Glow Plug Testing?

[diesel don]

I searched and can't find the simple method of testing glo plugs. I have a multi-meter for testing.

I suspect one is giving me fits.

Thanks

Don

[Pete Burton]

less than 1 ohm resistance is OK, more its NG

[rg2098]

Ohm's Law states the V=I*R (v-voltage, r-resistance, i-amps) or R=V/I. All electric heaters work on this principal of using a high resistace coil or wire to create heat, GP's being no exception. A 12 volt system would need a hell of alot of power flowing through to heat up those plugs at 1 ohm of resistance. I could be wrong, but I would tend to think that our GP's have extremely high resistance to heat up to over 1000 degrees that they do. Someone want to chime in and correct me if I'm wrong?

Hes right 1 Ohm or less...

I think you need to follow the equation to completion Adam.

1 Ohm or less = 12volts / XX amps....

So your car's system would need to produce at least 12 amps or high to keep it balanced. That's enough heat to ignite diesel, which perpetuates the the glow plug to contimuesly ignite the system. The combustion of the diesel heats the engine block, not the glow plugs...

[rg2098]

Your right I didn't take into account the volatility of diesel, and the primary function of the GP's in the first place.

Cheers....

[Eskimo]

Originally Posted by rg2098

Ohm's Law states the V=I*R (v-voltage, r-resistance, i-amps) or R=V/I. All electric heaters work on this principal of using a high resistace coil or wire to create heat, GP's being no exception. A 12 volt system would need a hell of alot of power flowing through to heat up those plugs at 1 ohm of resistance. I could be wrong, but I would tend to think that our GP's have extremely high resistance to heat up to over 1000 degrees that they do. Someone want to chime in and correct me if I'm wrong?

You have Ohm's law right, but I think you might be misunderstanding the consequences of it.

Consider that power is equal to VI (is equal to I^2 x R). If one has a fixed, low voltage (as we do with our 12V automotive systems,) and one needs (relatively) high power, then one needs (relatively) low resistance in order to get (relatively) high current. This is why cars need such big, fat cables to do any heavy work.

Ohm's law also explains why the starter motor circuit is sensitive to what might seem to be modest resistances from less than proper connections. The required current is high enough that the voltage drop across such a connection (Ohm's law works here, too) is significant. If someone says that the connections are fine because they "beeped them" for continuity with their meter, well, they haven't really said much.

As an aside, I think the resistance of the glow plugs increases with temperature, so it doesn't necessarily tell one directly what the "terminal" resistance during glowing is - I think it increases as they heat. The FSM has some graphs, if I recall correctly, showing how the temperature ramps up.


-- eskimo

[Brian Carlton]

Originally Posted by eskimo

As an aside, I think the resistance of the glow plugs increases with temperature, so it doesn't necessarily tell one directly what the "terminal" resistance during glowing is - I think it increases as they heat. The FSM has some graphs, if I recall correctly, showing how the temperature ramps up.

This is correct. If you have five glow plugs, each with a cold resistance of .5 ohms, each plug will flow 24 amps. 5 plugs in parallel will flow 120 amps. Since the fuse is rated at 80 amps, the plugs must immediately increase in resistance within seconds of the current application to prevent the fuse from melting.

[rwthomas1]

Heres the sure way: Remove glowplug. Apply 12volts to the terminal with ground attached to the body. Plug should glow red hot in just a few seconds. Not a dull red or just get hot. Red-orange hot like an electric range top element. I have had them test fine with a meter and not glow red hot. Only way to be absolutely sure if you have doubts is to pull them out and test them. RT

[Benzcrusher]

First, these 616, 617 engines operate the pencil glow plugs at 11 volts.

Remove the lead then use an OHM meter to test....

If it's been a few years or you have no idea of the last change, replace them...

[wolf_walker]

Originally Posted by Benzcrusher

First, these 616, 617 engines operate the pencil glow plugs at 11 volts.

Remove the lead then use an OHM meter to test....

If it's been a few years or you have no idea of the last change, replace them...

hehe.. Mine are original at 270K, could that be the cause of my little stumble right after cold start?

[diesel don]

You guys overestimate me. I needed to know the actual process. Like where to put the wires from the tester, etc.

However, RT gave me a good lead. I'll yank each one.

Thanks for all the info....

Don

[mplafleur]

Don,

You can test them in place. their resistance should be very low. On the order of 1 ohm, give or take, depending on the accuracy of the meter. Most handhelds aren't that accurate at that low of a range. Don't use a continuity beeper, most will beep with resistace as high as 40 ohms.

There is a connector that you can disconnect that will give you access to the plugs via the wire connecting to them. You can check the resistance from the socket in the connector to the engine block (or head). Check them one at a time, or check each plug at the threaded tip with reference to the block.

Good luck

[Old300D]

The pencil type are easy to test - I just pulled each one and applied 12V. If it glows it's good. The other type are more tricky, but once you remove the wires, using the multimeter it should register as a short. If it's not a short, it's no good. And when you pull it, the loop will be broken.

[dieseldiehard]

I suspect one is giving me fits.

Thanks

Don[/QUOTE]

One failure mode of a really bad GP is shorting, when the wire element inside touches the case where it shouldn't. I have seen this, it blows fuses. If you have blown more than one fuse you might have one shorting plug, and testing with an Ohmmeter may not be useful in identifying the bad one. Change em all! The adage "leave well enough alone" may not be applicable here, and can invite more problems, if one is bad then others are going to fail and in my experience its a good plan to replace them all when one or two have failed, and if you expect cold weather where you live, Fall is the right time to check for any bad ones. With as many daily drivers using GP's as you have, GP problems are bound to surface!

Be warned however, if the GP's are old, I mean over 3 to 5 years old and have seen lots of use then you might find that the pencil type GPs tend to get gunked up with carbon and they won't come out easily. Do not yank on one if it is stuck, it may come apart on you (not a fun thing!). Some strong solvent like Lubro Moly is useful here with a twisting motion, unscrewing the plug from the gunk if possible (use the search button looking for glow plug) but I have heard of other recommendations).
In case you are interested, the resistance element in a glow plug is made from tungsten wire, the same material used in incandescent lamps. The Resistance ratio, hot to cold, is 14:1 (meaning 1 Ohm cold and the equivalent to 14 Ohms hot, hot being defined as around 1200 degrees). The elements in a glow plug never get as hot as the filament in a lamp, only dull red is necessary to help ignite the fuel, so a ratio around 3:1 is more likely, therefore 5 GP's would draw around 24 Amps with 14 V on them (engine running with alternator charging). Starting current is around 60 Amps for a few milliseconds anyway, dropping repidly as they heat up. This causes stress cracks on the fuse on the GP timer, so the first time one blows, look closely before you remove it, if it has a hairline crack, its a failure normally attributed to thermal cycling, a known failure mode seen in fuses.
Dieseldiehard
1971 220 (gas) 4-spd manual 106441
1979 300TD w/ ’85 turbo engine 295530
1983 300D 243280
1985 300TD 217300
1987 300D 258230