Electrical

The basics

The system is conventional for its date: negative earth, Lucas alternator charging, and the usual Lucas fusebox and connectors behind it. None of that is inherently fragile, but age is. Half a century of damp, vibration and heat brings corroded earths and corroded connections, and those cause a remarkably wide range of intermittent faults: gauges that read low or not at all, lights that dim when something else is switched on, a fan that runs slowly or not when it should. Chase the earths and connections before condemning components; a great many "failed" Stag components turn out to have been starved of a decent return path. Clean, tight, bright earths are the single best thing you can do for a Stag's electrical health, and they cost almost nothing.

The starter, and what the hammering means

One symptom is worth knowing on its own, because it tells you where to look and most people read it the wrong way round. If turning the key produces a rapid loud hammering or chattering, sometimes described as a machine gun, and the engine does not turn, that is not usually a failed starter motor. It is the solenoid pulling in and dropping out, over and over.

The mechanism is documented by Lucas, and it was also set out on the Stag Owners Club forum in September 2026 by a member who offered it, modestly, as a theory. He was right. There is enough voltage to pull the solenoid in. The solenoid then connects the starter motor, which draws a very heavy current. If the supply or the earth is marginal, the voltage collapses under that load, the solenoid can no longer hold in, and it drops out. The load disappears, the voltage recovers, the solenoid pulls in again, and the cycle repeats several times a second. The hammering is that cycle.

If that is what is happening, the fault is in the supply and not in the starter. The suggested tests are cheap: run a jump lead from the battery negative to a good point on the engine to give a known good earth, and see whether the symptom disappears, or apply twelve volts to the solenoid directly.

The numbers, from Lucas

Lucas's own fault diagnosis manual gives the figures that turn all of this from an opinion into a test with a pass mark. They are for Lucas equipment generally rather than for the Stag specifically, which matters and is dealt with below, but they are the manufacturer's.

• Total voltage drop on the starter installation: 0.5 volts maximum. Lucas's general rule for most circuits is ten per cent of system voltage, 1.2 volts on a twelve volt system, and they name the starter circuit as the exception to it.
• Battery terminal voltage under load while cranking: about 10.0 volts on a twelve volt petrol engine. A low reading indicates excessive current in the circuit.
• Battery state of charge: at least seventy per cent, or none of the other tests mean anything.
• Voltage drop on the earth line with the starter switch closed: practically zero.

The half volt figure covers the whole installation, insulated line and earth line together. Measure across each connection in turn and the culprit shows itself.

Whether it is the solenoid, or the circuit feeding it

Lucas gives a test that separates the two, which is the question the hammering leaves open. Connect a voltmeter between the solenoid operating winding terminal and earth at the commutator end bracket, and close the switch.

• A reading slightly below the battery under load figure means there is negligible voltage drop in the switching circuit, so the fault is in the solenoid itself.
• A reading appreciably lower means high resistance or faulty connections in the switching circuit feeding it.

Lucas also specifies the switching circuit cable size, twenty eight strands of 0.30 mm, written in the period style as 28/.012 inch, and notes that some applications carry an additional solenoid fitted specifically to reduce voltage drop in that circuit.

The bonding strap

One item the archive has never mentioned. Because engines are rubber mounted, the braided strap between the engine block and the chassis carries the earth return for everything mounted on the block, the starter included. Lucas is blunt about it: a strap that is frayed or incorrectly fitted will have a serious effect on the performance of the starter and may even immobilise the vehicle. It is cheap, it is visible, and it is worth looking at before anything is dismantled.

Where it was actually found

The owner who raised that thread came back and reported the cause. It was a battery quick disconnect on the negative pole, corroded and dirty. Cleaning it cured the fault.

That is worth dwelling on, because a quick disconnect or battery isolator is something an owner adds, usually for good reasons, and it introduces a joint into the highest current circuit on the car. The same owner had the identical symptom two months earlier and traced that one to a negative battery cable that was not properly tightened. Two incidents, two connections, both on the earth side, neither of them original equipment.

The archive's general advice applies with particular force here: clean, tight, bright connections first, and be especially suspicious of the ones that were not fitted at the factory.

One case the thread did not solve

A moderator on the same thread describes the same symptom on the same type of starter, but triggered by heat: after a run, with the car stood a few minutes. In his case a booster pack did not help, which is the detail that separates it from a dirty terminal, and the car started normally when he returned to it later. He has already cleaned every connection, soldered the connector beneath the starter, fitted a relay and renewed the ignition switch.

That case is unresolved and the archive does not know the answer. It is recorded because a reader whose symptom is heat related should know that the obvious fix may not be the one.

One further observation from that thread is worth carrying. The same moderator notes that his original starter motor did the same thing, which is why he fitted the high torque unit in the first place. As another member put it, fitting an upgraded component often masks a system problem rather than curing it. The fault followed the car across a starter change, which is usually the sign that the starter was never the fault.

One caveat that matters. Lucas's manual covers Lucas equipment and not the Stag specifically, and it gives two different starter procedures, one for inertia drives and one for pre-engaged. The archive has not established which type the Stag left the factory with, and the owner in the thread above had fitted a non Lucas high torque unit. The test method holds either way, because voltage drop is voltage drop, but the figures are Lucas's for Lucas installations.

Sources: Lucas Fault Diagnosis Service Manual, Catalog No. XIA116, Lucas Aftermarket Operations, consulted as a copy circulated on the Stag Owners Club forum, and carrying no publication date. And the Stag Owners Club forum thread on a starter motor problem, September 2026, whose members are described by what they did rather than named. The archive has not established whether the Stag's original starter was an inertia or a pre-engaged type, and does not know why the heat related case above behaves as it does.

The cooling fans

The Stag relies on electric cooling fans, and a lazy fan, or its tired wiring and relay, is a genuine contributor to the overheating the car is famous for. Check that the fans cut in when they should, and that they cut in with conviction rather than turning over apologetically. Where the wiring is original and the voltage at the motor is down, a relay upgrade and a good dedicated earth let the fans pull full current and do the work they were fitted to do. It is one of the places where an electrical fault and a cooling fault are really the same fault, which is why an overheating Stag is worth testing with a meter before it is worth buying parts for. See the cooling upgrades guide for the cooling side of the same problem.

Sensible upgrades

A small number of electrical changes earn their keep. Electronic ignition in place of the points gives reliable starting and steady timing; relay-fed headlamps give brighter lights and put less load through the switch; and relay upgrades for the cooling fans, as above, address a real weakness. All three are popular, all three are worthwhile, and, importantly, all three are reversible, which keeps them in keeping with sympathetic ownership of an original car.

Wire colours, for the circuits on this page

Lucas used a standard colour code across British cars of the period: a main colour, usually with a tracer stripe of a second colour. The full code runs to several hundred combinations. What follows is only the handful that bear on the circuits discussed above, arranged by job rather than alphabetically by colour.

• All earth connections: black.
• Main battery lead, un-fused: brown.
• Starter relay contact to starter solenoid: brown with a slate tracer.
• Ballast terminal to ignition distributor: pink with a white tracer.
• Relay to radiator fan motor: black with a green tracer. Radiator fan motor to thermal switch: black with an orange tracer.
• Water temperature gauge to temperature unit: green with a blue tracer.
• Alternator to the no charge warning light: brown with a yellow tracer.
• Brake differential pressure valve to warning light or buzzer: black with a light green tracer.

The pink and white one is worth remembering. Lucas note that a ballast resistor does not always take the form of a porcelain block, and that on some applications it is a length of resistive cable instead. On a car where no block can be found, the pink and white wire is where to start looking before concluding that the car is unballasted.

Source note: these come from an uncredited compilation of the Lucas colour code circulating on the Stag Owners Club forum. It carries no publisher, no date and no catalogue number, so the archive cites it as a compilation rather than as a Lucas publication. The code itself is a widely published industry standard and the archive has no particular reason to doubt these entries, but anything critical should be confirmed against the wiring diagram for your own car, which varies between Mk1 and Mk2 and between markets.

Keeping it reliable

The order of work is always the same: clean, tight earths and connectors first; then a sound charging system; and then never ignore a flickering warning light or a gauge that has stopped reading, because on a Stag both are usually telling you something true. Confirm wiring against the diagram for your car, the looms and equipment vary between Mk1 and Mk2 and between markets.