Starter motor solenoid modification
Bob Owen (Blaze 1624) from Hampshire
provides a useful modification as part of a solution to a problem
starting a V8 when hot or when the starter solenoid auxiliary contacts
fail. (Jan 05)
Maurits Clement posted a query on the V8 Bulletin Board seeking help
with persistent starting problems he had experienced with his with
V8 for two years. Maurice explained "it starts well when cold,
even after some months in the garage, however when it warms up after
a long ride, it will not start again. Only after it cools down will
it work again. I had my starter replaced a year ago, so feel it should
be fine and was wondering whether, as the car is equipped with tubular
manifolds, could it be the starter gets too much heat from these manifolds?
Should I use some insulation?"
Just over an hour later I popped a response on the V8BB saying I had
the same problem with my factory V8 with the original exhaust system
with cast manifolds. The starter heat shield was missing - either
omitted from an earlier starter motor change or it had corroded or
dropped off. I fitted a new heat shield (supplied by Clive Wheatley)
and, as a belt and braces measure, lagged the exhaust from the manifolds
to beyond the gearbox. Starting when hot has not been a problem since.
It seems as though the heat from the exhaust system causes the starter
solenoid to stick or it reduces the magnetic pull below a critical
level. The exhaust lagging may also help to reduce under bonnet temperatures
when idling in traffic.
Heat shield for the V8, supplied by Clive Wheatley
mgv8parts. (Photo: Clive Wheatley)
Paul Hunt posted a note a day later noting "it is the
heat from the exhaust that seems to cause the failure of the starter".
When his starter motor failed the symptoms were a chattering solenoid,
often a symptom of a flat battery or bad connections, but not in this
case. His starter failed after 50,000 miles or so of daily driving.
The car had a tin heat shield which he replaced with the Clive Wheatley
unit. The new starter motor has been fine for the past couple of years
and around 6,000 miles. He felt that a starter motor failing after
only a year is not good and indicates the rebuild (and they are all
rebuilt units now) was not good as it should have been.
Paul also suggested that he would have a geared starter motor
now, but the units available at the time he had to replace his starter
had a very poor mounting arrangement to the adapter plate which bolts
up to the engine. So although he found the standard unit he fitted
was extremely quiet, cranked quicker and took less out of the battery,
he suggested Maurice might like to consider a geared unit now because
the mounting arrangements are much better now. Call Clive Wheatley
and discuss them. The only drawback is those units do not have the
coil boost function of the original but Paul feels that can be reproduced
with a relay, and you should not need it anyway given the lower current
drain and hence volt drop during cranking.
Nippon Denzo replacement starter motor is available for
My subsequent V8BB posting was directed to Paul's point about the
absence of ballast resistor shorting contacts, and suggested an alternative
to a relay is a diode bridge. This is smaller and cheaper, and
more reliable than a relay. It is wired up to the existing starter
relay using standard automotive style ¼ inch "Faston"
connectors. I have done this modification on my V8 because the solenoid
contacts had failed and removing the starter motor is dauntingly tedious.
With the standard battery-starter-ignition set up, the spark is too
weak to give a cold start if the ballast resistor remains in circuit.
V8 starter motor with solenoid alongside. (Diag:
Moss V8 Parts Supplement)
Details of the diode modification
This modification allows the use of a starter solenoid without auxiliary
contacts, or one with faulty contacts, to be used with the standard
V8 points based ignition system. It will take you about half an hour
to install the modification. For some background information see the
section at the end of this Note.
The bits needed are:
1 off 35A Bridge Rectifier, eg type KBPC3504, or equivalent. MAPLIN
Order code AR87U, cost £1.49. This is 28mm square and 10mm deep
with four spade connecting tabs and a central fixing hole.
1 off Snap-Lock Automotive cable connector:
car accessory shop or MAPLIN JR88V @ £0.19
3 off Lucar/Faston blue sleeve female crimp connectors (Receptacles
for ¼" automotive type spade connectors). Car accessory
shop or a 100 piece crimp kit with crimp tool is £4.49; MAPLIN
YP97F. Some "piggy-back" types would be useful.
useful background notes
An electronic device which only allows current to pass one way,
commonly a silicon based semiconductor. Often used as a "Rectifier"
which is a device for converting alternating current (ac) into
direct current (dc). Diodes in your alternator convert the ac
to dc for charging the battery, so obviating the need for the
commutator used in the earlier dynamo. The diode current rating
is the safe continuous forward current; the voltage rating is
the maximum reverse voltage before the diode breaks down and
passes reverse current. The normal forward voltage drop is around
1V and is only slightly dependent on the current passing.
Diode Bridge: Four diodes interconnected in a way useful
for rectifier circuits and often built into a single component
with four connections. Two connections are given the symbol
~ and the other two are + and -, corresponding to the ac inputs
(~) and the dc outputs (+ and -).
Relay: An electromechanical device allowing a small current
signal to switch a large current. The small current passes through
the coil of an electromagnet which attracts a pivoted iron armature
which operates electrical contacts. NB: Called a "Relay"
from its original 19th century development for "relaying"
telegraph signals. The Morse signal would become progressively
weaker as the resistance of the line increased with distance.
To make it able to operate the clicker at the far end the run
was split into sections each ending in a "Relay" which
acted as a slave operator key for each succeeding battery-line-relay
Solenoid: An electromechanical device with a plunger-like
iron armature within a coil. When current passes through the
coil the plunger is drawn in. The mechanical action can be used
to actuate something and/or to close electrical contacts. In
a starter solenoid the armature movement first causes the starter
pinion to engage with the starter ring on the flywheel (pre-engaged
starter) and then causes electrical contacts to close which
connect the battery to run the starter motor. Additional auxiliary
contacts may also be present for other purposes, eg to boost
ignition by shorting a ballast resistor.
Ballast Resistor: On many of the later (post 1970s) points
ignition systems the ignition coil is fed via a "ballast
resistor". For convenience this is not usually a physical
component but is made by making the coil feed wire a resistance
wire. The resistance is chosen so that, when the points close,
the voltage across the coil is 6 - 8V. The coil is designed
to operate properly at this 6 - 8V supply, ie supply adequate
spark energy for correct ignition. When the starter is operated
the cranking current is very large and is larger for a larger
engine (eg an MGBV8 takes more power to crank than a standard
B). The high current causes voltage drops in the cables and
within the battery itself, so the voltage available for the
ignition circuit may be 8V or less. To restore the spark energy
to at least it's normal level, the auxiliary contacts on the
starter solenoid are made to short out the ballast resistor
and feed the full (now 8V) battery voltage to the coil. The
car starts, the starter solenoid is released and the ballast
resistor is put back in circuit. If the ballast resistor remains
shorted for normal running the higher current would cause rapid
points wear and coil overheating. The ballast resistor is not
needed on modern cars as the electronic ignition system ensures
constant spark energy under all conditions.
Solenoid Aux Contact Failure: The auxiliary contacts
on the Lucas solenoid are not the best example of electrical
engineering. They can go intermittent and/or fail completely.
is an engine that cranks but won't fire. Checks on fuel and
sparks will show all is apparently well; but unfortunately there
won't be enough "welly" in the spark to ignite the
mixture. Sometimes the engine may start on the instant of releasing
the starter as the battery load comes off and momentum carries
the engine past a firing point. To check operation of the aux
contacts use a voltmeter or DMM: clip the black lead to a convenient
earth contact (bolt or connection of a black lead) and connect
the red to any of the fuse box connections. Crank for a few
seconds and note the approximate voltage, say 9V. Now move the
red lead to the "Sw" contact of the coil (White/Blue
and White/Light green wires). The cranking voltage should be
similar. If the cranking voltage is more than 1V below the fuse
box level then the solenoid aux contacts are not working. To
prove this, rig a temporary jump wire from the fuse box to the
coil "Sw" contact; if the aux contact is the problem,
the car should now start. Remember not to leave the jump wire
No 8 or similar ¾" Pan head self tapping screw to
fix the bridge.
1m (0.3m reqd) 10A equipment wire. Car accessory shop or MAPLIN
XR37S, white @ £0.29.
The tools you will need are wire cutters and strippers, drill
with 3mm bit, crimp tool (ideally), pliers, and a screw driver.
Mounting the bridge: Position the bridge in a convenient place
alongside the starter relay and mark the centre. Drill a 3mm hole
and fix the bridge with the No8 self tapping screw. Wiring: Refer
to the circuit diagram on the next page:
o Connect the "-" terminal of the bridge to a convenient
earth, eg join with the black wire on the starter relay.
o Connect one of the "~" terminals of the bridge to the
white/brown wire of the starter relay (white body, brown tracer)
o Find the White/Blue wire which is running in the loom passing below
the starter relay. It may be necessary to carefully cut away some
of the woven sleeve around the loom and tease out the White/Blue wire
with a hook, etc. Using the Snap lock connector, connect the "+"
terminal of the bridge to the White/Blue wire.
That's it! You can refer to the circuit diagram and use a DMM or continuity
tester to double check on the wiring in case the colours are non standard.
How does it work?
Refer to the circuit below: The lower left diode passes current into
the coil if the starter relay becomes more positive than the coil
"Sw" terminal - that is when the starter solenoid is energised.
Under normal operating conditions this diode has a reverse voltage
applied and so passes no current. The diode connected to earth, acts
as a clamp to any transient voltages caused when the starter relay
contacts open and current through the starter solenoid ceases abruptly.
Copyright reserved V8 Register & Bob Owen (2005)