The Triumph Dolomite Club - Discussion Forum

Hiya everyone!

Just a bit of advice please - I've got my wonderful new overdrive gearbox in which is working wonderfully - except sometimes the overdrive disengages, usually after about 10 minutes of running. I can usually get it back in by switching the switch back and forth.

Also, does overdrive usually engage a few seconds after you flick the switch?

Thanks!

In my experience of overdrives on Triumph and MG cars there is always a slight delay when you switch the O/D on. But it is slight - just a moment's hesitation. I think the delay is longer the more torque there is, and that backing off the throttle slightly will allow it to come on quicker. When switching off, the ones I have had switch out instantly.

If your O/D is switching out by itself, I would suspect the electrical connections first. The switch on the gear lever and the inhibitor switch in the gearbox. If either of these breaks contact momentarily the O/D will switch out.

Amazing, thank you for your help! I'll have a look at the connections and see how I get on.

After checking the wiring, double check the gearbox oil level. If it's a little low you might get issues.

I once had the problem where the O/D would drop out after a few miles and it turned out to be the inhibit switch was going open circuit when it warmed up. The switch was changed and the problem was solved.

If it helps, I converted my 1500 to overdrive and found that the switch operates the O/D unit within a split second. When it once stopped working altogether it was a plug and socket near the gearbox providing the electrical feed that was to blame. Unclipping it and cleaning the round pins and sockets as best I could has sorted it ever since - which was 2 years ago
Alan

The gearbox inhibitor switch is basically a push button that is operated when the selector moves into the 3-4 plane. Next time it turns itself off, try holding the lever over to the right and see if it comes back on. If it does the switch might need replacing or it might just need a washer removing from under it so it goes a fraction further into the gearbox.

So, although I haven't had the time to look at the problem directly, I've got more information from my work commute today haha! Pushing the gearstick over to the right doesn't change back into overdrive, making me lean more towards the dodgy wiring. 3rd overdrive is much harder to find than 4th, and today it only disengaged a couple of times in 20-ish miles of being used. As soon as I off/onn'ed the switch, it re-engaged without a hitch, so not a big problem. I'll clean up the connections and see where I end up

Check the wiring that goes down the gear stick shaft. The cable enters lower down and is a known trouble spot for shorting as the wire chafes and wears away.

I did wonder about this actually!! I was going to attach a piece of nylon to the top of the cable before I pulled it through, as I can imagine its a nightmare to push it back up?!

I think it unlikely to be the wires up through the gear stick shorting being damaged as normally when this happens you get a shock in your hand when you switch the overdrive out caused by the normal very high voltage that is induced in the wiring by the magnetic field associated with the operating solenoid valve collapsing. If you are replacing that bit of wiring my recommendation is replace it with a length of heat resistant ceiling light pendant wire as the two cores are contained in an outer covering which provides protection and make certain that this covering just comes through into the knob.

It could be the contacts inside the Overdrive switch in the gear stick. A spray with WD 40 might help.

Worth remembering that overdrive loom was just pulled off another old gearbox and untested. It may well be worth using new wires up the gearstick and checking the connections carefully.... Virtually all overdrive issues are electrical. (or low oil, or blocked filters which can be ruled out here)

Yes, very good point there Clive! Do I just use standard wire?! & wrap it all up? Do I need a higher resistance wire for the overdrive? I don't mind replacing it all, just need to know what to use haha! :')

Mine dropped in and out and was due to fatigued wiring around the base of the gearlever. If not that, the inhibit switch is probably the next item to check.

Just visit B &Q and purchase a ceiling pendant light fitting and use the twin cable or purchase some pendant lighting cable which will be heat resistant ( A plus I made some connectors for the switch by cutting down a couple of standard connectors if you do not have the special connectors.

See photo's