The MGB GT V8 was fitted with an AC Delco alternator at the factory, part number 7982707. This is a high-output unit driven by a vee belt from the engine crankshaft pulley, providing the charging current for the battery and the supply for the car's electrical loads. The AC Delco specification is V8-specific; the four-cylinder MGB used Lucas alternators of different specification, and the two are not interchangeable.
Alternator Operation
The alternator generates AC current in its stator windings, driven by the rotating magnetic field of the rotor. Internal diodes rectify the AC to DC for the car's 12-volt system. A voltage regulator (internal to modern alternators, external on some specifications) controls the alternator output to maintain correct system voltage regardless of engine speed. The output is fed to the battery via a heavy charging cable; warning is via the dashboard ignition warning light.
Drive Belt
The alternator is driven by a vee belt from the engine crankshaft pulley. Belt tension is critical, too loose and the belt slips, causing low charging output and belt squeal; too tight and the alternator bearings wear prematurely. Tension is adjusted by pivoting the alternator on its mounting bracket and securing the adjustment slot. Belts age and crack with service; renewal at major service intervals is good practice.
Mounting Brackets
The alternator mounts to the engine via V8-specific brackets that locate it correctly relative to the crankshaft pulley for belt alignment. Brackets are robust items that rarely need renewal except after damage. The pivot bolt and adjusting bolt should be checked for thread condition during alternator service work.
Charging System Faults
Common charging faults include worn alternator brushes (causing low or no output), failed voltage regulator (causing over-charging or under-charging), failed diodes (causing AC ripple in the output), and slipping or broken drive belt (causing no charging). Diagnosis usually starts with measuring battery voltage with the engine running at fast idle, a healthy system shows around 14 volts at the battery terminals.
MGOC Spares
MGOC Spares supplies V8 alternators, drive belts, mounting brackets and charging system service components for the MGB GT V8.