Overdrive cycling in and out issues Jim Livingstone had some issues with the overdrive cycling in and out of engagement and in May 2025 traced them to a low oil level in the gearbox caused by a leakage. Subsequent testing on local roads confirmed this which he described in a note released in May 2025. However, the cycling returned on the first long run with extended use of the overdrive. On revisiting the problem last month Jim discovered that adjustment of the inhibitor switch has proved a more long lasting solution. He has recorded the procedure he adopted in a further note. Both of Jim's notes are released here. How does a Laycock Type LH overdrive work? The Laycock Type LH overdrive fitted between the gearbox and propeller shaft is a self-contained gear unit which provides a higher overall gear ratio than that with the final drive. The overdrive gears consist of a central sun wheel and three planet gears which mesh with an internally toothed annulus. Fitted inside the annulus is a uni-directional clutch. A sliding clutch member is secured to the sun wheel and is free to move forward and backward on the sun wheel splines. Attached to a ball bearing, secured to the sliding clutch by a circlip, is a static thrust ring. The thrust ring is actuated by two hydraulic pistons and returned by primary and secondary return springs. An electronically operated solenoid valve, mechanical pump, relief valve and low pressure valve comprise the main components of the hydraulic system. We have an article on how the overdrive works which looks at the overdrive disengaged, so direct drive applies, and then with overdrive engaged with the diagrams alongside. See pages 3 to 5 of V8NOTE457. Link Overdrive components diagram Posted: 250715 Overdrive output shaft MGBGTV8 - renewing the oil seal in the overdrive output shaft Jim Livingstone had some overdrive issues with the overdrive cycling in and out of engagement and in May 2025 and traced them to a low oil level in the gearbox caused by a leakage. A visual inspection of the transmission unit revealed oil leaks were particularly evident around the output shaft and the sump cover. His plan, therefore, was to replace the oil seal on the output shaft and the gaskets in the sump and solenoid together with any O rings and hardware. The procedure to change the various seals is relatively straightforward but he found the information available in the official Manuals is sparse to nonexistent. He thought fellow V8 owners might appreciate the information he gathered from his research into the subject and provided a useful illustrated article as V8NOTE681. See the note. Link Inhibitor switch MGBGTV8 – readjusting the overdrive inhibitor switch (and a surprising discovery) In his previous note Jim Livingstone suggested that the cause of his V8’s overdrive cycling in and out of engagement was low oil level in the gearbox caused by a leaking output shaft seal. This conclusion appeared to be confirmed by subsequent testing on local roads. However, the cycling returned on the first long run with extended use of the overdrive. On revisiting the problem last month he discovered that the adjustment of the inhibitor switch has proved a more long lasting solution. He has recorded the procedure he adopted in this note. A surprising discovery was finding a rubber plug adjacent to the inhibitor switch when Jim was under the car. There was no reference to this in the workshop manual or parts catalogue so he assumed this was not an original part of the car. He decided to investigate if this tunnel aperture would be an easier route for the wiring or maybe even facilitate replacement of the switch should it prove faulty. He was able to check and readjust the overdrive inhibitor switch more easily. In his note Jim sets out the procedure he followed to check and readjust the overdrive inhibitor switch. From longer road testing that adjustment of the inhibitor switch has proved a more effective solution to the overdrive cycling in and out. See the note. Link See links to other articles on the Laycock overdrive. Information Gateway