House Calls with the RV Doctor
By Gary Bunzer
Dear RV Doctor:
I have an oldie, a 1973 Lifetime motorhome with a Dodge chassis. It looks as though the batteries are connected together with one 10-gauge wire from the alternator charging circuit going to some kind of box. A wire supplying 12 volts from the auxiliary generator and one from the batteries goes to the same box. Does this sound right? I just bought the motorhome and the first thing I had to do was rebuild a 5-foot section of the wiring harness because of a short circuit that melted that section. I am a good mechanic but don’t know anything about dual charging circuits. The box I am referring to is square in shape with aluminum fins on either side and three electrical contact studs that form a normal triangle. What exactly is this box?
RV Doctor: Wilfred, the device you have located is called a dual battery isolator. It is constructed with two diodes, which allow the alternator to charge both battery systems while keeping them separated. The two batteries should not be connected together for the simple reason that each battery is designed for a different purpose; one to start the engine, the other to power the 12-volt circuits inside the motorhome.
Typically, an automotive start battery is used to crank the engine, and a deep-cycle or RV/marine battery is used to power the house portion of the RV. By design, these two battery systems must remain separate. The dual battery isolator performs that function. The center terminal (or top of the triangle formed by the three studs) receives voltage from the output of the alternator, and each side terminal should be wired directly to each battery system.
To test the existing dual battery isolator, verify that its output voltage is above 13.5 volts (if lower than 13.5, replace the isolator), then connect an ohmmeter from the center terminal to each of the battery terminals. The meter should indicate continuity only in one direction from the center (alternator) terminal to each battery terminal. Reverse the test leads to verify it’s only in one direction. There should be no continuity whatsoever between the two battery terminals. The isolator is faulty if you can read continuity in both directions between any two terminals or if you have continuity between the two outside terminals. When replacing a dual battery isolator, be sure it is rated higher than the total output of the alternator.
Dear RV Doctor:
Can (or should) the 12-volt negative wires at the Parallax convertor station on the coach be grounded to the metal chassis of the RV? And won’t this be a problem? I’m doing some remodeling on the electrical system, and I noticed all the DC wires on a single lug block screwed to the wood of the floor. I thought this was odd.
RV Doctor: Yes, the 12-volt negative terminal block on every convertor should be grounded to the same metal component as the negative cables on the batteries themselves, usually the vehicle frame. It does nothing (other than keeping them in one place) to have them screwed to a wooden floor. In addition to the negative 12-volt conductors, the metal convertor chassis should also be “bonded” to the frame using an 8-gauge conductor. This conductor may be uninsulated and it also could be made of solid copper. The negative 12-volt conductors, however, must be stranded.
Now, with all that said, it is entirely possible that even though all the wires you see going to that single terminal strip are attached to the wood floor, one of them might actually go down to the frame, thereby making the connection and grounding those circuits. A simple continuity test with a volt/ohmmeter (VOM) will confirm this.
Dear RV Doctor:
My problem is that when I connect to the campground water system, my fresh water tank fills up slowly. This takes about 36 hours. Once the tank has filled, water overflows onto the ground, so I have to shut off the main water, drain the holding tank, and start all over again. I have been told that my fresh water pump may be faulty and allowing this to happen. I did notice that the main water line tees off to the pump. Any ideas?
RV Doctor: Craig, indeed it is possible that city water is somehow passing through the check valve located in the outlet of the water pump. Most pumps today have an internal check valve or backflow preventer, but some may be external and replaceable. Look closely at the outlet of the pump for an additional fitting that may thread into the pump itself. In some cases, it may be necessary to replace the pump head or even the entire pump to fully rectify this problem. Check with your owner’s manual and the pump maker to be sure. But you could always simply install another backflow preventer at the outlet end of the pump. A second backflow preventer will stop any water before it even reaches the first one.
Aside from the water pump, some motorhomes are equipped with a quick-fill valve that allows the fresh water tank to be filled via the city water connection. This manual valve may be slightly open or faulty, thereby allowing city water to overfill the fresh water tank. A close inspection of the fresh water plumbing system will reveal whether your coach is so equipped.