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Old 04-23-2017, 09:27 AM   #1
Join Date: Apr 2017
Location: Pismo Beach, CA
Posts: 49
Noticeable Buzzing from Power Inverter When Plugged in to Shore Power

Hello all, I'm having a strange issue with a new-to-me fifth wheel. When I purchased it, the batteries were completely dead. When we plugged it into shore power, none of the 12v accessories would work. I did some digging and found the power inverter wasn't plugged into its outlet under the coach, so I plugged that in and heard a fairly loud buzzing. I thought it was because there were no batteries, so I dismissed it initially.

Fast forward a couple of months to the point that we are starting to work on the RV to get it ready to use. I get batteries in it, and everything is good, except when we plug it into shore power with the power inverter plugged in. It's not that it isn't working, it is just buzzing like the dickens; very noticeable inside of the coach even. I don't know if this is related, but the microwave doesn't come on unless the coach is plugged into shore power, the generator doesn't power it on.

This sounds to me like the inverter is bad, but I wanted to see if there was something else I might be missing. Thanks!

EDIT: After doing a little digging on a replacement, this might be a converter instead of an inverter. I'll try and pull the unit out (only has a thick red, black, and the power cord) and get some info from it. If this is the case, would that only convert from 120v to 12v, or does it work the other way around as well?

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Old 04-23-2017, 09:58 AM   #2
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Location: North Central Vermont
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I am sure someone can figure it out better than I can. Typically, there is a charger/converter that charges the batteries and converts 120v to 12v for the DC stuff. That is usually hard wired, not plugged in. An inverter converts 12v from the batteries to 120v to power electric outlets and is optional, usually for a rig with solar and a large battery bank, but required for residential refrigerators. There should be some identifying information on the unit in question. The buzzing may be a bad cooling fan.

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Old 04-23-2017, 10:09 AM   #3
Join Date: Apr 2017
Location: Pismo Beach, CA
Posts: 49
I had initially thought it was a cooling fan. It is a WFCO 9875 (https://www.amazon.com/WFCO-WF9875-M.../dp/B004A2YSA2), which seems to be a very common model for large fifth wheels (2006 Fleetwood Gear Box 375FS).

Assuming that unit is bad or going bad, that is a fairly inexpensive fix at around $150 for a new PowerMax PM4 75a, but now I am wondering what is going on with the generator that it apparently isn't providing 120v. The microwave works when plugged into shore power, so it's likely not the breaker or outlet.

Also noticed that there is a whole-home water filter under the over, with no intelligent way to replace the cartridge. Guess my wife will get her wish for the oven coming out when she paints the cabinets!
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Old 04-23-2017, 10:13 AM   #4
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Is there a matching power outlet near the power cord area ?

Sometimes they don't use a transfer switch. You plug the power cord into the generator outlet.
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Old 04-23-2017, 10:24 AM   #5
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Originally Posted by twinboat View Post
Is there a matching power outlet near the power cord area ?

Sometimes they don't use a transfer switch. You plug the power cord into the generator outlet.
I was going to suggest to look for this as well. I'll bet there is a receptacle near the shore power cord that is fed by the generator. You would have to manually plug in the cord and then start the genset to get power. If this isn't he case, then some troubleshooting will have to be done. First being, to see if the genset is actually putting out power. Make sure the genset breaker is not turned off or tripped.
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inverter, power, shore power

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