Jaguar S-Type Battery Light On, Not Charging
On the S-Type I wouldn't start with a new alternator. The engine computer commands its output, so a generic unit can charge and still keep the lamp lit; and a bad battery earth or a deeply flat battery throws ABS, DSC and Gearbox Fault messages that look like three failures but are one. I'd measure the voltages and check the earth first.
How bad is it · Garage, carefully
Drive only to a garage — it can stall when the battery runs out
Without charge the car runs on its battery until the modules starve and the engine stops. Never test the alternator by pulling a battery lead with the engine running — the spike can destroy the alternator and the electronics.
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.
A poor battery earth — reported as ABS, DSC and Gearbox faults
Every module measures its supply against earth. Resistance in the battery negative lead or its body stud costs them a few tenths of a volt, and the most sensitive — brakes, stability, gearbox — start logging faults.
ABS fault, DSC not available, Gearbox Fault with limp mode, on and off for months. The battery tests fine; the instrument pack's service page shows less voltage than the battery terminals. The positive side does it too: a car can go completely dead with 12.6 V across the posts, the parking brake and airbag lamps flickering and the boot relays clicking.
Compare the voltage at a module connector with that across the battery posts — for example 12.2 V at the ABS module against 12.6 V at the battery. Then measure clamp to body: it should be close to zero ohms, not, say, 2.4 Ω. A meter on the posts proves nothing about the clamps — inspect the inside of each clamp, especially after a battery change.
Clean the clamp, the lead and the body stud to bare metal. With resistance at zero the cluster shows full battery voltage and the faults clear after a drive. Check the battery's capacity while you're there. On a 3.0 the clamps still carried deposits from the old battery; cleaned with a wire brush, the car started first time and the relays went quiet.
A loose plug in the engine-bay harness
The charging circuit passes through multi-pin plugs at the corner of the engine bay. A plug not fully home switches the charge on and off as the engine shakes it.
Now and then the charge stops: engine lamp, battery lamp, then many lamps, radio and A/C dropping out, a blank screen, a stall with a flat battery. A new alternator doesn't change it. Fuses half out of their sockets give the same pattern: odd messages after a start, then amber and red lamps with Restricted Performance.
Engine off: find the group of three plugs in the upper corner of the engine bay (passenger side on a US car) and press each home. Flexing one with the engine running while watching the voltage is safe; pulling a battery lead is not. While the engine bay is open, press home every fuse and relay in the engine-bay box as well.
Reseat the plug and secure it firmly with tape — the lamps and stalls stop. On a 2005 car the first four fuses in the engine-bay box were not fully seated; pushed home, the messages and lamps stopped for good.
A deeply flat battery after a jump — everything lit, parking brake stuck
A jump gets the engine going, but the alternator can't refill a deeply flat battery and hold the voltage the modules need. They misbehave until it's properly charged.
Straight after a jump: every lamp on, the electric parking brake won't release, no gear, and the engine may stop.
Disconnect the battery and charge it fully — for example, still taking 20 A after half an hour on the charger. If you do jump it, stop the donor car's engine before connecting, and use only a 12 V booster.
A full charge on a charger, then reconnect; the jump itself needn't have done any damage. After the battery has been disconnected, the gearbox can shift harshly and show Gearbox Fault until it relearns, in roughly 60 miles.
A replacement alternator that isn't the original type
The engine computer tells the alternator what to do. A generic unit listed for the car bolts on and charges but can't hold that conversation: the lamp stays lit and the voltage runs high.
Battery lamp permanently on after an alternator change; a steady 15 V or more when warm, which shortens battery life. A high figure just after a start isn't proof of a wrong unit: on a 2.5 or 3.0 built after mid-2002 the system holds about 15.3 V for the first five or six minutes, then drops to about 13.6 V.
Read the label — the original supplier is Denso. As a rough guide, the V6 switches between about 13.6 and 15.1 V; the diesel and 4.2 V8 between about 13.6–14 and 15 V, tapering lower when warm. Workshop figures for the system: about 14.2–15.1 V cold and 13.5–14.3 V warm, regulator range 13.6–15.3 V. The 4.2 and the diesel keep a variable 14–15 V.
Worth doing: have the original rebuilt with matching parts, or fit a good used original. A car sold with the battery lamp lit and a story about the alternator probably has a generic one.
A tired battery that reads fine at rest but sags under load
This car is very sensitive to low voltage. A battery that shows a healthy figure at rest can collapse once real current flows, and the modules log random faults and garble the display well before it refuses to start.
A wave of odd dash messages or a garbled radio display while driving; the car restarts fine, fails again a short way on, and one day won't start at all. The battery is around four years old. Gearbox faults, A/C and windows dropping out, or being stuck in Park with many lamps after a short trip all fit this pattern.
Charge it, then measure with a digital meter while cranking and under load, not just at rest. Below about 11.9 V it is on its way out — for example, a normal reading at rest falling to about 10 V under load. A borrowed known-good battery, or a jump, shows whether the electrics settle.
Worth trying: a new battery of the right size and capacity — an AGM is fine. During the swap keep the system up with a second battery on the leads, never a charger alone: its unloaded voltage wanders and can harm the electronics. If the car stands for weeks, use a maintainer or a disconnect.
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 the battery and the running voltage
Engine off, then running. A steady 15 V or more when warm is too high; under 13 V with the lamp on is not charging.
freeAfter a jump: charge the battery fully first
On a charger, battery disconnected. Many lamps after a jump are low voltage, not damage.
freeTest the battery under load
Charged, then while cranking. Below about 11.9 V it is on its way out; a borrowed good battery is the quickest proof.
freeCheck the battery earth
Clamp to body should read close to zero ohms. Clean to bare metal.
freePress home the engine-bay harness plugs
Engine off. The group of three in the upper corner of the engine bay.
freeCheck the alternator's make
After a recent change, a generic unit with a permanently lit lamp is the likely answer.
freeOnly then an alternator — a rebuilt original
Never pull a battery lead with the engine running to test it.
an alternatorThe fuses named above — where they sit
The highlighted position is the one the procedure talks about. Pulling a fuse is a stop-gap, never a repair: check what the circuit feeds before you pull it.
Circuit descriptions from the Jaguar handbook, 2007 edition, p. 7, 8. Drawing by jagaux.com from the factory electrical guide. Fuse layouts change between model years — read the legend on the lid of your own box before pulling anything.
Circuit descriptions from the Jaguar handbook, 2007 edition, p. 9, 10. Drawing by jagaux.com from the factory electrical guide. Fuse layouts change between model years — read the legend on the lid of your own box before pulling anything.
What it usually isn't
Don't spend money here first:
On jagaux.com
- Jaguar S-Type Gearbox Fault, Transmission FaultWhen the message is alone and the voltage is good, the selector is the usual cause.
- Jaguar S-Type Battery Drain, Battery Flat OvernightIf the battery empties while the car is parked.
- Jaguar S-Type Rough IdleIf the voltage is good and the idle still hunts or stalls.
- S-Type fuse boxes, 2002Other years from the same page.
- 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.