Bad synchro - replace trans - grinding gears

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by RepoweredRookie, Aug 16, 2022.

  1. RepoweredRookie

    RepoweredRookie Light Load Member

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    Hey Guys, I wanted to start this post as part of your troubleshooting process for grinding gears.

    I started a thread called “Clutch adjustment now replacement” but is was really about the transmission grinding when going from low to high gears. I almost cost myself $5000 because I’m impatient.

    Anyways if you are getting grinding going from low gears to high gears, try using your high/low selector switch at least 7-10 seconds before going into your high gears. This may allow a lazy solenoid time it needs to reduce/prevent grinding. It worked for me, and saved me $5000 by not needing a replacement transmission due to a bad synchro. It’s common to think a “hard to get in gear/grinding” issue may be a bad synchro, but once I allowed the stickshift selector more time, it stopped being hard to get in gear and grinding.
     
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  3. Judge

    Judge Road Train Member

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    Could save money by replacing the Slave gear and range selector switch.


    Don’t need replacement transmission.
     
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  4. kranky1

    kranky1 Road Train Member

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    Check the filter in the back of the transmission. It’s that coolie hat looking thing on the shift solenoid at the back of the transmission. You only have to have the input pressure to that solenoid drop by 1psi or so before it won’t shift right. A small air leak down there will do the same thing. You should pre-select range and splitter shifts on newer Eatons. The old ones, up to the 15 series I think, weren’t supposed to be pre-selected. You made the range or splitter shift when the main was in neutral, that ####s the new ones up. As @Judge said the shift solenoid itself may be bad. Lots to check down around the back of the transmission before changing it. In the end there may be damaged synchros back there, but that’s only in the auxiliary section. Those can be replaced without pulling the whole transmission out of the truck.
     
  5. RepoweredRookie

    RepoweredRookie Light Load Member

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    Hey thanks for the response Judge, I will dig into it and check the slave gear and range selector switch.
     
  6. Lazer

    Lazer Heavy Load Member

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    Wouldn’t hurt to change your transmission fluid/oil.
     
  7. RepoweredRookie

    RepoweredRookie Light Load Member

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    Thanks for the info kranky, I’m going out now to find that coolie hat filter. When I changed the trans oil last year it was nasty brown. Also, going to try selecting while in neutral to see if that helps. And knowing the synchros can be changed without removing the trans should be much cheaper than a reman trans. Thank you for the help!
     
  8. RepoweredRookie

    RepoweredRookie Light Load Member

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    Just truck spec:

    1999 Freightliner Century 10 speed Eaton, 12.7 detroit. 1.4 million miles.

    ***Meritor/Rockwell transmission-RM10155A2S021
     
    Last edited: Aug 17, 2022
  9. kranky1

    kranky1 Road Train Member

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    Eaton or Rockwell. Has to be one or the other, not both. The transmission in that truck may have been changed if the trans says different than the door decal on the truck. Doesn’t really change much, both transmissions work basically the same, and look similar at the back. Rockwell should have an air filter back there too.
     
  10. Hammer166

    Hammer166 Crusty Information Officer

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    And even if all the advice ^up there doesn't help, they make a kit to replace the range synchronizer. That's the usual mechanical cause of range shift grinding, no need to replace the tranny.
     
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  11. kranky1

    kranky1 Road Train Member

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    I’ve never had the auxiliary off a Rockwell. Only seen them on the floor but I’d imagine they’re similar to an Eaton as far as getting the auxiliary off it in the truck. He’s probably going to be into synchros. I aimed him at the air filter as that’s the root cause of a lot of tore up synchros and may have stopped it if they’re not badly damaged. Usually range shifts while coasting gets the rest, like pulling it out of 6th gear and punching that range selector down while they coast to a stop. That’s a big one on fleet tractors. Either way, I’d hate to see the guy into a transmission over auxiliary synchros, Eaton or Rockwell that should be an easy fix.
     
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