Fixing the SD70MAC 18250
on repairing the 18250 SD70 MAC and similar engines. 2000 era, dual
DC motors, LED lights with TMCC
have a roll of good electrical tape. Lots of cab wires are held with
tape in the shell. You will probably need to replace the old tape
with new when taking off the cab. One example of the wiring and tape
needed before replacing the cab. There are many more in the
cab has some long posts for mounting the cab. 2 screws in the front,
2 rear and 2 in the middle. NOT the 4 for the middle fuel
tank. Be careful removing these, not too much torque at
once. When first removed, use some plastic friendly oil when
putting them back in. Regular oil or automobile oil may make the
plastic swell and be tighter! In my opinion some of the mounting
points do not have enough support in the body. Reducing the
torque needed for the mounting screws helps to keep from cracking
the body there is a flat plate with 4 springs that is the contact for the
lights. See the right arrow in the picture below for the contact in
the frame. The long post that holds this often breaks. Since
the post is hollow a long, threaded screw can be used to put it back
together and re-enforce it when it breaks. I actually used a
slighter smaller, longer screw so the post didn't crack. The long
metal screw makes the post strong. This is usually the problem
when the headlight and ditch lights in the cab don't work.
problem that caused TMCC glitches and the engine to hang was the front
pickup was not working/ This caused power drops at switches and
control tracks. I noticed the lights flickering when this
happened. The pickup wire had too much stress and broke where
it soldered to the lug that screws in the top of the truck. I
finally added 2" of flexible wire and the pickup now works well and isn't
breaking again. The original wire was just too short once routed and
held with a zip tie so the cab could be assembled. This wire
connects with a bolt through the trucks, just behind the motor. The
left arrow in this picture shows the location. It has to come out
when removing the trucks also. To remove the trucks, the shell
has to come off!
needed to replace the traction tires, so the trucks had to come off.
To remove the trucks there is one bolt connecting the trucks at the
bottom. There is also the screw that holds the center pickup on that
also must come out. The screw in the center of the truck holds
it to the motor. During re-assembly, the center bolt goes in first.
the truck comes loose, be careful don't break the coupler wires that are
the truck off the center wheels come out. These are the ones with
the traction tires. This is the best way to get to the wheels to
replace the traction tires.
middle axle bearings have a flat spot that goes down to match the flat
spot in the truck. If not aligned, it wont go back together
correctly. Also replace the 20 year old gear grease.
the front truck you may have to remove the hood around the motor that is
held on with 2 bolts. This is so you can hold and align the motor as
you put the truck back on.
second problem with TMCC reliability I finally tracked down.
The trucks have 3 sets of wheels. 2 with flanges and 1 set
without flanges. The set without flanges are just for show and
overhang on turns. When going through the turn on O22 or 31"
switches, the wheel overhang will cause these wheels to just touch
the straight side, center rail. Sparks and a short.
Momentarily unless you stop in that spot perfectly. Still not
good for TMCC or the switch. I tested the engine running
without these wheels. Worked fine.
This closeup shows where the wheel actually touches the center rail in
don't have to remove the trucks from the engine to remove the front or
back wheel sets on the trucks. You can take the front or back wheel
sets off by removing 4 bolts holding the wheel set on from
underneath. This shows the inner truck removed.
drew up and 3D printed plastic non-flanged wheels to replace the metal
ones on the inner wheel sets. The ones that overhang.
the wheels are there, but they wont short when overhanging on a turn
through a switch. New plastic wheels mounted on the axle. Also
the old metal wheels.
the engine runs well and no more funny problems with TMCC.
Good power supply without intermittent shorts makes a difference.
you knock the engineer out of his seat, he can be re-attached with glue
once the shell is re-assembled. His window is open for him to slide
in with tweezers.
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Last update Jan 23 2023