Saturday, September 30, 2023


My generator only starts with RV engine running. What should I check?

Dear Dave,
Our Tiffin 32BA FRED carries an onboard diesel 6000w Onan generator in a compartment. It can activate with a switch on the generator itself but we’ve always used the remote switch on the dash. Last week, neither switch started the generator until I started the RV engine. There was just a clicking sound. Is/are there fuse(s) I need to check? Any other suggestions? —Tim, 2007 Tiffin Allegro Open Road front-engine diesel [FRED]

Dear Tim,
There are typically fuses from the 12-volt house batteries to the generator: some are glass-type in-line fuses, others are manual spade-type ones. However, if the starter for your generator clicks, I do not think it is the fuse, as a blown or open circuit fuse would provide no 12-volt power to the starter and you would not even get a clicking sound.

A clicking during start attempt is the starter not engaging far enough to turn the engine over, typically due to low voltage. That would mean your house batteries are not holding a charge. They may show 12.6 volts and most owners think they are fully charged and in good working condition. However, if they are sulfated, this could simply be a surface charge and they drop like a rock when a load is applied.

First thing to check

The first thing I would do is check the house batteries with a multimeter to see if they read 12.6 volts. Then check voltage at the generator with the multimeter. There is a positive terminal that typically has a + marking and a red 8 ga cable going to it. Some have a negative terminal as well with a – label, but you can just use the metal casing for the ground or negative side. You should read 12.6 volts or whatever you found at the house battery here. If you do not have any voltage, then there is an open circuit, which most likely would be the fuse.

Here is a generator from the 2015 Thor we worked on. It has a gas generator, but yours would be similar. Notice the red “boot” to the left side, which is the positive cable. We had a similar situation with this as it would not start without running the engine, which provides 12-volt power from the alternator to the generator.

The original issue was the house batteries were so sulfated they would not run anything. So we switched them out with two lithium batteries but still had nothing, no click at all. We tested for 12-volt power at the generator and got nothing. So we started backtracking to look for a fuse. The wiring diagram we got from Thor was useless, to say the least.

We were able to track the positive cable from the generator back into the battery compartment and found a relay on the back wall.

This was a spade-type fuse and the spade was out, creating an open circuit, i.e., no power to the generator. We pushed the fuse back in and it turned over.

Why did generator turn over with open fuse?

So why did the generator turn over with the engine running if the fuse was open? Looking at this fuse block, we see one positive cable connected at the right side which came from the house batteries. Power goes from the batteries through the fuse block to the red cable going to the generator. Notice the cable with the yellow connector. This is coming from the Battery Management System (BMS), which gets 12-volt power from the engine when running.

It can also charge the house batteries when running, but in this case it bypasses the fuse to supply emergency power to the generator. What can be deceiving is all the red protective spray on the cable ends, which makes them look like a positive cable. Notice the one to the left of the fuse block which looks like it’s red, but clearly connected to metal framework, which means it is negative.

What you should check

Back to your situation. If there is 12-volt power to the generator, try starting it and see what the voltage reads when a load is applied. I would anticipate it will drop significantly and that would indicate weak batteries. If it does not, then I would assume your starter is getting weak and requires more voltage than the batteries can supply. When you run the engine, test the voltage and it should be about 14 volts.

If you have no voltage at the generator, trace the positive cable back into the battery compartment and look for fuses. Since it clicks when trying to start, I would not think it would be a fuse. But I’ve learned never to say never when it comes to RVs! If you look closely at the battery photo above, you will see several red connections that are plug-in fuses. We checked those first, but none were bad—so I have no idea where they go.

 You might also enjoy this from Dave 

No power is coming from my RV’s onboard generator. Why?

Dear Dave, 
The Onan generator starts and runs perfectly, but when I start it, it’s not powering up the RV. —William, 2017 Fleetwood Flair LXE

Read Dave’s answer.

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

Read more from Dave here


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Dave Solberg
Dave Solberg
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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1 month ago

The first thing I would check would be for clean batt. terminals. Corrosion is difficult to see – so I would clean the posts and then try the start switch. As Dave said once you apply the load, even at 12.6, the batts. drop like a rock – same thing with dirty terminals.

1 month ago

Our 2022 diesel motorhome had a similar issue with it clicking but not starting. We had plenty of battery voltage. Took it to the Cummins dealership and they found a tiny fuse blown, which was right on top of the generator under a small panel. I’m not sure how many amps the fuse was but it was small and likely ran a relay.

1 month ago
Reply to  Tony

Thanks – I’ll check there, too.

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