Need help replacing charging circuit solenoid.

So this weekend we had a break in the rain (finally) and I got to look at the solenoid. Before I replaced it, I wanted to see if, when I pushed the switch to the MOM position, if I was getting full voltage across the solenoid.

So I disconnected the engine battery, and then I first measured voltage of the House batteries at the house post on the charging solenoid. It was like 12V. Then I had a helper hold MOM and I measured the voltage on the other side of the solenoid. It was the same.

So, the existing solenoid seems to be functioning fine.

So I went and had a look at the engine starting solenoid. And I discovered that the positive terminal nut was loose. Not flop-around loose, but I could pull on the battery cable lead and easily turn it on its post. So it's quite likely that I had a poor engine battery connection at the positive post of the engine start solenoid.

I pulled all the terminals off the post, wire brushed them, and put it all back together and snugged it down good and tight.

"Nope. When the rocker switch is in the middle position (it is 3-way), then the solenoid does not work at all. Rocker must be in MOM or DUAL to energize solenoid"

OK that explains it all. In DUAL it has to also close the solenoid which is NOT already on the run circuit like mine is.

It also explains why there is no Y in the solenoid's power line like I thought there must have to be.

BTW that DUAL would be handy for me, where mine is always in DUAL when the engine is running. I sometimes want to have no alternator charging on the way home so the AGMs will be low enough to accept a 20% charge rate, which they are supposed to get now and then. I need to install a switch in the line from the solenoid to house batts so I can turn that off before heading home.

When the rocker switch is in "OFF", the solenoid works from the run circuit and key.

Nope. When the rocker switch is in the middle position (it is 3-way), then the solenoid does not work at all. Rocker must be in MOM or DUAL to energize solenoid.

In key postion "ON" (but not in "START") you are already in "DUAL" (at least in my rig which has no rocker switch)

If you have no rocker switch, then I don't know how yours works.

With the rocker, it must mean you don't get "DUAL" just from powering the solenoid, you need to push "DUAL" too. But why should that mean the rocker needs to get any power from the run circuit to pass along to the solenoid, which is already in the run circuit?

The rocker is a 3-way switch. In the middle, it's off. When locked in Dual, it routes vehicle run power to the solenoid. When held in MOM, it routes house battery power to the solenoid.

The rocker switch will "stick" in the middle position (off), and in DUAL. It's spring loaded in MOM, so you have to hold it in MOM.

Does that mean the rocker itself needs 12v power to go from "OFF" to either MOM or DUAL?

12V power is what the rocker switch diverts to the solenoid. It's either getting 12V power from the house battery (MOM), or it's getting 12V power from the engine battery (DUAL) when key is in RUN.

All the 3-way switch is doing is letting you choose the power source for the solenoid. That's it. DUAL picks up power from the engine battery (if ignition is in RUN). MOM picks up power from the house battery. And of course the middle position is "OFF".

When the rocker switch is in "OFF", the solenoid works from the run circuit and key.

In key postion "ON" (but not in "START") you are already in "DUAL" (at least in my rig which has no rocker switch)

With the rocker, it must mean you don't get "DUAL" just from powering the solenoid, you need to push "DUAL" too. But why should that mean the rocker needs to get any power from the run circuit to pass along to the solenoid, which is already in the run circuit?

Does that mean the rocker itself needs 12v power to go from "OFF" to either MOM or DUAL?

Maybe this is a simpler schematic I drew up:

enblethen is right.

Hopefully this will put the nail in it:



"Battery Changeover" on the schematic is the rocker switch. I have labeled MOM and DUAL. The center common wire goes to the Charging Solenoid (they call it Battery Solenoid).

When in MOM, it's pretty simply - the Aux Battery (House Battery) is routed to the Charging Solenoid and gangs the batteries together. Done deal.

When it's in DUAL, power comes from the "Fuse Panel". This is a 4-pin yellow plug that simply plugs into a spot on the OEM vehicle fuse bus panel that is hot when the key is in RUN.

That's it.

All the rocker switch does is energize the Charge Solenoid either from the House battery (MOM), or from a RUN hot circuit in the vehicle fuse panel.

Steve

I might be confused!
Center common contact goes to the control wire
MOM goes to coach battery
Dual mode goes to the chassis battery

The emergency start switch does not interrupt the run circuit on the chassis in general; it's only controllign power to the solenoid that connects the batteries. One option for where it gets its power is from this run circuit, the other is from the house battery. The rest of the run circuit, and the key switch's control of it, is not changed.

The blower is stopped by the key switch alone. The combiner solenoid is also controlled by the key switch if the emergency start switch is not pressed (and, apparently, if the DUAL switch is engaged, provided you have such a switch...I don't on my motorhome). The idea is to use the existing circuit, controlled by the key switch, to also control the solenoid except for the odd circumstance where we want it controlled independently; so we put in a switch to select between the usual control via the run circuit from the key switch, or the house battery for what amounts to manual control.

If the emergency start switch is not pressed, then the batteries are indeed not connected together when the engine is cranking, and the chassis and all its appertenances is getting power solely from the chassis battery. Once the key is released they are (re)connected together, provided the DUAL switch is engaged if you have one, and the alternator charges both batteries.

If the emergency start switch is pressed, the two batteries are connected in parallel as long as it's pressed, regardless of the key switch position, and the starter and other chassis circuits can get power from both while cranking the engine. The rest of the chassis circuits are still controlled by the key switch as normal; electrically, this is precisely equivalent to hooking up jumper cables between the two batteries.

" The common connection on the switch goes to the solenoid's control wire, the normally closed to the chassis run circuit,...…"

If that then opens the run circuit (to stop the blower etc) then why, when turning the key to Start does that already?

(Assuming once the solenoid is passing house battery voltage to the engine side, the run circuit will just think it is getting engine battery voltage as usual.)

I only figured it out like 2 days ago. :)

Steve