't take this to the bank.... You are reading the voltage drop on the wire going to the glow plug. If it was designed right and is fairly new, no bad connections this drop could easily be less than a volt and won't light the bulb. Iffy test at best...

 

[HGM]

I would just add that you should check it to a good ground first, then the glow plug. I woud still ohm them out. If you have an amp meter, this works the best.