Troubleshooting the Starting
System
When the Starter doesn't engage or work at all
16-B
startingprobs.htm
NOTE:
It is very important that the reader of this be sure to read section 16-A!
To do that NOW: click on: 16-A
1. The basic starting system is exactly the same on ALL Airheads, and is very similar electrically, and in the parts utilized, to what is done on most cars and trucks. It is NOT exactly the same system as used on the Classic K bikes, Oilheads, etc... in that there is no solenoid and Bendix drive on those K bikes, they use a one-way mechanical clutch on the starter shaft; and, they use a remote high power starting relay, not the solenoid type as on the Airhead starters.
2. In all cases, the starter motor itself is a powerful electrically operated motor. There needs to be a means of passing very large amounts of current to the starting motor, and the switch on the handlebars and the start relay (on Airheads) are hardly capable. In the common Bosch system used on the Airheads, a very heavy duty solenoid operated switch is mounted on the starter motor. This solenoid-switch controls the large electrical current that operates the starter motor. When the starter begins to spin up, a mechanical gear device called a Bendix begins spinning as it is located on the rear of the starter motor on the starter shaft. The gear, via spinning forces and guide grooves, moves rearward and engages its teeth into the engine flywheel teeth, causing the engine to rotate. There are starter motors in existence that have the solenoid move, via fingers, the engaging gear, which is not moved by turning forces, but that system is found primarily on very old cars and trucks. Later model airheads, including the Oilheads, and K bikes, etc., may have a Valeo starter motor which has permanent magnets instead of field coils, and the Valeo may have a planetary reduction gear on its nose, which allows the starter to develop higher rpm and hence higher power. For the Airheads, the solenoid switch, which needs a fair amount of coil energizing current, is fed (to the solenoid coil, that is) via a small starter relay, located under the fuel tank. It is that small relay's coil that is turned on and off by the starter button on the handlebars.
3. When the starter button is depressed, assuming the key switch is ON,
and any handlebar KILL switch is centered (ON), power from the battery energizes the starter relay coil, which closes its
contacts, which sends a modest amount of battery power, via a black wire, to the male spade
COIL terminal on the starter motor solenoid assembly. The starter motor
solenoid COIL, now energized with a strong magnetic
field, pulls the solenoid 'slug' (various names for that) forward, and an
insulated pair of rather substantially large copper-brass contacts will be closed by that action. One of the
large contacts connects to the large gauge battery wire. The other large
contact connects to the interior of the starter motor. Thus, the solenoid switch itself
closes the two contacts, sending large amounts of battery power to the inside of the starter motor.
The amount of current drain can be large on a very cold engine with high
compression, and the INITIAL starting current can be VERY high until the starter
begins to rotate at a goodly rpm.
4. Expanding a bit on item 2., above: There are some variances in starter motor operation, on cars,
motorcycles, etc., but there are basically two types. In one, the solenoid
is strictly a mechanically operated electrical switch. The starter,
spinning-up, via its particular construction, sends the Bendix gear into the
flywheel teeth, and the engine is thusly rotated. When the engine starts,
the flywheel will be rotating much faster than the starter would be for the
starter power, so the Bendix is given reverse thrust by this action, helped
along in some by the teeth shape on flywheel and Bendix, and the Bendix
disengages. A spring moves the Bendix to home position. YOU, as the
operator, are expected to turn off the starter, after the engine is rotating
under its own power. In some
starters, the solenoid is connected mechanically to help the Bendix (or a non-Bendix
gear) move into
the flywheel, and to disengage it.
5. If the starter motor develops too much play in its bearings, it may
tend to bog down, and might even start to rub its armature against the
pole pieces. Worn brushes, grunge in general, and poor worn out solenoid
contacts, are other things that greatly reduce starter power output.
6. It is a very good idea to disconnect the battery before removing the aluminum cast cover over the starter area. Have it removed during replacement too! The reason for this is the potential (bad pun!) to short the cover against the battery terminal on the starter solenoid....and that connection needs proper orientation (to avoid shorts to the case/cover); and to be TIGHT;....something to check on before you replace the cover.
Very specific to the BMW airheads, is the following information on its starting system:
7. If the starter does not work correctly, you need to
determine if the problem is in the starter motor, and/or its solenoid; or,
someplace else. Just what your bike's symptoms are, may well
determine your proper approach to your problem.
8. It is vital that the battery be in good condition. You can
monitor its terminal voltage, which may give you some ideas, and see what the
terminal voltage does during cranking attempts. If the starter is very
bad, drawing HUGE amounts of current, that can drag the battery to under 11
volts. Thus, a true load test on the battery is done in a
well-equipped shop; but, monitoring the voltage whilst you try cranking in the bike
(assuming the starter functions at all) might tell you a bit. If the
battery wires are poor, corroded, etc., the voltage at the battery during
cranking may be much higher than at the starter. Monitor the
voltage at the battery terminal itself....is the battery OK during cranking
attempts? A battery CAN have good terminal voltage at rest, and fall
dramatically, during cranking attempt (usually a failed inter-cell connection
causes this).
9. If there is no tiny under tank click noise; then the starter relay, or
its sometimes troublesome socket connections, is suspect.
10. If the starter relay clicks but there is no major click/clunk from the
solenoid, you need to check the solenoid, be sure it is getting power to the
SMALL wire's terminal DURING cranking attempts.
11. Remove (pull off) the black small wire at the solenoid. Run a
temporary jumper wire, any small gauge wire is OK, from that male spade terminal
to the battery terminal next to it; or, to the battery +. If that causes
the starter to operate, where it would not before, then your problem is NOT in
the starter area. If the starter fails to operate properly, the starter or
solenoid is
at fault.
12. Various years and models of BMW Airheads have various types of
interconnections with the neutral switch and the clutch lever switch at the
handlebars. These can introduce complications in your testing. You
may need to
find out if power is going through the starter relay. The easiest way to
begin this process is to leave the black small push-on wire DISconnected at the
starter. Key ON, Transmission in Neutral, pull in Clutch lever, emergency
shut off switch centered (ON position). Push starter button: is
there power at the unfastened wire? If so, the problem is NOT in the starter relay.
13. Release clutch lever; still starts? Problem not in clutch lever
switch.
14. Transmission to any gear but not in neutral. Clutch lever pulled back.
Starts? problem not in transmission switch.
NO start? pull in clutch switch...starts? now you have more work to
do.
15. Depending on year and model, you may find the starter
not working unless clutch lever is pulled-in, even in neutral. You may
find your neutral light not working, nor, can you start in a gear, even with
clutch lever pulled-in. On some models it is possible for the
starter to run continuously unless the battery is disconnected if a certain
diode fails! If you
have a /5, you may hear funny cricket-like noises ...sort of ratcheting attempts
at firing up the starter. It is rather complicated to explain all the variations on the starter
system, but I will TRY.
NOTE!...for every Airhead, the large
heavy duty cable from the battery always goes to the starter solenoid switch,
and there is always that black small wire mentioned above at the solenoid. Everything that
follows deals with the starter relay and its various connections to the rest of
the bike; and, the complexities of that circuit section.
16. The /5, ONLY, had a peculiarity; and, the circuitry arrangement needs
understanding. The battery feeds power to the ignition switch, as in
all the airheads, but there was NO handlebars-mounted emergency cutoff switch. From the
ignition switch in the /5, the battery power fed a starter relay that has had various
names, including Starter Lockout Relay. It is located furthest forward on
the left of the backbone under the fuel tank. This is not just a simple
relay, but it incorporates a simple transistor circuit; which, when stock, is
unfortunately overly sensitive to temperature and to even slightly decreased battery
charge. When in the 'problem' area of temperature or battery
charge, the relay may chatter, or sound like a cricket, and may even power the
starter solenoid that way too. To an old time mechanic, it sounds just
exactly like a near dead battery, which it is usually not. The
transistor circuit design was to shut down possible relay operation (like in YOU
trying to start an already running engine!) as soon as the alternator
is spinning at engine idle. The idea is to prevent starter engagement if
the engine is already running. There are low battery and low temperature
problems with that circuit, and a modification is HIGHLY recommended, and there
is an article on this website specifically dealing with that problem, and the
fix.
The article is: slash5cricket.htm
Other than the added function mentioned, the /5 starter relay coil is
wired such that the grounding side of the coil connects to the starter
pushbutton switch, from whence it is grounded when the pushbutton is pressed. If the starter relay
on the /5 fails to operate, it can be corroded inside, often fixable as it is in
a metal can that can be carefully UNcrimped and things cleaned inside. If
the transistor fails the relay will not work, usually, unless the transistor is
shorted. The modification, which prevents problems at lower
temperatures, and/or slightly weak battery, is highly recommended.
There are NO complications in the /5, as to clutch switches, neutral switches in
the starter circuitry, etc.
17. After the /5, things started getting more complicated in the starter
circuits. The only thing that was simplified was the deletion of the
/5 starter lockout relay, in favor of a more standard relay...SOME of the later
ones of which
contained a diode.
18. The first change came in the early /6. The neutral
indicating lamp, which is green (and the GEN lamp which is red) both went
through a fuse, before connection to the ignition switch. That did NOT
continue, the fuse in THAT circuit, after this early /6. The
rest of the starter circuitry was the stone-simple starter relay, and starter
assembly. Thus, this early /6 is easy to troubleshoot.
19. The EARLY 1975 /6 models incorporated an emergency kill
switch....added between the ignition switch and the starter relay coil.
Sometimes these kill switches become corroded internally and act strangely
intermittent. So, these models were also easy to
troubleshoot. The starter relay coil in the 1974 /6 models also was
wired to the same fuse as the INDICATOR lights...so if those lights are not
working, chances are the fuse is blown!
20. It was in the late 1975 /6 models, and then carried forward into
1977,
that the beginning of complications came about that drive some mechanics or
would-be mechanics, crazy. BMW
added three things to complicate it all. Firstly, the starter
button, when pressed, no longer just grounded the starter relay negative side of
the coil to the frame. Whilst all the 5 speed transmissions, including the
early 1975 /6, had a neutral switch, the early models were NOT wired into the
starter section, like the later ones were.
Instead the path from that starter relay coil could take TWO directions:
ONE, if the neutral switch, located
on the transmission, was in the ON position, that grounded the coil, but through
a DIODE.
TWO, if the clutch lever mounted
switch was closed by pulling in the clutch, the starter relay could be operated
from the starter button, no matter if the transmission was in neutral OR
NOT. The diode was there to prevent the neutral lamp from indicating every
time the clutch lever switch is activated. These diodes have been known to
short circuit. If so, the neutral lamp will light up every time the clutch
lever is used. If the diode was OPEN circuited, the neutral switch will
not allow starter operation UNLESS the clutch is pulled in. The location
of this diode VARIES! For 1974-1980, except the R65, it
is UNDERneath the connection board in the headlight shell, where it can't be
easily seen. For 1979-1980 R65, it is part of the wiring harness, near the
VR. For all the others from 1981, it is INSIDE the starter relay!
THUS, the starter relay from 1981 is a SPECIAL relay.
21. In 1978, BMW changed things again. This lasted into 1980. BMW
added 2 diodes...and a switch! These models had an under-fuel-tank brake
master cylinder. BMW incorporated a low brake fluid level switch, which,
when closed, turned on a brake failure RED light. The switch was wired to
that lamp, and via a diode to the starter relay coil. This modification
did NOT interfere with the starter operation, even if the diode failed in the
open condition. If, however, that diode shorted (HIGHLY unusual), and at
the same time one had a failed (closed) switch, or the level was low, the
starter could fire up, mysteriously, all by itself, and NOT release!...unless
the ignition or kill switch was used. There was one other
complexity. BMW incorporated a diode in the headlight relay. The location of that relay varies with year and model. The
wiring and operation was such that when the starter was engaged, the headlight
relay coil dropped-out, turning off the headlight during cranking.
Depending on model, whether USA or European, this varied somewhat, and in some
the instruments and rear running lamp WERE left ON during cranking. Some had
various combinations. What is complicating the mess here, is that, at
least for USA models, the diode in the headlight relay was so connected that if
it SHORTED, then in some conditions of ignition switch setting, etc., it was
possible for the starter relay to fire up, and NOT be releasable, until the
battery was connected, and another starting attempt was made. Yes, the same
sort of thing like the 1978-1980 symptom of the shorted starter relay
diode. Thus the same SORT (nearly) fault could be had via a shorted
headlight relay diode or shorted starter relay diode!
Theoretically this problem could only happen in the PARK position of the
ignition switch. The fuse was
also now incorporated in the headlight relay section, and also fed the parking
light. Normally, if you used the starter button, even if the
fluid level was fine, the brake failure light would illuminate at every
start...this tested that lamp at each startup. It wasn't necessary to have
the fluid be low, then, to turn on that lamp.
22. We are now up to the 1981 models. No longer was there an
under-tank master cylinder, so the operation is slightly simplified. The wiring is exactly the same,
but withOUT the master cylinder switch and, of course, its
diode was eliminated. This lasted through the 1984 models.
23. There were several more variations and changes until the end of
production:
In 1985 and later, BMW put the starter button into the + power feed to the starter relay coil,
and the negative side of that starter relay coil went to the same diode and the
same neutral switch and clutch switch as before. By moving the
position of the starter button connection, BMW could incorporate the diode into the relay
housing. BMW also added another diode in the headlight relay area; this
time in series with its coil. The
starter worked the same, however. The GS and ST, and 1981-1985 R65 and R45 had slightly
different headlight relay contact arrangements; which in other models cut other
lights during starting besides the headlight lamp itself;...again, not a starter area to be
concerned with. [for the nerdy: the power for the instruments and tail light comes from
the ignition switch on these]. Earliest R65 models were different in this
regards.
11/29/2006: initial first draft and release
11/29/2006: correct typo editing error in h.
11/16/2007: re-number the items, combine certain sections.
Vastly improve clarity of descriptions.
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