Jaguar XJ40 Alternator Not Charging, Battery Light
On the XJ40 the alternator is rarely the whole story. The small feed that wakes it runs through the centre console fuse box, and a loose plug there stops the charge. I'd check that box first, then the cheap brush and regulator pack; on 1993–94 cars, the bulkhead jump-start terminal.
How bad is it · Garage, carefully
Drive only to a garage — it runs until the battery is empty
Once charging stops, the fuel injection and pump live on the battery and the car stops when it runs out. Overcharging is the more urgent case: above about 17 V the battery and the electronics are damaged.
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 centre console fuse box: plug loose or box out of circuit
The alternator needs a small ignition-switched feed to start charging, and on the XJ40 it comes through the centre console fuse box. After any work in the console its plug is easily left loose.
No charge at all after the console has been apart. Other things on the same box gone too — for example self-levelling, some lights and brake assistance.
Box fitted, plug firmly home, small fuses good. With the ignition on, the alternator's field terminal should show battery voltage.
Reseat the plug or refit the box; the charge comes back, even with the alternator untouched.
The brush and regulator pack on the alternator
On the Bosch unit the regulator and brushes are one small pack held by two screws. Brushes wear short or stick in their holder, often gummed with oil from a leaking hydraulic pump; a failed regulator can let the voltage run away.
Charging that only starts after a rev to about 2500 rpm, or comes and goes; voltage falling with loads; or above 17 V.
Measure at the battery at about 1500 rpm. Pull the pack and look: brushes nearly gone, stuck in the carrier, or the two slip rings glazed.
Clean sticking brushes and take the glaze off the slip rings with fine emery; fit a new regulator pack if the brushes are worn or the voltage runs high — at 18–19 V the battery is being cooked.
A failed diode: a rebuild, not necessarily a new alternator
One open diode cuts output by about a third — enough to lose the battery slowly in traffic with the air conditioning on. A failed diode trio stops the alternator exciting itself at all.
No rise above battery voltage at 1000–2000 rpm; flat after low-speed driving; whine on the radio. Far enough gone, the meter shows 8–9 V running and the engine cuts out each time it drops to idle.
Have it tested by an independent auto-electric shop; parts-store testers often miss a diode fault.
A rebuild, or just a new diode assembly — for example about $24.
The belt, or the crank damper behind it
A glazed, wet or slack belt slips under load even when it feels tight. The crank pulley's rubber-bonded outer ring can also let go, so the alternator is barely driven.
A squeal or whirr, lights dimming and the engine bogging with the noise; charging that fails under hard acceleration or drops out briefly when pulling away. Several new alternators that change nothing point at the damper.
About 4.5 mm deflection under firm thumb pressure midway. Chalk a line across the damper's ring and hub and see if it moves.
A new belt, or a new damper if the ring has slipped.
The load dump module and its wiring on 1986–92 cars
Earlier cars with the Bosch alternator carry a load dump module on the right-hand inner wing, below the airflow meter, wired to the alternator. A small wire to it can chafe through on the bulkhead and short, and a shorted diode inside it kills the charge too. Later cars use a different suppressor.
Charging that stops intermittently, then for good; the warning light staying on.
Follow the wires from the module: insulation worn through to bare metal, loose terminals, a poor earth. Unplug the module from the alternator and start the engine — if it now charges, the module is the fault.
Insulate or repair the chafed wire. A module with a shorted diode can be repaired with two new diodes if you can solder; a good one conducts one way only.
The bulkhead jump-start terminal on 1993–94 cars
With the battery in the boot, the charge reaches it through a jump-start terminal on the right-hand bulkhead. On 1994 cars the bolted joint sits in a small black box with a flip-up lid, next to the air conditioning expansion valve. It loosens, corrodes and runs hot, and the output never arrives.
A new alternator and battery changed nothing; the terminal hot after a quarter of an hour's running; weak starting too. Low system voltage shows up everywhere: dim lights, the ABS light slow to go out after starting, a rough idle.
Battery disconnected first. Clean the post, fit eye terminals, tighten from both sides — one person inside, one under the bonnet.
Clean and tighten the joint, and replace the terminal if it has been hot; with good connections the voltage at the terminal and on the dash gauge agree within a few tenths.
Loose connections at the alternator and the starter
The alternator's output cable joins the battery cable at the starter, and a plastic plug at the back carries the sensing and warning-light wires. A dirty joint or a half-seated plug cuts the charge while the alternator is fine.
Charge lost after an engine wash, a starter or alternator job, or nearby work; the gauge dropping to zero and recovering; about 12 V running from a freshly rebuilt unit.
Battery disconnected first — the heavy brown wires are unfused. Clean and tighten both ends; resistance from battery positive to alternator output should be tiny.
Clean and tighten the joints, refit any lead left off the starter, and push the alternator plug fully home with a little contact cleaner — a rebuilt unit often needs nothing more.
Engine and body earths
The charge current returns through the earth straps. A corroded strap under the car or behind the boot liner makes everything electrical weak.
Voltage collapsing with every start; dash and injection dying on the move.
Jumper from battery negative to the engine; if the symptom goes, fix the engine strap. Clean the battery-to-body earth in the boot.
Worth trying: a heavy earth from battery to block, a tight engine strap, a clean body earth.
Charge sagging only at idle: a sticky idle valve
At idle the alternator turns slowly. If carbon on the idle control valve lets the idle fall to about 600 rpm, the charge drops below 12 V the moment loads come on, and the alternator gets the blame.
Gauge falling to about 12 V and the engine nearly stalling with lights and fans on at idle, but 14 V as soon as it is above about 800 rpm; worst in Drive.
Watch the idle speed with the loads switched on. Pull the idle control valve from the intake between cylinders 2 and 3 and look for black carbon on the plunger.
Worth trying before an alternator: clean the valve; a seized one can be replaced.
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
Engine off about 12.5 V; at 1500 rpm about 14–14.5 V. The dash gauge can lie; trust the meter.
a multimeterHad the console apart? Check its fuse box
Box fitted, plug firmly home, small fuses good.
freeBelt and damper
Deflection, glazing, oil or coolant on the belt; a chalk line across the damper.
freeClean the cables at the alternator and starter, and the earths
Battery disconnected first. The plug at the back of the alternator fully home; on 1993–94 cars, the bulkhead jump terminal too.
freeOn 1986–92 cars, the load dump module
Look for a chafed wire to it on the bulkhead; unplug it and see whether the charge returns.
freeThe brush and regulator pack
Look at the brushes before buying an alternator.
about $26Then the alternator — by an auto-electric shop
Check the amperage before buying: 90 A and 115 A units look alike, and 1993–94 cars use a different make.
a rebuildWhat it usually isn't
Don't spend money here first:
On jagaux.com
- Jaguar XJ40 Rough Idle and StallingIf the idle sags and stalls even with the charging sorted.
- Jaguar XJ40 Battery Drain, Flat After a Few DaysIf it goes flat parked rather than on the move.
- XJ6 (XJ40) fuse boxesWhere the centre console fuse box sits — other years from the same page.
- Jaguar XJ40 Belt Squeal, Crank Pulley WobbleIf the main complaint is a screech at start-up rather than the charge.
- 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.