The MGB electrical system uses glass fuses to protect the circuits, and the fuse box is one of the most frequent sources of intermittent electrical faults on a car of this age. The holder corrodes internally, fuse ends oxidise in their contacts, and fuses can fail while still appearing intact to visual inspection. Cleaning and reseating all fuses, and inspecting the fuse holder clips for corrosion and loss of spring tension, is the correct first step before any deeper electrical diagnosis.
Fuse Boxes
Two fuse box types were used across the production run. The two-fuse Lucas 4FJ type was fitted from 1962 to 1969, with a single 35-amp fuse protecting most of the lighting and accessory circuits and a second fuse for the remaining circuits. From the 1970 model year, a four-fuse Lucas 7FJ type was introduced, spreading the load across four circuits for improved protection, a blown fuse now affects a smaller portion of the electrical system rather than disabling half the car. Both are available in reproduction and original Lucas specifications.
An inline glass fuse holder is available for ancillary circuits and aftermarket additions. Glass fuses are available in 10, 15, 25, and 35-amp ratings.
Voltage Regulators & Control Boxes
The charging system regulation differs between the three generations of MGB charging equipment. Dynamo-equipped cars to 1967 used the Lucas RB340 control box, which combines a voltage regulator and cut-out relay in a single unit mounted on the right-hand side of the engine bay alongside the fuse box. The cut-out prevents battery discharge back through the dynamo when the engine is at idle or switched off.
Early 16AC alternator cars from 1968 used a separate Lucas 4TR electronic control unit as an external voltage regulator. From the 1969 model year, the 16ACR alternator was introduced with an integrated internal regulator, eliminating the need for any external control box, if the car has a 16ACR or later alternator, the external regulator should be removed or bypassed to avoid conflict between the two regulators.
Relays
The four-terminal rectangular relay is the most commonly replaced relay on the MGB, serving the overdrive circuit on 3-synchro models, the starter circuit on early 4-synchro models, and the hazard warning circuit. The original metal-cased Lucas relay may be replaced by the later black plastic type, which was fitted from later production and is functionally identical. On cars with overdrive, the relay is essential for correct overdrive engagement, a relay that has developed high resistance across its contacts will cause intermittent overdrive operation, often temperature-dependent, before failing completely. Flasher units are available in two-terminal and three-terminal specifications for indicator circuits, with a separate hazard warning flasher unit for cars fitted with hazard lights from chassis 328801 onwards.