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Komatsu PC200-7 Will Not Crank

Discussion in 'Excavators' started by jlittle2212, Oct 14, 2015.

  1. jlittle2212

    jlittle2212 Well-Known Member

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    Pulled machine up to the shop and now it won't crank. Monitor comes on but Engine will not crank. Battery relay and throttle control will operate as you turn on the key. Just no fire. Key switch checked out good. I have power on the starter. Checked safety lever in the cab, seems to be functioning correctly and making contact.

    I have no book or schematic. Any help would be great!
     
  2. DK88

    DK88 Senior Member

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    disconnect alternator r terminal and see if it starts.
     
  3. jlittle2212

    jlittle2212 Well-Known Member

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    You referencing the 2 small wire connector? If so, tried to unplug and start, nothing.
     
  4. JDfan500C

    JDfan500C Well-Known Member

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    Hit the starter. Or pull it and hook it up to power and see if it spins.
     
  5. GregsHD

    GregsHD Senior Member

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    It must have a starter relay to activate the solenoid, I would jump the relay to see if it cranks and starts just to eliminate a few possibles. And if it starts when you do this, at least you can move the machine if needed!
     
  6. jlittle2212

    jlittle2212 Well-Known Member

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    The starter has power going to it.. The relay is inside the battery box, but there is two.. I'm unsure which is which. There's a few fuses in the battery box. I checked those as well.
     
    Last edited: Oct 14, 2015
  7. JDfan500C

    JDfan500C Well-Known Member

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    It dosent matter if there is power going to the starter. The starter dosent sound like it is cranking the machine. If there is power to the starter than you eliminate pretty much everything but the actual starter. Just saying.
     
  8. jlittle2212

    jlittle2212 Well-Known Member

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    Right, I understand. I'm figuring on maybe a safety switch somewhere or diode. Maybe one of the Komatsu gurus will chime in.. I was thinking on Komatsu, starting and stopping broke a ground to do so. I'm not for sure on that just seems I read that in a different post.
     
  9. GregsHD

    GregsHD Senior Member

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    I'm confused. So there is power going to the trigger wire on the starter solenoid when in the crank position??? Or just power at the battery stud?
     
  10. jlittle2212

    jlittle2212 Well-Known Member

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    At the power terminal on the starter...
     
  11. John C.

    John C. Senior Member

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    Have you made sure the hydraulic lock out lever is all the way in the off position? If you turn the key on, do you get a low oil pressure alarm on the monitor panel? Have you checked all your fuses? Is it possible that someone put a user code in the monitor panel? Pull out the two wire plug on the alternator and check for voltage at each terminal on the alternator with the key on engine not running. If voltage on both terminals the alternator has likely blown a diode inside it. This also what causes the low oil pressure alarm when the engine is not running.
     
  12. jlittle2212

    jlittle2212 Well-Known Member

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    The lever is all the way off. Checked the switch with ohm meter. Switch is operating correctly. There is no codes or alarms..have checked all fuses.
    Is there a diode for the starter behind the cab as the John Deeres? I had a similar problem with a 230 excavator.
     
  13. John C.

    John C. Senior Member

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    Did you check the terminals on the alternator with the key on?
     
  14. jey_c

    jey_c Senior Member

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    Check the starter itself
     
  15. GregsHD

    GregsHD Senior Member

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    I would do like I said in post 5 to start with, it may just be the relay itself... If you can't figure out where the starter relay is just try (carefully) jumping the solenoid trigger wire at the starter, if it starter/runs and all is well then you know it's not a password related lockout and more likely a wiring/relay problem.. Also have you verified there is no power to the trigger wire at the starter solenoid while in the crank position? Remember K.I.S.S.
     
    Last edited: Oct 16, 2015
  16. dogcyber

    dogcyber Member

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    Sounds like the starter safety relay.
     
  17. Adamjolt8813

    Adamjolt8813 Member

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    This is an old thread but hopefully someone may have an idea for me as well. I have a komatsu pc78mr-6. no start issue as well. I can turn the keyswitch on, and to the start position, but it will not crank. It activates the solenoid for the fuel shutoff as it should but no start, no power at start terminal at starter. And when I turn the keyswitch back to the off position, it stays on....If I pull the battery positive then it shuts off and stays off even when I reconnect the battery, until I turn the keyswitch back on. I can get power from the battery to the start post on the starter and get it to turn and start, then the unit works as normal. Do I have a relay sticking? If so where do I look?
     
  18. John C.

    John C. Senior Member

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    First question is has there been any recent electrical work done on the machine.

    So a quick explanation of how the system works. First off there is a battery disconnect that has two large terminals and two small terminals. You will have one of two types. Basically they only differ in that one type hooks to the positive cable from the battery and the other connects to the negative terminal of the battery and the other large terminal goes to ground. One of the small terminals will be hot when checked with a test light or volt meter and the other will be grounded. The small hot wire goes to the key switch and then back to the small terminal. Turning the key switch on sends current through a coil in the master switch which closes it and connects the batteries to the rest of the machine system. The alternator is the other main supply component to the system. There are three terminals to most of the Komatsu alternators, B terminal is the battery connection, the ground terminal connects to the case of the alternator and the R terminal goes to the computer. People usually think that R means regulator but in this case it doesn't. The R terminal will have no voltage when the engine is not running. It will have some voltage when the engine is running. Usually it will be charging voltage. Think of the R terminal as a signal that tells the computer the engine is running. The R terminal is hooked to a diode inside the alternator. That signal to the alternator has some logic. First off it is a safety latch that will disconnect a safety relay and prevent the engine starter from engaging when the engine is running. Second item is that the computer has an algorithm or logic piece that knows if the R terminal has voltage then the engine is running but there is no signal from the oil pressure switch so it starts making noise on the monitor panel. There are some twenty or more circuits in the machine that have something to do with coils. Coils can be a problem with computers because when you put current through the coil and then stop, a voltage can be induced that goes the opposite direction. To stop this from happening there are diodes in all the circuits related to a coil. As I recall in the larger Dash 6 machines there were somewhere around 17 diodes. They are all usually taped to the wiring harness. When a diode goes bad it provides another source of ground which means current flows and a coil won't drop out.

    Knowing all this should give you some clues to the problem. First symptom is the engine won't start. Does the engine oil pressure alarm come on when you turn on the key? Yes, check the R terminal on the alternator for current when the key is turned on. No, check it anyway to be sure. Now try to jump the starter by touching the battery connection of the starter to the solenoid terminal and the engine should start. If not, you need a starter safety relay or the starter needs a rebuild or replace. You can check the terminals of the starter safety relay by disconnecting the harness connector on it and checking for voltage when you turn the key to start. I don't remember which lead it is but as I recall, there are only three. Next item concerns the master switch itself. Turn the key on and then turn it off. If the master switch is still closed, go back and smack it with a big screw driver handle. If it drops out, you can take it apart and clean it and reinstall and try it again. If that doesn't work I would check all the fuses first and if all good then I would start looking for diodes. Just a warning, they are a pain. They are easy to check, finding them and getting them out of the harness connectors is the pain.

    You might want to start another thread and keep everyone informed as to your progress.

    Good Luck
     
  19. Michael Casey

    Michael Casey New Member

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    Did you figure out the issue with the PC 78? Mine is having the same issue
     
  20. Adamjolt8813

    Adamjolt8813 Member

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    Follow what John C. Said, check all battery connections...Yada, Yada, Yada. But if you do not see any issues, take out the vcm, it is the big control box behind the operators seat. I sent mine into flip, (flight systems industrial products), they repaired it instead of paying Komatsus $8000 for it. That was the problem with mine.