M-18 glow plug shorted

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Steve Walsh

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Oct 6, 2009
129
Newport Newport 28 MKII Jacksonville, FL
One of the glow plugs on my Universal M-18 completely shorted, causing the wiring to melt behind the engine panel, lots of smoke and plenty of headaches trying to repair and locate the cause. In stages, I replaced all the melted wiring behind the panel, ran new ground, amp meter and positive wires, new ignition and glow plug switch, trying to isolate the short, but the glow plug circuit still got extremely hot. What fooled me is that I could touch the tops of the glow plugs and they were hot, so I thought they were working, but could not start the engine. I noticed some deformation of the the wiring harness plugs where the positive and amp wires passed through them, which is why I ran new wires. There was no fuse at all, except on the solenoid start wire. I added a fuse to the positive wire from the starter solenoid post to the engine panel.

The advice and research from Mainsail, the Catalina forum and Cruiser's forum was a big help. I reasearched testing glow plugs and found that it is very rare that a GP will short to ground when they fail. Normally, just get weaker and finally quit working. $18.00 for two new NGK glow plugs at NAPA, and it started immediately.

I intend to get rid of the very long amp meter circuit and install a volt meter and a glow plug circuit relay or solenoid using Mainsail's upgrade procedure. Mainsail said these harnesses are dangerous and I'm a believer now. Please take the time to inspect your Universal wiring harness connector plugs and look for signs of hot wiring causing the plugs to deform. Here is a link to Mainsail's wiring harness upgrade: http://www.pbase.com/mainecruising/universal_wiring_harness_upgrade

Hope this helps,

Steve W
 
Maine Sail

Maine Sail

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Feb 6, 1998
11,677
Canadian Sailcraft 36T Casco Bay, ME
Steve,

Glow plugs by nature are "shorted" kind of like a light bulb. The resistive heating element is between + and -/engine block and gets hot when you apply 12 volts. The intent is that they heat up before the wire can.

If there was a short in the wires leading to, and before the glow plugs, that is different and should be easy to find & fix. Glow plugs are cheap to replace if you buy them from Kubota.
 
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Steve Walsh

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Oct 6, 2009
129
Newport Newport 28 MKII Jacksonville, FL
Stu, I was aware of the potential harness issues and inspected the wiring harness often, intending to get around to the harness upgrade. Never a problem or failure to start before last week. What I had never read or heard about was that a glow plug could completely short out, melting the wiring. My understanding was that a glow plug just got weaker and finally quit working, not a total dead short that threatened the (unfused, in my case) wiring. The only resource I found discussing this possibility was a Montana rancher in a Kubota tractor forum. Calder only describes how to test a GP, not that it can completely short out. If the PO or OEM had fused the positive wire to the panel, this would have been a non-event. It is fused now.
 
Maine Sail

Maine Sail

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Feb 6, 1998
11,677
Canadian Sailcraft 36T Casco Bay, ME
Steve Walsh said:
Stu, I was aware of the potential harness issues and inspected the wiring harness often, intending to get around to the harness upgrade. Never a problem or failure to start before last week. What I had never read or heard about was that a glow plug could completely short out, melting the wiring. My understanding was that a glow plug just got weaker and finally quit working, not a total dead short that threatened the (unfused, in my case) wiring. The only resource I found discussing this possibility was a Montana rancher in a Kubota tractor forum. Calder only describes how to test a GP, not that it can completely short out. If the PO or OEM had fused the positive wire to the panel, this would have been a non-event. It is fused now.
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Steve,

Can you elaborate on what exactly "shorted" out with the glow plug? Where was the burned wire? Where was it melted at the glow plug or at the panel end. It sounds like you had the typical high resistance found in this circuit often due to the trailer connector. It can happen on both of the "high amp" circuits, alternator and glow plugs, though the glows are less likely and happen less often due to how short the ON duration is compared to the alternator...
 
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Steve Walsh

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Oct 6, 2009
129
Newport Newport 28 MKII Jacksonville, FL
Maine Sail, after rewiring everything, the GP wire would still get extremely hot, but still no start. As a test, I did a direct positive (new) wire connection to the new glow plug switch to the (new) glow plug wire that attaches to the glow plugs. Same result, instant hot wires, top of glow plugs at connection very hot, but no start.

Researched internet and finally found a thread that said it is very rare, but possible for something internal in the glow plug to fail, causing a direct short, effectively connecting a positive wire to the block. Apparently they are supposed to fail gradually, getting weaker an not working at all, but a direct short should not happen. I pulled the plugs, ohm tested was "0" instead of 1.2-1.5 NGK spec, so that led me to a second test: attaching a positive wire to the GP lead and touching the side of the GP to the block. Results were huge sparks, immediate hot GP wire and the tip stayed cold. This confirmed to me that the GP had internally shorted out (not the normal resistance found on a good GP) and caused the original wiring to melt.

The new NGK glow plugs tested at the 1.2 ohm spec, installed them and got an immediate engine start after a 15 second pre heat, using the same new wiring, which stayed cool. BTW, all the terminal connector blocks were corrosion-free, probably since the boat has been in a freshwater slip since 1986 with a few trips to the Bahamas and Keys. One of the harness connector blocks showed signs of melting (not as bad as your pics) so I installed new wiring. All the gauges work, the alternator survived and I intend to install your GP cricuit solenoid upgrade and get rid of the amp meter, etc. Sorry this was so long, but I had to do this step by step, sort of a process of elimination, confirming what was good as well as the repairs. Sure would like to know why there was no fuse on that positive wire; there is one now.
 
Stu Jackson

Stu Jackson

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Feb 26, 2004
22,805
Catalina 34 224 Maple Bay, BC, Canada
Great news, Steve. All the best, you can now have a Happy Holiday. :)
 
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