Onan mobile Gen Set HDCAB/HDCAA start problem

I am thinking it could be loosing fuel pressure between the pump and the injectors. Enough cranking and fuel is finally getting into the cylinders.
It could be weak fuel pump.

Something is wrong with your battery or cables. You charge the battery for 45 min and then use the boost? If my house battery is dead I push the boost button and start the generator, you shouldn't have to wait for it to crank over.

I think I am reading OPs post wrong.
Does the genset turn over at all without the boost? If no, you have a problem in wiring between that battery and the genset.
Does this genset have a pre-heat for the start-up? If yes, could be issue with that system sucking the battery voltage down.

enblethen wrote:
I think I am reading OPs post wrong.
Does the genset turn over at all without the boost? If no, you have a problem in wiring between that battery and the genset.
Does this genset have a pre-heat for the start-up? If yes, could be issue with that system sucking the battery voltage down.

Our genset starts off the truck battery, if your having issues cranking, would be checking both the positive cables ends as well as a poor ground leads, making sure they are all clean and the wire is not rotted inside from battery acid. there should also be a ground strap from the genset to the base of the generator as well too to check.

The generator cranks over fine, it just won't fire up. We replaced the house batteries in February, could there be something wrong with them?

The Wife

Goes back to my original idea. It is loosing the fuel to the injectors. It cranks over trying to prime the fuel system. Cranking over enough will prime system, but is hard on starter.
You could try testing this theory by loosen fuel line to an injector, have helper crank over genset while you watch for fuel at injector. If you get air and no fuel, that would indicate fuel pressure is leaking.
Not sure if you engine shuts down via a fuel solenoid. If so equipped, check connections and make sure it is opening fuel.
Fuel pump?

Voltage at the back of the generator reads 13.6. When DH tries to start it the voltage drops down into the 9's. House batteries do not drop voltage when attempting to start.

you stated in previous post that "engine cranks over fine". Then batteries and connections are not the problem. have you checked the glow plugs on the engine? This Isuzu engine needs functioning glow plugs to start when cold. they frequently burn out on this engine, after a few years. to check you need to disconnect the connector strap and "ohm out" each plug. should read about 1 ohm on the plugs. something else to consider, if the coolant temperature sensor goes bad, the control board will not apply voltage to plugs for a suffcient length of time on cold start up. Should "pre-heat" for 10-15 seconds on cold start-up, depending on ambient temps. one more thing...air filter should be checked. a restricted air filter will not let sufficient air to engine to make compression heat to start engine. good luck with finding your solution

The problem was the Glow Plugs. Thanks for all the help!!! New problem I will post shortly.

glad you got to bottom of problem, and got it going