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MG TD TF 1500 - Battery Cut-off switch

I have just bought a battery cut-off switch with the dire warning that it should be used only with negative ground systems.

Why not have one for both positive and negative grounds?

What's the significance of not using it only with positive ground systems?

Gord Clark
Rockburn, Qué.
Gordon A. Clark

Gord, I dunno what kind of switch you might have but my first thought is that it has some provision in the switch to keep low amperage drwaw items powered while OFF. I am thinking of a clock for example.
Why not check it with an ohmeter to see if there is continuity between the mounting plate or frame and one terminal or the other when the switch is in the OFF position.
Otherwise, I agree that it should not make any difference which polarity is involved.
Jim Merz

I can't see how it would matter unless there are diodes, LEDs or transistors involved. I've only seen battery cut-off switches that had no electronics. In any event, it is best to break the earth connection rather than the non-earth connection when you hook it up.
George B.

I recently installed a battery cutoff switch on the positive terminal of my positive earth TD. It looks identical to Moss part no. 145-795. Because the hole in the battery cutoff switch is sized for the smaller negative terminal, I needed to widen the opening so it would fit on the positive terminal. I did this using a bolt and a couple of nuts.

Especially given the proximity of the metal surfaces of the battery box to the hot terminal (in my case, the negative terminal), I consider it essential that the earth connection, not the hot connection, be switched. If the hot terminal was switched, a wayward wrench could lead to unexpected welding in the engine compartment.

The battery cutoff switch works well. Spin the knob and the electrical connection is interrupted. A few more spins and the knob can be removed and placed in your pocket. Without the knob in place it is a significantly more difficult job for someone to hotwire your MG.

Larry
Larry Shoer

I am certain that Larry's explination is the reason for the disclaimer. Not everyone is cleaver enough to enlarge the hole for the positive battery post as Larry did, thus they would wind up with the switch on the hot side of the battery, shich in the case of this type of switch is a disaster waiting to happen. If the switch is being used as a theft deterent and one wants to keep the clock running while the switch is off, one only has to bridge the contacts with a 5 amp fuse. This will keep the clock running, but will blow if someone tries to start the car with the starter. Since I only use the switch to shut off power when I am working on the car, I do without the fuse and just reset the clock when ower is back on. Cheers - Dave
David DuBois

There are a variety of cut off switches – if the one Gordon has bought has a resistor and 4 connections in addition to the main battery cable it is one designed to switch off the engine in racing situations – made to FIA regs.
A simple cut off switch can break the circuit any way you want.
One suggestion, put the cut off switch where it is easily assessable. This way you will use it regularly.
Steve Taylor
Steve

On my TF (neg grnd car) I run the cut-off on poss. terminal to keep power from starter switch terminal..a great place to do some "welding by dropped wrench"!
I also have a hot lead to an aux pannel I have installed with seprate fusing and picked up a lead there to keep my clock running. This allows me to "quick dissconnect" for theft (but leave the clock) and when working under the dash can either have power for a work lamp or dissconnect as needed. I installed a "test-jack" like that found on the TD to my pannel but it is atactive only when turned on using an old Lucas switch that has a lamp to indacate power is on.
This has worked well for me. I did have to modify my cut off to handle the larger "poss" terminal and then
dipped the removable knob in RED tool handle coating so it is not mistaken for a neg earth connection by someon else. If you are going to put your cut off on a "hot circuit" this might be a real good idea so others can spot and not assume that "green knob" is a cold lead!
Cheers,
David 55 TF1500 #7427
David Sheward

David - Have you checked to make sure that there is no sneak path so that if somene pulls the starter know, starter current doesn' flow through the clock, causine it to operate in the incadesant mode for a couple of micro seconds on it's way to becomming a chunk of carbon?

Also to those concerned about having an anti-theft device for the T series cars, it already has two very effective anti-theft devices built right into them. First the starter knob - who ever heard of "pulling" on the starter button? Second the wonderfull "fly off" emergency brake. How many people do you know outside of the T series community can release that emergency brake? Add a small, unobtrusive switch that turns off power to the fuel pump and any wood be thief will think he has died and gone to auto theft hell.
Cheers - Dave
David DuBois

David,
Yesssir...current limiting diode & 1/8 amp fuse in the "clock line"!
Fully aggree on the hidden "fuel cut-off" switch...matter of fact, kept me from "stealing" my own car last nite, enngaged wrong switch....sputter put-put,cough...dead in the water!
"it already has two very effective anti-theft devices"
"3rd" one ...most auto thief's don't have the fund$ to replenish the oil reserve or they would be driving a Lexus instead of stealing cars!
4th...They will only get 2k miles (@most) on it before they have to reset the valves /points / or some little nusance!...by the time they get that done one of us would have followed the "Castroil trail" and traded them a bus ticket to get the old girl back!
More Cheers,
David 55 TF1500 #7427
David Sheward

I realized some twenty five years ago that the TF's natural defenses that you list were so invulnerable that I leave the key in the ignition when I park it. The only result is that in that period I have had the key stolen once. The possible value of of a TF key without a TF continues to elude me.
Bill McGee

Bill,
"The possible value of of a TF key without a TF continues to elude me.".............
Perhaps a well wisher or spouce trying to "help" you?
Cheers,
David
David Sheward

Okay. Okay !!

I've made my decision!

I'm going to mount my battery cut-off switch on the Negative side. But nobody gave me any comments as to what, if any risk there is, to installing one on the positive (ground) side as well.

There are several cut-off switches. the one Steve Taylor alludes to is,strictly for racing and is expensive($72). I can't see the benefit to the 'cheep' one (green knob) selling these days in the $6 range.

In Moss' catalogue they show both the Lucas and the 'El-Cheepo' (in the illustrated section) and again, they post the dire warning, and I quote " ... Sized for negative posts and should be used only on negative ground cars".

One chap suggested to me that there is a migration of positively-charged sulphate ions that, when the battery is disconnected, are attracted to the negative terminal, causing cationic action that causes a build-up of sulphates on the negative post. Despite being an electrical engineer, I have never heard of this.

So I have made my decision .... etc ....

Thank you all for your inputs. Got some great ideas. But on a slightly different tack - I can hot-wire a TD/TF in 5 seconds -faster even than in the movie, and it takes no tools! Common knowledge for most old-time T-Series owners.

Gord Clark,
Rockburn, Qué.
Gordon A. Clark

Gordon; You have to watch for those SULPHATE LIONS, especially when you start jumping fuses around. They GONNA GETCHA.
Sandy Sanders
conrad sanders

Gord - The only danger in mounting the type of switch you are talking about on the negative battery post of a positive ground vehicle is the greater chance of shorting the bare buss portion of the switch to ground. Sounds like the chap with the suggestion of the negatively charged sluphate ions was engaging in some highly creative writing. I would devinitely suggest that the bare portions of the switch, be it the grean knob or the Lucas type, be treated to some kind of insulation (Shrink sleeving, plastisol or bi-seal self vulcanizing tape) to avoid the possibility of shorting something to the negative side of the battery. There is also a cheaper version of the racing type switch that mounts through a bulkhead and can be wired, via short section of battery cable, to either side of the battery. Good luck - Dave
David DuBois

In case its of interest to anyone... one particular TF is 'disconnected' via:

- on the -ve terminal, the PO has fitted a 'redkey' style cutoff. He has cut a 1" hole on the firewall next to the starter solenoid. The key bit points down, towards the driver's toes - need to reach in above steering column to twist/remove the key.

- on the +ve (gnd) terminal, I've put in the simple bolt-on-terminal green-knob. Filing would have been a 'neater' job, but same can be acheived with a big hammer. There was enough stretch in the metal and the battery terminal does squish to make everything fit.

btw... we do NOT carry jumperleads in the car. Doing so just makes it too easy for any dropkick with a knowledge of car electrics to overcome all the protection you've installed.

(imho If your battery is in good condition and your ignition/engine is in good nick, you should never need jumperleads.)
Will

I used one of the green el cheapos, on the positive side, and it did corrode from sulphation. But, I thought this was due to overcharging the battery, not some "cationic action." I'm an electrical engineer and a physicist, and never heard of this until the thread above.

Ira
Ira Spector

I used the green "el cheapo" on the negative side of a positive ground car last winter. It worked great, no sulphate ions and a battery that worked in the Spring. What a concept.

FYI, I did put an "el cheapo" on the positive side first based upon input that you should break circuit to the earth side, I did I broke it.

David

David

I would be happy to send a digital picture that shows the technique I used to flare the opening on the Moss 145-795 (aka "el cheapo") battery disconnect switch. The battery switch can be installed over the larger positive terminal without damaging the battery or switch. So far, it was worked well for me.

Placing the battery disconnect on the earth side (whether positive or negative earthed), provides all the benefits of placing the disconnect on the hot side, plus additional benefits, too. The latter being that it will be much less likely one could create an accidental short or other electrical pyrotechnics.

If interested in the digital picture, please send an e-mail directly to me (LarryShoer@comcast.net).

Larry
Larry Shoer

Always disconnect BOTH when working on car. Can't be too careful when one's got leaky SUs.


re: white sulphur fur... we just rinse it off with boiling water then cover the terminal with vaseline.

Still the recommended prevention?
Will

It's a very good idea to fit a well engineered battery cutoff switch. I have been testing my new wiring and when I have fiished for the day have simply pulled off one of the battery terminals.

The other day my wife 'phoned me at work to tell me there was a cat locked in the garage - she could hear it purring. Turned out the purr was the SU pump - the loose batery lead had fallen and was making contact with the battery termial and fuel lines were not attached to the pump so it had ticked away, all by itself, for about two days. The pump body was very warm but it does it appear to be ok - I hope.


Jan T
Jan Targosz

What about the TF clock? I assume it requires a bypass low amp fused circuit to keep it running when the battery is isolated by one of the propriety switches. However, I can't see any power lead to it on the Workshop Manual wiring diagram, or is it spring driven?
John J
PS: I am picking up my "new" 1954 TF on Monday, so I can't study these finer points yet.
John James

John - The clock does require a low amperage fuse by-passing the switch to keep running.
Jan - the extened run of the fuel pump should not tax it in any way. When I restore pumps, I set them on a test stand and run them for at least 24 hours as an endurance test (in my case I have a temperature sensor attached to the coil body in case the pump stalls in the current on condition. The sensor is attached to a circuit that will shut power off in cases like that when the body reaches about 150° F)
Cheers - Dave
David DuBois

John and Dave,

I wear a watch. I know this is quite revealing and probably shocking (no pun intended) to a T-Series owner. So I will not disclose which terminal I am going to fit my new switch, to. (sorry about the preposition). I don't want anybody reading this to have heart palpitations! So keep cool, chaps!

For those of you going to Stowe in 2 week-ends, I will be wearing my WATCH and anyone wanting to, can have a look at it - no charge!

In the meantime, I will continue to adjust my TF clock almost each time I get into the car, just as I have been doing for almost 50 years! The serrations on the little knob are almost smooth.

Gord Clark
Rockburn, Qué.
Gordon A. Clark

Gord - I did the same thing for many years with our TD clock until I found out about Hal Kramer. Since he worked his magic on the TD's clock, I have reduced my contact with that knerled knob to about once a month. I say Hal worked his magic, it has to be magic for such a simple and rather primitive clock mechanism to keep time to plus or minus 10 minutes a month. If you want to send your clock to him, his e-mail address is halandjoan@aol.com
Cheers - Dave
David DuBois

As a young lad in the late 60's with an MG, one measure I used to prevent theft of my vehicle was to pop the distributer, remove the rotor and place it in a small plastic bag in my pocket!

Paul
51 MGTD
Paul Hinchcliffe

This thread was discussed between 06/09/2003 and 11/09/2003

MG TD TF 1500 index

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