Boost Switched Unexpected Stopped Working

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Boost Switched Unexpected Stopped Working

April 27, 2025, 01:30:47 pm

Team,

2000 U320 4210... was using the boost switch to help get it started to get the coach to the local truck garage to change out the chassis battery which is all but dead. Inexplicably, the boost switch stopped working at all. It even stopped lighting up the dash. House batteries are brand new. Any suggestions would be helpful.

In addition, what is the best way to get the battery out under the tag axle if I can't get it started. It's one giant interstate battery.

Thank you in advance

Re: Boost Switched Unexpected Stopped Working

Reply #1 – April 27, 2025, 04:29:37 pm

Go to the boost switch. Remove one of the large-gauge wire and ADD IT to the lug with the other large gauge wire.

Electrically the same as boost switch working.

You can also try to resurrect it by tapping on it while someone turns it on/off/on/off/on.

Re: Boost Switched Unexpected Stopped Working

Reply #2 – April 27, 2025, 07:05:28 pm

Your boost solenoid switch may be powered off your chassis battery (check your wiring diagram to verify). If so, and if the chassis battery is fully depleted, it may not supply enough juice to power the boost solenoid. If convenient, you could connect a temporary jumper from a house battery powered source (isolator?) to the boost solenoid activation terminal.

Catch 22. You need the boost solenoid to provide assistance to the dead chassis battery, but you can't use the boost switch because the chassis battery is dead.

Re: Boost Switched Unexpected Stopped Working

Reply #3 – April 27, 2025, 08:40:27 pm

Foretravel uses a 2 diode scheme to allow the boost switch to work with either battery bank dead. Here is the schematic from our '91:

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Re: Boost Switched Unexpected Stopped Working

Reply #4 – April 27, 2025, 09:18:45 pm

Thanx Peter! I learned something today.

Re: Boost Switched Unexpected Stopped Working

Reply #5 – April 28, 2025, 07:16:14 am

Good Morning,

So I jumped both 2 gauge wires together on the left post of the solenoid and nothing not even dash lights. But when I use the jumper cables and keep the 2 gauge wires where they are, I hear a click but no dash lights.

One of 2 gauge wires reads 14 volt the other is right around 4. Is there any safe place to try and jump the chassis battery. The coach has lowered to the point I can't reach the chassis battery.

Thanks again

Re: Boost Switched Unexpected Stopped Working

Reply #6 – April 28, 2025, 06:45:15 pm

I'm thinking the 14 V side of the boost relay is the house battery bank. With that voltage will your generator start? If you can get that running, then turn on your inverter/charger. This will charge the house batteries, then jump across the boost relay which should then also charge the chassis battery. Maybe you can get the chassis battery charged enough to start the engine? Once the engine is running the alternator should then charge both battery banks and you can remove the boost relay jumper.

If the house batteries are too low to start the generator, jump to the house batteries, start the generator, then go to the steps above.

Re: Boost Switched Unexpected Stopped Working

Reply #7 – April 28, 2025, 07:52:16 pm

Hi, I can get the generator started all day because the house batteries are new. When I was trying to loosen the lugs off of the isolator, it ripped the post right out. Not too sure if that would impact the starting of the coach, but I need to replace it. I will try to use the jumper off the isolator and connect to the 14 V how much battery side to the chassis battery side to see what happens. Right now I have a 42,000 pound driveway blocker.

Re: Boost Switched Unexpected Stopped Working

Reply #8 – April 28, 2025, 07:53:16 pm

The boot switch was working then all of a sudden it just stopped and I was using the house battery side to assist the chassis battery and starting the engine. All of a sudden, nothing like almost a fuse somewhere.

Re: Boost Switched Unexpected Stopped Working

Reply #9 – April 28, 2025, 08:07:30 pm

Short term, wire "around" the battery isolator to give you one large battery bank with alternator charging.

Combine the cable from the alternator B+ to both of the battery bank cables.

Do NOT start the engine if there is a chance that the alternator B+ has no charge path/is not connected to any battery.

Re: Boost Switched Unexpected Stopped Working

Reply #10 – April 28, 2025, 08:17:21 pm

To add to Brett's idea, look for a small additional wire connected to one side of the isolator. That will be the alternator "sense" wire. This should be connected to the combined battery bank wires for the alternator to function properly.

Re: Boost Switched Unexpected Stopped Working

Reply #11 – April 28, 2025, 09:19:59 pm

Do you have an air compressor at your house. If so you can make up some fittings to supply air to the air tanks and then use the hwh panel to raise the coach. If your chassis batteries are only 4 volts, then that might not work. You might have to get a jack under the axle and raise the coach enough to access the batteries. Sounds like the chassis batteries have totally failed.

Re: Boost Switched Unexpected Stopped Working

Reply #12 – April 29, 2025, 06:05:38 am

Thank you, All. So I know just enough to be dangerous. I'm guessing that I should connect the large alternator wire that was coming into the isolator to the battery bank wires with one large bolt, completely bypassing the boost solenoid?

There is no little wire on the battery isolator it is just standalone with no smart sensing. Looks like the original one from 2000.

If for some chance, this still doesn't work, is there a fuse or a problem elsewhere then I'm not thinking of. The switch upfront looks just like a regular two position rocker switch. It's either on or off. And then in between starts it all of a sudden stopped working.

Unfortunately, there's no way to get the battery started, but I do have an air compressor at home if there is a way that I can boost up the airbags to get the coach off the ground then I could probably access the chassis battery enough to get it replaced

Re: Boost Switched Unexpected Stopped Working

Reply #13 – April 29, 2025, 08:06:57 am

Yes, connect the battery wires at the isolator, you can use any post to connect all three wires together. Then try running the generator with the inverter/charger for an hour, then see if the engine will crank.

The boost switch mystery can be addressed later.

Re: Boost Switched Unexpected Stopped Working

Reply #14 – April 29, 2025, 09:14:05 am

So... the isolator fell apart trying to losses the bolts.

UGH!!!!

Re: Boost Switched Unexpected Stopped Working

Reply #15 – April 29, 2025, 09:15:24 am

Looks like the center stud would be a good place to combine: B+ from alternator, wire from chassis battery and wire from house battery.

Re: Boost Switched Unexpected Stopped Working

Reply #16 – April 29, 2025, 12:22:21 pm

Thank you... give me a few. Crossing fingers

Re: Boost Switched Unexpected Stopped Working

Reply #17 – April 29, 2025, 03:28:39 pm

The dash lit up momentarily but nothing else. House battery must be really dead

Re: Boost Switched Unexpected Stopped Working

Reply #18 – April 29, 2025, 04:11:47 pm

If battery voltage is too low, most smart inverter/chargers will NOT attempt to charge them.

You may have to hook up a "stupid"/automotive charger to get voltage up to where the inverter/charger will take over.

What does your inverter/charger monitor panel show for charging amps when the generator is running/you are plugged into 120 VAC?

Re: Boost Switched Unexpected Stopped Working

Reply #19 – April 29, 2025, 04:20:37 pm

Another option to using a dumb charger to get the voltage up to start a smart charger would be jumping with another battery to get charger started.

Re: Boost Switched Unexpected Stopped Working

Reply #20 – April 29, 2025, 06:40:53 pm

I tried a car charger at the back end of the battery cut off, but the distance may have been too far. When I turned it on to low charge, I could hear it resonate up in the front dash.

Would the closest ChargePoint be off of the isolator to the chassis battery? If I could just get the airbags up, I could change the battery. I'm still looking for that port to raise it.

Another day unable to move out of the driveway.

Re: Boost Switched Unexpected Stopped Working

Reply #21 – April 30, 2025, 07:55:44 am

If you make a male to male air fitting, you can plug an air hose from a air compressor to the air couplings that is on your coach. I don't know if all coach s were plumbed that way. This should allow you to air up your bags, except the hwh controller needs to have power. The newer coach's have a location on the coach where you can air up individual air bags.

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