Triumph · 1970–1977The Stag Archive

How-To & Repairs / Transmission

Overdrive won't engage — Laycock A/J-type troubleshooting

How to diagnose a Laycock overdrive that won't engage or drops out on a Stag. Overdrive is available in 3rd and 4th only. The vast majority of failures are electrical or a low gearbox oil level — check both before ever suspecting the internal hydraulics or a rebuild.

Tools needed

  • Multimeter (voltage and current)
  • Clean rag and a torch
  • Basic spanner and screwdriver set
  • Contact cleaner
  • Gearbox oil filler pump and the correct oil grade
  • In-line fuse holder and appropriate fuse (if none is fitted)

Parts needed

  • Overdrive inhibitor / isolator switch (if faulty)
  • Overdrive solenoid (if faulty)
  • Correct gearbox oil as specified by the workshop manual
  • New spade / bullet terminals as required

Warnings

  • Never engage overdrive in reverse or in the low gears.
  • A stuck pull-in winding is a genuine fire risk — always run the OD feed through an in-line fuse.

Steps

  1. 01. Confirm what you actually have

    The Laycock overdrive fitted to the Stag operates in 3rd and 4th gears only. Do not attempt to engage it in 1st, 2nd or reverse. If the switch is on but nothing happens in 3rd or 4th, you have a genuine fault to chase.

    Warning:Never force overdrive in reverse or in the low gears — you will damage the unit.

  2. 02. Check the gearbox oil level first

    Overdrive is hydraulic. Low oil starves the pump and will stop it engaging even when everything else is healthy. Check the level cold, on level ground, and top up with the correct grade of oil before doing anything electrical.

  3. Owner-uploaded photo slot
    Gearbox oil level check. Checking or topping up the gearbox oil level, on level ground.

  4. 03. Listen for the solenoid

    With the ignition on and the car stationary in 3rd or 4th, flick the overdrive switch. You should hear a distinct click from the solenoid on the side of the gearbox. No click means an electrical problem; a click with no engagement points inside the box.

  5. 04. Verify 12V at the solenoid, in 3rd/4th only

    With a multimeter, confirm 12V at the solenoid feed when the OD switch is on AND 3rd or 4th is selected — and no voltage in other gears or with the switch off. This proves the loom, column/gearlever switch and inhibitor/isolator switch are working.

  6. Owner-uploaded photo slot
    Meter reading at the solenoid. Multimeter reading 12V at the OD solenoid with 3rd/4th selected and the switch on.

  7. 05. Test solenoid current draw

    A healthy Laycock solenoid draws roughly 1.8A. A much lower reading suggests a failed pull-in winding; a much higher reading suggests it is stuck on the pull-in winding, which is a fire risk (see warning). Compare against the workshop manual's specification.

    Warning:SAFETY: the solenoid has a heavy pull-in (engagement) winding and a light holding winding. If it sticks on the pull-in winding it will draw high current continuously and can overheat and melt the supply wire. Fit an in-line fuse on the OD feed if none is present.

  8. 06. Clean or renew the inhibitor / isolator switch and connectors

    The inhibitor switch on the gearbox and the bullet connectors in the OD circuit are common failure points. Clean them, remake any tired terminals, and renew the switch if it fails the voltage test above. This cures the majority of "dead" overdrives.

  9. Owner-uploaded photo slot
    Inhibitor switch cleaned or renewed. The inhibitor / isolator switch on the gearbox after cleaning or renewal.

  10. 07. Service or renew the solenoid

    If the solenoid clicks but the unit still won't engage, or if the current draw is out of spec, the solenoid itself is suspect. Service or renew it per the workshop manual. Only after all of the above should an internal rebuild be considered — typically £500–600 through a specialist.

Related faults

No directly linked fault-finding entries yet.

Further reading

Figures and procedures should be confirmed against the official Triumph workshop manual; for safety-critical or specialist work, consult a specialist.

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