One year old bad glow plug - PeachParts Mercedes-Benz Forum

Biodiesel300TD · #1 02-21-2011, 02:57 PM

Quote:

Originally Posted by spark3542

Glow plugs are like light bulbs. They either work or they don't, and there's no set mileage or age when they go bad.

This isn't totally true. As GPs get old the resistance starts to increase and you get a less than optimal heat from the plug. They do burn out like light bulbs but they don't necessarily burn at the optimum for their whole life. If you bench test and old GP you'll often find that it burns bright red to bright orange. A new one will be extremely bright orange to almost white. New ones will often leave sunspots in your eyes, whereas the older ones won't. Don't touch them though, they're HOT, obviously, something like 1600F.

spark3542 · #2 02-21-2011, 06:46 PM

Quote:

Originally Posted by Biodiesel300TD

This isn't totally true. As GPs get old the resistance starts to increase and you get a less than optimal heat from the plug. They do burn out like light bulbs but they don't necessarily burn at the optimum for their whole life. If you bench test and old GP you'll often find that it burns bright red to bright orange. A new one will be extremely bright orange to almost white. New ones will often leave sunspots in your eyes, whereas the older ones won't. Don't touch them though, they're HOT, obviously, something like 1600F.

I figured my statement would trigger this response, but I still stand by it. Perhaps heat areas do not stay optimum throughout the life of the GP, but 99% of GP-related questions on the forum are a go/no go issue. If you're reading megohms, your GP is bad, if you're reading an ohm, there's 144 amps going through that plug on a 12v system. Ohm's law holds...If there's 144 amps going through your GP, it's turning into heat. No other option.

Diesel911 · #3 02-22-2011, 12:03 AM

Quote:

Originally Posted by spark3542

I figured my statement would trigger this response, but I still stand by it. Perhaps heat areas do not stay optimum throughout the life of the GP, but 99% of GP-related questions on the forum are a go/no go issue. If you're reading megohms, your GP is bad, if you're reading an ohm, there's 144 amps going through that plug on a 12v system. Ohm's law holds...If there's 144 amps going through your GP, it's turning into heat. No other option.

The Fuse is 80 amps. I would expect the Fuse to burn long before it reaches 144 amps.

Also the Glow Plug in my picture that is heating mainly in the middle could case a miss on that particular Cylinder because the Heat would be trying to heat up the Cylinder Head and the Coolant (because that is what surrounds the Glow Plug in that area of the Plug) instead of heating the Air in the Prechamber.
Even though the Glow Plug has only partially failed it may functionally mimic a dead Glow Plug.

spark3542 · #4 02-22-2011, 08:06 AM

Quote:

Originally Posted by Diesel911

The Fuse is 80 amps. I would expect the Fuse to burn long before it reaches 144 amps.

It was a math exercise. If the fuse is 80a, then the resistance of the glow plug has to be more than 2.178 ohms (if the battery is at 13.2v) to not blow the fuse. I'm not makin' this stuff up...Ohm's law holds everywhere.

funola · #5 02-22-2011, 08:51 AM

Quote:

Originally Posted by spark3542

It was a math exercise. If the fuse is 80a, then the resistance of the glowplug has to be more than 2.178 ohms (if the battery is at 13.2v) to not blow the fuse. I'm not makin' this stuff up...Ohm's law holds everywhere.

Huh???

funola · #6 02-22-2011, 10:03 AM

Quote:

Originally Posted by spark3542

I figured my statement would trigger this response, but I still stand by it. Perhaps heat areas do not stay optimum throughout the life of the GP, but 99% of GP-related questions on the forum are a go/no go issue. If you're reading megohms, your GP is bad, if you're reading an ohm, there's 144 amps going through that plug on a 12v system. Ohm's law holds...If there's 144 amps going through your GP, it's turning into heat. No other option.

Do you have a weak battery in your calculator perhaps because those numbers are way off!

spark3542 · #7 02-22-2011, 12:24 PM

Quote:

Originally Posted by funola

Do you have a weak battery in your calculator perhaps because those numbers are way off!

Yup, sorry bout that...I had watts on the brain when I typed amps. The concept is still there...resistance determines the amps, amps determine the heat generated.

Diesel911 · #8 02-22-2011, 12:34 PM

Quote:

Originally Posted by spark3542

Yup, sorry bout that...I had watts on the brain when I typed amps. The concept is still there...resistance determines the amps, amps determine the heat generated.

When I tested some Glow Plugs one at time with an Amp Meter the Meter pegged out and as the Glow Plug heated up the Amps dropped to around 16 amps and stayed there.
It only took about 2 seconds for the Amps to drop to 16 amps.