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Thread: 1943 Cat D4 2T pony motor

  1. #1
    Join Date
    Oct 2011

    Default 1943 Cat D4 2T pony motor

    Hi guys,

    I'm new here, just joined. I picked up a cat D4 2T two weeks ago and been having a blast on it pushing dirt around. Everything works great except for the pony motor. I have been pull starting it with a 4x4 truck which is getting to be a pain. The guy I got the dozer from claimed it just needed a distributor which had 2 teeth missing on the gear. His story is that the crank had too much up and down play due to worn bearings which caused the distibutor gear to break. I pulled the top of the pony motor off and the crank doesnt even ride on thedistributor the cam gear does. The cam gear has no play so his story is not correct. I found out that number two cylinder has 0 compression so Im going to rebuild the motor. I a little worried that even if I get the pony motor running that maybe it wont be able to turn the diesel motor over, is there any pattern failures on these cats I should be aware of? Also what is the procedure for starting a D4? I have 3 levers. A start/run which seems to be a de-compression lever. A short spring loaded lever ( im guessing starter engaguement, and a 3rd long lever which switches front to back have no idea on this one. Thanks in advance for any help.

  2. #2
    Join Date
    Apr 2007
    Elkhorn, WI

    Thumbs up Levers

    Read this Thread, halfway down is an explanation on the Levers and their functions.


  3. #3
    Join Date
    Jan 2009
    Kalamazoo Michigan



    It's common for Pony Motor Bearings to wear out (but it will never affect the magneto), new bearings are still available from Caterpillar. Your Cylinder with no compression could be due to a valve stuck open.

    The smaller, horizontal lever coming out of the bell housing is the Starter Pinion Engagement Lever. The larger vertical lever is the Pinion Clutch/Brake Lever. The other lever as you suspected IS a compression release.

    The Pony motor is geared to the Starter Pinion. To start the Diesel you first start the pony and idle it down. Open the Compression Release and then pull the clutch/brake lever to the rear to stop the pinion (and keep it from grinding) and then pull up on the pinion lever to engage the pinion with the diesel's flywheel ring gear. Once the pinion is engaged, push the pony idle knob in, rev up the pony and then push the clutch/brake lever forward to crank the diesel. When the oil pressure builds up you can close the compression release. After cranking the diesel for a minute or two you should be able to feel some heat on the diesel head, then you can open the throttle and the diesel should start to fire. Once the diesel fires the starter pinion will disengage (hopefully). If the diesel fires and quits, go back and engage the pinion as before and crank the diesel some more with the throttle closed. It may take several attempts with the pony motor to get the diesel engine started but you'll soon get the hang of it.

    Good luck with your new toy!
    Last edited by zootownjeepguy; 10-23-2011 at 07:31 AM.
    Rich Salvaggio
    D2 5U9917
    '46 Willys CJ2A Farm Jeep, '39 Buick sedan, '49 International KB-7, '37 Allis Chalmers WC, Cushman Scooter(s)
    Antique garden tractors & outboard motors
    Other rusty old junk comes & goes without warning.

    The 2 most useful tools to have in your shop are a Crystal Ball and a Magic Wand

  4. #4
    Join Date
    Jan 2007
    Kerrick, Minnesota


    Actually, it is possible for pony crankshaft issues to damage the mag. I remember seeing an old Caterpillar service bulletin that edb posted on here a year or so ago regarding that very problem. If either the pony flywheel is allowed to run loose on the crankshaft or the radial clearance of the crank journals in the bearings becomes excessive, it will build eratic vibrations in the geartrain that will transfer from the cam gear into the mag gear, usually resulting in missing mag gear teeth or a broken armature shaft. Since the mag is the weakest point in the system it usually suffers the damage.

    I had this very thing happen on the first D2 I owned, my old 5J. I bought it from the previous owner with broken mag teeth and a dead pony, I replaced the mag gear and had it running good for about ten starts then it broke the mag gear again plus bent the armature shaft in the mag. I rebuilt the mag again and replaced the gear yet again, and this time found the pony flywheel was running loose on the crank, the tapered end was worn and the flywheel wouldn't stay tight. I replaced the pony motor with a good one and never had any problems again.
    D2 5J2115SP
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    Minneapolis-Moline prototype crawler X253
    1945 Farmall H

  5. #5
    Join Date
    Nov 2006
    Corralitos, Ca.


    Make sure you have some backlash clearance for the Mag drive gear.

  6. #6
    Join Date
    Nov 2006


    I'm with Tony on this, I have seen it as well. I would certainly inspect for main bearing wear as well as endplay on the pony motor. If there is up/down play on the crank, you'll be looking for another mag gear sooner than you want.

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