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RV Doctor ColumnRV Doctor #334 Dear RV Doctor, I have an oldie, a 1973 Lifetime motorhome with a Dodge chassis. It looks like the batteries are connected together with one 10-guage wire from the alternator charging circuit going to some kind of box. Another 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 is rebuild a five foot section of the wiring harness because of a short circuit that melted that section of the harness. 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 with three electrical contact studs which form a normal triangle. What exactly is this box? Wilfred Taylor, (Elizabethtown, KY)
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 totally 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 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) receives voltage from the output of the alternator and each side terminal should be wired directly to each battery system. The key is to keep each system "isolated" from each other; hence the name. To test the existing dual battery isolator, 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. Another method of battery separation is accomplished by the use of "smart" devices such as the Sure Power Industries Smart Solenoid. These devices incorporate a high capacity, electronically controlled solenoid switch within a well monitored charging system. The Sure Power Smart Solenoid comes in two varieties; one begins charging the auxiliary system only after the engine battery has reached a minimum 13.2-volts. The other couples the two systems together in parallel when either battery has reached this pivotal voltage. Until then, the battery systems are kept separate. This is a much better alternative than a standard solenoid, but I personally like to see each battery system completely isolated from each other all the time.
Dear Gary, my problem is that when I connect to the campground water system, my potable water holding tank slowly fills up. 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 allowing this to happen. I did notice that the main water line tees off to the pump. Any ideas? Craig Hugh, (Eatonville, WA)
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 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 if your coach is so equipped.
Dear RV Doc, I have a question but first some background on the question. A friend showed me the starter from his old DuoTherm 65930 furnace. The starter has three thin probes; the first is attached to a insulated spark plug unit, which I assume is the lead from the capacitor. Very near to this one, is the second one which is attached to the frame. It has to be the ground, right? The third just looks to be the grounding probe for some solenoid. But here's my question. The third probe, is raised above the burning area, therefore does the gas act as a conductor or is the flame the conductor? If either of the answers is yes, then it sure is something I never knew about in my 68 years of life! Edward Elliott, (Victoria, BC)
Wow Edward, you sure dug up an oldie-but-goodie! It just proves a point I've been preaching for years; that well maintained appliances and motorhomes can literally last for years and years! What your friend showed you was the electrode assembly of the furnace. There's an electrode assembly found in virtually every DSI (direct spark ignited) appliance. Some electrode assemblies only have two probes, but many of the older ones had three. As you guessed, one probe is the high voltage ignitor probe that receives the spark from the printed circuit board. When the voltage jumps the gap between this probe and the center, ground probe, it ignites the LP that comes into the burner. The third probe is called the "flame sense probe" and indeed it "senses" the presence of the flame by measuring the micro-amps produced by the flame and sends this current back to the printed circuit board, letting it know the burner is still lit. If the flame were to blow out for any reason, it's this flame sense probe which tells the board to turn off the gas solenoid. This is but one of the safety devices incorporated in the LP appliances. It's important that all three probes are kept clean and free of corrosion and carbon buildup. It's something that must be checked annually. Also important is the gap spacing between the three probes. Though some appliances may differ, a general rule of thumb is that the gap between the ground probe and the flame sense probe be twice as much as the gap between the ground probe and the spark probe. You certainly do not want that spark jumping to the flame sense probe. You'll be replacing the printed circuit board if that happens! I've always asserted that if the electrode assembly is kept clean and gapped correctly and the LP pressure is set correctly, that the majority of furnace operational problems would be eliminated.
Dear RV Doctor, can (or should) the 12-volt negative wires at the Parallax converter 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. Mike Purdy, (Boise, ID)
Mike, yes, the 12-volt negative terminal block on every converter should be grounded to the same metal component as the negative cables on the batteries themselves, usually the vehicle frame. It does nothing, (but keep them in one place), to have them screwed to a wooden floor. In addition to the negative 12-volt conductors, the metal converter chassis should also be "bonded" to the frame using an 8-gauge conductor. This conductor may be uninsulated and it could also be a solid copper conductor. The negative 12-volt conductors, however, must be stranded. Now 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 VOM will confirm this.
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