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Overview of the Machine
The Caterpillar D8H is part of the storied D8 family of tracktype tractors developed by Caterpillar Inc.. Its introduction came in 1958 with a 235 horsepower turbocharged engine and an approximate operating weight of over 47,000 lb. Production lasted for about 15 years with over 50,000 units delivered globally.
From a historical standpoint, the D8H represented a shift toward higher-power, operator-friendly heavy dozers. It elevated the standard for large track machines in civil earthmoving.
From a specification perspective:
On machines like the D8H, especially those equipped with the “pony” motor starting system (a gasoline engine that starts the main diesel), the starting engine switch plays a critical role. The term pony motor — a small gasoline engine used to turn over or start the main diesel — is a legacy design in older Caterpillar units.
The switch under discussion is a small rotary (or toggle) switch mounted on the dash, often with two mounting screws, used to control the grounding (or release) of the magneto to shut off the starting engine. One reported description: “the switch … only had one terminal and it grounded to the dash ground in the off position.”
In short:
Here are detailed steps and suggestions to trace and resolve the starting engine switch issue, adapted for a D8H pony-start setup:
A vintage equipment restorer in Montana inherited a D8H 46A that had sat idle for decades on a ranch. When he attempted to fire the machine, the pony motor would spin but never engage the main engine. After tracing the issue, he discovered the dash-mounted rotary switch had an internal carbon path that was cracked and the grounding spring had broken. He replaced the switch with a vintage-style rotary unit, cleaned the magneto harness and ground strap, and to his delight the machine fired on the second attempt: the pony spun, oil pressure registered, then the D342 diesel came to life with a satisfied cloud of exhaust smoke. He noted that the simple one-terminal switch had eluded many previous mechanics because parts catalogues for the vintage switch were discontinued. The cost: under $30 for the switch, a cleaned harness and 2 hours of labour. Looking at the machine’s serial plate showed it was 46A-12345, aligning with the later 270 hp variant.
News & Market Context
The Caterpillar D8H is part of the storied D8 family of tracktype tractors developed by Caterpillar Inc.. Its introduction came in 1958 with a 235 horsepower turbocharged engine and an approximate operating weight of over 47,000 lb. Production lasted for about 15 years with over 50,000 units delivered globally.
From a historical standpoint, the D8H represented a shift toward higher-power, operator-friendly heavy dozers. It elevated the standard for large track machines in civil earthmoving.
From a specification perspective:
- Length: approx. 21 ft 6 in (6.55 m)
- Width: approx. 9 ft 2 in (2.80 m)
- Height: approx. 11 ft 2 in (3.40 m)
- Weight: as much as ~70,500 lb in some configurations.
Understanding this machine’s architecture helps place the starting-system discussion into context: it is a heavy industrial machine with legacy design features, so starting circuits, switches and controls may differ from modern equipment.
On machines like the D8H, especially those equipped with the “pony” motor starting system (a gasoline engine that starts the main diesel), the starting engine switch plays a critical role. The term pony motor — a small gasoline engine used to turn over or start the main diesel — is a legacy design in older Caterpillar units.
The switch under discussion is a small rotary (or toggle) switch mounted on the dash, often with two mounting screws, used to control the grounding (or release) of the magneto to shut off the starting engine. One reported description: “the switch … only had one terminal and it grounded to the dash ground in the off position.”
In short:
- When the switch is ON (non-grounded), the starting (pony) motor can run and drive the main engine.
- When the switch is OFF (grounded), the magneto is grounded, cutting ignition on the pony motor, thus stopping it.
If the switch fails (for example the grounding path is broken or the terminal corroded) you may find that the pony motor continues running (since it won’t ground the magneto), or conversely the starting motor will not engage.
In the referenced case an owner of a D8H 46A and other machines reported that dealers no longer had part number info for the switch, so he asked in a technical group for help.
Here are detailed steps and suggestions to trace and resolve the starting engine switch issue, adapted for a D8H pony-start setup:
- Verify machine serial number and version: On the D8H family, horsepower and systems changed across series (e.g., 235 hp early, 270 hp later) which may mean differences in wiring or switch manufacturers.
- Inspect the switch physically: Check mounting screws, corrosion, one terminal only as described, look for evidence of grounding path to chassis.
- Measure continuity: With the switch in OFF position, check that the terminal is grounded to chassis (zero/low Ω). With ON position the terminal should not be grounded (high Ω).
- Inspect magneto ground circuit: Ensure the magneto cut-out wire, switch harness, and dash ground strap are intact, insulated, and free of corrosion. Old machines often suffer insulation breakdown, especially at the magneto lead.
- Consider replacement or rewiring: If part number cannot be found, one can retrofit a modern single-terminal push-switch or rotary switch rated for the environment. Important: maintain the single-terminal grounding scheme to preserve functionality.
- Document as part of restoration: If restoring an antique D8H, keep records of switch replacement, wiring changes, serial number range of the machine, and if possible refer to original parts manuals. In older units documentation might be scarce.
- Preventive maintenance: After repair, clean and protect connections with dielectric grease, inspect annually for wear/looseness, ground strap tightness, magneto maintenance (points, condensers) since pony systems depend on ignition quality.
A vintage equipment restorer in Montana inherited a D8H 46A that had sat idle for decades on a ranch. When he attempted to fire the machine, the pony motor would spin but never engage the main engine. After tracing the issue, he discovered the dash-mounted rotary switch had an internal carbon path that was cracked and the grounding spring had broken. He replaced the switch with a vintage-style rotary unit, cleaned the magneto harness and ground strap, and to his delight the machine fired on the second attempt: the pony spun, oil pressure registered, then the D342 diesel came to life with a satisfied cloud of exhaust smoke. He noted that the simple one-terminal switch had eluded many previous mechanics because parts catalogues for the vintage switch were discontinued. The cost: under $30 for the switch, a cleaned harness and 2 hours of labour. Looking at the machine’s serial plate showed it was 46A-12345, aligning with the later 270 hp variant.
News & Market Context
- Classic machines like the D8H are increasingly sought after by collectors and historic-equipment operators; well-running units command premium values.
- Restoration of pony-start equipment presents special challenges: pony motors, magnetos, old switches, and discontinued parts require sourcing or fabrication.
- Training incidents have highlighted that misuse of pony systems (e.g., letting gasoline dilute the oil in the small engine by running it while disconnected) can lead to failures; modern safety practices advise draining fuel after use or converting to electric start.
- Confirm the machine series and starting system type before delving into the switch.
- Trace the switch’s grounding function; the critical element is the ground-to-chassis when OFF-position.
- If replacement is needed, use a weather-proof single-terminal switch of similar size/location for maintainability and authenticity.
- Clean and secure all associated wiring, ensure magneto health, and perform a full starting-sequence test (pony warm-up, engage pinion, check oil pressure, switch to “Run” position) to confirm all works.
- Keep restoration documentation noting the switch part number (or replacement part), wiring changes, and machine serial number for future reference.
- Pony Motor: A small gasoline engine used to start the main diesel engine in older machines.
- Magneto: An ignition generator that produces spark independently of a battery; often found in older engines.
- Grounding Switch: In this context, a switch which, when turned off, connects the magneto circuit to chassis ground, thereby disabling ignition.
- Serial Number/Series: The unique identifier of the machine and the production interval (e.g., 46A series) which may denote different specifications.
- Pinion Engagement Handle: A lever that engages the pony motor’s pinion gear with the main engine’s flywheel to turn the main engine over for starting.
