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MG MGB Technical - fitting amp-meter to roadster

Can anyone advise how to wire an amp meter into my mgb roadster 1970 c/b, or show me on a diagram
LESLIE BARRAS

Hi Leslie,

You need to disconnect all wires, except the battery wire, from the starter solenoid, and fit one heavy wire to it. The other end of this new wire goes to the ammeter. the disconnected wires need to all be joined and a new second heavy wire needs to connected to them. This goes to the other end of the ammeter. See attached diagram. The wires with red dots are the ones that need to be disconnected.

Herb


Herb Adler

A much better instrument to use, that doesn't require running large amounts of current into and out of the passenger compartment is a volt meter. A volt meter can be wired with much smaller gauge wire, making it easier to route and is a much safer installation. Cheers - Dave
David DuBois

I second Dave, the best word of advice about fitting an ammeter is "dont". As its a series instrunment if it or any part of its circuit goes open or high impedance you have no electrics. A voltmeter tells you a lot more as well.
Stan Best

Another vote for the voltmeter.
Dave O'Neill 2

I wouldn't fit an ammeter myself. I don't like the idea of all that current going to one point behind the dash where you can't see it. If a short should occur you have full battery power, un-fused, in wires thick enough to carry it - recipe for a fire methinks.
Mike Howlett

Another vote for a voltmeter.

I bought a cheap gizmo from ebay in which one half gives air temperature in or out of the car, the other half gives a constant voltage reading. Just needs a live and earth via thin wires. As Stan says, very informative.
John Bilham

Another vote for don't do it, fitting an ammeter on an alternator based system is asking for trouble and a voltmeter will give you a much better indication of the state of the system.

Bob
R.A Davis

Why not add a current monitor, not intrusive and you just need to slip the monitor ring over the wiring.

Not sure if anyone sells them, if not then this may be worth someone investigating!
Pat Gregory

"Not sure if anyone sells them" :o)

Another vote for a voltmeter over an ammeter, if you really must fit another gauge to worry about. But as John Twist has said: "Except in the RAREST of circumstances, the ignition warning light indicates any problem with the charging. AMMETERS properly connected into alternator circuits provide at least two more connections which can corrode and cause the alternator to fail. Buy some driving gloves instead."

Even with a voltmeter there are questions on where it should be connected, and unless you are going to connect it to the battery cable on the stud it isn't going to be displaying battery or alternator voltage anyway. It's essential to have the meter on an ignition switched source so the typical thermal gauge isn't discharging the battery when parked, and highly desirable to have the meter on a fused source in case there is a problem with the meter and its wiring. The green circuit meets both of those requirements, but is usually a volt or more lower than battery voltage on most MGBs. The white circuit is slightly better but the meter would require its own in-line fuse. The battery cable at the solenoid is the most accurate, but would need its own ignition relay as well as in-line fuse. Even then meters vary, I have seen a digital instrument (reading down to tenths of a volt!) read 2 volts lower than two analogue test instruments connected to the same point. And it still isn't going to tell you whether your car will start next time.
PaulH Solihull

"Why not add a current monitor"

I was thinking along the same lines until I realised it will only work on AC, not DC. If anyone knows better, I'm all ears.
Peter B

http://www.hallsensors.de/

The Chinese have already done it. Hall effect relies on resistance changes caused by the magentic field of the measured current. There may be other vendors, this was 1 st hit on 12v hall effect.
Stan Best

"...unless you are going to connect it to the battery cable on the stud it isn't going to be displaying battery or alternator voltage anyway. It's essential to have the meter on an ignition switched source so the typical thermal gauge isn't discharging the battery when parked, and highly desirable to have the meter on a fused source in case there is a problem with the meter and its wiring."

That could be done with a relay if it is really desired, but of course, that adds to the complexity of the whole exercise.

Good find Stan. Those sensors look to be rather large in size (with 20 mm opening), but that might not be too bad to put around the main battery lead.

It would appear that there are several ways to monitor the battery current/voltage without having to bring heavy current carrying lines into the passenger compartment for those who are interested. Cheers - Dave
David DuBois

My first time on the MGB Tech site, Thank you all for comments.
It looks like the vote carries for the Volt Meter.
Cheers to All

Mr Bee
LESLIE BARRAS

Welcome to the BBS.

There is a wealth of knowledge here, so if you need to know anything, ask away.
Dave O'Neill 2

Thanks for your post Paul re observed voltages on the different circuits. Since we are mainly concerned with battery health off charge and alternator output does it matter what the drop is to the green circuit so long as we have an intelligent observor. You would fully charge the battery and confirm it was say 12.6v with your DVM, note the displayed voltage, eg 11.6, then fire the car up and say you get 14.5v at the brown wire note the reading, say 13.5. Then you are set to use it in service, or am I missing something. I always try to use ratiometric rather thzn absolute calibrations, you cant always achieve it, but its so much more convenient if you can.
Stan Best

I've been running a Smiths ammeter for over with 25 years with no ill effects. It is very sensitive to power fluctuations and gives me real time information as to what is drawing current. It is an invaluable tool in monitoring my electrical system. While voltmeters are all the rage these days, I find them to be lacking when I'm trying to isolate an electrical problem. Using at least 10 gauge wire and soldering the connections is a must. RAY
rjm RAY

We, as well as some manufacturers in the past must be risk takers. I remember my old morris mini minor having an ammeter, and I think my old TR6 had one also. Mike
J.M. Doust

Mike
I don't think anyone is saying you can't fit an ammeter but I think a couple of points need to borne in mind. Your old cars that that had one fitted (not the TR6) would probably have had a dynamo. The charging current from most dynamos was marginal at best and considerably less than even the smallest alternator. Adding extra cable and joints (and an ammeter) to the circuit is extra things to go wrong. That coupled with running un-fused power cables through the bulkhead is also a potentially worrying area. A short to earth of one of these cables will almost certainly result in a fire.

It's also worth considering the extra cable between the alternator and the battery, unless it's sufficiently heavy gauge, will drop a voltage. As most alternators sense the charge voltage internally this could lead to the battery not being fully charged. In the early days of alternators Lucas had a separate voltage sense connection that went straight to the battery to overcome this problem.

On balance I think there's as much chance of causing a problem as there is of solving one by fitting one of these.

As long as the ignition warning light goes out after starting the engine I'd stop worrying about it and enjoy the driving instead.

Bob
R.A Davis

Spot on Bob.
J.M. Doust

Leslie-
I have both a voltmeter and an ammeter in my B, but I now feel that the ammeter is a bit much. You've heard everybody else's solid arguements against installing one. I have to agree with them. Don't do it, install a voltmeter instead.
Stephen Strange

This thread was discussed between 03/02/2011 and 08/02/2011

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