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MG MGB Technical - Overdrive on Secondgear

In simple terms can i get overdrive on secondgear on my MGB GT. Yes i know the problems but would like to have it, well try it. Simple instructions please i am no engineer.
David Angel

The only way to get this would be to bypass the switch on the transmission that breaks the circuit to the overdrive solinoid when not in third or forth gear, it is mounted on the gearshift lever extension housing. If you do this you will have overdrive in every gear. If you forget and try to back up you will damage the overdrive. You could install a relay in the circuit so that when the reverse lamps are turned on it would disable the overdrive. To do this bypass the switch as above and put the normally closed contacts of the added relay in series with the solinoid. Connect the relay coil so that it is energized when the reverse light switch is energized. This will give you overdrive in all forward gears. Using the overdrive in low gears is not a good idea as the torque available in 1st & 2nd will shorten the life of the clutch on the OD sliding member. This is expensive.
John H

There is a switch on the geabox which switches out the o/d circuit when the car is not in 3 rd & 4th. There should not be o/d on second. To get at this switch is easy when the car is on a ramp. A hand up the LHS of the box wil find the adjuster. Or, cut a hole in the side of the transmision tunel. I think there were notes on this on the Chicago MG site.
Bob Marshall

Agreed. Give it a go if you wish but remember that transmissions *multiply* torque. Depending on your year, your ratio in 3rd is likely 1.27; in 2nd it might be 2.21, which means you're putting 61% more torque through the OD in 2nd than in 3rd, for the same engine output. That means you're exceeding the overdrive's design torque capacity by more than half. Plus, the OD oil pump is pumping more slowly than the designers were expecting, further increasing chances of slippage. Bottom line: in 2nd OD, don't floor it. Ever.

Anyway, to answer your question: simply bypass the lockout switch. The lockout uses the same kind of switch that the backup lights do. The reverse one is in back, where it's easy to get to. Can't miss it. The lockout is on top of the tranny, up near the shifter, against the top of the tunnel. If you start shouting cuss words when you're trying to get your hands onto it, you've found the right switch. Just short across it. Then, you'll have overdrive in any gear you want.

This will also allow you to run your OD in first and reverse, which is not good.

The reverse issue can be handled using a relay on your backup light circuit. Run OD solenoid power through the normally closed side of the relay, with the relay triggering open when the backup lights go on. Most generic automotive relays can do this. Five bucks, problem solved.

The real problem is actually not reverse, then, but first gear: there's no switch available to detect when you're in first! 1st gear ratio is somewhere around 3.64, delivering a staggering 266% of the torque delivered in 3rd, for a given engine output. A single jackrabbit start in 1st OD will likely cause spectacularly interesting behaviour on the part of your overdrive.

Therefore, it would be wise to order a replacement overdrive unit at the same time that you perform this modification, so that you have a spare available after you toast your current overdrive.

Have fun with it!
Sam Good

As has been said, a notably bad idea almost sure to cause gnashing of teeth (yours and the transmission's)when you inevitably bugger up the OD unit.

To do it right you need another isolater switch - and to modify the trans case for a boss for it, as well as the appropriate ramp on the shifter rod.

All that could be done, but the factor that really indicates it is a bad idea is that the pistons in the OD are not designed to function in second gear and if you get it on in second you will eventually damage the OD unit. The OD used in the MGB GT V8 had stronger springs and I can't recall if they also had suitable piston sizes to accomodate 2nd OD, but that might be an option assuming you could find a spare V8 OD unit.

If 2nd is really too low for you, the better options are to use a close ratio gear set, or go the 5 speed route.
Bill Spohn

Lets say someone installed the relay to keep the overdrive off while in reverse - if overdrive were left on in first gear, and the car rolled back at a stoplight, would that be enough to damage the OD unit?
Erick Vesterback

I cant cause damage, how would the o/d know the difference between being on the overun and rolling backwards. Once its engaged in 3rd/4th you can forget about it. OD 2nd is a nice gear, I had a switch problem and shorted it out once, you can do this under the bonnet. I swapped the problem interlock switch out promptly though, it was just too scary.
Stan Best

Remember that the OD is a hydraulic unit, not strictly electric. Hydraulic power is provided by a pump that goes when you're rolling. When you stop, no pump=no pressure=no overdrive within a few seconds of stopping. Rolling back a bit at a stoplight won't be enough to build the pressure needed to re-engage the OD. If you roll back so promptly that the pressure has not dissipated and the OD is still engaged, well, you'd still have to roll backwards with enough force to break something.


Overrun while rolling forward is in fact quite different from rolling backward. Think of the speed forward or backward as a positive or negative number - a bigger negative number is in fact a lower number. Similarly, the overdrive engaged in reverse, viewed from this perspective, would make the overdrive's output speed 'lower' than its input, right? Well, the unidirectional clutch allows the output to spin at the same speed or higher than the input. In reverse w/OD, the output speed is 'lower' than the input - the clutch's purpose is to prevent this from happening, so it engages harder to try to bring the output 'up' to speed, which further 'slows' the output, which puts yet more force on the clutch which tries to 'speed up' the output. This, my friends, is a feedback loop, and opposing forces will increase until something slips or breaks.

Hope that was semi-comprehensible.

-sam
Sam Good

Overrun forwards and driving in reverse are completely different activities as far as the OD is concerned. You would have to be rolling backwards in 1st gear (or any forward gear) at several mph for there to be any chance of damage, and even then I'm not so sure.

You can't bypass the OD lockout switch from under the bonnet - unless your arms are long enough and thin enough to reach down past the bell-housing and over the top of the gearbox! You may be thinking of the manual switch, and then only if the switch is on the dash or the column.

American cars did have an extra switch on the gearbox for a few months in 76 or 77. This operated whenever the gearbox was in 2nd or 4th, whilst the standard switch operated in 3rd and 4th as usual. The standard switch operated the OD as normal, the output of that switch also being wired through the additional switch to give a '4th gear only' signal to operate the TCSA. Maybe this was unreliable or maybe it was cost reduction, but for whatever reason this extra switch was soon dropped and the original switch modified to work in 4th gear only, and the output of that used for both OD and TCSA, which is when American spec cars ended up with OD in 4th gear only. Wiring these two switches in *parallel* would give you OD in 2nd, 3rd and 4th. But the torque reversals in 2nd can damage the OD, which is why it was limited to 3rd and 4th on the MGB, and 4th only on all but the earliest V8s. Triumphs had a stronger OD unit and *did* have OD on 2nd.

Paul Hunt 2

I wrote an article 14 years ago on how to bypass the od lockout switch and wire in a power cutoff relay in the reverse lights circuit for an MG publication. If used judiously there should be no problems. If not failure could well result. That said I have been running my LH od with no ill effects. On occasion I have left the od in the on position when coming to a stop. When accelerating from a stop the od engages at 15 MPH. RAY
RAY

This thread was discussed between 10/07/2007 and 15/07/2007

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