Diesel RVs: Re: [diesel_mercedes] Re: glow plug problem

I never pull the plugs to test. Just pull the plug connector at the
relay, and check ohms from this plug to ground. Each glow plug is
numbered on the plastic plug, though small type. As others said, at
least 2 will stand out as bad; good being "0" or close, and bad
something else. The test is simple, replacing #5 is a pain.

Rob
'85 300D
Garden Grove, CA
==

On 2/12/13 10:28 AM, diesel_mercedes@yahoogroups.com wrote:
> Re: glow plug problem Posted by: "Bogy Wan Kenobi"
> polespearbogy@yahoo.com polespearbogy Date: Tue Feb 12, 2013 10:07 am
> ((PST)) Let's see: 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. 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. 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. 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. 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. 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. 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. But unscrewing each plug is the
> most sure way for the novice to test his glow plugs. That's it. I'm
> out of tricks. Bogy. --- On Tue, 2/12/13, bmwsid
> sidcranston@yahoo.com> wrote:
>> >OK, Had a beautiful day
>> >yesterday. I tested each glow plug right after turning
>> >the switch on. Each one was getting 12 V.
>> >I then tested with an ohm meter between the engine, and the
>> >electrical connector with the wire on it, and most of them
>> >were '0', one or two were .1
>> >Was it necessary to unplug the other end of those wires
>> >first? I didn't, because it was on there pretty good,
>> >and I was leery about breaking anything. Does it just
>> >pull straight away?
>> >It is very clean in there, so there is not a corrosion
>> >problem, and the fuse was nice and shiny, and tight. Car
>> >still wouldn't start.
>> > Sid, near Niagara Falls
>> >
>> >--- Indiesel_mercedes@yahoogroups.com,
>> >Bogy Wan Kenobi wrote:
>>> > >
>>> > >If the dash light didn't come on you should suspect
>> >that at least two plugs are dead. And maybe three. There is
>> >a relay and control box under the hood on the left inner
>> >fender well near the left headlight - about 12 inches back
>> >from the headlight. It should b3 under a plastic form fitted
>> >cover. It has two plugs - one with smaller-ish wires and one
>> >with 5 larger-ish ones. The plug with the 5 larger wires is
>> >the feed for the glow plugs themselves. You will need to
>> >unplug that plug to test the glow plugs individually. Their
>> >resistance is near zero ohms to begin with so an ohm meter
>> >will only help detect an open plug. A shorted plug will
>> >likely blow the big fuse at the "rear" end of the relay. The
>> >best way to test them is with a DC ammeter under power. Each
>> >working plug should pull between 8 and 12 amps. A wire from
>> >the battery through an ammeter and to each plug, one at a
>> >time, is the best way to find a bad glow plug. But be sure
>> >to disconnect the feed plug
>>> > > first.
>>> > >