Jaguar XJS Alternator Not Charging, Battery Light
The original Lucas alternator is the usual culprit. But a slipping crank damper, the warning-lamp wire and loose connections mimic it with a healthy alternator — I'd check those first; they're free.
How bad is it · Garage, carefully
Drive only to a garage — it runs until the battery is empty
The car runs on the battery for a while, then the ignition and injection starve and it stops wherever it is. A voltmeter pegged high is the more urgent case: at 17–18 V the battery boils and the electronics are at risk.
Which one applies to yours depends on your symptoms, your model year and what has already been done to it.
Likely causes, most common first
Read the check cost before the odds: a free check that rules a cause out beats a likely guess that costs money.
The Lucas alternator: diodes or regulator
Age and V12 heat kill a diode or the regulator. One dead diode leg can leave it fine by day and short at night with the lights on. The 1987–88 units are marginal even when healthy.
Lamp dim at idle, brighter with revs; voltage sagging with headlamps and wipers; or the gauge pegged high — a failed regulator.
At the battery terminals, 1500 rpm or more: roughly 13.5–14.4 V. A parts-store test shows output and bad diodes.
Replace it — a Land Rover 100 A unit fits the original position; GM or Delco conversions work too. External rectifiers stop repeated diode-pack failures.
The warning-lamp wire
The alternator is woken by current through the charge lamp and its brown/black wire. A blown bulb, a weak LED or a broken wire means no charge until revved hard, or never.
The lamp doesn't light at key-on, or the voltmeter stays red until about 2000 rpm.
Key on: the lamp must light. If not, fix the bulb or wire first.
Repair the bulb or wiring. If a failed internal diode back-feeds the ignition, an in-line diode in the brown/black wire stops it; a resistor beside the lamp covers a blown bulb.
Loose connections or a slipping belt
Charge current runs through heavy cables and the firewall posts to the battery in the boot. A loose nut or slipping belt cuts the charge with a healthy alternator.
Charging that comes and goes; more voltage at the alternator than at the battery; a glazed or squealing belt.
Battery off first — the posts are always live. Tighten the alternator output nut, the firewall post nuts, the battery posts and the earth strap.
Tension the belt; clean, grease and tighten the joints, including the starter's to the right-hand post. An extra heavy cable from alternator to bus is a workaround.
Idle too low for an added electric fan
At a low idle the alternator can't keep up with a big fan.
The lamp flickers only at idle with the fan running.
Raise the idle slightly and watch the lamp.
A higher idle — for example 650 rpm with a 16-inch fan.
The crankshaft damper slipping
The alternator runs off the damper's outer ring, bonded to the hub with rubber. When the rubber lets go, the alternator barely turns and a new one changes nothing.
Weak charging after a new alternator; a squeal.
Chalk a line across ring and hub, run the engine; a split line means it slips.
A rebuilt or new damper. Worth ruling out before buying another alternator.
The order to check them in
The point of the order is money: each step either rules a cause out or makes the next one worth doing. Green is free, amber costs a little, red is the one to arrive at last.
Measure at the battery, at 1500 rpm
About 13.5–14.4 V; around 12.5 V means no charge. Never pull a battery cable with the engine running.
a multimeterKey on: does the charge lamp light?
If not, fix the bulb or its wire first.
freeChalk the crank damper
A split line means the ring slips.
freeTighten every heavy connection
Battery off first: alternator output, firewall posts, battery posts, earth. Check the belt.
freeThen the alternator — bench-tested
Land Rover (Marelli-type) units fit the original position; the later Bosch 115 A works; GM units go in the air-pump position with enough belt wrap. Test any rebuilt unit before fitting.
from about $150What it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJS Overheating, Cooling Fan ProblemsIf you're adding electric fans, or the car also runs hot in traffic.
- XJS fuse boxesOther years from the same page.
- Jaguar XJS Electrical Problems, Bad EarthsIf lamps run dim or switches burn: voltage lost in earths and Lucas connectors.
- Jaguar XJS Instrument Cluster, Gauges Not WorkingIf the voltmeter reads low but the battery measures fine, the gauge or the cluster earth may be the fault.
- Jaguar XJS Headlights Not WorkingIf only the headlamps are dim and the charging voltage is fine: the switch and wiring carrying the lamp load.
- Jaguar VIN decoderConfirm your exact model year, engine and factory options before ordering parts — free.
Ratings, fuse numbers and torque figures vary by model year and market. Read the legend on your own car before pulling anything, and use the figures in the manual for your car, not the ones in a video.
Check your VIN: exactly which car you have and which bulletins apply to it.