Replace Big Boy with Blue Sea ML-ACR

[pwhittle]

I cleaned our Big Boy relay about 12 months ago, but it recently stopped working again.

Rather than spend any more time on it I replaced it with a Blue Sea Sytems ML-ACR. This is a simple device that does one thing: connects or disconnects the house and chassis batteries.

The features I like about it are
- will charge both sets of batteries from the alternator or output from the inverter/charger
- auto mode as well as manual off and manual combine override via the remote switch or on the relay itself
- LED status indicator on the remote switch that replaces the Battery Boost switch in the dash/side panel
- takes no current to hold it in either position, so does not run hot
- has high current ratings
- sealed from the elements
- senses charging on either bank of batteries to combine in Auto mode

I had previously replaced the diode based isolator and battery boost relay with the ML-ACR on our 1994 Signature and it operated well.

I have helped Vanwill replace the same setup on his 2000 Dynasty and Mecdatlanta on his 2002 Signature.

In those cases the ML-ACR replaces the isolator, the Battery Boost Relay, and the Lambert style or Trik-l-Amp or Trik-l-Start solution to keep both sets of batteries charged.

For our coach the ML-ACR replaces the Big Boy relay and renders the circuitry that drives the Big Boy unnecessary.

Mechanically the ML-ACR is a similar size as the Big Boy relay and we used 1/8”x3/4” copper bar to connect the ML-ACR to the existing wiring.

We also ran a pair of wires from the RRB to the dash for the remote switch. We could have run one wire and reused the existing wire from the Batt Boost switch.

The switch also needs fused power and ground which we picked up from the FRB.

Before the switch


After the switch


Remote switch


Copper bars


Manufacturers page

https://www.bluesea.com/products/762..._-_12V_DC_500A

ML-ACR on Amazon

Blue Sea Systems ML-ACR 12V DC 500A Automatic Charging Relay with Manual Control https://www.amazon.com/dp/B001VIXLRO..._DQuYDb6AVKC8Z

Copper bar on McMaster Carr

https://www.mcmaster.com/#8964K5

Many thanks to Mecdatlanta for the help. It certainly helps to have a second person when running wires through the belly of the coach.

[Burgy]

Great Post was looking to this as well, I already have used one of the spare wires Monaco ran from FRB to RRB and there are 4-5 more I can hi jack as well...

Really hate the constant draw it requires and the hum oh the big boy until the next cleaning of the contacts.

[pwhittle]

I found a set of 4 unterminated wires near the tow wiring plug. When I used a toner on it, i thought I found the other end behind the dash, but they did not measure out with a multimeter.

[153stars]

That's a beautiful job right there looks better that factory .Replacing my isolator with one is on my short list. I haven't heard of one problem mentioned about one and its supposed to have lifetime warranty.
I have tons of unused wires to dash area haven't looked for the other ends though. 95 vintage I'll bet they are not that generous anymore.

[Twomed]

Great writeup thanks...my most unfavorite part of Monaco electric system. Cleaning and/or replacing is just an unecessary design feature.

[Burgy]

the harness i use was in the FRB.. other end was tied up behind the RRB in loop with no ends on it.

[pwhittle]

Originally Posted by Burgy

the harness i use was in the FRB.. other end was tied up behind the RRB in loop with no ends on it.

Both ends unterminated?

Paul

[Burgy]

no, the on in the FRB was a six pin I think

[vito.a]

Wow, what a great job. Looks like it came from the factory.

[mmulvenna]

Originally Posted by pwhittle

I cleaned our Big Boy relay about 12 months ago, but it recently stopped working again.

Rather than spend any more time on it I replaced it with a Blue Sea Sytems ML-ACR. This is a simple device that does one thing: connects or disconnects the house and chassis batteries.

The features I like about it are
- will charge both sets of batteries from the alternator or output from the inverter/charger
- auto mode as well as manual off and manual combine override via the remote switch or on the relay itself
- LED status indicator on the remote switch that replaces the Battery Boost switch in the dash/side panel
- takes no current to hold it in either position, so does not run hot
- has high current ratings
- sealed from the elements
- senses charging on either bank of batteries to combine in Auto mode

I had previously replaced the diode based isolator and battery boost relay with the ML-ACR on our 1994 Signature and it operated well.

I have helped Vanwill replace the same setup on his 2000 Dynasty and Mecdatlanta on his 2002 Signature.

In those cases the ML-ACR replaces the isolator, the Battery Boost Relay, and the Lambert style or Trik-l-Amp or Trik-l-Start solution to keep both sets of batteries charged.

For our coach the ML-ACR replaces the Big Boy relay and renders the circuitry that drives the Big Boy unnecessary.

Mechanically the ML-ACR is a similar size as the Big Boy relay and we used 1/8”x3/4” copper bar to connect the ML-ACR to the existing wiring.

We also ran a pair of wires from the RRB to the dash for the remote switch. We could have run one wire and reused the existing wire from the Batt Boost switch.

The switch also needs fused power and ground which we picked up from the FRB.

Before the switch
Attachment 266494

After the switch
Attachment 266495

Remote switch
Attachment 266506

Copper bars
Attachment 266496

Manufacturers page

https://www.bluesea.com/products/762..._-_12V_DC_500A

ML-ACR on Amazon

Blue Sea Systems ML-ACR 12V DC 500A Automatic Charging Relay with Manual Control https://www.amazon.com/dp/B001VIXLRO..._DQuYDb6AVKC8Z

Copper bar on McMaster Carr

https://www.mcmaster.com/#8964K5

Many thanks to Mecdatlanta for the help. It certainly helps to have a second person when running wires through the belly of the coach.

What did you do with the PCB and its wiring that controlled the BB?

[wamcneil]

Nice upgrade! I wish I'd run an extra wire back there while I had the chance...

[pwhittle]

Originally Posted by mmulvenna

What did you do with the PCB and its wiring that controlled the BB?

I left it in place and taped up the purple wires.

Paul

[Ivylog]

I realize older rigs did not use the BigBoy so a BlueSea (rated 500 Amps (continuous) / 700 Amps (intermittent) is a good replacement for those. A BigBoy is capable of more than double that as the contacts are the same as a Delco MT50 solenoid which can handle two 950 amp batteries...1900 amps. You can even see the difference in the smaller size of the lugs in your second picture. I would not replace a BigBoy with the BlueSea, especially if you have a 500+ HP engine.

I carry a spare starter solenoid ($40) and like this style because you can easily clean the contacts because the small wires are separate from the plate that holds the contacts unlike the $250+ BigBoy.
https://www.dbelectrical.com/product...E&gclsrc=aw.ds

[pwhittle]

Hi Dick,

The ML-ACR is rated at 1450A cranking (30 seconds).

https://www.bluesea.com/products/762..._-_12V_DC_500A.

I picked our motorhome up today after a month of sitting unplugged. Our chassis batteries are marginal. It would not start the big ISX-15, so I connected the house batteries with the ML- ACR and it cranked right up.

I need to replace the chassis batteries soon.

Paul