06-09-2010 Ask Mike! Archive Page
Master Switch

Good morning Mike

My 1961 Debonair 8DC had a hot wire smell in the cockpit yesterday when I had all the running lights on and the landing lights...and the strobe.  This smell was observed once before at night when I had all the lights on...the power (battery) switch was very hot...I could not touch it (on the back side) the toggle itself was not hot...when I looked at the underside of the switch the yellow connector has melted to a tight little ball of plastic around the WHITE wire.  There are two WHITE wires and both have the plastic on the crimp connection looking this way.  On other days I have felt the battery switch and it is very hot but I could keep my finger on it for a while...

I think I should change out the battery switch and add solder to the connection (on the wire - connector), put on some heat shrink around the connect joint and reattach the new switch.  This is what I think should be done...the wires are two short to cut and re-crimp on a new connection.

The question is...is this OK to do and where can I get a new battery switch...

What else should I check?

Ken

Ken;

There is something that is causing the heat.  The master relay circuit is the only thing on this switch, takes roughly one amp of current to keep the relay closed under normal conditions.  This should not matter how much of a load there is on the electrical system.  (This is provided the aircraft wiring has not been altered in an unacceptable manner.)

Two sources of the heat that come to mind are the connectors to the switch - screws loose causing arcing and excess current or internal switch arcing, but this would most likely be accompanied by flickering lights and avionics as this would chatter the relay.  The other would be a short in the relay windings itself.  Check current draw on the circuit by putting a DMM selected to ammeter in series with the circuit.  Should be roughly one amp.

Yes, not much of a service loop on the original installations.  Serial number for that N# would indicate old style panel, has yours been modified and if so, whose panel is it?  You will need this info to find a switch part number, unless it is visible on the existing unit.  Soldering the wires to the terminals may be an option, but if there is any corrosion in the wire, or other contamination, such as melted terminal insulation, in the wire, the success of this may not be good.  If the insulation has melted but you still have a good mechanical crimp on the wire, adding heat-shrink for insulation may be the better option.

Yes, replace the switch, but I suspect you will find a relay is also needed.

Gear Green,

Mike

Thank you Michael.

A "floating" Beechcraft panel from 1963 was added to 8DC.  (Beech unit with Beech drawings and STC).  I keep the battery charged with a Napa 1.5 amp battery conditioner but yesterday the amp meter was up to about 20-25 amps.  The JPI says it only has 11.8-12.0 volts when I turn it on (even with the battery conditioner on).  So I bought a new battery...but it reads the same with that battery.  If I leave the power on and start the motor but don't turn the alternator on right away the voltage drops to about 10.5 and the fuel flow monitor resets or stops working and yesterday the auto pilot (S-Tec 60-2) would not go to the "ready" stage and/or stay in the "ready" mode.

I think I will start with your recommendations and see what happens...thank you for your advice.

Ken