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MG MGA - battery switch current rating

I'm thinking of installing one of those master battery disconnect switches on my '56 roadster. In checking around on the internet, there seems to be a number of switches that look essentially the same but have an assortment of different current ratings. Can anybody tell me how high a current rating would be required for an A? Thanks.
Andy Bounsall

Andy - The highest current that would be encountered would be the starter draw, but I don't know right off hand what that would be. I have had one of the cheapie switches with the red flag like key in our MGB for a couple of years and had no problems with it so far. Good luck - Dave
David DuBois

What do you want to use the master disconnect switch for? In the standard, unimproved MGA, there is zero current draw with the ignition switch in the off position. If it is for catastrophic shut-off, the switches at the lower price end will not interrupt a high current flow.

If you have upgraded some of the electricals to modern.... then there may be justification in its use.

While I don't use one, myself, I think that any of them will work. I would be very surprised if the current load of the starter in the MGA whould be much more than that of any V8 (North) American car.

Chuck

Chuck Schaefer

Chuck makes a good point. Almost all switches and relays have basically two current ratings, and that is the switching current spec, and the hold current spec. If you are planning to use the switch to interrupt the high current that would be caused by a pinched battery cable, or stuck starter switch, possibly after a wreck, then a premium switch is necessary.

However, If you plan on using the switch as a theft-proofing measure or service isolation switch, such that it will only be switched under very little load, then a less expensive switch, such as the one Dave descibes would be adequate. The one with the red plastic key (like Dave's) is the one I have for my midget.

Best of luck - - Alec
Alec Darnall

Chuck

I have a battery disconnect and for me it serves 3 functions:

1. I never fully trust these old electrics, so it's peace of mind to kill the car dead in the garage. The garage is built into the house with interconnecting door so it lessens a fire hazard.

2. It's an additional anti-theft device when parked in a public area.

3. It's a quick way to kill the electrics when I want to do maintenance work.

Actually I have a 4th. I keep 2 x 12 volts batteries in the racks and I have 2 isolators - one for each. I only ever run one battery at a time, isolating the other.

Steve
Steve Gyles

I use one like this. http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=92688

I use it mainly for the reasons Steve listed, but I do have it mounted where it is easy to get to from the drivers seat. It would probably not pass the requirements for a disconnect for racing purposes, but feel it adds a little margin of safety. I don't see any reason it wouldn't interrupt a high current flow. It is basically the same design as the starter switch, a metal disk pressed against two metal studs.

Jeff Schultz

Yes, all 3 reasons that Steve mentioned. Thanks for the replies. Now I know what I'm looking for, I'll see if I can find one locally.
Andy Bounsall

Jeff

Where did you locate your switch?

Thanks

Larry
Larry Hallanger

Larry,
I know I bought it locally, but I really can't remember where. We do have a Harbor Freight locally, and I have seen them there, but I think I bought mine at a local tractor/farm supply company.

Jeff
Jeff Schultz

Jeff,

I think Larry was asking where in the car is it located? You said "easy to get to" - but where?

thanks.
Joe Cook

Joe,
Sorry, of course he did. I must have been half asleep last night.

I mounted mine on the vertical plywood bulkhead behind the left edge of the passenger seat. I drilled a hole thru the plywood as close to the mounting flange for the gearbox tunnel as I could. It is right beside and about 1/2" lower than the top screw on the gearbox tunnel. I then cut a small piece of 1/2" plywood, drilled a hole thru it, and mounted the switch to it. This spaced the switch so that it barely protrudes though the carpet. I used wood screws thru the plywood bulkhead into the plywood spacer to fasten it to the bulkhead. I mounted it so the red key is straight up when installed, and just pushing on the top of the key will disconnect it.

Jeff


Jeff Schultz

Jeff

Joe is correct, and your explanation is good. I hadn't thought about a "spacer" to recess it in the way you have done. Good idea! Now all I have to do is get it mounted and to figure out some way to keep the key removable without losing it ;-)

Thanks

Larry
Larry Hallanger

Steve, and all, These are good reasons. I just wanted to point out the issue with a cut-off switch being turned off under a short ciruit. The only wire that really bothers me is the one running from the battery to the starter switch. If this one fails.......

The best reason I see stated is that of switching of the battery while performing any service work.

I'm not sure of the trade offs between the reliability if 50 year old Lucas vs. new Chinese stuff. I don't want to get into the Chinese bashing. Have been to China many times over the last couple of years on business and I do have a high respect for some, not all, of the manufacturers there. But I'm likewise not afraid of the old Lucas regulator that makes a disconnect itself.

For anti-theft reasons, why not just add a hidden toggle switch under the dash in the ignition line?

Chuck
Chuck Schaefer

Chuck

The ignition circuit through the key and a hidden cut-out switch under the dash is dead easy to by-pass. You just need to remove one fuse from the fuse block and re-insert it between the 2 sets of fuse terminals. A good tip if you ever lose your key.

The anti-theft aspect was a bi-product of my wanting an easy disconnect for maintenance.

Steve
Steve Gyles

Andy, if you're still running a generator (dynamo) then the simple single circuit switch will work. If you have up graded to an alternator there are special switches for those applications. A bit more expensive, but will protect the alternator if the switch is switched off while the engine is running.
Bill Young

Steve, Very true, but not every possible car thief will know the wiring details of the MGA. A simple wire with alligator clips will bypass your switch too. Add one from the coil terminal to the starter switch and push start the car. And away you go. For every plan to protect this simple car, there is an equally easy way to get around it.

Again, the reason of disconnect for servicing is a good one.

Chuck
Chuck Schaefer

Chuck

My battery earth (I am negative earth) is isolated with the switch. The would-be thief would have to put the crocodile clip on the battery -ve terminal and the the other end of the wire to earth to by-pass my cut-out. The position of the battery makes this bit of a chore. Probably easier for him to tow the car away!

Good fun these thought provoking views.

Steve
Steve Gyles

Steve, probably easier just to take somebody elses car. <g>
Andy Bounsall

Since he's gonna be near the battery to get the car anyway, why not just jump the switch?

Now what will most likely happen, is I'll be out somewhere, lose my key, and won't remember which fuse to jump where, nor have access to a wiring daigram!

P.S. what will you do when you lose the red battery disconnect key?

Peace,
Chuck




Chuck Schaefer

I had a key made on the lath of bullit stock. I have had problems with the red keys, the pin at the end breaking off so the key will not work. my alum. key is smaller and fits in my pocket when I park my car. I installed the switch for all the reasons listed above.
JEFF BECKER

This thread was discussed between 15/11/2005 and 19/11/2005

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