Big MO 400M Battery help

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Big MO 400M Battery help

Postby D_Shack » Thu Jul 13, 2017 8:27 pm

:!: I have a recently acquired an early 60s Big MO 400M with a 4 cylinder LPG Engine. The previous owner said it hadn't been run in a year. The tractor has one 12v battery with 500 cranking amps and a full charge but when trying to crank over the engine, the engine turns slow for about a quarter of a revolution, stops and then the positive battery terminal begins smoking. I've checked and cleaned all leads to both the battery, the starter and the negative ground to the frame with no different results. QUESTION 1 is does anyone know if this tractor requires a second battery or a larger capacity of cranking amps to spin the engine consistently? QUESTION 2 is should this early 12v tractor have a positive ground as well and if so from where to where would it be located? Any help would be greatly appreciated.
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Re: Big MO 400M Battery help

Postby G Brown » Fri Jul 14, 2017 6:09 am

In 1960 MM still used positive ground.All gas or LP only used one battery. Diesels used two. LP are higher compression motors than gas. 500 cca is a little small. Pull plugs out and get motor turning freely.Add a bit of oil in cylinders then put plugs back in.
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Re: Big MO 400M Battery help

Postby D_Shack » Fri Jul 14, 2017 7:26 am

Thanks G Brown..... I appreciate the info. I pulled the plugs yesterday and added MMO to each cylinder. I have a 1000 cca Optima battery I'm going to try today. Should the battery be grounded only on the negative terminal to the frame though as it is now? That's an area I have no knowledge about how the positive ground system works.
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Re: Big MO 400M Battery help

Postby G Brown » Fri Jul 14, 2017 8:39 am

Battery was normally located on right rear axle on fender mount. Battery was originally positive ground which just means positive post to chassis. Many were changed to negative ground. You should be able to tell as positive battery cable end is bigger than negative. generator has to be changed to make negative ground system work,
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Re: Big MO 400M Battery help

Postby D_Shack » Fri Jul 14, 2017 9:42 am

G Brown, then mine must have been converted due to the + cable going from the + battery terminal to the starter motor and a + lead coming off the alternator. As for the battery, it is located underneath the seat centered in a nice but older battery frame mount. According to the engine data plate the "Date" is Sept 1960" so I assume that means it is a year model 1961. As for the chassis data plate located on the right side gear box, it has been removed long ago. Perhaps by the military or someone along the path of ownership afterwards so I can't tell much about the year and model of the tractor unfortunately. I appreciate your knowledge and taking the time to pass it along to me. I love the old tractor and hope to get her back to work soon for me.
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Re: Big MO 400M Battery help

Postby G Brown » Fri Jul 14, 2017 10:05 am

I have a Big Mo 600. They were all positive ground and had generator not alternator. Alternator came out around 1966-7. You get more cranking power with negative ground so many were changed over.
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Re: Big MO 400M Battery help

Postby D_Shack » Fri Jul 14, 2017 10:11 am

Good to know. Mine definitely has an alternator versus an old skool generator. Any idea what would be a good battery CCA for mine considering it is a '61 4cyl LP version?
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Re: Big MO 400M Battery help

Postby G Brown » Fri Jul 14, 2017 11:12 am

If it was mine 1000. Have U302 LP and that is what i use.
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Re: Big MO 400M Battery help

Postby rvirgil_KS » Mon Jul 17, 2017 7:33 am

Since it has an alternator it must be positive ground. Generator could be polarized either way, but alternator, unless specially modified must be negative ground.

Starter itself doesn't care if positive or negative ground, it will turn same direction either way. Positive or negative ground configuration should make no difference on cranking power.

With negative ground, -- post on the coil goes to side of distributor. If + and - post to the coil is reversed it can cause hard starting when engine is cold.

If battery cable terminal smokes after cleaning the connections, replace the cables. You probably have internal corrosion in the cable that you can not see. The battery sits a ways from the starter, don't use cheap wimpy cables.

You may need to pull the starter back and clean the starter to frame mating surface.
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