I'm really worried I might damage something on my 1999 Fleetwood Discovery 37V diesel pusher. The generator absolutely refuses to start unless I run the main Cummins engine for several minutes first, and I've already tried using the boost battery switch which seems to do nothing.
I have brand new batteries - two 12V for the chassis and four 6V house batteries properly wired. Everything else works fine, but this generator issue has me concerned. I want to understand: 1) Why won't the gen set start from its batteries without the main engine running first? 2) Which battery bank does the gen set actually use - chassis, house, or a separate gen set battery?
I'm 80 years old and hesitant to crawl under the coach to trace the gen set 12-volt cable. Can someone help me understand what might be happening with my generator's electrical system?
Your generator likely uses the chassis battery and may have a weak connection or failing starting battery. The main engine alternator charges the chassis battery when running, explaining why the generator starts after running the engine.
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Your 1999 Fleetwood Discovery 37V generator is most likely connected to your chassis batteries, not the house bank. The fact that it starts after running the main engine strongly suggests the generator battery (likely one of your chassis batteries) has developed an internal fault or high resistance connection that prevents it from delivering the high cranking amps needed for generator startup, even though it appears "fully charged" on a meter. Running the Cummins alternator provides the extra voltage boost needed to overcome this resistance. Your boost battery switch may not be working properly, or the generator may be wired to only one specific chassis battery that has gone bad.
In your Discovery 37V, the generator typically draws power from the chassis battery system, usually the auxiliary chassis battery. Even though your batteries show as "fully charged," they can still fail to provide sufficient cranking amperage due to internal cell damage, sulfation, or corroded connections. This is especially common in batteries that have sat unused or been deep-cycled improperly.
When you run the main engine, the alternator provides 13.2-14.4 volts directly to the system, which is higher than your fully charged battery's resting voltage of around 12.6 volts at room temperature. This extra voltage compensates for the internal resistance in your failing battery or poor connections. The boost switch you tried connects the house batteries to assist the chassis batteries, but if there's a wiring issue or the switch itself is faulty, it won't help.
Another possibility is that your generator has a dedicated battery mounted near the generator itself (common in some Fleetwood models) that you haven't discovered yet. These are often small Group 24 or Group 27 batteries mounted in a compartment near the generator, and they're easy to overlook during maintenance.
Start by testing your chassis batteries under load. With the engine off, use a battery load tester on each chassis battery individually - they should maintain at least 10.5 volts under a load equal to half the battery's CCA rating for 10 seconds. If either battery fails this test, replace it even if it shows 12+ volts at rest.
Next, check all battery connections at the chassis batteries. Remove, clean, and retighten all terminals. Look for green corrosion or loose connections that could cause voltage drop. Pay special attention to the heavy cables running toward the generator area.
Test your boost battery switch by having someone activate it while you measure voltage at the chassis batteries with your multimeter. You should see voltage rise when the switch is engaged. If not, the switch or its wiring has failed.
Locate your generator's battery connection point. In your Discovery, follow the heavy positive cable from the generator starter solenoid (usually mounted on the generator itself). This will lead you to either a chassis battery or a dedicated generator battery. Check this connection point for corrosion or looseness.
If you find a separate generator battery, test it the same way as the chassis batteries. These often fail because they're forgotten during regular maintenance and sit discharged for long periods.
If you're uncomfortable working around batteries or electrical systems, call an RV technician. Battery testing involves risks including hydrogen gas exposure and potential electrical hazards - always wear safety glasses and ensure adequate ventilation. Incorrect connections can damage expensive components.
Also call a professional if you discover the generator wiring runs through areas you can't safely access, or if the boost battery switch needs replacement. These switches often require removing dashboard panels or accessing tight spaces that are challenging for DIY work.
Consider having the entire 12-volt system professionally inspected if multiple electrical issues arise. At 24 years old, your Discovery may have multiple aging components that could fail soon, and a comprehensive electrical survey could save you from future roadside breakdowns.
Finally, if load testing reveals that multiple batteries are failing despite being "new," you may have a charging system problem that's damaging batteries. This requires professional diagnosis of your alternator, converter, and charging circuits.
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