M.L. Toys
M.L. Toys
HobbyMasters M.L. Toys
Anything that doesn't fit somewhere else & general forum updates
Hey guys, I've been searching through the MPW forums and trying what has worked for others but so far, I'm not having much luck with this repair. I've "inherited" a Barbie Mustang that works fine when empty in slow speeds, but when it has the weight of a child in it, one wheel spins while the other barely or doesn't move.
  • To trouble shoot I've taken the entire car apart screw by screw.
  • Taken the gear boxes apart - all the gears look good with no broken teeth and plenty of grease.
  • Pulled the shifter assembly apart to look for corrosion on the connections - Connections look clean, but I cleaned them again gently with a wire brush just to be safe.
  • Re-attached the motors to the gear boxes and connected a test power source - everything is moving in the correct direction. Slow Reverse, Slow Forward, and high speed forward.
  • In slow reverse/slow forward, one motor clearly turns faster. At first I thought that could be the issue, but after reading the forum I understand that in slow speeds the motors run in series and that's how they are supposed to perform.
Anyone have any suggestions of other things to test before I re-assemble the car and test it with weight in it to see if it was just a bad connection that I accidentally fixed during tear down/rebuild?

Thanks in advance!

Motor 1
IMG_6361.jpg (36.41 KiB) Viewed 962 times
Motor 2
IMG_6362.jpg (33.35 KiB) Viewed 962 times
Gearbox 1
IMG_6363.jpg (55.6 KiB) Viewed 962 times
Gearbox 2
IMG_6364.jpg (54.34 KiB) Viewed 962 times
Shifter Assembly
IMG_6369.jpg (39.47 KiB) Viewed 962 times
Does this happen in high gear? Any strange noises from the gearboxes? I see you have 19t motors. Can you verify which gearboxes you have or post a clear picture showing the numbers on the outside of the gearbox housing?
If your problem only happens when in slow forward and reverse then it's normal (wheel with least resistance will spin). In high gear both motors/wheels should spin same speed regardless of weight or terrain.
I only tested the low speeds before taking everything apart, so I cannot say about high gear at the moment.
I did not hear any strange noises from the gearbox.
Here's a gearbox test video where you can see the drastic speed difference in low gear.
I'll take a pic of the gearbox housing with part numbers as soon as I get home today and post up on this thread.

If the gears, motors, and transmission are functioning as they should, then it sounds like the "tires" are the issue since they are not providing even resistance for both motors.
Grab the gearbox output gear that is spinning faster (apply a little resistance) and the other one will speed up. When in low the 12v from the battery is shared between the motors (series circuit). If there is any more drag or resistance on either motor/gearbox then the opposite one will spin faster. If the motors are mounted properly, the gearboxes are lubricated and there is no binding then everything is good to go. One motor may be a little weaker than the other or one gearbox may be a little tighter but I'd roll with it as long as it was working and the motors are staying relatively the same temp. Try applying some drag to the faster spinning gearbox and see for yourself. Hopefully there is no problem. If they do not spin same speed in high then there is a problem.
You could always swap gearboxes to the opposite side/motor and figure out if it's just the gearbox or not. You may be able to turn them by hand (power off) and feel if one has more resistance than the other. If the motors don't sound like they are laboring you should be fine.
Please let us know what you find.
I'll try the gearbox resistance test shortly and report back. I'm also going to try using a slow motion video to capture the rotation on high speed to see if I can tell that both motors are going at the same speed. I've tried turning the gearboxes manually and the resistance is about the same between the two gear boxes with the motor attached.

In the mean time, here are the gear box covers as promised:
Gearbox Cover 1
IMG_6374.jpg (50.62 KiB) Viewed 928 times
Gearbox Cover 2
IMG_6375.jpg (59.84 KiB) Viewed 928 times
Reassembled the frame, motors, gearboxes, 4 wheels and steering for testing...

She works! Tested with a boost pack and the weight of two 6 year olds. No problem at all using all speeds and directions. I have no idea what the fix was but taking it apart and putting it all back together seems to have done the trick.
Stripped down and working!
4850C2AB-41AA-4B52-85A1-8F47AA595DD5.jpeg (1.22 MiB) Viewed 914 times
Next up, clean & prep for paint!

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