Overdrive Issues

[Paul Harvey]

Finally got my overdrive working again by replacing the solenoid relay which had gone 'temperamental.' Everything seemed fine until I discovered overdrive was now also working on first gear! Without the engine running and the gearbox in neutral I can also get the solenoid to 'click' by engaging the overdrive switch/lever. Having heard horror stories of trashing the gearbox if the overdrive engages in reverse I wondered if anybody could tell me what they think the problem is and whether the car is safe to drive!

[Tony Millward]

Finally got my overdrive working again by replacing the solenoid relay which had gone 'temperamental.' Everything seemed fine until I discovered overdrive was now also working on first gear! Without the engine running and the gearbox in neutral I can also get the solenoid to 'click' by engaging the overdrive switch/lever. Having heard horror stories of trashing the gearbox if the overdrive engages in reverse I wondered if anybody could tell me what they think the problem is and whether the car is safe to drive!

 

 

Hi Paul,

 

Check your isolating switches on top of the gearbox, it sounds like they are either not connected properly, badly adjusted or faulty. DO NOT use reverse until you are positive the overdrive is actually switching OFF. It won't trash your gearbox but it will bugger your overdrive.

 

Cheers

 

Tony

[ianc]

The isolator switches, on top of the gearbox, ensure that there is no path to earth when the selector levers are in neutral, in 1st gear or in reverse gear (i.e. switches are "open" under such conditions).

HOWEVER! If the wire from the relay coil running under the gearbox cover to these switches should come adrift and touch the gearbox or some other piece of metal which is earthy, the relay can be activated at any time, as the isolator switches will have been bypassed. So you need to find the false earth connection somewhere along that wire from the relay coil to the isolator switches.

As advised, DO NOT REVERSE WITH OVERDRIVE ENGAGED - YOU COULD BURST THE OVERDRIVE CASING!

Ian Cornish

[Paul Harvey]

Thanks Ian and Tony for your advice.

[Peter Harper]

I had a problem with randem overdrive connecting in, and traced the fault to the operating lever was shorting out against the indicator wires at the base where they are tight and close together as they come down the steering column.

 

Peter

[Horse]

No doubt you have solved your problem by now, but for what it is worth I had the same experience earler this year but resulting from a defect different from those already referred to. In my case, the short was between the connecting pins on the overdrive switch at the head of the column and the metal case of the adjacent indicator switch. These are in very close prximity and any looseness in the nut holding the overdrive switch in position can allow the switch to move such that the connections are permanently shorted. I settled for insulating tape as a precaution in case the switch should come loose again in the future.

[ianc]

Horse - a short at the switch (as you have described) should NOT cause the overdrive to operate in any other than the gears permitted by the isolator switches on top of the gearbox (i.e. 2nd, 3rd and 4th). But, with such a short, the car would engage overdrive as soon as one of those gears was selected e.g. when accelerating, you would go 1st, o/d 2nd, o/d 3rd, o/d 4th - missing out the directs in 2nd, 3rd & 4th. In other words, it's the same as leaving the overdrive switch in the on/selected position all the time.

 

If overdrive gets selected in 1st (or reverse), it has to be a short to earth which is causing the relay to remain energised when it should have been released. This problem has been described earlier in the thread.

 

Ian Cornish

[Horse]

Ian,

 

Thanks for your interest, but sadly it does. Perhaps I explained myself badly? Although I have not physically traced the wiring, as shown in the circuit diagram I have the overdrive selector switch is fitted between the live from the overdrive relay and the gearbox interlock switches, effectively removing the gearbox interlock switches from the circuit. If this diagram is correct, a short to earth at the switch will activate the overdrive relay and hence the overdrive solenoid. I proved this to myself by intentionally shorting the switch against the indicator switch case and switching the ignition on and off - the relay clicked in every time regardless of gear selection. After insulating the indicator switch case overdrive selection was normal, and has been for the last 500 miles. Perhaps with my car (build date March 65) the metal case of the indicator body provides the path to earth?

 

Regards,

 

Horse

[ianc]

Horse - sorry, I misunderstood you.

You are correct in saying that a short to earth (bodywork) at the overdrive selector switch will have the effect of over-riding the isolator switches on the gearbox cover, so overdrive would be engaged regardless of which gear (if any) had been engaged. And this would mean that overdrive could be engaged with the gear lever in neutral or in reverse - the latter could be catastrophic, as discussed above.

Ian Cornish

[TR 2100]

Perhaps with my car (build date March 65) the metal case of the indicator body provides the path to earth?

 

Maybe, if it's a Monday morning or Friday afternoon car!

 

AlanR

Edited August 11, 2010 by TR 2100