coils issues

Discuss fixes, upgrades and modifications to your M37

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ZGjethro
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Re: coils issues

Post by ZGjethro »

tinmedic wrote:I got my NOS coil from VPW and it was perfect, no issues at all.
That is where I ordered mine from. At a premium price though. The old one which was in my truck when I bought it lasted for seven years with me, and who knows how long before I got the truck. I just hate having to buy a spare for the glove box at $120
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Re: coils issues

Post by ZGjethro »

Darn it! VPW is out of NOS coils. Chromie Chinese crap coils should be here soon
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carolinamv
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Re: coils issues

Post by carolinamv »

I bought a NOS coil from VPW on 7/18.
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Re: coils issues

Post by ZGjethro »

You must have gotten the last one. I talked to a guy there and he said they are out of them. My two Tiawanese replacement coils showed up today
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Re: coils issues

Post by sturmtyger380 »

I am going to start taking my meter with me to the flea markets. I bought a coil at a low price at one and got home and it was no good. I asked the gut at the show if they were good or not. His response - 'Gee, I don't know.' Yeah right. That's the last time.
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Re: coils issues

Post by w30bob »

Bringing a small multi-meter with you is a smart move. But to be fair, most of the vendors at the meets these days are simply parts peddlers. When you run across a dirty old distributor (hopefully with a coil in it) you almost never know the history of it..........heck, most people don't even know HOW to test a coil. You can usually get them for a pretty reasonable price, but it's always buyer beware.

Now if you buy from a vendor who has all his parts all cleaned and shined up, and is asking premium prices, that's a different story. But of all the coils I've run across at meets I don't ever recall a seller looking me in the eye and telling me the used coil he was selling was definitely good. They all said they either didn't know or weren't sure. For a few bucks I'll take chance, but anything more than that and I'll pass. But having one of those small multi-meters on me in the future might change that.

NOS is the way to go, but supplies are dwindling and even new NOS coils are OLD, and the sealing system that keeps the oil from leaking out wasn't designed to last forever. If the Chinese or Taiwanese coils are junk then the market will have to drive a change. If the Jeep/MB/M38/M151 crowd, who seem to be driving the military vehicle market these days, starts bitching about the quality of the coils...........the vendors will act. I hope. :shock:

I suggest that when we call Midwest or VPW (or any MV part vendor) for other parts that you mention your dissatisfaction with the repro coils currently available. At some point someone will realize there's a market (and money to be made) from having a better coil made.

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Re: coils issues

Post by ZGjethro »

My reproduction coils showed up yesterday. They are a Newstar product. They both read 11.5 ohms on the primary, and 11.19 Kilo-ohms on the secondary. I have a hard time believing they are both bad, but those readings are way off of what the made in USA coils are supposed to read. I have one in the distributor now, and hopefully tomorrow I will get a chance to install it.
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Re: coils issues

Post by isaac_alaska »

ZGjethro wrote:My reproduction coils showed up yesterday. They are a Newstar product. They both read 11.5 ohms on the primary, and 11.19 Kilo-ohms on the secondary. I have a hard time believing they are both bad, but those readings are way off of what the made in USA coils are supposed to read. I have one in the distributor now, and hopefully tomorrow I will get a chance to install it.

Did the coil bottom out on the distributor housing just slightly before the screw tabs seated? Ideally, when you tighten the screws, the screws should press the coil against the housing to aid in heat transfer. If there's any gap, a piece of thermal tape or thermal silicone from radio shack or similar should do the trick
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Re: coils issues

Post by ZGjethro »

Isaac, , I'm not sure if the coil bottomed out. I will check and see how the other new coil compares to the old coils to see if it should be in contact. An aluminum disc would probably be a suitable heat sink also I would think.

How snug is the cap supposed to be screwed to the distributor body? That seems like the only variable concerning how hard the distributor cap is pressed on the rotor. The carbon button was worn off the cap when I pulled it out, and it was not that old
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Re: coils issues

Post by w30bob »

Isaac,

I wouldn't worry too much about the bottom of the coil touching the housing to transfer heat, that's not the primary heat transfer path. The coil is sitting in an air-tight housing that's bolted to a hunk of cast iron (the engine) that runs around 200 deg F when fully warmed up (the temp the thermostat is rated for is the temp it starts to open, not the temp when it's fully open). The coil shouldn't be so hot you can't touch it.........but the engine when fully warm can't be touched........so sinking the coil to something that's hotter than the temp you want the coil to be isn't going to work. If the thought is the distributor housing itself would sink the heat to the air surrounding it.............there's also not much airflow around the distributor body due to all the mud guards around the engine.......and the M37 wasn't designed to spend much time at speeds more than a crawl. The only real airflow in the engine bay is from the hot air being pulled thru the radiator via the fan. There's just not a lot of cooling air to be found around the distributor.

The earliest M37s used a 6 ohm coil with a wafer capacitor mounted directly underneath it in the coil well in the distributor. So the capacitor sat between the bottom of the coil well in the distributor body and the bottom of the coil housing. The capacitor was made of a ceramic material, which is an insulator, obviously not a good heat sink. Had the designers wanted the bottom of the coil body to sink to the bottom of the distributor housing well they would have put the capacitor somewhere else.

That design went away pretty quickly and the coils were manufactured with the capacitor mounted internally, as they are now, but the design of the distributor housing and the well the coil sits in didn't change in any way. So..........the primary heat transfer path for the coil is through the vent lines attached to the distributor body, and it's the ambient airflow thru those vent lines that do the majority of the cooling. That's why making sure those vent lines are working properly is so important. Any cooling done by the coil body being in contact with another metal surface, or from air around the distributor body, would be secondary. At least that's how it looks to me..........I most certainly could be wrong (wouldn't be the first time). :mrgreen:

regards,
bob
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Re: coils issues

Post by isaac_alaska »

You have a good point there. When I get my truck back together in Fairbanks I'll grab some photos with the thermal camera. Although when I do in sure ambient temp won't be more than 50 F so maybe mis leading
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Re: coils issues

Post by ZGjethro »

Good points Bob. I compared the new and old coils, and the distance below the mounting screw plate seems identical on the Made in USA coils and the Taiwanese coils. I'm not going to worry about the heat sink issue.
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Re: coils issues

Post by carolinamv »

Is running the vent lines necessary? Could you unlook them if your not going to ford the truck or do they push air through the distrib??
And is one and air flow in and one line a out?
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Re: coils issues

Post by Elwood »

carolinamv wrote:Is running the vent lines necessary? Could you unlook them if your not going to ford the truck or do they push air through the distrib??
And is one and air flow in and one line a out?
If you're using the military distributor housing with the internal coil, you need those vent lines, even if the truck never sees a drop of water. If the vent lines are not installed and routed correctly, the coil will have a short life.

I don't think the lines are directional, in that the air must flow through the distributor housing in a certain direction. But the fittings that screw into the intake manifold are directional, in that one is designed to scoop air from the direction of flow through the manifold, and the other is designed to return it into the airflow.

Photos of the two manifold fittings can be found here: http://www.g741.org/PHPBB/viewtopic.php ... ent#p53922
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Re: coils issues

Post by isaac_alaska »

If you route the vent fitting in the distributor more towards the rear of the vehicle to the intake fitting more upstream, and the fitting in the front of the distrubutor to the intake fitting more near the carburetor, the lines don't have to cross. But functionally it doesn't matter. On several I've seen the intake fittings have been replaced with wrong fittings that don't have the tubing that sticks inside the intake elbow to pull in and suck out air.
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