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Master Disconnects and where to install them
From Ron Francis Wiring

Over 28 years of Street Rodding and as a wire harness business owner, I have overheard and been in many conversations with friends and customers regarding the best placement of the vehicle’s master disconnect switch.

(1) Should it be placed in the positive or negative battery cable?

(2) Should it be placed as close to the battery as possible?

(3) Should it be mounted in an area that can be accessed easily in case of failure?

(4) Should it be protected from the weather?

The answer to these questions will differ greatly depending on whom you talk to. A lot of questions seem to get generated when speaking about this one very important part of your vehicle’s restoration. Hopefully, some of the answers to those questions are noted below.

Answer (1) Until the invention of the electronic gadget, the master disconnect could be installed in either the positive or negative battery cable. Both lead to the power source and both essentially cut off the electrical flow from either terminal and would render the battery "disconnected".

Digital radios, radio memory circuits, CD players, Electronic fuel injection computer memory circuits all require power to retain memory when the master disconnect switch is shut off. While these circuits operate immediately when current is applied, they operate much more effectively if power is kept applied.

Shutting off the battery by use of a Master Disconnect shuts off these memory banks. In order to keep them "alive", the memory circuits must be fed with an isolated power source which can bypass the "disconnected" battery cable. This is best done with a fused circuit as in the Ron Francis Wire Works ""memory bypass circuit" found on the AP-97 Advantage Plus wiring panel. This fused circuit is powered directly from the battery and is NOT shut off by the master disconnect switch which, in this case, must be in the positive cable. This is a great improvement over the alternative which is to "restart" these electronic circuits every time the car is pressed into service

To reverse this "bypass circuit" theory and install the disconnect in the ground circuit instead, is next to impossible. One would have to totally isolate the grounds for the circuits for these accessories to make this happen. Doing so might be possible as long as nothing in the vehicle has lost its ground and feeds back though this isolated ground you created.

The modern automobile electrical system is expected to be recharged at periods of no longer then a few weeks. By this, I mean that any new model car has enough draw on the electrical system that the battery will go dead in possibly less than a month if stored and left unused. Older vehicles which this hobby / sport remembers could sit all winter, or as much as 6 months, and draw virtually no current at all if properly wired.

The constant flow of electricity to certain circuits in your vehicle keeps your favorite radio stations ready and the driving patterns of the driver in the memory banks of the electronic fuel injection and other equipment involved.

Answer (2) Electrically I see no reason to mount the switch close to the battery, as long as the switch is mounted securely and the studs on the back of the switch are protected from touching ground. The MS-1 Master Disconnect switch from Wire Works is supplied with tight fitting PVC caps that help protect these studs from both accidental grounding and corrosion. The shorter the distance between the battery and disconnect, the shorter the unprotected distance that might be physically damaged in case of an accident, etc.

Answer (3) The owner of the vehicle or the installer must determine this answer. A lot of people use this switch as an anti-theft deterrent as well as just a Battery shut off switch. Allowing easy access to the switch allows it to be easily found and defeated if you are using it as an anti-theft system. On the other hand, if you need to get to the switch to disconnect the battery in an emergency situation, easy accessibility can be critical. From past experience, using a quality switch have proven very reliable and can be mounted in a unaccessible area, if you so desire.

Answer (4) These switches are weather resistant, not weather or water proof! To be safe, mount it in an area that is protected from water spray and other harsh conditions. Use dielectric grease on the connections to minimize any corrosion.


If you are going to use a "memory" circuit for any electronic circuits, it would be best to put the disconnect in the positive cable to eliminate any risk. It’s a long walk home without those creature comforts known as "electronics".

We continue to add many articles to our Technical Knowledge base in our HotRod M.D.... Check out the latest articles from Ron Francis wiring and others... you’ll find it very useful.

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