Wednesday, August 10, 2022


Ask Dave: Two questions: Microwave mystery and WD-40 on holding tank cable

Dear Dave,
I have a 2015 KZ Durango Gold Mod #370 fifth wheel and have two completely unrelated questions. We have been full-time RVers for three years now. I have questions about our microwave and about lubricating the holding tank cable.

Question #1: We discovered 3+ years ago that our full-size Samsung refrigerator did not run on battery power while attached to our Ram 3500 but the microwave did. An electrician buddy of mine discovered that the circuit was factory wired incorrectly. He ran a new wire down from the microwave electrical box to the refrigerator box. Problem solved. Now, while going down the highway, the clock on the microwave gains minutes. On a 6-hour run, it may gain 10-12 minutes. Short drives, fewer minutes, 2-5. Why would this happen?

Question #2: I have been spraying down the cable behind the handles for dumping the tanks with WD-40. This has worked remarkably well for the past three years, but I know inevitably the valves will have to be changed. I looked behind the utility board and it appears the cables terminated under the rig above the fully enclosed shroud. How tough is that to drop and can I just work from the middle of the rig forward and leave the front attached to do this alone? Thanks, Dave. —Dennis

Dear Dennis,
I am assuming your Samsung refrigerator is a residential compressor-driven model that runs on 120-volt power and is designed to typically run on an outlet wired to the distribution panel and shoreline power. The only way it can run off 12-volt power is through an inverter and a huge battery bank. You indicated the microwave does run off battery power while attached to the Ram 3500. However, I would believe technically it is plugged into an outlet that is attached to a smaller inverter in your fifth wheel.

Here is how a unit like yours is typically set up…

120-volt power comes into a distribution center from shoreline power or a generator. Circuit breakers are installed and wiring is routed to 120-volt appliances such as the refrigerator, roof air conditioners, and various outlets inside the rig. These all need power from the shoreline or generator to the distribution center to work. Your house batteries are 12-volt deep cycle. They have automotive-style fuses with wiring to provide power to the lights, roof vents, water pump, and other 12-volt DC operations.

I would also assume you have an inverter installed in your rig. It takes 12-volt power from the batteries and provides 120-volt power to smaller appliances such as outlets for the TV and, evidently, your microwave. Here is a schematic of a Winnebago with a 1,000-watt inverter that is wired to several outlets in the kitchen area and even an exterior outlet.

If you had an electrician friend run a wire from your microwave outlet to the refrigerator, it will draw power from the house batteries and drain them fast! Most inverters will have an electrical wire such as this one, so when you are plugged into shoreline power, it will sense the presence of 120-volt power and pass it through.

Get a wiring diagram

I would suggest getting a wiring diagram for your rig and finding out what type of inverter you have before going any further. I doubt your house batteries and engine alternator can keep up with this type of power draw. And I don’t believe this is a recommended “fix.”

You will need to find out the draw from your refrigerator and the size of your batteries to do the math. I would also suggest taking a look at Mike Sokol’s articles about residential refrigerators and power draw.

Why is the microwave gaining minutes?

The microwave gaining minutes driving down the road with this type of electrical configuration is a mystery. I would suggest researching the proper connection and power consumption first.

Question on holding tank cable

As for the second part, Valterra and other valve manufacturers do not recommend WD-40 as it will remove the originally applied lubricant and eventually fail. They recommend a dry lube such as CRC or other silicone that is applied wet and then dries. If you lubricate it properly and add valve lubrication by Thetford a couple times a year for the inner seal at the spade valve, you should not need to replace it.

Many trailer manufacturers use the same type of pull handles with a long cable that disappears into the false underbelly or behind a shroud. Check out the recent Q&A here and the great comments regarding drilling a hole in the case of the valve and applying lubrication. I am not familiar with your configuration of the service center and shroud that you have referred to. However, if you can get to the main valve body like the one shown here, you should be able to lubricate the cable or shaft at the body.

Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club.

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3 months ago

I have a 2018 Hemisphere 5th wheel with a residential fridge wired to an inverter with 2 deep cell batteries.

With freezer full and fridge items already cold I can go 8 hours in 85 degrees. You would be surprised how little the compressor actually runs if everything is all ready cold.

3 months ago

I would offer a comment on the difficulty of dropping a portion of the underbelly shroud. I found that ratchet straps, extended from I-beam to I-beam, provide support for a sagging underbelly. I don’t see why these straps, judiciously placed, cannot be used to control the shroud’s drop and allow you the access you need. Once the job is complete, you should be able to raise the shroud with the ratchets to the extent you can tuck the shroud back behind the I-beam lip. I left my straps in place.

3 months ago

You advised getting a Electrical Schematic for a fix.
I have a 2021 39’ Bighorn, I asked the dealer I purchased it from ( they didn’t have one) and they advised me to contact Bighorn, I did and They said “ WE DON’T HAVE ONE.” How can this Company wire a 5th wheel without this? They must think I’m going to build my own RV, I just want to correct lights and electrical receptacles.

3 months ago

I am a little surprised a qualified electrician would make such a modification to a system without knowing all the specs of that system and the possible implications of the change on attached appliances (eg: potential damage to the refrig compressor, battery drain, etc.)

Jumping to a conclusion that the factory wired it incorrectly was the first mistake. It was wired exactly as it should be. The jumper he installed should be removed and a correct solution for the refrigerator installed, which may mean some $$$ for a second inverter and a battery bank.

Bob p
3 months ago

SIL had a 42’ 5th wheel with what looked like a residential refrigerator but actually was gas/electric. He wanted to eliminate running with gas going down the road, he found the electrical requirements of the fridge, bought a full sine wave inverter to more than handle the load, hooked it up to where the fridge ran off the inverter all the time. When driving his dual alternator truck easily handled the load, when in camp the converter kept the battery charged. I don’t know if it was right, but it lasted 2 yrs before he sold it, the new owner says it’s still working 2 yrs later.

Bob S
3 months ago

The inverter is likely not a pure sine wave output. A square wave or modified square wave inverter will cause the digital clock to run fast. There also may be a risk of damage to the refrigerator compressor motor if it is not run with pure sine wave power.

Don Woods
3 months ago
Reply to  Bob S

Digital clock time depends on power frequency maintaining exactly 60 Hz. If running fast, the frequency provided by inverter is greater than 60 Hz.

Bob Palin
3 months ago
Reply to  Don Woods

Exactly, this happened all over Europe a few years ago, the power grid had a fault that slightly changed the frequency and clocks everywhere gained time. I wouldn’t believe my mother in England because I believed the frequency was too closely regulated, but she was right (I hate it when that happens, and it’s all too often!)

Pierre Woody
3 months ago
Reply to  Don Woods

Exactly what I thought, once again, Dave didn’t pick up on it!

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