Master Disconnect Switch

34ford · March 10, 2004, 10:47:35 PM

I have my master disconnect switch installed in line on the positive side of the battery cable which goes all the way up to the starter. Also the wire from the alternator (which is a one wire) goes to the same terminal on the starter as the battery. My question is, is the positive the right wire to switch? If the car is running and something goes bad and I want to kill the power quickly and shut off the master switch I'm thinking the power will not be coming from the battery but alternator will still be sending voltage to the main fuse panel wire which is also on the same terminal on the starter so the motor will still be running and all things powered up. Both the alternator and the main harness wires have fuseable links in them. I have a seperate #10 wire from the battery going to a small 4 fuse panel to work the remote door unlocks and the receiver when the master disconnect is turned off and the doors on the car are closed and windows up. I do also have a manual release for the doors incase all power is lost or a fuse pops for the remote entry.
If I would put the switch in the negative side and still had the 4 fuse panel hot wouldn't it feed a ground back thru it. All my grounds go to a terminal strip on the main fuse box (which is a Haywire unit) and a #10 wire back to the battery .

Hope this is clear. Thanks for the help

Bob

model a vette · March 10, 2004, 11:50:41 PM

I've run my switch in the negative side for 20 some years but I don't think the car will shut off if I work the switch with the engine running.

There are some new style switches which have two pairs of terminals. One pair for the positive battery cable and another pair for the alternator wire going to the battery. The pairs are isolated from each other.

The reason for the isolation is to stop the engine from running off of the alternator if the switch just cut the battery cable.

Bruce Dorsi · March 11, 2004, 12:09:54 AM

Quote from: "34ford"My question is, is the positive the right wire to switch?

If the car is running and something goes bad and I want to kill the power quickly and shut off the master switch I'm thinking the power will not be coming from the battery but alternator will still be sending voltage to the main fuse panel wire which is also on the same terminal on the starter so the motor will still be running and all things powered up.

Bob, IMHO, I would leave the master switch on the positive lead.

I agree with your thoughts that the alternator will continue to supply power with the switch open, if left as wired. .... However, connecting the alternator output to the BATTERY side of the master switch should open all circuits, and allow emergency shut-down as desired.

Also, be aware that an electric cooling fan, when coasting down, can supply enough current to keep the ignition energized if precautions are not taken to prevent this. ....A relay or diode, will prevent this condition.

I don't believe putting the master switch in the negative cable will do what you want. ..... The alternator case is grounded, as well as the ignition circuit, through the engine block. .... I believe the engine will continue to run, even if the negative cable is removed from the battery.

enjenjo · March 11, 2004, 01:29:58 AM

You also have to consider most cars now day have one or more computers in them. If you switch the ground off, the current can find a groung through the computer, and make bad JuJu.

You can use a relay to isolate the alternator if you shut the master disconnect off. Mack trucks use one.

SKR8PN · March 11, 2004, 06:33:40 AM

Quote from: "enjenjo"You also have to consider most cars now day have one or more computers in them. If you switch the ground off, the current can find a groung through the computer, and make bad JuJu.

You can use a relay to isolate the alternator if you shut the master disconnect off. Mack trucks use one.

I agree with Frank,cutting the ground side,makes for some very bad JuJu,when it comes to engine controllers..............On my truck,I ran a hot cable from the Bat, straight to the starter,then on the Alternator,I ran that feed to a 125 amp FUSE,then to a kill switch,then on to the fuse panel,which then powers everything else up.Including the start circuit. When I hit the kill switch,it kills everything,even with the cooling fan on.The only thing that remains HOT is the feed to the starter.

34ford · March 11, 2004, 11:05:41 AM

Guess I will leave it as is. Just started thinking about how things were hooked up. I do have the electric fan on a relay. With the key off and the master off there is no power to anything so should be ok. The key is not on the colmun so it won't lock anything up if I need to turn it off. Thanks for the information guys.

Does anyone know of any good sites with tech articles on wiring? Have a friend looking for some to learn some of the basics. He wants to use the frame

for a ground and told him it's best to use a lead off the battery to a central ground block, especially with a glass car.

thanks again

bob

purplepickup · March 11, 2004, 12:34:06 PM

This seems like a good time to plug Skip Readio's book "How to Do Electrical Systems: Most Everything About Auto Electrics".

Skip is a long time member here but he's pretty busy and probably hasn't seen your question.

As far as websites go, here's a couple:
http://www.nesstronics.com./
http://www.autoshop-online.com/auto101/electext.html

Ed ke6bnl · March 11, 2004, 04:58:11 PM

Quote from: "Bruce Dorsi"
Quote from: "34ford"My question is, is the positive the right wire to switch?

If the car is running and something goes bad and I want to kill the power quickly and shut off the master switch I'm thinking the power will not be coming from the battery but alternator will still be sending voltage to the main fuse panel wire which is also on the same terminal on the starter so the motor will still be running and all things powered up.

Bob, IMHO, I would leave the master switch on the positive lead.

I agree with your thoughts that the alternator will continue to supply power with the switch open, if left as wired. .... However, connecting the alternator output to the BATTERY side of the master switch should open all circuits, and allow emergency shut-down as desired.

Also, be aware that an electric cooling fan, when coasting down, can supply enough current to keep the ignition energized if precautions are not taken to prevent this. ....A relay or diode, will prevent this condition.

I don't believe putting the master switch in the negative cable will do what you want. ..... The alternator case is grounded, as well as the ignition circuit, through the engine block. .... I believe the engine will continue to run, even if the negative cable is removed from the battery.

I do have mine connected to the ground no fancy things on the vehicle but moved up to an alt. The engine will continue to run and at full charge if I were to accedently turn the master neg. swith off. Ed ke6bnl

34ford · March 11, 2004, 05:59:37 PM

George,

Thanks for the links. Think I will order one of Skips books. Know he is the resident electrical guy and thought he might reply. Didn't know he was a famous author. Did get a kick about your pucker comment in the brake thread.

bob

Mac · March 11, 2004, 07:50:36 PM

I have heard that a one wire GM alternator won't properly charge -especially a remote located (like trunk mounted) battery unless it's wired in a certain way with adequate size wire.

I'm planning on a trunk battery and thought I'd be simply running 8 ga. from the alt. to the batt. terminal on the starter. Wouldn't that be good enough?

Hope I'm not hijacking the thread.

TIA

Bruce Dorsi · March 11, 2004, 09:13:49 PM

Quote from: "Mac (54Jag)"I have heard that a one wire GM alternator won't properly charge -especially a remote located (like trunk mounted) battery unless it's wired in a certain way with adequate size wire.

I'm planning on a trunk battery and thought I'd be simply running 8 ga. from the alt. to the batt. terminal on the starter. Wouldn't that be good enough?

There should be no problems doing it that way.

If you are using a high-output alternator, you may want to use a heavier wire.

It's also a good idea to install a fuse in that wire, as Jim suggested further up in this thread.

Depending on pulley sizes and idle speed, you may find that the self-energizing alternator may not put out a charge when first started, unless the throttle is "goosed." ....Once it is energized, it will stay energized, even if the rpm's drop back to idle.