Diesel RVs: [diesel_mercedes] Re: glow plug problem

--- In diesel_mercedes@yahoogroups.com, BStromsoe wrote:
>
> Bogy, this is worth posting in our permanent archives.
>
> Â
> brian from laverne, ca
> Mary (195K)Â Martha (280K)
> 1983 w123 300d's
>
>
> ________________________________
> From: Bogy Wan Kenobi
> To: diesel_mercedes@yahoogroups.com
> Sent: Tuesday, February 12, 2013 10:07 AM
> Subject: Re: [diesel_mercedes] Re: glow plug problem
>
>
> Â
> Let's see:
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> Yes you should have pulled the plug before doing a resistance test. What you were testing is all the plugs together and if one is good then that is the one that you are testing. If, say, two of them have opened up, then the remaining three will still show a connection. You have to resistance test them individually. And even then a low resistance does not mean they will work. The only real way to test them is by measuring the current they use, and that has to be measured for each individual plug.
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> Also, the fact that they are getting 12 volts does not mean that any current is flowing. It only means that the supply wire is intact. Remember that a disconnected battery shows 12 volts across it's open terminals. But there is still no current flowing. 12 volts at an open glow plug doesn't tell you anything useful.
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> By the way, most electrical problems are not short circuits, or shorts. Most electrical problems are open circuits. That's when the wire breaks, or the transistor stops conducting, or the light bulb filament breaks, and no current flows. This is how glow plugs commonly fail. They burn out and open up. They get too hot and the resistance element inside them melts or evaporates or fizzles away and opens up. In the rare case that they melt and "short", the fuse would blow from too much current flowing.
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> When all the plugs are working, you can turn the key on with the door open and watch the dome light dim down a bit. This is because they will pull enough current to materially affect the battery voltage, and the light will dim down a bit. Then, when the relay turns off (opens the circuit) the dome light will get a little brighter. Now one glow plug may or may not have this effect. But if it does then that is an indication that it is pulling current and may be good. Again, the only real way to tell for sure is with a current measuring meter.
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> So here is a simple and uncalibrated way to measure current: get a small (1 inch or so) and relatively weak bar magnet and "attach a thread to each end". I put that in quotes because you can tape the middle of a long (24" or so) piece of thread to the magnet. Hang the magnet by the two threads, forming the shape of the letter "V". Hold it over the wire under test, at 90 degrees to the wire, and one inch above the wire. Then have a friend turn the key. The magnet will want to align with the wire if current is flowing. The more current flowing, the stronger it will 'snap' into alignment with the wire. This is how large DC ammeters are made. Except that they attach the magnet to a spring and an indicator needle. But you should be able to tell the difference between good glow plugs and bad ones by the difference in response.
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> Otherwise, you could pull them all out and energize them to see if they get hot. It is actually fun to watch. Or you could turn the key on when the engine is cold and touch a crayon or candle to the exposed end to see if they have gotten hot. (CAUTION: BURN WARNING! TRY AT YOUR OWN RISK!) The good ones will get hot and melt the candle.
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> Okay, here is one more possibility. Unplug the bigger glow plug wire plug at the relay. Then you could touch a hot wire from the battery to each individual glow plug, one at a time, and observe the size of the spark produced. Weak sparks mean low current and bad plugs.
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> But unscrewing each plug is the most sure way for the novice to test his glow plugs.
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> That's it. I'm out of tricks.
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> Bogy.
>
Thank you, Bogey. I will mess with it again tomorrow. It is supposed to be nice weather-wise again, so iI can work on it.
I will keep you posted.
Sid