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ScottG's avatar
ScottG
Nomad
Oct 04, 2017

Onan 6500 won't turn over

Working on a 1990 Onan 6500Watt genny. Either the starter is fried or the eng is seized because when you hit the start switch, it pulls a good set of battery's down to 10V and draws around 400 amps.
Note that all connections have been disassembled, inspected and cleaned so the 12V side is good.
Same batterys can spin the big 454 without breaking a sweat.

Is this a common failure?
What is generally the cause?


Thanks,
Scott
Class A
  • wolfe10's avatar
    wolfe10
    Explorer
    Oct 04, 2017
    Reach up from underneath with a long screw driver into the fan and see if the engine turns over manually.
  • romore's avatar
    romore
    Explorer II
    Oct 04, 2017
    That doesn't sound good at all. Does it have oil in it? Remove the spark plugs and starter, can you move the flywheel with a screwdriver in the ring gear?
  • ScottG's avatar
    ScottG
    Nomad
    Oct 04, 2017
    It does have nice clean oil.
    I'll check to see if I can turn it by the fan blades. The starter seems to be in back?
    Can't really get to much with it in the couch and I'm not as flexible as I used to be:)
    First i have to trouble shoot his WH.

    Thanks!

    Scott
  • wolfe10's avatar
    wolfe10
    Explorer
    Oct 04, 2017
    Scott,

    With most installations, there will be an opening directly under the fan (for air flow), so that should not require much to access.
  • ScottG's avatar
    ScottG
    Nomad
    Oct 05, 2017
    wolfe10 wrote:
    Scott,

    With most installations, there will be an opening directly under the fan (for air flow), so that should not require much to access.


    Thanks, so the fan is hard or strong enough to turn the engine with?
    Never did get a chance today to look at it. We ended up resealing his roof, fixing the water heater (bad ECO) and doing a bunch of other little stuff that was needed.
  • ScottG's avatar
    ScottG
    Nomad
    Oct 06, 2017
    OK, so I stuck a big Crafsman screwdriver in the fins and after a lot of effort, the eng turned about half a turn. So I tried the starter - it turned another half a revolution and just about hopped out of the compartment it stopped so fast.
    Then I couldn't get it to budge. Even bent the screw driver trying.
    I think it's seized. Might explain why the oil was so clean - seller was scamming buyers.
    Anyone need an anchor?
  • bounder39zman's avatar
    bounder39zman
    Explorer II
    Oct 07, 2017
    I have seen these engines build up so much carbon on piston and combustion chambers in cyl. heads that it essentially "locks up"
    when the piston is moved to top dead center. removing heads and cleaning carbon fixes the issue, In fact ONAN listed this procedure as a regular maintenance item in their service manuals. And you may have altogether different problem, such as broken connecting rod that allows engine to move a little bit before locking up. If you have a couple inches clearance at generator end, you can turn engine over with a 3/8" allen wrench inserted in rotor thru-bolt

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