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Kliwks · 05-06-2025, 08:45 AM

This is a followup to my request for schematics. 2018 Dynamax Force HD 37TS. When the coach is plugged in everything works. When I unplug shore power, anywhere from 5 to 15 minutes I lose the 12 volt supply to the main solenoid and the solenoid drops out. This provides power to the battery disconnect solenoid as well. So when the main solenoid drops out, I loose 12 volt power to my lights and pretty much everything that runs off of 12 volts. Battery disconnect switch does not operate, battery disconnect solenoid does not pull in. If I plug the coach back in the 12 volts is restored to the main solenoid and it send power to the battery disconnect solenoid and everything works, including the disconnect switch inside the coach. Any electrical wiz's out there with any insight on this issue? It is a strange one.

Skip426 · 05-06-2025, 07:16 PM

Have you load tested the coach batteries ?

Kliwks · 05-07-2025, 04:49 AM

I have not. I still have 12.7 on house batteries when I loose the 12v on the solenoid. I will try to start the generator after the solenoid drops out. I would think if they can start the generator they should be good enough to pull in the solenoid. That being said, They are original batteries. Thanks for the reply.

Skip426 · 05-07-2025, 10:58 AM

I can't see enough of the " main solenoid " to figure out it's type .
Does it look like the photo below ?

Also info on testing the disconnect .

Skip426 · 05-07-2025, 11:02 AM

Or perhaps one of these?

Kliwks · 05-08-2025, 05:02 AM

Not the best picture. This main solenoid was replaced last year because it was bad. It would engage, but not pass the 12v.

Kliwks · 05-09-2025, 04:20 AM

Followup on my solenoid issue. Began troubleshooting again yesterday. Decided to test the two 250A fuses that come off of the batteries. (See pic) And sure enough, one of them was bad. I had a replacement, and this resolved the issue. The one burning question is, why did it take anywhere from 5 to 10 minutes for the problem to show up after you unplugged from shore power??? If a fuse is bad then the problem should be immediate. Anyway, thanks for the responses. See you on the next problem.

Skip426 · 05-09-2025, 09:39 AM

I've only ever seen that fuse type used in inverter power supply circuits , and I have no idea why 2 would be necessary .

How many watts is your inverter ?

Kliwks · 05-10-2025, 04:30 AM

Not sure as this is my friends rig. I was just doing my part the help him fix it. He's not a real computer guy, so I posted for him to try to get his issue resolved. He does have a residential fridge, so I assume its a pretty big inverter.

Skip426 · 05-10-2025, 10:24 AM

I wonder ; given the fuse set up ; if there are two inverters , one inverter only for the residential fridge, and an inverter/charger for the rest of the RV and battery charging .

Still would not explain the delay in 12 volt drop out ; unless inverter low battery protection is involved in the shut down.

Anyhow good to hear you got things working ; don't forget to get more spare fuses.

Kliwks · 05-11-2025, 05:29 AM

Only one inverter. But there was this little black box labeled Bi-directional relay delay. We were able to verify that the 12v that feeds the solenoid runs through that box. I’m sure that has something to do with the delay. Yes, he’s already ordered 2 of those fuses. $100 each. Ouch!!!