The Midget electrical system underwent one of the most significant production changes in November 1967: polarity was switched from positive earth to negative earth at chassis numbers GAN4-60460 (Midget) and HAN9-72041 (Sprite). This change affects the compatibility of virtually every electrical component, dynamos, starter motors, coils, gauges, wiring looms, and battery must all be matched to the correct polarity. Quoting the chassis number when ordering electrical components is essential to ensure the correct polarity specification is supplied.
Wiring Looms
Replacement wiring looms cover the full production run from 1964 to 1980. Each loom is specific to a narrow date range, reflecting the numerous changes to switches, instruments, and ancillaries made across production. Main harness, front sub-harness, and rear sub-harness are supplied separately. Looms are available in PVC-bound and period-correct braided cotton-bound finishes for owners choosing between practical durability and concours authenticity.
The exact chassis number must be quoted when ordering, looms are not interchangeable between adjacent production periods.
Dynamo and Alternator
Early 1275cc Midgets (12CC, 12CE, 12V586F engines) were fitted with a Lucas C40 dynamo; later 1275cc and all 1500 cars received an alternator providing higher output and improved reliability. When replacing a dynamo, it must be polarised to match the car's earth polarity before first use, failure to polarise will damage the dynamo and the regulator. A dynamo-to-alternator conversion is one of the most popular electrical upgrades, providing increased output for modern accessories and eliminating the regulator. The MGOC Spares blog article "Roger Parker Q&A - MG Dynamo to Alternator Conversion" provides detailed guidance on the conversion process.
Starter Motor
Two Lucas starter motor designs were used on A-Series Midgets: the M35G (radial brushes, side commutator) and the later M35J (axial brushes, face commutator). Both are physically interchangeable within the A-Series range. The 1500 uses a different starter motor mounted on the opposite side of the engine. Brush kits are available for both A-Series types, and reconditioned exchange starter motors provide a freshly rebuilt unit in return for the worn original.
Ignition
The ignition system uses a Lucas 25D4 distributor on the 1275cc and a 45D4 on the 1500. Electronic ignition conversions are available for both: the Lumenition Optronic uses an optical trigger fitted inside the original distributor body, while the Lumenition Magnetronic uses a magnetic trigger in the same application. The 123 Ignition is a complete replacement distributor with selectable advance curves, available in both positive and negative earth versions, this is a particularly popular choice for owners seeking a comprehensive upgrade with tuneable advance characteristics.
Lighting
Front and rear lamp assemblies changed between chrome-bumper and rubber-bumper cars. Halogen headlamp conversions provide significantly improved visibility over the original sealed-beam or pre-focus units. LED bulbs are available for indicators, side lights, and tail lights, an LED-compatible flasher unit may be required to maintain the correct flash rate, as the reduced current draw of LEDs can cause a standard thermal flasher unit to flash too slowly or not at all.
Switches and Gauges
Dashboard switches, gauges, and the ignition switch position all changed across production periods. Gauges are specific to both production date and earth polarity, a positive-earth gauge fitted to a negative-earth system reads in the wrong direction, so the polarity of the car must be confirmed before ordering replacement instruments. The Smiths dial face number should be quoted when ordering to ensure the correct specification is supplied.
Fuse Box
The original Lucas fuse box uses glass fuses in a simple open-backed housing. Replacement fuse boxes, fuse holders, and spare fuses are stocked. The fuse box connections and earth points should be cleaned and tightened periodically, corroded fuse box terminals are a common cause of intermittent electrical faults that mimic component failure, leading to unnecessary replacement of perfectly serviceable parts.
Battery and Earth Strap
The battery polarity changed once during production (positive earth pre-November 1967, negative earth from GAN4-60460 onwards). The physical terminal layout was then revised again with the introduction of the 1500, because the Triumph engine has its starter motor on the opposite side of the block to the A-Series, requiring the battery cables to reach different points in the engine bay. The earth strap is one of the most overlooked maintenance items, a corroded earth strap causes difficult starting, dim lighting, erratic gauge readings, and poor charging performance.