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Home » Workshop » Mechanics » Overdrive Units

What is an Overdrive Unit?

An overdrive unit is simply an additional gearbox mounted after your standard gearbox containing an extra ratio. It is engaged by an electrical solenoid, which allows a clutchless gear between 2nd, 3rd or 4th gear and overdrive. Its primary purpose is to lower the engine’s revs when cruising at speed, although it had racing uses allowing a ‘quick change’ when blasting up a short straight, which is why you can often use it in lower gears.

Which classics have them?

Given today’s high-geared cruising on motorways any classic that is ‘overdrive-equipped’ is much-prized amid the classic car fraternity.

They were a common fitment in sports cars such as Ferrari, MG, Healey and Triumph TR, saloons such as Rover, Sunbeam, Jaguar and Vauxhall, and commercial vehicles such as Land Rovers and Ford Transits. Possibly the final cars to be offered with overdrive instead of a five-speed gearbox were the Volvo 240 and 260 series from the 1980s.

What goes wrong with them?

They share the same oil as the gearbox. So, they wear similarly, but can also suffer electrical faults. On Triumph TR’s you often find that the internals of the switch break down, Healey’s often suffer from a fault on the inter-lock, which stops you engaging the overdrive at the wrong engine speed and they all suffer with poor earths.


  • Overdrive simply won’t engage
  • Slow to engage or disengage
  • Sounds like it is slipping
  • Thumps when going into gear
  • Makes a horrible noise


We are able to check and repair the electrical systems, adjust solenoids, remove units and have them remanufactured.  We also have a full range of the correct gearbox oils to make sure that they perform reliably.

Home » Workshop » Mechanics » Overdrive Units
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