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FLD 120 Master switch?

Posted By andyg Friday, January 31, 2020 11:58 AM
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andyg
 Posted Friday, January 31, 2020 11:58 AM
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It doesn't seem like an old truck to me but it kinda is. I have a 1989 FLD 120 that is used to tow my traction engine. Sometimes I notice that sound of random relay clicking when I walk by the truck in my barn. I have no idea what is going on here but I'm thinking that a master disconnect switch could solve the problem. When i look at the truck I see that there are 3 large cables running from the (4) batteries directly to the starter. To power the electrical system there are 3 or 4 wires (about 10 ga) going from the starter to the firewall. Putting a switch/switches in the large cables between the starter and batteries dies not seem practical due the the very large amperage requirement. I could put a switch between the starter and firewall though. Has anyone done this? Any advice?

Evansville, IN
Geoff Weeks
 Posted Friday, January 31, 2020 12:44 PM
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I hate to say it, but what your considering is masking a problem. I may come back and bite you in the way of a fire while driving down the road.
A Cranking motor can draw close to 1800 amps with a cold engine. If you are going to go ahead with the idea anyway, I would isolate the power after the starter. You have to make sure that there are no other power paths. Backfeeds can cause all kinds of headaches.
Another hazard is if the switch fails while driving, it can cause a voltage spike that can burn out any electronics, like the alternator, and radios.
Far better to get to the bottom of the problem.
Friday, January 31, 2020 12:45 PM by Geoff Weeks
andyg
 Posted Friday, January 31, 2020 3:05 PM
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Thanks Geoff, I didn't explain my idea very well. I would put the switch in the small wire between the cab and the starter (like you suggest). I would NOT try to put a switch between the starter and the batteries. I understand your concern about the switch failing but I don't have clue where to start looking to the cause of the phantom relay clicking.

Evansville, IN
steeltracs
 Posted Sunday, February 23, 2020 4:05 PM
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buy the high amperage Flaming River or Cole Herse disconect switches and put them on your cables from the batteries. Get the relay problem fixed too.

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