One year old bad glow plug - Page 4 - PeachParts Mercedes-Benz Forum

TnBob · #46 01-07-2013, 07:59 AM

A DT-830 is a cheep sturdy DVM that more than meets the needs we have to work on MB and Studebakers.

We need DC volts to 20
We need ohms to about 1000
We need a milli volt AC scale if we are working on a funky alternator issue.

A search in any auction site for " digital multimeter " will come up what what you are looking for. Currently on ebay over 9000 listings came up.

whunter · #47 01-07-2013, 09:31 AM

Quote:

Originally Posted by cooljjay

what I am now understanding about glow plugs, that even if they light up. They may not be good. They aren't like a light bulb, but more like a resistor. With age and voltage their values drift thus not producing the correct heat. So the only way to test them, is to ohm test them...if the reading isn't to that of a new one...its worn...also I've read that the glow plug relay produces 8-10v mine produces 10v....your battery produces 12-14v thus making the plug brighter and producing more heat....this is why people tell you to run a jumper cable to the plugs to see if that will start the car more voltage equals more heat..

FYI:

Trying to OHM a heating element designed for high temperature operation (1000° - 1300°C = "2372° Fahrenheit") at ambient temperature gives a deceptive (FALSE) reading.

A new glow plug reads 0.7 OHM.
A BAD glow plug can depending upon failure read 0.7 OHM, or anything from open circuit to dead short.
I have many used glow plugs, for show and tell + hands on test at tech sessions.

IMO relying on an OHM test is playing Russian Roulette with a loaded Glock.

All of my used glow plugs are tested:
# With the HOT electrode in the jaws of a vice.
# Battery/charger ground attached to the vice.
# Positive applied to the wire nut threaded pole/terminal for 15 seconds.
After you have tested one good glow plug, and personally seen how bright/hot it gets, there is no mistaking weak/bad ones.

Please read the data in the following thread for further data.

Glow plugs link thread All diesel models
Glow plugs link thread

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Walkenvol · #48 01-07-2013, 11:04 AM

No disagreement on bench testing being the best, but depending on the vehicle it can be a lot of work to remove the glow plugs.

I assume a professional like yourself working on a customer's car would normally replace all GP's for a preheating problem. An individual may want to only replace the defective GP.

Is there an installed GP testing method you believe to be reasonably accurate?

t walgamuth · #49 01-07-2013, 11:19 AM

Based on the bench test methodology I believe the old plugs are all good. I will try again and use the time period you describe.

Brian Carlton · #50 01-07-2013, 11:28 AM

Quote:

Originally Posted by Walkenvol

Is there an installed GP testing method you believe to be reasonably accurate?

Removal of the plug to check heating pattern is the defacto standard.

However, it's a lot of work and isn't warranted unless you demand absolute performance in temperatures that are frequently below 0°F.

I've used the method with the ohmmeter for the last 12 years and have never failed to identify a bad plug. If the resistance is between .7 and 1 ohm, I let it go. If it's open or greater, I change it.

Fortunately, I have never encountered a plug that wouldn't perform but had proper resistance readings.

ah-kay · #51 01-07-2013, 11:47 AM

Quote:

Originally Posted by Brian Carlton

Removal of the plug to check heating pattern is the defacto standard.

However, it's a lot of work and isn't warranted unless you demand absolute performance in temperatures that are frequently below 0°F.

I've used the method with the ohmmeter for the last 12 years and have never failed to identify a bad plug. If the resistance is between .7 and 1 ohm, I let it go. If it's open or greater, I change it.

Fortunately, I have never encountered a plug that wouldn't perform but had proper resistance readings.

x2.

I am a newbie, so it is only in the last 5 years, not 12. If it had a 'dead short' then the fuse would either blow or the GP will become open circuit when the massive current went thru the next time. As for GP measurement, any run-of-the-mill cheapo would do to measure resistance.

whunter · #52 01-07-2013, 02:43 PM

Quote:

Originally Posted by ah-kay

x2.

I am a newbie, so it is only in the last 5 years, not 12. If it had a 'dead short' then the fuse would either blow or the GP will become open circuit when the massive current went thru the next time. As for GP measurement, any run-of-the-mill cheapo would do to measure resistance.

I don't worry about dead short glow plugs, they are rare and obvious.

False OHM readings are common, I have at least 50 in my junk box that all OHM between 0.5 and 1.0, but are obvious junk when tested in the vice.
I never throw out glow plugs, even total trash, worst case the shell can be re-used to make a reamer or compression test adapter.

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