Jaguar XJ40 Brake Problems, ABS and Brake Accumulator
On the XJ40 the usual cause of ABS and BRAKE lights, a pump that runs on every press and a pedal that goes hard is an accumulator sphere that has lost its gas charge, then the pump relay and the pressure switch. I'd count pedal presses with the key off before buying anything; a pull, a hot wheel or a shake under braking is the calipers and discs instead. Which parts apply depends on the year: cars up to about October 1989 have Girling power hydraulics and an overvoltage relay in the boot, later ones the Teves electric pump.
How bad is it · Garage, carefully
The brakes still work, but get it fixed now
Without pressure you lose assist and, on Teves cars, the rear brakes rely on that pressure too, so stopping distances grow. A dragging caliper overheats the pads and disc. If the pedal is rock hard or sinks to the floor, don't drive it.
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 accumulator sphere has lost its gas charge
The sphere is a nitrogen spring that stores pump pressure between brake applications. Once the diaphragm leaks it fills with fluid, so the pump reaches its cut-out almost at once and the stored pressure is gone after a press or two. The lights come on, the pump runs briefly on every press and assist fades.
ABS and BRAKE lights flick on as you brake and go out again; the pump whirrs for a few seconds after every press; a double click from behind the knee bolster; the pedal goes hard after very few presses with the key off.
Let the pump charge and stop, switch off, then count presses until the pedal goes hard. On a Teves car 25–30 or more is healthy and under 15 means the sphere. Off the car, push a blunt probe into the oil port: a charged sphere won't let it in.
A new sphere. Teves cars (1990 on) take JLM1907, also sold as AC Delco 25528382 or Bosch 0265202070 for around $100; 1988–89 cars use a larger, different sphere (JLM11659). Key off, pump the pedal 20 times or more (fluid can still spurt), unscrew with a strap or chain wrench while holding the block, refit snug only — the O-ring seals it. No wheel bleeding; the pump recharges it in 30–60 seconds. Avoid substitute spheres from other makes: one built for a different system, re-threaded to fit, cracked its diaphragm within weeks and vented nitrogen into the reservoir — foam in the fluid and a pedal kicking back at the first inch. On early cars the smooth sphere can be extremely tight: with the block off the car and clamped, a steel-band filter wrench tapped round with a hammer moves it where a strap wrench won't.
No power to the pump: the yellow pump relay, its fuse or its plug
On Teves cars the electric pump is switched by a relay whose contacts carry the full motor current. Burnt contacts or a cracked solder joint inside it mean the pump never starts; contacts welded shut mean it never stops. A dirty in-line fuse holder or a loose plug at the pump motor does the same as a dead relay.
Pump silent at key-on, both lights on, a rock-hard pedal with no assist — often straight after work under the dash or on the blower. Or the opposite: the pump keeps running and the fuse blows.
The pump relay is the yellow-base one among the three ABS relays on the left-hand blower housing, behind the knee bolster. Check for 12 V at the pump plug with the key on. As a test only, bridge pins 30 (brown/orange) and 87 (brown/blue) on the relay base: if the pump now runs and the lights go out, the relay is the fault.
A new or good used relay; cleaning the pump's in-line fuse contacts behind the footwell fuse box; unplugging and reseating the pump motor plug and the ABS computer plug. Relays tapped or reseated often come back for a while, so treat that as a diagnosis, not a repair. For reference, the pump relay is DAC4506 and the main relay DAC7151; the ABS computer relay, where fitted, is the same five-pin type as the main. Later cars have only two relays, main and pump.
The combined pressure and warning switch
The gold switch on the pump unit does two jobs: it tells the relay when to start and stop the pump, and it lights ABS and BRAKE when pressure is low. When its pump-start contacts stay open the pump never runs, even with a good sphere and relay. If they stick the other way the pump runs on until the motor cooks.
No assist and both lights on, a new sphere already fitted, pump silent at key-on — yet bridging the relay makes it charge and stop normally. Or fluid weeping round the switch plug.
With the relay in place, ground the white/pink wire at pin 4 of the blue 5-way plug on the switch: the pump should start. If bridging the relay works but the switch never calls the pump on its own, the switch is the fault. Normal cold sequence: ABS light out at about 32 s (around 1400 psi), BRAKE at about 35 s, pump off at about 42 s.
A new or good used switch. A complete used pump, sphere and switch assembly is a cheap way to cover all three when the pump motor is also tired.
Early cars: the ABS overvoltage relay in the boot
Cars up to about October 1989 (Bosch ABS, VIN up to 594575) protect the ABS computer with an overvoltage relay. The heavy coil frame cracks the solder round its pins, and the computer drops out — ANTI LOCK FAILURE on the VCM with brakes that otherwise feel normal. Damp in the boot corrodes its pins too.
ANTI LOCK FAILURE that stays on after start or comes and goes, sometimes after rain; normal pedal and no pressure warning. The relay can click and still be faulty.
Left side of the boot, behind the liner under the fuel filler: a row of relays under one cover, the overvoltage relay is the larger one on a green base. Look at the board under a magnifying glass for ring cracks round the pins. Bench test: 12 V to 31B and 86 should give continuity between 30 and 30a.
Resolder every pin, even ones that look fine — a second pass sometimes finds the bad joint — and clean the socket. A new relay is the quick way out. 1990-on Teves cars have no such relay.
ABS light only: a wheel sensor, its lead or the ABS computer
With the ABS light alone and a normal pedal, the hydraulics are fine and the computer has found an electrical fault. A rear sensor lead snapped at full suspension travel, a dirty or weak sensor, or a computer corroded by water in the boot are the usual finds. The light comes on at start for the static test, or only once the car passes about 3 mph or 19 mph for the moving test.
ABS light without the BRAKE light, often coming on a little way down the road; on some cars the speedometer dies below about 20 mph at the same time. A light that is on for a while, off for a day, then back can be mechanical: a rear hub flopping on a worn lower trunnion moves the sensor against its ring. A car that has stood for years can come back with every sensor slightly under spec.
Read the blink codes: brown connector in the front left of the boot near the ABS computer, bridge black to brown/pink, key to II, first code after about six seconds. 21 is the main valve and rides along with any other code; 33/43 point at the right rear sensor, 34/44 the left rear (open or short, then air gap). Measure each sensor at its plug and compare with the others. Bridge only the two wires named — a stray short at that connector can blow the ABS fuse, after which the readout points at the main relay; check the fuse before buying the relay. With rear sensor codes, grab the rear hub and rock it before replacing the sensor.
A spliced sensor lead with more slack, new rear sensors, cleaned sensors and toothed rings, or a good used ABS computer when it has been wet. Clean and reseat the computer plug first.
After brake work: air, or dirt pushed back into the ABS valve block
Pushing caliper pistons back with the bleed screw shut forces old, dirty fluid backwards into the ABS valves, and a grain of dirt can hold one shut or open. The result is a pull, one caliper that gets no fluid, or a pedal that will not firm up however much you bleed.
A pull to one side or a long, soft pedal that started right after pads, calipers or hoses; one bleed nipple that gives nothing.
Crack the flex hose at each end to see where the flow stops. Make sure the calipers are on the correct sides with the nipples on top. On Teves cars the rear circuit is fed by the pump, so bleed the rears with the ignition on. On early non-Teves cars, rear lines run dry after caliper work fill from the valve block end or with a pressure bleeder, and the rears bleed with the engine running. If air keeps coming back at one wheel, it may be drawn in round the bleed nipple itself.
A full bleed with plenty of new fluid, which can flush a sticky valve; a few hard stops on loose gravel to exercise the ABS. A valve that stays blocked means a used ABS unit or its valve set. Next time, open the bleed screw before pushing pistons back. A wrap of PTFE tape on the bleed nipple, or speed bleeders, stops air creeping in. Only the circuits you opened need bleeding.
A pull or one hot wheel: a sticking caliper piston, dry guide pins or a collapsed hose
Each XJ40 caliper has a single piston and floats on two guide pins. Rust on the exposed piston or seized pins stops one side letting go or gripping properly, and the car pulls. A flexible hose that has collapsed inside acts as a one-way valve and holds that brake on.
A pull under braking, sometimes a pull the other way while cruising afterwards; a burning smell and a hot wheel; a snatch on the first press after the car has stood; one wheel with far less brake dust; the inner pad worn more than the outer.
After a short drive, compare the wheels by hand or with an infrared thermometer — the dragging one is hottest. With the wheel off, each guide pin should slide by hand; on Teves cars they come out with a 7 mm Allen key, upper then lower. Push the piston back and watch whether it returns freely.
Clean the pins and refit with high-temperature copper grease; polish piston corrosion with oiled 1200-grit paper and fit new seals; a new flexible hose if the caliper releases once the hose is cracked open. If a piston will not retract at all, check it is the right part — a longer one from another caliper will not go back. The two bracket bolts are locked with wire: re-wire them or use a locking compound.
Early cars: low power fluid, or the charge valve and its switches
Before the 1990 model year the brake assist runs on green mineral oil (HSMO) from an engine-driven pump, shared with the self-levelling rear. Low oil, or a sticking charge valve or switch at the accumulator, leaves the accumulator short of pressure. The VCM then shows Low Brake Pressure, often with Power Fluid Low.
Low Brake Pressure at traffic lights that clears after a few seconds of driving; Power Fluid Low at the start-up check; the red marker showing in the reservoir sight bubble.
Green sleeve in the sight bubble means full. On 1987–89 cars the light can normally stay on for up to a minute above 1500 rpm; longer, or needing much higher revs, points at the charge valve or switch. A charged accumulator gives roughly nine slow presses with the engine off; about four means it is finished. As an example, one 1989 car read about 950 psi of boost against 1160–1740 psi expected until the charge valve was cleaned. Two pressure switches sit on the early accumulator block and look alike; when one weeps, identify which before ordering — the high-pressure (charge) switch is the upper one, and the part numbers differ by a digit.
Top up with the green mineral oil, never DOT 4 — the power reservoir still feeds the servo even after the self-levelling rear has been removed — and drive above 1000 rpm for a minute or two; look for weeping at the pump hoses; clean or replace the charge valve, the low-pressure switch and the non-return valve with new O-rings; a new sphere if the count is short.
BRAKE light with PAD on the VCM: the pad wear sensor
The same BRAKE warning covers low pressure, low fluid and worn pads. A pad sensor plug with spread sockets, or a lead chafed through by a worn disc, flashes the light with the hydraulics perfectly healthy.
BRAKE light flashing or on with a normal pedal, often straight after new pads; PAD in the VCM window when you press the button while it is lit.
Press the VCM button while the light is on: PAD means the sensor circuit, FAIL means low fluid or low pressure — and a FAIL can hide behind PAD until the pad fault is cleared. The sensors form one loop, so pull the push-on plug at each wheel and meter it to find the open one. Cars from VIN 594576 have two detachable sensors, left front and right rear; earlier cars have four built into the pads.
Reconnecting a sensor plug left off at the pad change, new pad sensors with the pads, or a repaired lead where it has rubbed bare on the disc edge. Bridging the two wires at the sensor plug clears the warning while you wait for parts. On early cars, pad work can also disturb the wheel-speed lead that runs beside it.
Pedal pulsing or judder: disc run-out, pad deposits or a pad hanging up
A disc that runs out by more than a few thousandths of an inch, or that has uneven pad material baked on, pushes the pads back on every turn. A seized guide pin that loads only the inner pad distorts the disc and gives the same shake. A pad that is slightly undersize can twist in the caliper and hold the brake on.
A rhythmic pulse through the pedal and a shudder through the car under firm braking, worse from speed; a brake that stays on at the lights until you drive off or stab the pedal; a scraping clunk from one front wheel.
Measure run-out with a dial gauge: about 0.004 in is the limit. Check the guide pins before blaming the discs. Ventilated discs go on the front, solid ones on the rear — a rear disc will not seat on a front hub. The normal ABS pulse at very low speed does not shake the car.
Skimmed or new front discs with new pads, the guide pins cleaned and greased. Tighten the wheel nuts progressively in a star pattern rather than one at a time. After storage, a rust film clears with a few gentle miles.
Fluid going down: reservoir grommets, the master cylinder rear seal or the sphere O-ring
The plastic reservoir sits on the master cylinder in two rubber grommets, and DOT 3 or the wrong fluid softens them until they weep. The rear seal of the master cylinder leaks down the servo or booster, and a failed O-ring under the sphere can dump the reservoir in one go once the pump builds pressure.
A low fluid warning, fluid or black rubbery residue round the base of the reservoir, a wet trail down the booster, or a sudden spray of fluid over the engine bay with an empty reservoir.
Wipe everything dry and look again after a day: under the reservoir, behind the master cylinder where it bolts on, and round the sphere and pump unit. On 1990-on cars the brakes and the pump share one reservoir of DOT 4.
New grommets and DOT 4; a seal kit or a new master cylinder if it leaks at the back — some kits come with the wrong grommets, so keep good old ones; a new O-ring under the sphere, fitted every time a sphere comes off. Then a full bleed.
A sinking pedal: master cylinder or servo cylinder leaking inside
If fluid bypasses the seals inside the master or servo cylinder, the pedal creeps down under steady pressure and the pump keeps topping up a system that never holds. On the hottest days it may not reach cut-out at all.
Pedal slowly sinks at a standstill with no external leak, or drops further on some presses than others; pump runs often and takes a long time to charge; one circuit gives almost no fluid when bled. A master cylinder that will not release can also leave the brakes on with the pedal sitting low.
Pull the ABS return line off the reservoir, let the pump charge, then hold the pedal down firmly. Fluid flowing back out of that line points at leaking ABS valves; nothing flowing while the pedal still sinks points at the master cylinder seals.
A new or resealed master cylinder, or a good used servo cylinder, then a full bleed.
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.
Press the VCM button while the light is on
FAIL means low fluid or low pressure; PAD means a pad wear sensor; ABS on its own, with a normal pedal, is an electrical ABS fault and the brakes are fine.
freeCheck the fluid
Brake fluid reservoir on 1990-on cars; on earlier cars also the green power fluid reservoir — green in the sight bubble means full. If the level keeps dropping, look under the reservoir, behind the master cylinder and round the sphere.
freeListen at key-on
On a Teves car the pump should run for about 40 seconds and stop, with the ABS light out first and BRAKE a few seconds later. Silent pump: go to the relay. Pump that never stops: relay stuck or pressure switch.
freeCount the presses with the key off
After the pump has stopped, switch off and press until the pedal goes hard. On Teves cars 25–30 or more is good, under 15 means a new sphere. Early cars give far fewer — around nine when healthy.
freeIf the pump is silent, test the relay
12 V at the pump plug with the key on; reseat the pump plug; bridge pins 30 and 87 on the yellow relay base for a moment. If that runs the pump, fit a relay.
freeABS light only: read the codes
Blink test from the connector in the front left of the boot. On cars up to October 1989 resolder the overvoltage relay first.
freeIf it pulls or shakes, find the dragging caliper
After a short drive compare the wheels — the hot one is dragging. Wheel off, the guide pins should slide by hand; inner pad worn more than the outer means a sticking pin. For a pulse in the pedal, check the pins before the discs.
freeFit a new sphere
Depressurise with 20 or more presses, key off; strap wrench, hold the block, refit snug; no bleeding needed.
about $100 aftermarketPressure switch or the whole pump unit
Only once the sphere and relay are proven good and the switch fails to call the pump.
about $600 new, far less usedWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ40 FUSE 1 or FUSE 2 FAIL WarningIf the VCM shows a FUSE message alongside the brake warnings.
- Jaguar XJ40 Blower Motor Not WorkingThe ABS relays share the left-hand blower housing — if the lights came on after work there.
- Jaguar XJ40 Cranks But Won't StartIf it won't start after a brake job — check the inertia switch.
- XJ6 (XJ40) fuse boxes and relaysWhere the relays and fuse boxes sit — other years from the same page.
- Jaguar XJ40 Dashboard Warning LightsFor the other warnings that share the message display.
- Jaguar XJ40 Rear Suspension ProblemsIf the self-levelling rear is also playing up.
- Jaguar XJ40 Power Steering Problems, Steering RackPower steering leaks, noise and heavy steering on the XJ40 have their own 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.