The charging system

An E-Type that keeps flattening its battery is telling you about four things in a fixed order: the belt, the generator and its control, the wiring, and whatever is drawing current while the car sleeps. The order matters, because each step is cheap and the last one is the only expensive one. Where a car is already showing a charging symptom, the archive works through it as a step by step charging failure diagnosis.

What your car actually has

Almost every published figure for charging depends on which Lucas equipment is bolted to the engine, and the E-Type carried three different systems in seven years. Identify yours before reading any figure, including the ones below.

• The earliest 3.8 cars of 1961 carry a Lucas C45 dynamo with an RB310 control box, which has three screw-in terminals. The change point is by chassis number: roadsters to 850091 right hand drive and 875385 left hand drive, coupés to 860004 and 885020.
• From 1962 the 3.8 carries a Lucas C42 dynamo with an RB340 control box, from roadster 850092 and 875386 and coupé 860005 and 885021.
• The 4.2 cars of 1964 onwards, Series 1 and 1.5, carry a Lucas 11AC alternator, type 23526, twelve volt and negative earth, rated at 45 amperes. It works with a 4TR control box, a 6RA relay and a 3AW warning light control.

These are supplier catalogue listings rather than a Jaguar parts book, and the archive has not seen the factory publication for this. The chassis number breaks are quoted because the catalogues quote them, and a car that has been rebuilt may not match its own chassis number in this respect.

Before any test means anything

Lucas are explicit that two conditions come first, and that testing a charging system without them is wasted effort. The battery must be at least 70 per cent charged and in good condition, because a sulphated battery produces a low charge rate and a battery with a shorted cell produces a high one, so a faulty battery will make a healthy charging system look faulty in either direction. And the drive belt must be right: Lucas call for 13 to 19 mm, half an inch to three quarters, of play under moderate finger pressure at the longest run of the belt. A slack belt will not charge. An over-tight one damages the bearings.

Lucas's hydrometer figures for establishing the battery's state are 1.110 to 1.130 for discharged, 1.230 to 1.250 for seventy per cent charged, and 1.270 to 1.290 for fully charged.

The dynamo cars, and the RB340

The RB340 is a current-voltage control box, and its voltage regulator setting is temperature dependent, a detail that catches people who test on a cold morning and again on a hot afternoon and conclude the box is drifting. Lucas give the open-circuit checking figures at about 3,000 rev/min as 14.5 to 15.5 volts between 0 and 25 degrees Celsius, and 14.25 to 15.25 volts between 26 and 40 degrees. A regulator found stable but slightly outside those limits is reset to the nearest limit. One more than half a volt outside is treated as faulty rather than adjusted.

The cut-out is checked separately. The points should close within 12.7 to 13.3 volts, and as engine speed is reduced again the ammeter should show a reverse current of three to ten amps before returning to zero as the points open.

The current regulator setting depends on the dynamo. Lucas's table gives the C42 a nominal 30 amperes, plus or minus one. Limora, listing the RB340 box for exactly this dynamo, describe it as an RB340 with 30 ampere, which is an independent confirmation of the same figure from a different kind of source.

And a figure worth having whatever the system: with the ammeter reading 10 amps, the voltage drop measured on the supply line between the dynamo output and the battery should not exceed 0.75 volts. More than that is resistance in the insulated circuit, which is a connection to find rather than a component to replace.

The alternator cars, and the 4TR

The 11AC is a battery-excited machine, which has a practical consequence: its field depends on the battery through a relay for its initial excitation, so Lucas note that a flat battery means no charge even if the engine is started by towing. A self-exciting machine behaves differently, which is why advice that works on a later car does not work here.

Lucas give the 11AC a maximum output of 45 amperes measured at about 3,000 rev/min, which matches the 45 ampere rating the supplier catalogue gives for type 23526. The 4TR regulator setting for a twelve volt system is 13.9 to 14.4 volts. The 3AW warning light control is checked at its AL terminal against earth at about 1,500 rev/min, where it should read 6 to 8 volts on a twelve volt car.

The warning light that glows but does not go out

This is the symptom owners describe most often and guess at most freely. Lucas name three causes and no others: internal high resistance in the ignition switch, dirty cut-out contacts in the control box, or a fan belt slipping badly enough to matter. It is worth knowing the mechanism, because it explains why the light is not a charging gauge. The lamp has battery voltage on one side through the ignition switch and the dynamo on the other. As the dynamo comes up to speed the two voltages approach each other and the lamp fades; at almost the same moment the cut-out points close at around thirteen volts and short the bulb out entirely.

Lucas add the caution that follows from that: the light going out does not by itself prove the charging system is functioning correctly. It proves the two voltages met. A car can pass that test and still be undercharging.

Polarity, and why it matters before you touch a meter

The alternator cars are negative earth, and the supplier catalogues list the 3AW replacement module in separate positive earth and negative earth versions, which tells you plainly that both existed across the range. Lucas's test procedures assume negative earth throughout and say so. Establish which your car is before connecting anything, because on a positive earth system the supply side of the coil is the negative terminal and a meter connected on the negative-earth assumption will read backwards.

The drain

With everything off, a current meter in series with the battery shows what is being drawn at rest. A significant draw on a sleeping car is a fault to find, not a battery to replace. This is the step most often skipped and the one that explains most batteries that go flat over a fortnight rather than over a night.

Replacing the battery

Battery location and hardware changed during the E-Type's life, so the first step is to identify what is in front of you rather than assume it matches a generic picture. The six-cylinder cars carry the battery in the engine bay, mounted on a tray or frame against the bulkhead or inner wing. The V12 cars moved the battery to the boot, partly to free space around the wider engine and partly to alter the weight distribution; it sits in its own tray with a hold-down rod and wing nut.

The awkward holding clips

The six-cylinder cars used a bakelite tray with a top frame or clamp held by short rods and wing nuts. Reproduction clamps and modern, slightly oversized batteries do not always agree, and owners often report that seating the frame squarely without the metalwork touching the terminals takes more patience than the job appears to deserve. The rods are short, the angles are tight, and the battery must finish upright with the vent tube still connected. It is worth test-fitting the new battery with the terminals covered before connecting anything.

The V12 boot installation swaps the frame for a single hold-down rod and a small wing nut, which is easy to drop into the boot trim. The battery has to be lifted in and held steady while the rod is threaded, and the boot floor and trim leave little room for fumbling.

What to check before buying a replacement

• Case size and terminal layout. Some markets used batteries with the positive and negative posts on opposite sides; the cable reach and clamp design must match.
• Capacity and cold-cranking output. The V12 in particular needs a battery that can turn the engine over reliably. Confirm the figures against the handbook or workshop manual for your car.
• The vent tube, especially on batteries mounted inside the boot or passenger compartment, where escaping gas needs a route outside.
• The earth or ground strap condition, because a new battery will still struggle if the return path is corroded.
• Tray corrosion or cracks. A loose battery will fail early and can damage the surrounding paintwork or wiring.

Once the battery is in place, the real test is that it stays put when the car accelerates, brakes and corners. If the clamp is loose or the case is the wrong size, it will work loose and the terminals will arc.

A note on upgrades

Dynamo-to-alternator conversions and uprated alternators are common and reversible, and reproduction 11AC-pattern units are sold at 55, 70 and 80 amperes against the original 45. They are worth considering when a car has gained electric fans, pumps or other loads its original system was never asked to carry. What they also do is invalidate every figure above, because the figures belong to the equipment and not to the car.

Common questions