Chicagoland MG Club
Ammeter Installation for 1971-1972 MGB
On 17 July 2011, Les Partington in th UK wrote:
Year 1972 -- Model MGB GT 1800 -- I intend fitting a amp meter and not sure of the point to brake into the circuit and the location".
The ammeter was a fairly common accessory for MGs prior to 1968. These cars had up to 22 amp generator, and the entire vehicle current load was limited accordingly, so a +/-30 amp ammeter would suffice.
Starting 1968 the MGB was fitted with an alternator. With progressively higher current capacity in the alternators an ammeter needs wider scale and progrressively larger connecting wires to carry the higher current. These wires pass from the engine bay into the passenger compartment to connect to the meter on the dash panel. Any accidental short to ground in one of these wires effectively connects the battery hot terminal to chassis ground, which in turn will melt insulation on the ammeter connecting wires, may melt neatby wires, may even start an electrical fire that can burn rapidly with PVC insulation as the fuel. Safety device could be an in-line fuse equal to the maximum scale reading on the ammeter. As such, ammeter is not a popular accessory in later model cars with alternator.
The MGB alternators were not so large as in some more modern cars, limited to 42 or 45 amps maximum output. So an ammeter with 50 or 60 amp scale would suffice, depending on what size temporary electrical load might be applied (like an air horn compressor for instance).
For electrical connection you need to find the point where the alternator output and all electrical power to the car is tapped from the main battery cable on the input side of the starter solenoid. Connect the ammeter in series with this wire. In this manner the ammeter will pass and read all current for everything in the car except the starter motor (which itself will draw hundreds of amps when cranking the engine).
For the later model pre-engage starter the alternator output wire and the vehicle power supply wire both conect to the main battery cable at the starter motor. You would need to disconnect these two wires from the starter input terminal, connect them together with one wire for the ammeter, and connect the other ammeter wire to the original terminal post (the battery cable). See diagram velow.
Written by Barney Gaylord
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