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Old 05-02-2022, 06:26 AM   #1
Join Date: May 2022
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Issue with Fuel Gauge/Generator

Hey y”all,

I have recently acquired a 2001 Forest River Class A (34 ft) on a workhorse chassis. The generator in question is an Onan Marquis Gold 5500.

When I purchased it from the guy he mentioned the fuel gauge is stuck on Full, and that he couldn’t seem to get the generator working - not sure if it’s related). He mentioned that he had attempted to have the fuel gauge repaired twice in the past few years with both attempts being unsuccessful so he “gave up” and simply calculated his mileage and how much to pump for each journey basically.

It’s not “the end of the world” by any means for us to have a faulty fuel gauge but I am starting to believe it is effecting the generator. I could be entirely wrong, and I actually hope I am entirely wrong, as I was told the following from a friend:

“Well there has to be an interlock switch a set of contacts that has to be closed for it to enable if the fuel float isn’t working then it’s probably not closing the switch it and therefore your generator won’t turn on. It Doesn’t-go by what the needle says on the gauge itself inside the RV”

As of now if I go to start my generator it tries to turn on, it blows exhaust out and all but the moment I take my finger off the start button (on the generator itself as the inside RV button doesn’t seem to work) it just cuts out and dies.

My friend also advised there may be a way to bypass the switch to test the generator, but I would need a wiring diagram and I have not the slightest clue where to obtain one. If anybody does have a schematic of the float / generator would be highly appreciated as well.

I guess all in all I was hoping for some general advice on what to check / if the above information makes sense AKA is it possible that the reason my generator isn’t working simply due to the faulty fuel gauge (that’s stuck reading Full)?
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Old 05-02-2022, 07:44 AM   #2
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The thing to do first is to make sure you have above a quarter tank of fuel, put in 20 gallons or so to be sure you have fuel in the tank.
The fuel pickup line within the fuel tank does not reach all the way to the bottom of the tank. This is done to insure the generator does not consume all the onboard fuel and leave you stranded. The fuel gauge being on full all the time is not an issue in running or starting the generator, and should be addressed as a separate issue.
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Old 05-02-2022, 11:46 AM   #3
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Thanks. That’s exactly what I thought but somebody local tried telling me otherwise. I appreciate your reply on the matter.

Addressing the fuel gauge issue is still something I want to do. If anybody has any advice on where to start I’m all ears!
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Old 05-04-2022, 07:33 AM   #4
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I would suggest replacing the fuel pump in the fuel tank especially if the MH has a lot of miles behind it. The fuel pump comes with a new fuel gauge tank unit. Since some fuel pumps in earlier chassis have failed at around 75,000 miles, you would be avoiding a roadside engine failure and eliminating the fuel gauge tank unit as the reason for the gauge failure. As Dieselclacker stated, the generator starting issue is affected by fuel level and certainly not by the fuel pump.
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Old 05-04-2022, 07:43 AM   #5
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Get a copy of the generator manual. Not sure on your specific model but in my case I have a 7.5KW diesel generator, and have the ability to get fault codes from my generator. If there is a problem it will point me in the right direction as to what is wrong.

In my case I just press the stop side of the switch for ~5 seconds and it will give me a two part blink code if there is a problem. It will also tell me if there are no faults with a different blink code.
Again, not sure if your model generator does this or not.
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Old 05-09-2022, 12:14 PM   #6
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Many of the Onan generators will stop running if there is no power generation. Will fire up while cranking but will immediately stop if you let go and no power is detected. Depends on which Marquis model you have. My 1992 Marquis will keep running even if no power is generated. New models will not. Might be time to drop your genset and do some serious maintenance. New plugs and a carb rebuild at the very least. One issue that's common on the newer Marquis is a belt the connects the engine to the generator. If broke, it will not keep running. On older models if can be a slip ring that needs to be cleaned. On all models it can be a faulty regulator.

As others have indicated, there is no connection between the engine fuel system and the generator. The generator has its own fuel pump and pickup tube in the gas tank. Nothing about your fuel gauge could affect your generator.
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Old 05-09-2022, 12:27 PM   #7
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It would be interesting to know what the previous repair attempts on the fuel gauge involved .
As mentioned by exrench, the tank sending unit is on the side of the fuel pump in the tank .

If the sender or the pump assembly has already been replaced then wiring or dash unit have to be tested.

Cummins Onan should provide wiring info for your gen set .

Or you can check out this site for just about any manual you may need.
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Old 05-10-2022, 05:39 AM   #8
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Here's a web site that's been very helpful on older Onan generators:

No mater what, you will need to know the generator model number and serial number.
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Old 05-10-2022, 06:03 AM   #9
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If you do your own work, check the gas gauge for voltage, it should vary with the fill level. Disconnect a wire to see if it drops to empty, if not it's mechanically stuck. Mine gets wonky after I (over) fill the tank but eventually straightens itself out. The advice to keep a 1/4 tank or better for your genset should be followed, otherwise it will never start. Changing the fuel filter isn't a bad idea and cleaning the carb might help. For me having a reliable genset is priceless. Another thing I noticed about my W24 is pressure in the gas tank, somehow, I think it effects the gauge and once in a while my "check engine" light comes on. Opening the gas cap slowly and keeping it really clean has helped with the gauge and the light. I think the float gets less of a pounding, but I can't confirm that, I just know it works for me. I have a 05 Adventurer on a W24 chassis, 75-gal tank and 7000-watt Onan genset. I hope you get this stuff squared away, especially the genset.
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Old 05-10-2022, 06:41 AM   #10
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I doubt the problems are related. For the genset you should be able to run it independently from the fuel tank by disconnecting the fuel line near the genset and using a jerry can of fuel to feed it. I'm not sure about that one but lots have the fuel pump right at the genset so you could disconnect the line there to feed it from the jerry can and eliminate the fuel issue. I have run older Onan's that way so I don't think there is any electrical interlock on them. You would need the schematics to know for sure though and with them it should be easy to figure out how to bypass it.
For the fuel gauge check your grounds, if you have a bad ground typically they read over full.
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Old 05-11-2022, 04:44 AM   #11
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Someone is confused about the problems being related, there are two separate issues, the gas gauge doesn't work and the genset doesn't work. Before you do the jerry can trick just check for fuel at your generator. You were given good advice about fuel levels in your gas tank and a link to get manuals for your generator, I would follow that good advice before you start taking your coach apart.
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Old 05-11-2022, 06:18 AM   #12
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Originally Posted by Tilted Kilt View Post
Someone is confused about the problems being related, there are two separate issues, the gas gauge doesn't work and the genset doesn't work. Before you do the jerry can trick just check for fuel at your generator. You were given good advice about fuel levels in your gas tank and a link to get manuals for your generator, I would follow that good advice before you start taking your coach apart.
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Old 05-11-2022, 07:18 PM   #13
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Thanks for the advice everyone. I’ve been busy as heck dealing with more urgent matters (leaks) and haven’t had a chance to get to this yet. I can confirm that the RV has barely any gas in it (as the previous owner only put 20L of gas in it he said for me and thinks it’s near empty — something I thought he would of mentioned earlier but it is what it is).

So, first thing I’m going to do when I have time to is fill the tank at least half way to see if it’s as easy as that. If this isn’t a fix, then I will dig deeper. There are less then 40 hours on this generator for an ‘01 model, and only 18 thousand miles on the MH itself.. also seems that everything was extremely well kept (that I can see anyway) around the gen.. so it very well could be the lack of fuel itself.

I appreciate everyone’s comments.
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Old 05-27-2022, 08:09 PM   #14
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When you press the bottom of the start switch do you hear a clicking sound. This is the start switch at the generator.
That clicking is the fuel pump priming or pumping gas into the carb. If you don’t hear anything you have a bad fuel pump. It is located under the generator and s easy to change. Just did it on mine. I paid 22 bucks for the pump and 18 for the associated filter on Amazon just search for your generator fuel pump.
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