1
Position for the special tool used to
withdraw the instrument cluster.
W124
Instrument
cluster
problems
2
Trip recorder
ver the last year
or so the trip
odometer on my nine
year old 300D had
developed a nasty
habit of not quite The cluster partially withdrawn prior to removal
resetting when the of the electrical connectors and speedo cable.
relevant button was
pressed on the instrument cluster. This problem manifested
itself with all the digits at zero except for the tenths digit
which would stubbornly remain jammed between zero and
nine tenths of a mile. A very loud click from the speedometer
head every tenth of a mile was audible in this condition
resulting from the mechanism failing to index the jammed
tenths-digit.
I had become accustomed to giving the reset button a
severe ‘twang’ in order to restore normal operation,
however, one cold January morning, after filling with fuel,
this treatment failed to work so immediate action was called
for to prevent damage to the speedometer head.
The instrument cluster is pushed in from the front and
retained by friction. Exactly the same principle is used on the
W123 series. On the W124, however, it is much more difficult
to apply sufficient pressure to the rear of the cluster in order
to facilitate its removal (I guess a contortionist midget
wouldn’t have any problems). A special tool (140 589 02 33)
will allow the cluster to be withdrawn quickly without
dismantling any of the trim panels and, as luck would have
it, I had already purchased a pair from my dealer. The tools
are inserted between instrument cluster and dashboard and
then rotated to engage a hook behind the cluster (see photo
one). The cluster can then be gently eased out of the
dashboard.
As the cluster comes free it is most important not to strain
the speedometer cable. As soon as there is sufficient room to
pass a hand around to the rear of the cluster the speedometer
cable should be released from the head by unscrewing the
knurled nut. In addition, the 15-way connector can be
released at this stage to enable the cluster to be withdrawn
further in order to improve access (see photos two and
three). The instrument cluster can then be manoeuvred to a
position that allows the rest of the electrical cables to be
disconnected. Unlike the W123, the oil pressure gauge is
electric on the W124 so there is no risk of oil leaking over the
upholstery as the cluster is withdrawn and no spanners are
needed.
The usual Mercedes-Benz attention to detail applies to the
electrical connections, all connectors are noninterchangeable and if there are accessory warning lamps
fitted these are all numbered, so there is no need to record
by Jeremy
Stevens
O
5 0 T H E M E R C E D E S - B E N Z C L U B LT D
3
The 15-way connector and speedo cable after the
cluster is withdrawn.
4
where all the wires go for re-assembly. I
found that the steering wheel had to be in
the straight-ahead position so that the
cluster could be eased free of the car.
Once the instrument cluster is
removed, check that the rubber retaining
bushes are all firmly attached to its
exterior and that they haven’t been left The rubber retaining pieces – in need of
behind in the dashboard opening or
dropped out as the cluster was removed.
All four on my cluster fell out or were loose and were glued
back into position using Evostik impact adhesive (see photo
four). To dismantle further, the remaining screws for the
various instrument sections should be removed. There is
also a screw through the dimmer rheostat (white ceramic
body, see photo five). The rheostat plugs onto the back of the
cluster and is the first item that should be removed. Then,
after unclipping and removing the low washer water level
warning lamp and its lead, the central speedometer head can
be removed (see photo six).
With the speedometer head removed I found that the trip
recorder reset mechanism worked perfectly when directly
operated. However, there was an awful lot of lost motion
due to distortion of the plastic parts when the reset button
itself was operated and the whole linkage used. I decided
that the problem was due to excessive friction in the tortuous
operating linkage (see photo seven). A tiny spot of 3-in-1 oil
was applied to the metal parts of the reset linkage and the
return spring slightly stretched for good measure. This made
all the difference and I was confident that normal operation
was restored.
Re-assembly is a straightforward reversal of the
dismantling procedure, but there are a couple of points
worth noting. Firstly, whilst it was not, strictly speaking,
necessary to remove all the retaining screws for the three
instrument sections, doing so will greatly facilitate removal
and, more particularly replacement of, the speedometer
head into the instrument cluster. The clock and rev-meter
can be eased up slightly whilst the speedometer head is
manoeuvred into position. Secondly, use a small amount of
rubber lubricant on the four retaining rubbers prior to
pushing the cluster back into the dashboards as this will
greatly reduce the amount of force required to push the
instrument cluster fully home.
Electrical problems
With the cluster removed it is worth touching on a couple
of electrical issues. This is the second time that I have had to
remove the instrument cluster in the four years that I have
owned the 300D. On the first occasion, about two years ago,
this was on account of total loss of instrument and cabin
switch illumination, including the auto-transmission gear
indicator and heater controls. Bizarrely, the ashtray
illumination remained. The problem started as being
intermittent, but quickly developed into a
permanent fault. Faults of this nature
always have a habit of occurring at the
worst of times and, on this occasion, not
only was I in the deepest, darkest part of
the Hampshire countryside, but also had
my father-in-law in the car. He lost no time
in rubbing salt into open wounds by
telling me how his Nissan never gave any
trouble whatsoever and so on. It was most
d of gluing.
5
The instrument cluster prior to disassembly. The
red fuse-holder can be clearly seen.
disconcerting having to cope with
invisible instruments compounded by a
lively family discussion concerning the
relative merits of car manufacturers!
When I examined the circuit diagram,
I was immediately able to eliminate the
lighting switch itself and fuse three from
the suspects’ list, due to the fact that the
ashtray illumination had remained
functional. The ashtray illumination feed
is taken from the 58R right hand lighting
supply (grey/violet lead) at pin 13 of the
15-pole connector to the instrument cluster. On the
instrument cluster the lighting feed is taken from the
connector through a dimmer rheostat before feeding the
cluster’s twin lamps and then to the four-pole connector on
the cluster. The rest of the cabin illumination circuit is
supplied via a grey/blue lead from pin two of the four-pole
connector. Sure that the power was reaching the instrument
cluster but going no further, I was thus able to concentrate
fault finding on the cluster itself. The dimmer unit could
have been at fault but in my case proved to be innocent. It is
simply a variable resistor and can be easily checked across a
12-volt battery when wired in series with a 2.2W 12V lamp.
The resistance should vary smoothly as the dimmer is
rotated as indicated by a corresponding smooth variation in
lamp intensity. If the lamp extinguishes at any point as the
control is rotated then the dimmer is faulty and should be
replaced. I now suspected a hair-line crack in one of the
printed circuit traces running to the dimmer unit so decided
to bypass the two possible traces using conventional PVC
insulated 7/0.2 wire soldered to the connector pins at each
end of the printed circuit traces.
Time has shown that this repair afforded a permanent cure
to the problem. Whilst the cluster was removed, I took the
opportunity to check the bulbs and replace any with signs of
blackening as a precautionary measure.
It is not generally known that from May 1987 a fuse was
mounted on the rear of the instrument cluster to protect
against short circuits in the instrument illumination circuit.
This special fuse is mounted in a red plastic holder, which is
otherwise identical to a cap-less bulb holder. This fuse
protects the dimmed instrument supply output from the
instrument cluster to the cabin (ie loads connected to pin two
of the four-pole connector) and if it fails the instrument
cluster remains illuminated whilst the rest of the cabin
illumination does not. Never be tempted to short out a failed
fuse, if the fault is still present the excess current flow could
burn out the affected circuit traces of the instrument cluster.
My favourite trick for diagnosing circuit faults where a fuse
has blown is to temporarily replace the fuse with a lamp. In
the case of the cabin illumination circuit, a 21W lamp will do
the job nicely. A continuing short circuit will be immediately
apparent if the lamp glows at full brightness, whereas the
normal load would cause little or no illumination of the
lamp’s filament.
6
Speedo head fixing screws and dimmer unit
removed.
7
The trip-recorder reset linkage.