THE BIG NIGGLE
Your classic problems solved
Many common faults can be fixed at home – find out how
The overdrive on my Triumph TR5 won’t engage. It is very frustrating because long distance cruising isn’t nearly so relaxed, running in direct top gear without the overdrive. I’ve heard that overdrives are complex and need an expert to deal with any problems. Is there anything that I can check at home?
NIGEL’S JOB CARD OVERDRIVE FAULTFINDER
I’m showing how to diagnose and fix common overdrive problems that either prevent the unit engaging, or stop it disengaging. Since overdrives are actuated electrically, and engage through a hydraulic mechanism, we’re going to look at both electrical and simple hydraulic faults.Virtually any electrical problem can be dealt with at home, as can some of the simpler hydraulic issues. The key is to be methodical.
FAILURE TO ENGAGE 1 Oil level If the overdrive won’t engage, it may just be due to low oil level in the gearbox. Remove the gearbox level plug and, if oil doesn’t dribble out, top up with the recommended grade of oil for your classic.
2 Electrical connections Examine the wiring around the gearbox and overdrive which leads to the solenoid. Fix any lose connections and try again. The oily, untidy wiring shown here doesn’t inspire confidence.
6 Sticking clutch Occasionally the overdrive’s cone clutch can stick to the brake ring in the engaged position. A sharp tap with a soft-faced hammer on the black brake ring usually frees the clutch.
3 Solenoid problems The solenoid which actuates the overdrive can fail. On A- and D-type overdrives, the solenoids also need to be adjusted correctly. Check whether the solenoid clicks when operating the overdrive switch inside the car.
4 Solenoid adjustment Adjustment of the solenoid operating arm on A- and D-Type overdrives is critical. Get it wrong and the overdrive will either not engage, or worse still fail to disengage. Incorrect adjustment can also burn out the solenoid.
5 Oil filter The overdrive’s hydraulic oil pump is protected by a mesh filter, which can block and prevent engagement. On J-type units the filter is housed behind this rectangular plate on the underside of the overdrive body, A- and D-types have filters retained inside brass plugs.
7 Sticking solenoid Sometimes a solenoid can stick, preventing it from engaging or disengaging. This J-Type solenoid’s internal rubber O-rings had hardened and it was sticking in the engaged position when hot.
8 Consult an expert Stripping and repairing an overdrive really is a job for an expert. If our easy electrical/ hydraulic fixes don’t help, there may be no option but to remove the unit and take it to a specialist for overhaul.
EXPLAINER Overdrive faults explained
1 Failure modes
There are two possible failure modes: won’t engage or won’t disengage. Failure to engage is most c
