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MG MGB Technical - Reverse Inhibitor – Reverse Logic Switch

I am little confused as to what the reverse inhibitor switch actually does – does it prevent the overdrive operating in certain low gears and reverse. See paragraph 2 of the description below – is this the ‘gotcha’?

If this is the case has anyone fitted the Revington reverse logic switch?

(https://www.revingtontr.com/product/rtr1001k/name/logic-switch-overdrive-all-types

Not only would it solved this potential problem of reversing with overdrive in but it looks to be a good thing to have anyway ( edited from Neil Revingtons website) :

Logic overdrive switching device. This clever device allows for fast and sequential overdrive operation. Overdrive cannot be left in inadvertently, reducing strain on the overdrive.

When changing gear with the overdrive left engaged, overdrive drops out automatically. Fits all electrically operated overdrives and suits both positive and negative earth.

This device can be used on all electrically operated overdrives where inhibitor switches are employed on the gearbox to prevent the overdrive operating in certain low gears and reverse.

The modus operandi is this: Whatever gear or state of overdrive the car is in when the ignition is turned off, the overdrive will be off when you turn on the ignition again. When overdrive is required (in an overdrive enabled gear of course), flick the switch. When a non-overdrive gear is required from an overdrive gear, flick the switch. If a fast change is required from say 2nd overdrive to non -overdrive 3rd, just move the gear lever to 3rd and the logic device will disengage overdrive for you as you pass through the gear shift gate. This applies to any gear change up or down the gearbox where the start point was an overdrive gear.


Mike Dixon

Talk about complicated! When operating correctly the MGB overdrive will only operate on 3rd and 4th gears. As the lever crosses the gate from the 3-4 plane to the 1-2 plane the inhibitor switches it out. As reverse is to the left of the 2nd gear position, it can't operate in reverse either. Simples.

Of course, if the manual switch is left in the on position, the O/D will kick in as you change from 2nd to 3rd, and will remain on as you change from 3rd to 4th. This is no problem for the O/D unit and will do it no harm.

The unit you describe would allow you to change from 3rd O/D to direct 4th seamlessly, but since 3rd O/D is almost the same ratio as direct 4th, why would you want to do that?
Mike Howlett

Lol - TR owners must be mad to spend that much on something that a simple latching relay will do!

O/D 3rd to direct 4th involves neither MG inhibitor switch so I can't see how the unit knows to drop O/D out?

Maybe TR boxes are different?
Chris at Octarine Services


Thanks , I think I have been a little worried by the dire warnings here and elsewhere about engaging reverse with the overdrive in. Why the concern if the inhibitor works?

I agree the logic switch is a perhaps a bit OTT, nice though it would be to go from O/D 3rd to Non O/D 4th so easily, I think I can live without it. It was the reverse protection from OD in that was the main attraction - but it seems all being well, I have that already.
Mike Dixon

If you did go from O/D 3rd to direct 4th, you wouldn't notice any change had happened because they are just about identical ratios. It would be like changing from direct 4th back to direct 4th. There's no point.
Mike Howlett

MikeH,
I thought the same as you about going from 3rd o/d to direct 4th from when I had a B with o/d but someone on here pointed out this wasn't quite so.

Overall (earlier) gear ratios (0.802:1 o/d)
(3rd - 5.369:1)
3rd o/d - 4.306:1
4th - 3.909:1

Overall (later) gear ratios (0.82:1 o/d)
(3rd 5.4:1)
3rd o/d - 4.43:1
4th - 3.909:1

Assuming (dangerous I know) that the factory WSM version I have is correct with its figures.

The beauty (and downside with slowing) is that you engage and disengage the overdrive with load on which means not lifting off the accelerator (or fully) and not needing to use the clutch pedal so you have quicker changes and maintain revs more, keeping the power/torque more. Very useful for going from direct third to o/d third on a B to maintain momentum.

Those that insist on lifting off (fully) are missing the point, and dipping the clutch to save strain.

Going from 3rd o/d to direct 4th with a dash mounted o/d switch is a hands-free operation in the wrong way though.


Nigel Atkins

Had a B with overdrive that i ran in hillclimbs years ago and for sure 3od is different to 4
Ran a 4.55 axle
started off in 2nd gear(1st was uselessly spinny) with the od turned on but inop because of the blockout, wound up in 2nd then pulled the gearlever over towards 4th and it would pick up 2od, wound that up then into 3 then 3od and 4th
then had 4od to drive home which was reasonable, about the same as a normal non od B in 4th gear

from 3-3od-4 are fairly equal splits, maybe 3od is a little closer to 4 than it is to 3 but that's an advantage when you're getting up in the gears
William Revit

I built one for myself years ago, kits just a couple of transistors and a relay. That price is taking the mick. I got fed up changing out of an OD gear, then changing back up to 3rd having forgotten to turn off the manual switch, and getting a jolt as it engaged at an inappropriately low speed and another as I tuned it off. The circuit locks OD out until I turn the manual switch off and back on again. Mine just uses the existing manual switch so the box is out of sight, with push-buttons something has to be mounted somewhere.

The TR may be different but on the MGB going between 3rd and 4th does not trigger it as the reverse inhibitor switch on the gearbox remains operated as it passes through neutral, and it is that switch that triggers the circuit. There was a chap selling one for the MGB a while ago and he made the same mistake in his description.

OD 3rd makes a very useful gear for spirited twisty bits and long inclines, particularly with the LH. The D-type does have OD 3rd a little closer to 4th.

People change to a 5-speed, but I would never voluntarily do that especially with the 4-cylinder having OD on 3rd and 4th. The CB manual switch near the steering wheel is easy enough to reach and the RB on the column is even easier, and anticipates the later paddle changes that many set such store by. The last version on the gear-lever was the worst incarnation, IME.
paulh4

Even with a close ratio box od 3rd has the edge on direct top even though the difference is only 5%. On one particular hill climb it is consistently worth half a second.
Paul Walbran

Either I've got false memories again(?), or I'm a hobbit, or you guys have arms and hands the size of apes as I can't remember being about to flick the dash mounted o/d switch (door end of the dash) with my right hand still on the (original 15.5") steering wheel so to go from 3rd o/d to 4th direct meant left hand on gear lever and right hand to o/d switch so hands off steering wheel and I didn't have chins, boobs or bellies then to hold the steering wheel straight as I done so. Or am I imagining this?
Nigel Atkins

I had my od switch (on-off press button) mounted just above the rh steering wheel spoke and hit it with my thumb--used the horn circuit to operate it and had the horn button in the old od hole-------worked for me but then probably wouldn't suit everyone
willy
William Revit

Flicking off is an easy reach for me, switching is more is a stretch but still possible. With my circuit if I want to go from OD 3rd to 4th I just move the lever to the left and back again while in neutral i.e. before going into 4th, and OD locks out.
paulh4

I run a CB B and have both the dash mounted switch and a gear leaver switch, both will operate the OD but you have to use the same switch to de-activate it as used to activate it. I only ever use gear lever switch as it allows me to switch out the OD while changing down. The only exception to this is when I’m wearing gloves in cold weather with the top down, the gear leaver switch is a bit difficult to operate, in these cases I use the dash switch.

The only issue I’ve had with the gear lever switch, which was fitted by a PO, is they used ordinary flexible wire. I didn’t think this was suitable to handle the continual movement of the gear lever so I replaced the section that flexes with traficator wire. This is extremely flexible and is designed to handle the movement.

Bob
R.A Davis

On the recent Targa Time trial Rally found that going from 3rd od to 4th standard a pain as I had to swap hands in my 67. Agree with Paul Walbran that 3rd od is a very useful gear as 80mph ( speedo not accurate ) is easily achievable
Rod
R J Pemberton

Bob - your switches must be in parallel, it would have been easier to wire them in series. That way either switch could be used to disengage, but both would have to be on to engage - horses for courses.

The gear-lever switch wiring is susceptible to shorting out which as the circuit wasn't fused from the factory can cause significant wiring damage in the main harness as well as the gearbox harness, and to the ignition switch. I always recommend fitting a fuse regardless of which type of switch is fitted - dash, column or gear-lever, typically where the main and gearbox harnesses join, although it can be fitted at a dash switch as well. D-type ODs have two unfused 12v supplies so both need to be protected.

I've not had a gear-lever switch car but both my MGBs and others I have worked on have suffered from shorts at the pump (mine before my time), which has damaged the rear and main harnesses. So I always recommend fusing those as well, again typically where the main and rear harnesses connect.
paulh4

Totally agree 3rd to 3rd o/d is a joy and very useful, no lifting keep your foot in flick the switch, keep momentum. Very useful in UK for joining a fast dual-carriageway or motorway from a slip-road or for overtaking or on traffic free twisting A or B roads.
Nigel Atkins

Paul, having the switches in series is a good idea but as say you’d need to ensure the one you weren’t planning to use was switched on. Perhaps the real answer is 2 way switches, that way either switch could engage or disengage OD. The only down side is it might look a bit out of place on the dash and fixing it to the gear lever could be a challenge.

Putting a fuse in the OD circuit was the first thing I did, I also put a relay in the circuit, it wasn’t strictly necessary as the load doesn’t justify it but I don’t like switching inductive loads with ordinary switches. I do understand the relay is inductive and yes I could have used a diode, but although I haven’t measured it I’m sure the relay is only a fraction of the OD solenoid’s inductance.

Bob



R.A Davis

Couple of these, no problem.

Or a rotary switch under the knob ...


paulh4

A fuse sounds like a good plan - what size for the overdrive switch?
Mike Dixon

I've just used standard 17 amp rated 35 amp blow glass type in in-line holders for both as there are two spares in the fusebox. No point in proliferating different values, and the wiring is standard gauge so doesn't need a lower rating.
paulh4

This thread was discussed between 07/01/2020 and 10/01/2020

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