The voltage regulator (control box) manages the dynamo's output to maintain charging voltage regardless of engine speed and electrical load. Alongside the regulator on the bulkhead are the fuse box, indicator relay, flasher unit, and starter motor switch, the principal control gear for charging, lighting protection, and starting. The control box specification changed at chassis TD/8142 (June 1951), creating two non-interchangeable generations.
Voltage Regulator (Control Box), Two Specifications
Two control box specifications are catalogued. Early TDs to chassis TD/8141 used the Lucas RF95/2, a combined regulator and fuse unit with nine terminals and integral fuse provision, supplied with its own cover. From chassis TD/8142 (June 1951), the Lucas RB106/1 was fitted as a separate five-post regulator (without integral fuses) alongside a separate SF6 fuse box, splitting regulation and circuit protection into two components. The RB106/1 covers the majority of TD production and all TFs.
Both regulator types are supplied with their respective covers.
Polarity Sensitivity
The control box must match the car's electrical polarity, positive earth (factory specification) or negative earth (if converted). The cutout relay inside the control box (the mechanical contact set that disconnects the dynamo from the battery when the dynamo's output drops below the battery voltage) is polarity-specific, and a wrong-polarity control box will not regulate correctly or may fail to disconnect the dynamo at idle, causing the battery to discharge through the dynamo armature.
Fuse Box and Fuses
The Lucas SF6 fuse box was introduced from chassis TD/8142 alongside the RB106/1 regulator, replacing the integrated fuse provision in the earlier RF95/2 control box. The SF6 is a two-fuse type with screw terminals, protecting the principal electrical circuits from overload. Two fuse ratings are used, 35 amp and 50 amp glass cartridge fuses, both available individually. Spare fuses should be carried, a blown fuse on the road immobilises the car until replaced.
Indicator Relay and Flasher Unit
Cars with direction indicators (NA-spec TDs from chassis TD/22315 and all TFs) require an indicator and brake light relay plus a flasher unit. The relay is a four-screw-terminal unit handling the higher current of the indicator and brake light circuits, switched from the lower-current dashboard switches. The flasher unit produces the clicking sound and pulse rate, catalogued in two terminal configurations: the original screw-terminal type (period-correct) or an alternative spade-terminal type for cars converted to spade connectors.
Starter Motor Switch and Hardware
The starter motor switch is a cable-operated solenoid-style switch on the bulkhead, energised by the starter pull on the dashboard. The installation comprises the switch, a coupling connecting the starter cable to the switch, two rubber terminal covers, and a mounting bracket. When the starter pull is pulled, the cable operates the switch, connecting the battery to the starter motor solenoid through heavy-gauge cabling. Switch components are common to TD and TF, with model-specific starter cables differentiating the installation.
Diagnosis and Service
Charging problems should be diagnosed in stages: confirm the dynamo is producing voltage at its own output terminal, that the control box is regulating correctly, and that the loom is delivering charge to the battery. A control box that fails to cut out at low engine speed (cutout points stuck closed) will discharge the battery through the dynamo at idle; one that fails to cut in (points stuck open) gives no charge despite the dynamo working. Both can sometimes be remedied by cleaning the contacts; severe pitting requires replacement. The starter switch itself rarely fails internally, but the coupling and bracket can corrode and benefit from renewal during any electrical refresh.
Ordering Considerations
The regulator is determined by chassis number, RF95/2 (with integral fuses, 9-post) for TDs to chassis 8141; RB106/1 (separate, 5-post) for TDs from 8142 and all TFs. Regulator covers are specific to each type. The SF6 fuse box and 35A/50A fuses are common from TD/8142 onwards. The indicator and brake light relay is required on cars with factory or retrofitted indicators.
Flasher unit terminal type (screw or spade) is customer choice, screw is period-correct. Starter switch components are common across TD and TF. Polarity must be specified for any regulator order.