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  #1  
Old 01-01-2014, 01:24 PM
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Glow Plugs 1985 300SD

Hello Everyone,

I have done some research and did a few checks on my car, but once again it is getting pretty cold around here, 5 degF for the next few days, and I need to try and get this figured out. Lately, the glow plug light has been working intermitently, now it hasn't worked in the past few days.


I have done the following checks:

1. 0 ohms on every glow plug, from relay to glow plug
2. Voltage at each glow plug wire output on the relay (12.42 V)


When I unplug the glow plug wires from the relay, the light on the dash comes on like normal. When I plug the wires back in, the light does not come on in the dash. I tired this multiple times with the same outcome.

Is it possible that there are 1 or 2 glow plugs which are shorted out?

I am wondering if the relay is an issue or if there are other ground wires which might need to be cleaned? I don't have an electrical schematic at this point in time.

Any help is greatly appreciated.

Thanks,

Keith

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  #2  
Old 01-01-2014, 02:05 PM
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Where, exactly, are you measuring the reported zero Ohm resistance?

If you do not attempt a start, how long does your GP relay remain closed?
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  #3  
Old 01-01-2014, 03:01 PM
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Is this resistance test being performed with a digital or very high quality analog meter? What resistance range is the meter set to when the zero ohms reading is obtained?

If you are correctly reading the resistance on these glow plugs, then the reading suggests that all of them are bad.
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  #4  
Old 01-01-2014, 03:36 PM
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I have found that the best way to check the glow plugs is this: Take a jumper wire and connect one end to +12v. Remove the glow plug wire harness from the relay, and touch the other end of the jumper wire to each of the glow plug terminals in the plug. If it makes a little spark, the plug is good. No spark, the plug is bad. It is just that simple. I have found using a meter to be very unreliable. Even some plugs that check good with a meter, are bad with this test. Of course the best way to check is to remove them and apply 12v directly, and I do that anyway after replacing the bad plugs just to be sure....Rich
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  #5  
Old 01-01-2014, 03:57 PM
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Quote:
Originally Posted by mopart2 View Post
Hello Everyone,

I have done some research and did a few checks on my car, but once again it is getting pretty cold around here, 5 degF for the next few days, and I need to try and get this figured out. Lately, the glow plug light has been working intermitently, now it hasn't worked in the past few days.


I have done the following checks:

1. 0 ohms on every glow plug, from relay to glow plug
2. Voltage at each glow plug wire output on the relay (12.42 V)


When I unplug the glow plug wires from the relay, the light on the dash comes on like normal. When I plug the wires back in, the light does not come on in the dash. I tired this multiple times with the same outcome.

Is it possible that there are 1 or 2 glow plugs which are shorted out?

I am wondering if the relay is an issue or if there are other ground wires which might need to be cleaned? I don't have an electrical schematic at this point in time.

Any help is greatly appreciated.

Thanks,

Keith
Reading this again, I'm left with the impression that you tested only the leads to the glow plugs, and found no resistance. That, if it is the case, is fine, but it sheds no light at all on the condition of the plugs themselves.

The voltage reading at the relay pretty much indicates that it is functioning. A quick test of resistance through the plugs may or may not turn up a problem, but that's really the important step. Check resistance between each glow plug and ground, looking for readings over 1 ohm, zero, or infinite resistance, as evidence of a bad plug.
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  #6  
Old 01-01-2014, 05:30 PM
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Quote:
Originally Posted by ROLLGUY View Post
I have found that the best way to check the glow plugs is this: Take a jumper wire and connect one end to +12v. Remove the glow plug wire harness from the relay, and touch the other end of the jumper wire to each of the glow plug terminals in the plug. If it makes a little spark, the plug is good. No spark, the plug is bad. It is just that simple. I have found using a meter to be very unreliable. Even some plugs that check good with a meter, are bad with this test. Of course the best way to check is to remove them and apply 12v directly, and I do that anyway after replacing the bad plugs just to be sure....Rich
That's a good test except it permanently burns and leaves a mark on the surface of the terminal where the spark occurs.
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  #7  
Old 01-01-2014, 05:39 PM
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Quote:
Originally Posted by mopart2 View Post
...........
Is it possible that there are 1 or 2 glow plugs which are shorted out?

I am wondering if the relay is an issue or if there are other ground wires which might need to be cleaned? I don't have an electrical schematic at this point in time.

Any help is greatly appreciated.

Thanks,

Keith
Glow plugs very seldom fail by shorting out. If that were to happen, the glow plug harness will melt and possibly start a fire. It is more likely one of more of your glow plugs have failed in the open position. Not getting a glow plug light is a diagnostic tell tale sign that you have one or more burned out glow plugs.
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  #8  
Old 01-01-2014, 08:23 PM
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Quote:
Originally Posted by funola View Post
That's a good test except it permanently burns and leaves a mark on the surface of the terminal where the spark occurs.
I have not experienced that. In fact, the spark is so small that it is barely visible. I am sure it is not enough to damage the terminals in any way.
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  #9  
Old 01-01-2014, 10:25 PM
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Rather than directly connect the glow plug to power (which should be done through a fused circuit for safety), I use a test light to B+. The light should light brightly for all five plugs. Dim or off indicates faulty plug. Note that each plug must be tested independently of the others in the harness.

HTH
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  #10  
Old 01-01-2014, 10:32 PM
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The Ohm Meter is suposed to set at 200 Ohms on the Meter setting/Dial
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  #11  
Old 01-02-2014, 07:12 PM
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Everyone,

Thanks for the quick response. I forget what setting I was on for Ohms, but I will re-test and I ordered 5 new plugs so I can just swap them out when I pull them out for testing.

Thanks,

Keith
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  #12  
Old 01-02-2014, 07:40 PM
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Ive had a glowplug short out in one of my cars.The strip fuse kept popping till I tested to see which gloplug was bad.Put a new plug in and all was good..
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  #13  
Old 01-04-2014, 10:52 AM
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Originally Posted by ftwchopper View Post
Ive had a glowplug short out in one of my cars.The strip fuse kept popping till I tested to see which gloplug was bad.Put a new plug in and all was good..
You're right, I forgot about the fuse. I have never experienced a shorted glow plug. My guess is that kind of failure is 1000 to 1 or higher. Do you still have the shorted glow plug? I'd love to examine it.
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  #14  
Old 01-04-2014, 10:56 AM
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Originally Posted by mopart2 View Post
.......I forget what setting I was on for Ohms, but I will re-test and I ordered 5 new plugs so I can just swap them out when I pull them out for testing.

Thanks,

Keith
Use the lowest ohms scale (least sensitive) if you want to get a more accurate reading. If you pull the old plugs out, the best way to test them then is to power them up and watch them glow. If they are in the engine, the best way to test them is power them up individually with an ammeter in series and measure the current.
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  #15  
Old 01-04-2014, 10:57 AM
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Quote:
Originally Posted by mopart2 View Post
Everyone,

Thanks for the quick response. I forget what setting I was on for Ohms, but I will re-test and I ordered 5 new plugs so I can just swap them out when I pull them out for testing.

Thanks,

Keith
If you are going to Pull them out test them individually on the battery. The Tips of the Plugs should start to glow first.
If the Start to glow in the center of the Heating Element that is also a bad Glow Plug eve if the Ohm reading is good.

Look in the DIY section and find some way to Ream or other wise clean out the Carbon from the Glow Plug Holes.

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