Posted: Mon May 23, 2011 12:33 pm Post subject: Battery Cut-off switch
I have a battery cut off switch to install on a M38a1 still in 24v. I have conflicting opinions of where to install it, Pos side between the battery box and starter terminal or the Neg side between the frame ground and the battery box. I have only seen switches on the negative side, but the instructions for a switch I say online showed a diagram with the switch between the Battery and the ignition switch, not showing the starter at all.
Joined: Feb 28, 2011 Posts: 148 Location: Escondido, CA
Posted: Mon May 23, 2011 3:12 pm Post subject:
The battery switches on all the fire apparatus I've seen are on the positive side. They are an A, B, A&B, Off selector for a dual battery setup. Electrically, it doesn't make any difference.
If the switch is between the battery and the starter, or between the battery and the frame, it must be capable of conducting the full current of starting (hundreds of amps). The one you saw online may have been a light duty switch intended to just prevent starting by interrupting the starter solenoid and ignition circuits and not really a battery cutoff switch. _________________ Jim M.
1952 M38 son-father project
Discovering more worn out parts, one assembly at a time
Joined: May 14, 2009 Posts: 972 Location: South Dakota
Posted: Mon May 23, 2011 5:49 pm Post subject:
The idea of it in the ground wire between the battery and it's main ground is that all possible shorts are stopped. If it is in a box where it's locked up, they can't hot wire your rig either. They were standard equipment on a Austin Healey 100-4, and 3000. If you have a small system drain that runs the battery down over time, the switch stops that too. Good safety device when working with tools around the electrical system. I like them. Peace of mind. John
A quick tutorial on the electrical theory with battery current flow. The electrons pass the ball from negative to positive, which means the current flows down the negative battery cable to the frame then thru the block (via the motor mount ground strap) then thru the starter to the battery again. The frame current also flows thru each accessory then back to the starter terminal via the regulator and back t0o the battery. This is why the battery ground connection is the favored connection for cutoff switches. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Battery cut-off switches were installed on Dutch built M38A1s, on the right side of the dash, I have it on mine. It did prove useful once already. Need pics?
Joined: Feb 18, 2009 Posts: 43 Location: Seattle, WA USA
Posted: Wed May 25, 2011 3:50 pm Post subject:
I installed my battery cut off between the two batteries. Then, when I'm away, I have two small 12v trickle charge maintainers hooked up to each battery since they are now 12v and not 24.
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