Overdrive isolator switch. Your advice please.

[Sundowner]

Hi,
Although I've wanted to and needed to in order to replace the overdrive isolator switch, I've been unable to get beneath my Sprint for about 12 months now. So I saved up enough readies to get the thing to a mechanic with a hoist, so he could do the job.
As we had earlier figured out, the old (Lucas) switch was not working. The same test was performed on the new (Lucas) switch with success. So in it went. The old switch had 2 washers underneath it and the new one has just one. But on the first test drive, it wont go into overdrive. Can you offer me any suggestions please? I'm desperate to get this right as I haven't had any SPRINT fun for about a year now, and the car is still in the hands of the guy with the hoist.
I hope you can help.
Cheers,
Rob

[gmsclassics]

The answer to your question Rob is a), it is compressed to allow current to pass when in 3rd or 4th. Same principle with the reverse switch on the side.

What puzzles me is how you can change these from underneath? I've had to change these a number of times over the years and have always done it from inside the car, by removing the gearbox tunnel. That way you can test the circuit easily as the number of washers needed is part of adjusting to ensure the circuit is working correctly. I've always had to set these up individually, presumable to account for account for differing wear in the selector rods. I should note that while the old, original switches after 40 plus years are notoriously unreliable, I have found that the new ones don't have a particularly long life, particularly when I was racing.

You should be able to hear the click of the solenoid operating (with the ignition on) from inside the car, even easier with the tunnel removed! Incidentally I have found the solenoid to be very reliable.

The photo below illustrates the wiring / current flow. I rewired this one as I had also fitted an additional (new type) switch so I could use overdrive on second (not all gearbox covers have this hole). In this case the wire though into the gearlever and up to the switch was shorting.
So, with ignition on, the green supplies power to the reverse light switch and to the gearlever switch.
With the gearlever switch on, power travels down the yellow to both overdrive switches.
If the car is in 3rd or 4th (or in my case 2nd as well) it completes the circuit and power flows along the blue down to the solenoid on the overdrive unit.
So in practice when in the appropriate gear, flicking the switch on top of the gearlever sends power to the solenoid.
For reverse, when in that gear, power is connected to the brown wire along to the reversing lights.

Hope this helps

Geoff

[Sundowner]

Thanks Geoff,
That is most helpful.
We've checked out the solenoid and the gear lever switch and the wiring between them. All OK, but when the switch was removed it was not operating.
What is it exactly that the ball in the switch bears on?
I noticed in the parts book, there is a drawing of a (sort of) cam, pinned to the shaft to operate an isolator switch. (might be reverse). I assume from the drawing that it is a roll pin holding it in place. Is there any history about these breaking or bending, making the cam ineffective?
Again, Thank you.
Cheers,
Rob

[gmsclassics]

Photo shows the three selector rods, the centre one is the one that operates 3rd and 4th with neutral in between, the position it remains in when 1st, 2nd or reverse are selected. You can see they are all cylindrical rods. If you look carefully down the hole that the OD switch screws in you can see a machined flat that has 45 degree edges. (note that picture is taken with gear partially engaged so the step down to the machined flat can be seen more easily). The centre of that flat is where the switch lies when neither 3rd nor 4th are selected, so the ball on the end of the switch is out. I think it doesn't touch the rod at all in this position. When the gearlever is moved into either 3rd or 4th the selector rod moves to the position where the rod is the full circle and the ball on the switch is pushed in. The 45 degree angled part helps this movement allowing the ball to move into the switch. When installing the OD switch it should be done with either gear selected and washers added so that the switch is ON in the middle of the range to allow for heat expansion of the components. Those flats on the rods aren't shown in the parts book.
The exact same method using the cutout in the rod is used for reverse, however where the facility exists to add overdrive function to second gear, the actuation is on top of the brass forked selector rather than the rod so the appearance is different looking down the hole. You can see that protrusion on top of the fork in the parts book. Same principle though.

Hope this helps, let us know how you get on. Geoff

PS The inspection cover allowing sight of the 3 rods isn't on all the gearbox covers I have, just on this late model one. I also have covers with differing numbers of holes with the thread for the OD switch. This ranges from one (reverse only) to five, the latter being on a big Triumph saloon 'box that has been converted to use on a Sprint.

[GrahamFountain]

I think that answers a related question that came up before, about whether the overdrive is de-selected when moving between 3rd and 4th and re-selected on entering the other gear.

Some have claimed it is, implying it's selected by two notches, one for 3rd and one for 4th. I've never been able to feel that happen. But if the selector rod is operating the switch all the time the lever is over to the right, the overdrive will stay engaged when moving between (just) those two gears, and you won't feel it kick back in.

That is, unless you come out of 3rd or 4th, move the lever leftward, e.g. to the centre or between 1st and 2nd, and then go back right to engage 4th or 3rd. So I should be able to test that by centreing the stick between 3rd and 4th while OD is engaged next time I get out of traffic enough.

Or is that putting a C in OD?

Graham

[Sundowner]

Hope this helps, let us know how you get on. Geoff

Its sure does help Geoff,
Thanks again, in bucketsful.
Success? I'll have an answer for you tomorrow.
Cheers,
Rob

[tinweevil]

I think that answers a related question that came up before, about whether the overdrive is de-selected when moving between 3rd and 4th and re-selected on entering the other gear.

Some have claimed it is, implying it's selected by two notches, one for 3rd and one for 4th. I've never been able to feel that happen. But if the selector rod is operating the switch all the time the lever is over to the right, the overdrive will stay engaged when moving between (just) those two gears, and you won't feel it kick back in.

That is not correct. There is only one notch and it is aligned to the switch in the neutral position for that rod (box in neutral, 1, 2 or R). The switch is pushed (enabling overdrive) by the two un-machined areas either side of the notch, two un-notches if you will. Thus OD is de-powered as you move from 3 to 4. In practice it takes a while for the field in the solenoid to collapse, for the hydraulic pressure to change and the clutch to disengage so you won't feel it unless your changes are leisurely. If you're making leisurely changes you're unlikely to be using 3OD.