I have a 1988 honey motorhome with a 5.0 cck onan generator there is a single wire with a diode the wire goes under the sheet metal on the engine the other end goes to the positive lead on the solnoid this wire is draining my battery can you tell me what this wire is and where it goes.

check oil level in the sump
if the oil level is correct check for a problem oil level sensor circuit
it is there to stop the engine when the oil level reaches critical level

check and see if it gas a kill switch some were on it follw a black wire out of coil see were it goes maybe to a kill switch look for it

If it stops after 5 minutes whether under load or not, and assuming the air filter, plugs, and fuel filter are new and or clean. The fuel pump is suspect. If it is in a motorhome try conencting it to an external fuel source that is at the same height as the generator. Then higher than the generator. If the problem persisits replace the fuel pump. Or just check fuel pump pressure with a gauge. BE CAREFUL do not use an automotive fuel pump Onan have special low pressure pumps, so use OEM part

Assuming your generator has the battery charger option?
Measure the battery voltage at the battery lead in the generator, without the unit running, or the motorhome ignition on. It should read approx 12V after the unit has been sitting a while. Then start the generator, it should rise to approx 13.8 with a good battery. If it does not then the stator windings and charge rectifier in the generator should be checked. If the voltage is 13.8 approx with the generator then you problem lies further upstream, and possibly with an isolation solenoid. If you generator does not have a battery charge winding/option then charging comes from the line voltage via a motorhomes inverter, check the inverter.

Is your generator wired in to an automatic switchover in the motorhome, or do you have external plug in on face of generator, or near it. The power from the generator needs to be attached one way or another to the motorhome. It may be as simple as plugging the cord from motorhome into it, depending on what set-up you have.

Hooked in to motorhome via automatic switchover, or does the power plug of motorhome have to be plugged in to receptacle at or near generator?

Yes it wil not let you run your motorhome out of fuel they set it up that way so you dont strand yourself you may have a air bubble in the fuel line to the generator try blowing air generator fule line back into the tank

try the company web site

You MAY have blown the rotating diodes on the rotor.

This would require servicing.

I have an account in the Buffalo NY area .. they can help you with this .. call 716-668-2666 after 9:00am EST tomorrow (sat)