Cole Hersee Continuous Duty Solenoid
You might be wondering what the heck this is, but the odds are there is one like it somewhere in your rig. In my current rig, it's in the Battery Control Center (BCC) — a black box in the compartment above the propane tank. On some rigs, it's in the engine compartment.
What this solenoid does is connect the house batteries and the chassis battery in the engine compartment. There are three reasons for connecting them. One is so the chassis battery will charge while you are plugged into shore power. Another is so the house batteries will charge when the engine is running and you're *not* plugged into shore power. The final reason (on most rigs), is so you can press a toggle switch on the dash to connect them so you can start the rig from the house batteries if the chassis battery is weak or dead.
It's not a question of "if" this solenoid will fail — it's a question of "when". The OEM solenoid is usually not a very good one. It will definitely last until the rig is off warranty, but after a few years, you're on borrowed time.
There's also a fatal flaw in the way it works. If the solenoid fails while you're on shore power (and the odds are that's when it will go bad), you won't know it until the chassis battery is dead. When you try pressing the toggle switch on the dash to start the rig, nothing happens because the solenoid isn't working.
Mine failed recently and the first symptom was that the lights on my dash stereo started blinking frantically and there was no way to stop it. Apparently, some engineer designed the stereo to warn you when it receives low voltage. Unfortunately, at that point the chassis battery was too weak to start the rig.
The bottom line is that it's a good idea to carry a spare. Don't be fooled into buying (or letting a repair tech. install) a solenoid that is not rated for "continuous duty".
The Cole Hersee continuous duty solenoids are solidly built and are well-respected in the RV community. The less expensive one above has copper contacts, like the OEM solenoid. The more expensive model has silver tungsten carbide contacts and should last considerably longer.
If yours is buried at the back of the BCC like mine, replacing it can be a challenge. Be sure to disconnect the ground wire from both the chassis battery and the house batteries before you even take the cover off the BCC. Then, you need to label, then disconnect, the three large cables that go to the outside of the box and loosen the nuts on them so they have some play. Then, take the nuts off of the vertical bolts going through the silver bus bars inside the box. Finally, remove everything in front of the solenoid, put in the new one and put everything back where it was. It's not as bad as it looks at first — just keep track of where everything goes. Taking some pictures along the way can help.
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— Bob Ray