Allison Fault Code

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P0842 / DTC 42-12

Check Trans Light / Harsh Downshifts

System

Pressure Sensing

Symptom

Check Trans Light / Harsh Downshifts

Diagnosis & fix steps

LOGIC: Main Modulating Pressure Switch Circuit Low (Grounded). ACTION: Disconnect the main transmission external 16-way internal harness bulkhead plug. Measure switch state key-on engine off; check the internal ribbon harness for thermal insulation wear shorting against the aluminum valve body casting.

Red Seal field insights

Ribbon Harness Wear: High-mileage Allison internal flexible printed harnesses soak in hot fluid, becoming brittle and developing shorts to the valve body case.

Same code keeps coming back?

That's a program problem, not a part problem. Book a 30-minute uptime review with Grant, a Red Seal Heavy Duty Technician.

Book an uptime review

Read before you touch anything

  • This is general reference information, not a repair manual. It is for trained, qualified technicians.
  • Fault codes, meanings and procedures vary by model, serial number and software version. Confirm the code and the procedure against the manufacturer's service manual before acting.
  • Follow the OEM service manual, torque specs and your site's safety procedures: lockout/tagout, block raised equipment, relieve hydraulic, fuel and air pressure, and let hot systems cool.
  • You use this information at your own risk. Adventure Mechanical and AM Fleet Integrity accept no liability for damage, injury, downtime or loss arising from its use.
  • Not affiliated with or endorsed by any manufacturer. Brand names are used only to identify the equipment a code applies to.

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