[xj40] Brake pad wear sensor mystery- long!

XJ40
1

As we know, on the '90-on Teves equipped cars, there are
only two brake pad wear sensors - front left and rear right
wheels.

So, in the course of a ‘major service’ on my '91, I came to
the bit where I remove the front wheels to check the brakes
and re-grease the caliper slider pins (etc.). For
convenience, I started with the front right wheel - no
particular reason except it happened to be the one nearest
to my work bench in the garage. I noticed that both inboard
and outboard pads were worn down evenly, both approaching
the 4 mm minimum.

Oh, well, no point in proceeding further until I get a set
of new pads and then I can complete the service on both
sides - so back on goes the front right wheel and the car is
parked in the garage. I ordered the pads from David Manners

  • JLM1829#, genuine OEM Jurid pads in a plain brown wrapper
  • but there was a slight delay in them being shipped so only
    today were they delivered.

So again, off with the front right wheel, open bleed screw,
push back fully the caliper piston, clean everything up,
copper grease the guide pins and fit the new pads. Sure
enough the old pads measured just about exactly 4 mm each.

On to the front left.

Mmmm! The outboard pad is down to about 2 mm but the inboard
pad is down to 1 mm at the most and the pad wear carbon
‘slug’ is starting to crumble, so much so that I was
surprised that it had not already generated a ‘BRAKE’
warning on the dash because and in trying to remove it, the
wire loop came apart. Not to worry, I’ll get a new sensor
and in the mean time fit the new pads with the wire ends of
the old sensor soldered together to make the circuit so that
I can use the car without a false ‘pad wear’ warning on the
dash. Easy-peasy! :slight_smile:

I removed the sensor wire at its connector to the car
harness and took it to the bench to do the soldering job.
Before re-fitting it, out of curiosity I switched on the
ignition, fully expecting the ‘BRAKE’ warning to remain
illuminated on the dash and the VCM to declare ‘PAD’

NOTHING! The lights went through their check cycle and all
of them went out including the ‘BRAKE’ warning icon. How odd

  • so I re-fitted the soldered sensor wire and grounded the
    bare wires. Still no warning.

I’m not too worried about that because I reckon I inspect
the brake pads at every service so I will see if they are
getting near their wear limit,(or wearing unevenly, left and
right) but I do wonder what would have happened if I had
driven the car another few hundred miles while waiting for
replacement pads.

I’ve no idea why that BRAKE warning doesn’t work for the
front left pad sensor. The warning light is good and comes
on when there is inadequate brake pressure as it should and
then the VCM shows ‘FAIL’ as it should.

The moral of the story for '90-on owners - when checking the
front brakes, always start with the front left! :slight_smile:

Oh, and in case you are wondering no brake pull or any other
problems despite the uneven wear left to right sides.

Very strange.–
Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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2

In reply to a message from Bryan N sent Tue 10 Jul 2012:

Bryan

I assume the wiring set up for your sensors is similar to mine
except that you only have two sensors, however suspect that the
system is similar - ie the two sensors are actually ‘in line’ so
that if one goes down then you get the alarm but, again as per
jaguar normal design, it does not actually tell you which one has
caused the alarm (but like the BFM setup !)

What happens to yours if you do the same check that you carried out
on the front on the one you have on the rear ?

If the alarm comes up then its probably something to do with one of
your connections/wiring to the front sensor, if it does not come up
then there is something on the entire circuit and/or the Dashpack

Try bypass the circuit by connecting wire to the two pins on the
multi plug to the dashpack which serves the brake sensor circuit
(on mine those are the orange/brown(pin 2) and orange/slate (pỉn 3)
wires to the B54 multi plug (cannopt remember what colour plug that
is) - howver yours is probably different anyway.

If shortcircuiting those pins has no alarm and when not short
circuited still no alarm then its the dashpack - if no shortcircuit
the pins and alarm comes up then the wires are probably
shortcircuited somewhere between the dashpack and where they split
to go to front and return from the back

Nigel–
1987 XJ40 3.6 Auto Sov SAJJHALH3AA512874
Singapore, Singapore
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3

In reply to a message from Nigel Snowden sent Wed 11 Jul 2012:

Nigel,

Thanks for your comment.

I did consider doing what you suggest by uncoupling the
right rear pad sensor to see if that flagged a fault
message. Just out of curiosity, had I have worked on the
front end first and then the rears I would have done that
but since I had already finished at the back end I wasn’t
that interested in pulling off the wheel again to prove
something which is now only of academic interest to me.

I figure that at my current rate of usage of the Jag, if I
get another 34,000 miles out of the new front pads before
they are down to limits as I did with the old pads, it will
be another 10 years or so before I need to worry about them

  • and either the Jag or myself or both may not be here then! :slight_smile:

It has occurred to me that a possible explanation of its
failure to flag a pad wear fault could be that the wiring
harness upstream of the sensor may be trapped and damaged so
that the two wires are shorted together, thus completing the
loop before it gets to the sensor itself - hence no warning.
Thinking about it some more(!), I do remember that when I
first took delivery of the car in '96 with 25,000 miles on
the clock, the dealer had just fitted new front pads. In the
200 mile ‘gentle’ drive from the dealer to my house, there
was so much brake dust generated by those new pads that it
was impossible to tell where the Lattice Alloy front wheels
ended and the tyres began - they were totally black. The
front discs had apparently been red-rusty after the car
stood with suspension damage in a field for some months
before the previous owner traded it in and they just lobbed
in new pads - I guess on the basis that they would
automatically clean up the disc in use!

Knowing now the devious working practices of that particular
dealer who had serviced the car during the previous five
years for the one previous owner, it would not surprise me
to find that they had overcome previous brake warning
problems by inhibiting the pad wear warning system.–
The original message included these comments:

What happens to yours if you do the same check that you carried out
on the front on the one you have on the rear ?

–
Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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4

In reply to a message from Bryan N sent Wed 11 Jul 2012:

Crown of the road.
Stateside, tyres and brakes wear more on the right front.
Years ago when we were more of the highly crowned 2-lanes,
it was worse, and we even set up the caster different to
drive the car left a bit.
'cross the pond, left side.–
1964 MKII 2.4L 1950 Ford Tudor, 1950 Ford F-1, 1949 Willys
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5

In reply to a message from TOC sent Wed 11 Jul 2012:

Good debating point - but why should the road camber affect
the brakes?

Tyres I can more easily understand but I carefully measure
the front tyre wear on my car during every service after
suffering lousy dealer-induced handling and uneven tyre wear
problems when I first bought the car. I eventually corrected
both of those problems by tweaking the Toe-in until both
improved to my satisfaction and now the car steers virtually
‘hands-off’ on all but the severest camber roads.

As a matter of interest, both my front tyres have now run
for nearly 28,000 miles and the wear levels are exactly
the same on both sides (which seems to discount any effect
of road camber - and we have plenty of that on some of the
roads I travel) the remaining tread depth across both tyres
being :-

Centreline = 4.5mm, and measured from that centreline:-

55mm outboard = 4.0mm : 90mm outboard = 2.0mm
55mm inboard = 4.25mm : 90mm inboard = 2.25mm

The tyres BTW are Pirelli P6000(J) 225/55 WR 16(J) on
stiffened SportsPack suspension.–
The original message included these comments:

Crown of the road.
Stateside, tyres and brakes wear more on the right front.
Years ago when we were more of the highly crowned 2-lanes,
it was worse, and we even set up the caster different to
drive the car left a bit.
'cross the pond, left side.

–
Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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