The MGB was built with a positive earth electrical system from 1962, converting to negative earth in 1967 with the introduction of the MkII, a change that has significant implications for any electrical work on early cars and must be confirmed before ordering any electrical component. Current was supplied by a Lucas C40/1 dynamo on most cars prior to 1967, normally painted in the red engine colour, with a Lucas control box fitted to the right-hand side of the engine bay alongside the fusebox. The alternator became standard from the MkII, offering significantly higher output and better charging performance at low engine speeds.
Alternator History
The alternator specification changed several times. The 1967 to 1968 cars used a 16AC alternator with a remote-mounted 4TR electronic control unit.
From 1969, a 16ACR alternator with built-in regulator was fitted. In March 1972 the 16ACR received a modified regulator and surge-protection device, and in February 1973 it was replaced by a 17ACR type.
The final change was to an 18ACR. The 16AC alternator used British Standard termination which is now obsolete, a conversion kit is available to convert the wiring to European termination to accept replacement ACR alternators.
Replacement Components
The C40 dynamo is available as a new negative earth unit. The dynamo cooling fan and C40 brush set are available for servicing. The 16/17ACR alternator is available as a new 55-amp unit and on a reconditioned exchange basis. An alternator conversion bracket kit is available for owners fitting an alternator to a car originally built with a dynamo.
Rear mounting brackets are available in three specifications, dynamo bracket for 18G/GA/GB engines, early alternator bracket for 18GD/GG/18V engines, and post-1976 alternator bracket, reflecting the different engine and mounting arrangements across the production run.