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RV Doctor ColumnRV Doctor #309 Dear RV Doctor, I have an isolator problem on my RV. On a recent trip the isolator burned out, so my neighbor, a truck mechanic, got me one from a truck shop. This one works find but on a recent trip we lost charging again. This turned out to be the 20-amp fuse that is positioned just before the isolator. We had a big rain storm on the trip and there was rust and moisture in the fuse and it did not have continuity. I sealed the fuse and put it in its place and then put silicon caulk around it. Think this will work? Doyle Heimann, (Westmont, IL) Doyle, I'm a little confused as to the location of the fuse. Is it on a typical fuse panel? Or is it an "in-line" fuse holder? And if it is "just before the isolator" it is certainly undersized. Alternator outputs may range from 65 amps to over 200 amps depending on the make and model. A 20-amp fuse does little good on either side of the isolator circuitry. It will blow as soon as you start the vehicle. You may want to eliminate that fuse altogether, or install a heftier one if you really want fuse protection at that point. If so I would opt for a sealed, weather-proof, in-line fuse holder instead of a typical fuse panel of some sort. The marine industry uses some very fine, heavy current fuse holders. They are typically not used in the RV industry, so you may have to visit a marine-type service facility to obtain one. My guess is the isolator burned out because it too was undersized. Be sure to install an isolator with a current-carrying rating higher than the output capability of the alternator. When sized properly, no fuse is required to protect the isolator. If you still feel protection is needed, try using an automatically resettable 12-volt circuit breaker, again sized accordingly. That way if a surge from the alternator approaches the limit of the diodes in the isolator, the circuit opens, but after cooling will reset so alternator charging can continue. Dear Gary, I purchased an RV last year. It's a 1989, in good shape. However, there are a few things to fix. When I'm traveling I find that the odor from the bathroom overwhelming. I've put water in the toilet bowl to see if the seal is gone and the water disappears immediately. There seems to be a seal problem. Do I have to buy a kit or should I replace the toilet? Is it a question of a seal? I'm sorry I don't have the manufacturers name available because the RV isn't at home. Gilles E. Landry, (Azilda, ON) Gilles, indeed it appears the main bowl seal is no longer viable. At that age, it's probably long overdue to have all the seals replaced in the toilet. Most toilet manufacturers produce a packaged seal kit with all the parts and instructions provided. It's not the most pleasant task to perform, but if you are handy with common hand tools, it is relatively easy. The pros will charge anywhere from one to one and one-half hours to do it for you so you can save some bucks if you do it yourself. Toilet repair kits are reasonably priced and are readily available at any well-stocked RV supply store. You will, however, need the brand name and model to obtain the correct replacement kit. To aid in evacuating holding tank odors, you may want to take a look at a product on the market called the RV 360. My first impression was positive. Learn about them at: www.rv-360.com. Dear RV Doctor, I have a question about the placement of fifth wheel hitches in pickups. Why are they placed directly above the axle, and up to 1" ahead of the axle and not placed up to 6" to 8" ahead to get some weight on the front steering axle like they do on semi tractors? My plan in the next year is to start designing and building a fifth wheel travel trailer. I would like to know why and where some of these specifications come from. I would appreciate the input. Fred Keith, (Birch Run, MI) Great question Fred. There are two reasons for the recommended hitch location in the bed of the pickup; weight and turning requirements. Obviously you know about the weight distribution; behind the axle would be a definite no-no. Six to eight inches ahead of the rear axle would be good as long as you do not hit the rear of the cab with the corners of the trailers on tight turns or while backing into a site. The narrower the trailer, the further forward you can place the hitch. Here's my recommended formula for hitch placement: The first step is to measure the width of the trailer. Then divide that number by two, and add two more inches to the total. Next, measure back from the front of the bed and mark that previous total. This position is the centerline location of the hitch. Here's an example: Your fifth-wheel trailer is 96" wide at its widest point. Half of that is 48"; adding two inches equals 50". Measure back from the front of the bed 50" (remember, not the back of the cab, but the front of the bed). This method will ensure you will have plenty of clearance on tight turns. Dear RV Doc, I was interested in the question from Ray Fowler in an earlier column about AC polarity. You didn't explain what to do if indeed we discover the polarity is wrong. Usually it is late in the day and no one wants to move on to a different RV park. What to do? Jim Ammons, (Pueblo West, CO) Jim, if reverse polarity is discovered and you do not wish to relocate, simply do not plug into the park power. Doing so could be potentially dangerous and harmful to you and your on-board components. Notify the park manager and see if another nearby site is wired correctly. It is entirely possible that only a few sites are affected. If not, stay on battery power until you can move. Of course, if your RV is so equipped, a generator and/or inverter can provide A/C for your use. Remember, most parks have restrictions about running a generator after certain hours. Never compromise the safety factor! Reverse polarity can be lethal. Always measure the voltage and check the polarity before you connect your RV at every park.
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