Tuesday, November 28, 2023


Ask Dave: When I exercise the generator, can I leave the shoreline cord plugged in?

Dear Dave,
I understand that I should exercise my generator at least 2 hours per month, and at least at 50% load. If I am hooked to 30-amp shore power and start up the generator to exercise it with AC on, is that sufficient / correct? In other words, is it okay to leave the RV hooked to shore power while exercising the generator? Or should I disconnect while exercising the generator? Thanks for your response! —John, 2018 Leisure Travel Van Unity TB

Dear John,
According to the specifications for your 2018 Leisure Travel Van, the generator options were a 3.2 KW Onan LP or 3.6 KW Onan Diesel.

What an Automatic Transfer Switch does

Since your Mercedes drive train is a diesel, most owners opt for the diesel generator so they can use the diesel fuel in the chassis fuel tank. What I do not see in the specifications is a listing for an Automatic Transfer Switch (ATS), which will sense when your shoreline power is connected to the campground source or running the generator.

If your rig has an ATS, the generator is wired directly to the relay as well as the shoreline cord. If the generator is running, the relay automatically switches to that source of power. So, yes, you can leave the shoreline cord plugged into the campground source while running it.

A “J” box is an outlet wired to the generator

If you do not have an ATS, then you have what is called a “J” box. That is an outlet wired back to the generator, and you need to plug your shoreline cord into the outlet to get power from your generator to the distribution center.

In this situation, you can still leave your shoreline cord plugged into the campground source while exercising the generator. The power will just go to the box and not to the distribution center.

Gasoline-powered generators are prone to varnish conditions if not exercised often. Owners should stay away from ethanol blends of more than 10%. However, diesel and propane generators do not have such issues. Onan recommends running once a month at 50% load for 2 hours. According to the service manual:

Exercising a generator set drives off moisture, relubricates the engine, replaces stale fuel in fuel lines, and removes oxides from electrical contacts and generator slip rings. The result is better starting, longer engine life, and greater reliability.

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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Dave Solberg
Dave Solberghttp://www.rv-seminars.com/
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. He has been in the RV Industry since 1983 and conducts over 15 seminars at RV shows throughout the country.



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Jesse Crouse (@guest_177381)
1 year ago

If I have learned anything in 53 years in the mechanical trades is “any piece of equipment can fail just when it should not”. So out of an abundance of caution I always unplug from shore power when switching on the generator. We all know ATS never fail.

Spike (@guest_177287)
1 year ago

Another thing to keep in mind is to not have any heavy draw items, like an AC, running during the transfer. If you are on shore power and have these kinds of things running, shut them down and then do the transfer. Once the transfer is complete and the genset has run a few minutes you can restart those items. This allows things like AC’s to go through their normal stop and start procedures and gives the genset a few minutes to warm up before putting a big load on it.

Last edited 1 year ago by Spike
Leonard Rempel (@guest_177281)
1 year ago

When will you do a test on a REDARC DC-DC charger? I currently use the 50 amp version which also doubles as a solar charge controller.
This small unit also eliminates the need for any type of generator.
With California proposing legislation to ban portable generators, this seems like a fantastic option.


Tom H. (@guest_177350)
1 year ago
Reply to  Leonard Rempel

Interesting. I wonder if Mike Sokol has any comments about this charger?

Left Coast Geek (@guest_177361)
1 year ago
Reply to  Leonard Rempel

pulling 50 amps off most vehicles 7 blade RV connector isn’t going to work real well. I added a similar Victron Orion-Tr SmartCharger 12/12-18, which is an 18 amp DC to DC charge controller that can be configured for lithium batteries. 18 amps at 14 volts into the LFP can require like 25 amps input at a dragged down 10 volts, which is what in fact I see with my older F250 longbed diesel pickup by the time its made it to the back of my trailer where my electricals are.

Left Coast Geek (@guest_177362)
1 year ago

oh, and 50 amps at 12-14VDC isn’t going to run any sort of air conditioner, which is the main justification for a discrete generator.

Greg S (@guest_177271)
1 year ago

If the RV is equipped with generator autostart along with the automatic transfer switch, make sure autostart is enabled. When it comes time to exercise the generator, go out and flip the power pedestal breaker off. The generator should start with the loss of power.
Now you have exercised the generator and made sure the autostart system is working as well.

Dave (@guest_177273)
1 year ago
Reply to  Greg S

Unless you keep the RV in a building, in which case you must NEVER enable auto Gen start for hopefully obvious reasons.

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