Big Boy Relay Thumping Noise

[rdjl972]

Been a while since I asked for help here in this Forum. Always had good luck, so here goes.

I'm having an issue that started about a month ago with a thumping sound in the right rear portion of our 2008 Monaco Camelot 42 footer. It only seems to happen at colder temps. In tracking down the source of the noise I discovered that it's the Big Boy relay in the rear run electrical bay. I've been checking forums, including this one, and really couldn't find any guidance of what the issue might be that causes the thumping sound, or what can be done to correct it. Some say to replace the relay, some say to remove the relay and clean the contacts, and some say to just live with it. But if it hasn't thumped for the last seven years, and it's just now making the sound, why should I ignore it and "live with it" now? That doesn't seem right.

To give a bit of background, this only happens when hooked to shore power (nothing when dry camping and the batteries are isolated), with both the inverter and the charger side of the inverter turned ON, and only when it's pretty cool outside. During the day with warm temps ... nothing ... it's quiet. When I switch the charger side of the inverter OFF, the thumping sound stops. Now, I replaced the house batteries about a year ago with 300 amp AGM's X4 because the original batteries were shot, and haven't replaced the chassis batteries at all. They seem to be performing just fine. They're group 31P with two maintenance free Diehard's in parallel. Total CCA's to crank the engine is 1500. Now they're also about seven and a half years old. Is it possible that the Big Boy relay is trying to charge them and they're not accepting a full charge because of age, so the relay is cycling continuing to try and charge them?

Last tidbit of info is this ... the control board just behind the Big Boy relay has three green LED's on it and all three are lit.

So what do you guys think? Does anyone out there have any guidance that can help? I'd appreciate any help that any of you can provide.

Thanks,

Rick

[manholt]

Does it also happen on generator with invertor and charger on?

Other thought is dead cell.

[huffypuff]

Is that the relay solenoid on the back wall above the batteries?

[rdjl972]

OK. I promised an update when - and IF - I was able to figure out what happened. It turns out that the Big Boy relay will make the "thumping" sound whenever it's trying to charge a battery that's low on charge. In our case, that was the engine start batteries - two group 32 maintenance free batteries in parallel. These are our older batteries - 7 years plus - so all I had to do was replace the two start batteries and the problem went away.

BUT ... I'd like to give credit, where credit is due. In this case Mark and Chris at M & M Audio mnmaudio@roadrunner.com were an excellent source of information about the Big Boy relay. They explained everything I needed to know and more. They even mentioned stuff I hadn't even thought of. What a great help they were and I wholeheartedly recommend them to anyone having battery and/or charge problems. Send them an email and see what you get.

Thanks again to everyone that helped me.

Rick

[rwilli]

Rick,

Please post your findings as inquiring minds want to know. I have a 2008 Dynasty and we also go through that annoying thumping coming from the rear electrical box relay. This happens ocassionally and I am assuming your assessment is correct--the thumping appears to be connected to low chassis/house battery banks. We even added a trickle charger on the chassis battery to aide in the charging process--but this actually seemed to exaerbate the thumping. However, once the coach has been connected to shore power for a few days all becomes quiet and stays quiet.