AVIATOR pOinls - American Bonanza Society

ELECTRICAL PROBLEMS
Again this month, I am covering some common electrical problems
that ABS members have asked me about over the years.
Troubleshooting alternator-out light(s)
One of the more common calls for advice goes something
like this :"My alternator-out light is on, but the alternator seems to
be working just fine:
The early Bonanzas had generators and therefore did not
use an alternator-out light. so this discussion does not apply. The
P model and the S model had alternator options, but the warning
light was an over-voltage indicator; so this discussion does
not apply to them either.
Starting with the 1968 model year for most Bonanza models,
the alternator had an aux post added and an "alternatorout"
light was added to the pilot panel. It was attached to a
relay that turned off the light whenever the voltage at the aux
post of the alternator exceeded a threshold voltage (about half
the normal alternator voltage).
The alternator-out relays proved to be problematic; so in May
1969 Beechcraft issued Service Instruction No. 0180-364 that
replaces the relay and a resistor with a transistor-based alternatorout
sensor (PIN 36-380000-1). Note: This was not a mandatory
change, so your aircraft may still have the original relay,
The wire from the altematoI aux post was protected with an
AGC3 fuse, usually mounted on an alternator fuse block on the
engine side of the firewall, pilot side. If the fuse is burned out or
any corrosion exists on either the fuse holder or the fuse itself,
this can cause the alternator-out light to give a false indication.
The fuse is supposed to protect the wire coming from the
alternator-aux terminal to the alternator-out sensor. Since most
of the wire is unprotected from the alternator to the fuse block,
in August 1981 Beechcraft issued Service Instruction No. 1157
that replaced the fuse with an inline fuse mounted near the alternator
aux tenninal. It also changed the fuse to an AGCS.
This exposed the fuse to the incoming airstream into the
engine, so moisture could easily get into the inline fuse and
accelerate corrosion, causing the false alternator-out indication
to occur more frequently.
To help solve the problem, Beechcraft issued Service
Bulletin No. 2016 that replaced the inLine fuse with a small
printed circuit board (PCB) that was mounted directly on the
alternator aux post. The only component mounted on the PCB is
a fusible resistor that perfonned the same function as the fuse.
The PCB attaches to a wire via a screw and washer, which was
in tum butt-spliced to the existing wire to the sensor. (See photo)
There are some advantages to using the PCB mounted to
the alternator aux post. For instance, it protects the entire wire
to the sensor and the fusible resistor is fully encapsulated and
therefore well-protected from corrosion. Unfortunately, there
are problems as well.
The PCB is located in a high-vibration area of the engine
and is subject to cracking. or the solder lands separate from the
PCB. Of course, if this happens, the sensor may get a false
alternator-out indication.
Beechcraft then changed the design to eliminate the PCB
and simply mounted a fusible resistor with leads instead. This
seems to give good reliability, but the vendor that supplies
Beechcraft with the part stopped making them; so Beechcraft
returned to the PCB design.
Know how your airplane is wired
With all the possible updates, one of the first things you
need to do is to figure out how your airplane is currently wired.
If you have the PCB, inspect it for any cracks or to see if the
solder lands have lifted off the surface of the PCB, In all cases,
check for signs of corrosion in any of the connections and
clean as required.
Then check the voltage of the input to the alternator-out
sensor with the engine running and the alternator on. You should
see half of the alternator regular output voltage at the aux tenni-
Alternator with fusible resistor (yellow arrow) on a printed circuit board.
RAPID LIST PIICE$
Replacement 14-volt alternator-out sensor
(PIN 36-380000-1) - $196.
The 28-volt version (PIN 36-380000-3) - $122.
The Baron uses replacement alternator-out
(PIN 36-380000-11) - $119.
A replacement PCB kit (PIN 55-3025-1 S) - $108.
Use your ABS credit card for a discount on RAPID's list prices.