elec test help on Onan gen

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I know there must be a common since answer to my problem.
My gen doesn't want to start. As in "starter engages-disengages" What describes my problem the best. Working my way thru the trouble shooting checklist,
(A)battery state of charge, (12.77)
(B)try starting gen with vehicle running (didn't work),
(C) clean battery cables and battery posts...Here is where I could use some help. Only one end of cable I can get to without pulling the genny. Appears the cable has a wire bracket clamp (better known as a P clamp)that holds the cable, so cable doesn't rub on other stuff I guess. I really don't want to drop the genny just to test a cable, big hassle to get it out and back in.

So I'm thinking how to test this... without the pain of pulling the genny. Thinking about running a regular auto jumper cable directly from battery to genny. In other words I'll by pass the cable being used now (that might be bad) and see what happens. My concern is the other electronics I might not see. Is there anything I could screw up doing a test like this? On genny's, the cable goes directly from battery to genny or is there anything in the cable routing such as a fuse or relay or ?


(I didn't post genny info because this is a elec test question, really not model specific).

Thanks

[bounder39zman]

have seen this problem numerous times, most often on Onan 4KY (Microlite/Microquiet) . Usually caused by battery voltage dropping under load of starter motor, due to high resistance in cable connections, or weak capacity of battery . what is causing starter to "engage-disengage" is starter relay contacts closing, then dropping out (opening) due to low voltage. corrosion in cable terminals where terminal/lug connects to cable can be an "unseen" problem

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opnspaces wrote:
Does the starter engage the flywheel and start cranking the engine and then mysteriously just disengage before the generator starts? Or does the started just click, you hear the gear fit the flywheel, and then it immediately retracts and shuts down?

Can you put a meter on the starter battery terminals and then watch the display while you're cranking the starter over to see if it hits 6 volts? Either that or as MrWizard suggests put a battery on the ground right by the generator and run some jumper cables to the terminals on the generator.

"start cranking the engine and then mysteriously just disengage before the generator starts"?
describes it best.

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D.E.Bishop wrote:
I'm going to ask a really important question, have you read the manual? There was nothing wrong with my Onan engine, oil level, battery charge, damaged wires... I finally deciphered the little set of flashes on the display. Mine wouldn't start because the generator was not outputting. So you may find the answer in the manual. If it's not there, what have you lost, nothing but a few minutes.

Yep have read owners manual and service manual. Service manual not that helpful as I wish. Covers about a dozen things, then for "faults" says to see authorized deal.

[D_E_Bishop]

I'm going to ask a really important question, have you read the manual? There was nothing wrong with my Onan engine, oil level, battery charge, damaged wires... I finally deciphered the little set of flashes on the display. Mine wouldn't start because the generator was not outputting. So you may find the answer in the manual. If it's not there, what have you lost, nothing but a few minutes.

[Sam_Spade]

Chris Bryant wrote:


But, don’t really understand starter engage disengage.

IF....the voltage falls low enough it will disengage.

In addition to the "normal" things that can cause the voltage to drop....like loose connections or bad solenoid or weak battery....a locked up engine or rotor will draw a HUGE amount of current and the voltage will go WAY down.

If that is the problem, jumping in a second battery probably is NOT a good thing to do.

[opnspaces]

Does the starter engage the flywheel and start cranking the engine and then mysteriously just disengage before the generator starts? Or does the started just click, you hear the gear fit the flywheel, and then it immediately retracts and shuts down?

Can you put a meter on the starter battery terminals and then watch the display while you're cranking the starter over to see if it hits 6 volts? Either that or as MrWizard suggests put a battery on the ground right by the generator and run some jumper cables to the terminals on the generator.

[MrWizard]

Yes, most generator installs cable runs directly from battery or nearby terminal block to the generator

No electronics between those points

12v battery up front, or 12v battery sitting on the ground beside RV
It is still only 12v battery

You can not harm anything if you connect the jumper cables correctly

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[Chris_Bryant]

You can certainly jump to the battery input on the genset, and don’t forget to jump the ground as well.
You can also just measure the voltage while trying to crank.

But, don’t really understand starter engage disengage. Does it run and not crank the genset? That’s usually a broken part in the starter, but some models are more prone than others.

[MEXICOWANDERER]

Maybe disengages too fast?

Starts cranking but then spins-out before the engine starts?

[Sam_Spade]

Tom_M wrote:

If gen cranks over you do not have a battery problem.

That simple fact is not conclusive proof.
Neither is the resting voltage at the battery.

As was mentioned, a locked rotor is a possibility too.

I recommend that you find a Cummins/Onan dealer and let them diagnose the problem. In my experience, they are very competent and reasonably priced.

[Tom_M1]

It would be helpful if you did state model number and coach information.
Is coach gas or diesel? If diesel, is gen diesel or propane?
What does "starter engages-disengages" mean?
Does gen crank over?

If gen cranks over you do not have a battery problem. Check if you are getting fuel. If gas or propane check if you are getting spark.

I have an old Onan gas gen that I use for home backup power. It failed to crank over one time. Turned out that a chunk of carbon had broken loose and blocked the piston. Had to pull heads and clean out the carbon build up.