BCC – Coach battery disconnects immediately after its connected

This one had me going for a while.

While I was working on broken exhaust studs, I had the Southwind sitting for a couple months with the batteries physically disconnected and no shore power. After reconnecting everything and getting ready to apply power for engine testing,  I noticed the coach connect/disconnect switch wasn’t working properly.

The battery CONNECT / DISCONNECT switches are a momentary contact type. When I pressed the “CONNECT”,  I could hear a distinct “CLUNK” as the solenoid pulled in.  But when I let my finger off the button,  I heard the solenoid CLUNK again and the solenoid disconnected..  (scratching head).

I started looking at the BCC (Battery Control Center.)  I have an early model RV Custom Products BCC with circuit board CB-115 Rev B.  This BCC uses a permanent magnet type disconnect solenoid for the coach and chassis batteries.  The internal coil is momentary energized by the push button switches. Depending on the polarity of the 12 volt that’s applied to the coils,  the coils magnetism either pushes a permanent magnet contact bar to open the contacts, or pulls the permanent magnet contact bar to close the contacts.  The permanent magnet then holds the bar in the last selected position.

I initially thought there might be something on the BCC board that’s causing the disconnect to kick in. but early version BCC doesn’t have any automatic connects or disconnects incorporated. (NOTE 1)..   I removed the BCC board to see if there was any damage or anything shorted that would cause this.  Everything appeared OK.  While I had the board removed,  I used some small test clips and an external battery to work the solenoid.  It worked perfectly.  When I momentarily energized the coil one way,.  it would connect and stay connected.  When I reversed the battery polarity and momentarily energized the coil again,  it would disconnect and stay disconnected..  (scratching head)

I traced the circuit board pads and the wiring against the BCC schematic.  The only way to energize the coil was through the CONNECT / DISCONNECT switch.  I put the BCC back together and moved to the switches.

Thinking it might be some weird internal short in the switch,  I disconnected the two wires on the “DISCONNECT” side of the switch.  Same thing.  When I pressed the CONNECT,  it would THUNK and connect,  when I released the switch it would THUNK and disconnect.

I used a voltmeter to see if there was any residual voltage on the coil wires that would cause it to disconnect. It was normal with no voltage on the wires when the switch was off.  How is it disconnecting??  (scratching head)

I completely disconnected the CONNECT / DISCONNECT switch.  Using my two jumper test wires,  I jumped the wires to momentarily CONNECT.  I heard the THUNK,  when I removed the jumper,  it stayed connected.  I reversed the polarity, when I touched the wires again,  THUNK,  it disconnected..  I repeated this several times,  and it worked perfectly with the jumper wires.  Something in the switch was causing it, so I bought a new switch.  (DPDT – MOM – OFF – MOM)..

When I connected the wires to the new switch, it worked perfectly.  EXCELLENT !!!!!!!!

THEORY – I suspect the contacts in the 19 year old original switch were a little dirty and not making good electrical connection.  When the switch was pressed,  it wasn’t applying the full voltage to the coil, so the coil wasn’t developing a nice strong magnetic field to pull in the permanent magnet contacts.  It was strong enough to make contact,  but not strong enough to pull the permanent magnet plunger all the way in.  Then when the switch was released,  the magnet would return to the OPEN position.

NOTE 1 – My version of BCC has a feature that prevents the chassis battery from disconnecting while the engine is running..