BAC Forum
General Boards => General Discussion => Topic started by: Shane Froman on September 30, 2025, 07:19:39 PM
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Hello everyone - New to this group and have a new to me 07 Contessa.
A recent issue has started when I disconnect from shore power, the Big Boy Isolator kicks in within minutes isolating and killing power from the chassis. At this point I have no power to my alarm system or keyless entry, among other things. I find this odd as I would think these things should still function as long as the battery has some juice. All batteries are new and I tested both, house and chassis to ensure there was plenty of amperage.
So questions are -
Is it correct that keyless entry would not work any longer if the big boy isolated the chassis battery? Do I have another problem?
What tells the Big Boy when to cut the power and at what battery level? (is it the relay module?)
Pics attached
Thanks in advance for the help!
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Shane,
Access "Coach Assist" on the BAC website. It's under the "Technical" tab. You will find complete writeups on the "BIRD" and "Big Boy" units in the "Electrical Systems" section. The lock system is powered by the chassis batteries. You will also find the wiring diagrams for your 07 in Coach Assist. It sounds like your battery voltages may be low.
Steve
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Shane,
Unless an issue exists, the Big Boy relay should never isolate the chassis battery bank from the chassis circuit system.
Check your chassis battery disconnect switch to see if it is passing current.
The purpose of the Big Boy relay is to join the chassis and house battery banks when the BIRD relay (or PCB) ‘thinks’ that charging is taking place. This allows both battery banks to be charged from a single source (invertor/charger, alternator, solar). The Big Boy should drop out and disconnect the chassis and house battery banks from each other when the voltage goes below the threshold, indicating that charging has ceased.
-Scott
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Like Scott, I also am not sure how the BB would cut off chassis power. You say it “kicks in” after a few minutes of shore power disconnect. Perhaps you are hearing some other device, like the transfer switch, though that’s normally within seconds, not minutes. Is the battery boost switch on your dash (armrest panel?) off? Perhaps the BB or BIRD are miswired?
Loss of chassis power to anything would send me first to the streetside forwardmost electrical bay, wherein I’d check the power busses; and then the solenoid’s posts with a voltmeter, initially with the ignition key off and then check again with it on. Given its reputation, the one thing I’d suspect is the ignition solenoid there. Some owners have replaced it with the wrong one, or remounted it’s diode backwards.
But you say you do have power to all chassis components while on shore power?
Joel