HELP! OD prob tracked down, but I need advice

[Sundowner]

Hi Folks,
I eventually extracted the digit from the fundamental today and attended to the overdrive problem on my Sprint.
Our investigations indicate that the gear lever/knob switch is OK and so is the solenoid. But the problem lies between the two and its looking like its the gear selector switch on top of the gearbox....OR....the wiring attached to it.
I need your advice on three things if you don't mind........
What is the part number of the switch?
Where do I begin to look for one? (I couldn't find it listed on the Brothers or RobSport sites)
What is the easiest method of extracting the switch if its stuffed? (without removing the gearbox) My idea would be to drop the gearbox down and try to access the switch from under the bonnet.
Of course it still may be a wiring issue, as going by feel, the wires attached to the switch are incredibly close to the floor and may well have rubbed through.
Here's hoping you can help me out, yet again.
Cheers,
Rob

[Sundowner]

Could it be the same item as a reversing light switch?
#BAU1074A or SMB499 ?
Rob

[xvivalve]

Could it be the same item as a reversing light switch?
#BAU1074A or SMB499 ?
Rob

It is indeed.

Probably easiest to remove the transmission tunnel, it’s a ball ache, but simpler and cleaner than dropping the ‘box.

The switch ‘just’ screws in....but they can be tight...

[xvivalve]

It is an inhibitor switch.if once you get access to the top cover, you see a large headed bolt screwed into a cast boss, buy another switch and replace the bolt with it. Wiring it up in parallel will then give you overdrive on second, which is very useful around town . Some ‘boxes don’t have the boss drilled and tapped, but that’s easily remedied but best done with the cover on the bench!!

[Sundowner]

Thanks for that.

I've just ordered one from an MG specialist in Adelaide who is an agent for Moss. It'll be here on Monday.
Are these things known to fail? They look pretty simple and bullet proof to me.

Good advice about the OD on second. I like it.

Thanks again,
Rob

[tony g]

While you have access to the switch, before removal put an Ohm meter across the 2 terminals (wires removed) and select 2nd to 3rd and watch if the meter changes. In 2nd it will read open (usually 1` on most meters) and in 3rd and 4th it should read 0 showing continuity. This will tell you if the switch is opening /closing.

Tony

[Sundowner]

Thanks Tony.
I believe I've written that before......... more than once.
Rob

[soe8m]

There are plastic washers under the switch. These are to adjust the height of the switch onto the selector shaft. When you take one washer out the switch will probably work again. When you don't find any anymore and you replace the switch be sure the new switch doesn't bind internally and pushes itself broken inside. When fitting a new switch just fit about 2mm (fibre) washers under the switch and measure if it switches. If not then go thinner every time till it switches properly.

Jeroen

[NickMorgan]

There are plastic washers under the switch. These are to adjust the height of the switch onto the selector shaft. When you take one washer out the switch will probably work again. When you don't find any anymore and you replace the switch be sure the new switch doesn't bind internally and pushes itself broken inside. When fitting a new switch just fit about 2mm (fibre) washers under the switch and measure if it switches. If not then go thinner every time till it switches properly.

Jeroen

That is useful information Jeroen. My TR jumps out of overdrive on the overrun. If I pull back on the gear lever the overdrive switches back in again, so I was assuming it is an inhibitor switch that needed adjustment. I was wondering if it was wear in the linkage, too, but your response sounds like there may be an easier solution.

[Sundowner]

Thanks Jeroen.

Thanks again to all who answered.
I'll let you know how I get on after the switch arrives, and I get motivated. As I mentioned before, it may well be the wires rather than the switch.

Cheers,
Rob

[gmsclassics]

Yes, those inhibitor switches are not the most reliable, especially under extreme temperatures I’ve found. Of course not forgetting that if the gearbox oil level gets very low, overdrive won’t work either. I’ve dug out a photo of the wiring I replaced on the race car just for interest – this replaced the original that had gone hard, failed in many places and I got fed up with fixing.
Another point is check the new switch, after fitting with washers as Jeroen says, works when the gearbox is hot as well as cold. I made that mistake on an old Jag – worked fine when fitted and tested in the workshop but stopped after driving about 10km by which time the gearbox was hot and expansion affected the switch positioning.

Wiring is self explanatory with green as power from ignition, brown out to reversing lights, yellow live when gearlever switch is on and blue live to solenoid when either of the inhibitor switches are activated.

[Sundowner]

Thanks Geoff,
A pictures worth a thousand words.
Cheers,
Rob

[GrahamFountain]

Of the two switches on top, which appear to be wired in parrallel, which one does what?

Graham

[xvivalve]

Of the two switches on top, which appear to be wired in parrallel, which one does what?

Graham

Rear most does 3 & 4, front does 2, side activates reversing lights.

[GrahamFountain]

Oh well. I was hoping for a separate 4th, as I don't really want it on 3rd (or 2nd).

Graham