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MG MGB Technical - Fitting an Amp meter to 80 B

I am fitting a Smiths amp meter from an earlier B to a 1980 B. I will be placing it on the dash where the clock is normally located.

The issue is regards to the wiring. Based on my, abeit limited, electrical knowledge I am thinking of running wires from the ignition light to the amp meter for powering the amp meter.

My understanding is that the wires from the ignition light are connected to the battery and alternator. When the current from both is equal, the ignition light stays off. When current is unequal, current flows through the warning light turning it on. The Amp meter will be able to tell me which device is putting out power and which one is not.

Suggestions would be appreciated.


Thanks,

John
J Long

The ammeter has to go in the heavy brown wire which goes down to the starter solenoid - it measures the current flowing to and from the battery and this wire is the mian feed to the battery.

You will have to cut into the wire and use equally heavy guage wire to connect the two ends to the two erminals on the back of the ammeter.
Chris Betson

John,

What you are wanting to do won't work. For an understanding of ammeter circuits, see:

http://members.aol.com/danmas2/ammeters.htm

After you've installed your ammeter, you'll understand why manufacturers stopped using them as alternator outputs increased.

What is the scale reading on the early B ammeters? +- 30 amps I would guess. If your alternator is capable of much more than that, you will want to add the ammeter shunt described on the above web site. And, as Chris said, you'll want to use LARGE wire for this, capable of carrying the total alternator output.
Dan Masters

Whilst alternators are capable of high amps - this doesn't actually go to the battery (unless the battery is really flat) so a +30 / -30 ammeter is OK
Chris Betson

Ammeters are bad news. Think of the facts-two high current wires running into your passenger compartment and out again. Everything that your alt puts out to charge your battery must go through these wires and the meter. Two more locations for bad connections to be created to eventually heat up and catch fire, buried in your dash and console. Our little cars have enough electrical problems without adding more. BTW, if the meter goes bad or the connection comes loose, you loose all charging to your battery and likely damage your alt as well. Just is not worth the risk
gerry masterman

Chris wrote:

"Whilst alternators are capable of high amps - this doesn't actually go to the battery (unless the battery is really flat) so a +30 / -30 ammeter is OK."

You're right, but if you're like most of us, sooner or later you'll leave your lights on and run your battery down away from home. After a jump start to get you home, your alternator may charge at a rate much higher than 30 amps. Better to be safe than sorry, and shunt the ammeter. It's not hard to do, and gives you that added margin of safety. On the other hand, an ammeter can probably stand a small overload without damage for a short period. Inside the ammeter, there is nothing between the + and - terminals other than a short piece of solid wire, about 12 ga or so, formed into a loop. The magnetic field formed by current through this loop moves the needle, so if the needle isn't bent, and the overload isn't excessive, the ammeter should survive ok. 12ga wire can withstand a pretty hefty current in such a short length.

Gerry wrote:

"Ammeters are bad news."

You're right, it wasn't just a cost cutting measure that prompted manufacturers to drop the ammeter. I just installed a 60 amp meter in a friend's car as part of a complete wiring replacement/upgrade - what a pain in the a#$! I prefer an ammeter (or both ammeter and voltmeter, actually), but I'll stick with a voltmeter.
Dan Masters

John - If you want to monitor your battery/alternator condition, you are much better off installing a voltmeter. You can get one from Smith Gauges, web site: http://www.gower-oaks.co.uk/
or VDO, web site: http://www.vdona.com/performance
The voltmeter is much safer to install and gives you a much better indication of what is happening with your battery and alternator. I installed a voltmeter in my 66 MGB a couple of years ago and this summer i noticed that the battery voltage was lower than it should be and did not go up when the engine was reved. I was able to replace a defective alternator before it stranded me on the road. By watching the voltage on the battery, I was able to continue using the car and charging the battery at home when it started to get too low to operate on.
Good luck - Dave
David DuBois

Hmmmm. Should of consulted with you guys before I got the B Smiths amp meter ("ammeter")off of E-Bay. Oh well, you can never have too many MG parts laying around.

Dave (and others), I took your advise and contacted Gower-Oaks about getting a Smiths Classic range 0-15 vlt voltmeter.

Alrighty then, now how do I hook up the voltmeter to 80 MGB?

John
J Long

John - (after all these different threads, it is begining to look like this is our private communications link). I brought a 16 gauge wire right off the back of the ignition switch to the meter. The 12 volts can be picked up anywhere that switched 12 volts is available, but the closer to the actual switch, the more accurate the reading will be. Just be sure that the 12 volt side of the meter is well isolated from any ground in it's vacinity. It woudl not hurt to put some shrink sleeving over the terminal to insure that it is well isolated.
Good luck - Dave
David DuBois

This thread was discussed between 13/07/2002 and 14/07/2002

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