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Upgrade MTH SP AC-6 Cab Forward Page 2 of 2

This 20-3025-1 Southern Pacific AC-6 Cab Forward was listed in the MTH 1998 Volume 1 Catalog.

Boilerw MTH Box

Boiler Chassis Left

Boiler Chassis Right

The upgrade was done in April 2012 using a MTH PS2 Upgrade Kit with the included Steam Locomotive Upgrade Kit Installation Manual - 4th Edition.  The page numbers refer to the pages in that manual.  When following the Installation Manual the color illustrations in the website manual are helpful.  Page 28 of the MTH Cab Forward Operating Manual shows the location of the 4 body mounting screws for boiler removal.


(No drilling or grinding is required - just a bit of razor sawing/filing on smoke unit board.) The fun part is adding nine more harness wires and keeping that nice open space between the boiler and chassis.
Secure wires to boiler and motor with premium grade 3M electrical tape. (about $8 a roll)
Use the 3V battery to test lights, smoke fan, smoke heater, etc. before installation.
During the installation keep the shell near for visual check fitting.

Note part locations for reinstallation.
P. 26  The spacer for the harness socket is not needed. 

Boiler Back stripped

The additional red wires are not needed if the existing roller pickup wires test okay.

Boiler Front Stripped

P. 29 Connecting The PS-2 Harness To The Smoke Unit
After smoke unit modification, test the fan direction with the charged tender battery.

Boiler Constant Voltage Board

Front of Boiler Before Upgrade

Boiler Front Inside

After the PS2 harness is installed, the fun part is routing all the wires. The tach reader wires can be routed similar to the existing motor wires.

Boiler Harness Installed

P. 23 Determine Engine Motor Type
There is a Pittman motor that requires a 1/16" Allen Key for flywheel removal. 
P. 24 Installing Tach Tape Onto Motor Flywheel - Center tape on flywheel.
P. 25 Mounting Tach Reader To Tach Reader Bracket - Remove tach reader spacer as shown.

Boiler Motor Out

Test original motor wires routed along chassis for good insulation and continuity.  If okay, keep the original motor wires connected and routed to the harness board area in the back.
The Tach Reader harness wires must be lengthened. 

Boiler Wiring Under Motor

P. 27   Connecting Headlight Bulb & Wire Harness
The cab must be removed to install the PS2 Headlight bulb. Remove 4 screws for cab removal. The two center screws hold the backhead in place.

The cab must be removed to install the PS2 Headlight bulb. Remove 4 screws for cab removal. The two center screws hold the backhead in place.
Boiler Cab Screws 
Secure Wires in Boiler above Motor.  Sleeve the wires secured by metal hold down strap. The metal strap will cut insulation.
Boiler Front Wires Secured 

P. 31 Connect Power To The Constant Voltage Board
The inductor can go into the boiler shell.  The board does not need to be removed from the boiler. 
 Boiler CV Board w Inductor

Remove smoke switch from front spring contacts and use the cut red wires as jumpers to join the spring contacts together.  Tuck the soldered insulated splices under the motor.

Boiler Motor and Jumper Wires

Motor Wire Complete  Motor Wires Tucked Under 

P. 27 Connecting Motor, Ground & Pickup Wires - (in back near circuit board)
The original motor wires can be attached to the harness near the rear of the locomotive. Original motor wires are Brown and Black. Connect the white harness wire to the brown motor wire and connect the yellow harness wire to the black motor wire.
Tie the harness ground lead to a rear gear box screw. The harness red pickup roller wire can be spliced to the existing pickup roller wires.
Both gear box covers must be tight.

Now what?  Six wires must be routed to the enclosed boiler area while keeping a low profile.

Now What?

Boiler Chassis Open Space

Test system before proceeding with wire management.

Engine and Tender Test

The kit plastic wire wrap sleeving is too large for the smoke and headlight harness wires over the gear box. If small black thin flexible wire sleeving is unavailable, use premium electrical tape for low profile.
Fabricate 3 wire hold down straps using metal tabs similar to the ground lead tab. Strap tabs must be metal so gear box screws can be as tight as possible.

Boiler Wire Back Sleeving 

Secure sleeved/wrapped wire to hold downs with 22 gauge solid black wire used as tie wire.
The metal plate behind the back gear box is bent up slightly on both ends to take pressure off wires entering and exiting below the metal plate.

Boiler Chassis Wires Secured

Double check wire placement before attaching boiler.

Boiler Ready to Close

Lost Clear Space under Boiler

Boiler to Chassis Space after Upgrade

P. 34 Loading The PS-2 Sound File Into The Upgraded Engine
Use Premier AC-6 Cab Forward 20-3349-1  (2008 Vol. 1) sound file.

MTH Loader Files work better if you put them in their Default folders in Windows Program Files.
Be sure you have the latest version of the MTH DCS Consumer Loader. Before downloading the sound file, specify saving it to the Desktop. In Windows Vista, see file in Windows Explorer after downloading and look under Compatibility Files (Window top bar). When you find the file after downloading, go to Properties and click Unblock under Security at the bottom of the page. You can then move the file to your MTH Files in the MTH DCS Consumer Loader folder.

If your TIU has the RS-232 computer input socket use a RS 232 to USB Converter (mine is from MCT Corp.). The sound file download will take about 40 minutes with that connection.

Be sure remote batteries are good so you can shut off smoke as soon as download is complete.
After sound file is installed, name engine, shut down, and remove power.

Here is the finished Cab Forward with smoke coming out of the back of the engine. Cab Forward Front w Smoke

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