How do I use the Dual MOM Battery switch?

[Detect15]

Sent: 9/9/2003

My 78 Winnebago Brave has a dash switch labeled BATT.  It is a toggle switch that can be switched to the "Dual" position or on the right it can be switched to the "Mom" position, but returns to a neutral position unless switched to the "Dual" position.

I have no owners manuals, and am not sure what position this should be in while running the engine to charge the coach batteries.  Also what position to leave the switch in while hooked up to A/C.

Another related question is that I had my Onan Gen Set rewired because it was sending too many volts to the electric fuel pump, causing the fuel pump to burn out, while running.  I seem to have plenty of juice in the coach ie..lights, heat but since the Gen Set was rewired, the batteries don't have enough juice to start the generator, unless I'm plugged in to A/C.  Prior to having the generator rewired, I had no problem starting it with battery power.

I'm confused, your replies would be greatly appreciated. Thanks

Earl 

[Boat Nut]

Sent: 9/9/2003

The use of the "DUAL/MOM" switch is often asked question.

1. If parked, (ignition off) it makes no difference how the switch is selected.
2. If driving, without the generator running, select "Dual" position. That will also charge the coach battery.
3. If driving, with the generator running, select the center "Off" position. Then the engine alternator will not be fighting the generator to charge the batteries. Sometimes the alternator looses the battle, and sacrifices it's diodes.
4. If attempting to start with a low engine battery, pressing & holding in the "MOM" position while cranking will give a temporary jump start from the coach battery (which is hopefully in better condition).

I don't know an answer to your Genny problem, but it sounds like the mechanic did more harm than good. Find another mechanic, to correct the work of the first.

Chuck

[Photoman]

Sent: 9/9/2003

On my '72 C20, my battery switch is marked dual on the top position, batt on the center position, and mom (momentary) on the bottom position.

If I've had the switch on the dual setting to charge both batteries on the way home from a trip, and I forget to switch it to batt, or center position when I park, both batteries will drain down over a few days time to where there isn't enough
juice left to start her. I don't think it's supposed to work that way, but that's the way it's always been since dad bought her in 1972.

I don't have a generator, so that doesn't enter into the equation. I wrote myself a reminder on a note card & taped it to the dash so it hangs over the ignition switch (yeah, the ignition switch is on the dash on the Chevy chassis) so I can't pull the key out without seeing the reminder note. I also wrote on the note card to unplug before driving off. Bet I'm not the only idiot that's ever done that?

Hard to find extension cords over 100 miles long. LOL Photoman

[Myk-dK]

Sent: 9/10/2003

On some of them they are wired incorrectly & will still hold both batteries together with the ignition off, & if you park for 3 or 4 days the amp draw from the solenoid will kill the batteries. The solution is to wire the switch so it gets its power from a wire thats switched by the ignition.
mike

[DaveVA78Chieftain]

Like the posts above say, the DUAL-NORM-MOM switch should not work when the Ignition switch is off.
Power to the DUAL post on the switch should come from the Ignition switch in order to prevent depleting both batteries by the battery relay when parked.
Additionally, the MOM post should come from B+ so that it gets maximum voltage when starting if needed.  Even so, if the starter battery does not have enough juice left in it to engage the isolation relay the MOM position will not work.
As far as low voltage electrical problems, be aware that on a Dodge chassis, many of the shared B+ leads for starting/Charging are joined (soldered) together at the rear of the engine under the electrical tape.  That joint is subject to corrosion which can cause issues.
Here is a simplified drawing of how this all is connected on a Dodge Chassis.
Dave Bailey

[ClydesdaleKevin]

Awesome diagram!!!

Kev