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MG MGB Technical - Question for RB OD owners

Hello all,
I have just bought an overdrive transmission for my '77 B. It's a later unit because it has the side oil fill.

The shift knob has the OD switch on top, but the shift lever looks like a standard one and any wires that may have been there are gone.

1) Are the lever interchangable?
2) How are the wires routed?
3) Is there a nice alternative to using this knob switch, as I really like my current wooded shift knob?

Thanks in advance!!
David
David Steverson

David. The same lower portions of the knob are used with both the standard, four speed transmissions and with the overdrive transmissions. As you note, the cap differs between the two transmissions.

The wires go down the cut out area of the gear lever and are rather thin. I believe I remember some form of device to secure them in the track--something like a rubber tube type device. But, it has been a long time since I used the standard set up.

Yes, perfectly possible to use an alternative means of controlling the over drive function. I like to mount a toggle switch on the upper, left hand side of the dash. There is a blanking plate there which can be removed from behind--there are four "legs" that secure the blanking plate into the dash. Squeeze them together slightly and remove the plate from the dash. Drill a hole for the toggle switch to fit into, wire up the line from the engine compartment to the switch, thence to the over drive inhibitor switch. Make sure you use some of the shrink type tubing on the wire to switch connections to insulate them from anything which might touch them. I found it worked best when I used a little black, silicone seal to securely hold the blanking plate into the dash.

All of the four syncro transmissions interchange without problem. If you are looking for originality, the side fill transmission is correct in a rubber bumper car and the dip stick type in a chrome bumper car. But, either will work fine in either style of vehicle.

Make sure that you purchase the longer speedometer cable for the over drive transmission. The current one will not work. Also, make sure you remove the new cable from the outer sheath and lubricate it before installing it. The retailers do not tell you that you have to do this, allowing you to purchase a second cable from them after the first one seizes due to lack of lubrication.

Les
Les Bengtson

I actually prefer the switch on top of the knob. When I installed an OD in mine, I machined up a replacement custom base for mine, having lost the original somewhere along the line. Then I used an original type replacement cap switch on my new base. It's a perfect placement for switching with a flick of the thumb without removing your hand from the shifter.

You could always make a new walnut base or modify yours to put a switch in the top of it. You will want to switch it frequently enough that reaching up to the dash will be annoying.

You should also be sure to use the 3rd/4th gear lockout switch on the transmission to insure that it only activates in those gears.
Gerald O'

Les and Gerald,
Thank you for your helpful comments!

Gerald,
I'm not familiar with a "3rd/4th gear lockout switch". Is that one of the two switches for wires on the transmission?

As is obvious, I've never had an overdrive.

David

David Steverson

The first and last thing to remember is that you MUST have a gearbox lockout switch in the circuit or if anyone reverses the car and the manual switch has been left on you could easily destroy the OD.

You are better off without the gear-lever switch, the wires of which are prone to fraying and shorting out and as they are unfused burn the wiring. They can be fused but personally I think the mid-term column switch was the best as it allows you to flick OD in and out without removing your hands from the wheel and as such anticipates the much-vaunted paddle changes of recent performance cars. I use that feature far more then the need to change gear and switch OD at the same time.

The switch on the left-hand side of the gearbox is the lock-out switch, that on the right is the reverse light switch. If the gearbox you have came from a pre-Feb 77 rubber bumper car then it won't have the gearlever switch or wiring as they used a column manual switch. But if the gearbox is from a 75 rubber bumper it should have yet another switch which operates in neutral for the sequential seat-belt system!

If you have a car built after Feb 77 your gearbox should already have two switches, the one on the left operating in 4th gear only, to operate the Transmission Controlled Spark Advance system. This switch was also used to control the OD on those and later cars which had it, which was why they had OD in 4th only. If from before July 76 it may only have one switch for the reversing lights. But if built between those two dates it could have *three* gearbox switches, the one on the left operating in 3rd and 4th for the OD (where provided), and also feeding an extra switch on the front which operated in 2nd and 4th, the output of this being used to operate the TCSA in 4th only.

First thing you need to do is determine whether your wiper switch has the OD function or the flick-wipe function. I thought this change occured in late 76 for the 77 year but some diagrams indicate it didn't change until the 78 model year. You then need to determine whether you have TCSA or not, and if you want to keep it if you do. Finally you have to determine which of the three (at least) possible gearbox switch arrangements you have. Only then can you decide what wiring you will have to do!
Paul Hunt

Paul!
Thank you for taking the time to provide a concise explanation!

Both my B (built 5/77) and the newly acquired overdrive transmission have the same 2 switches.

My wiper/washer switch is just that, apparently the flick-wipe to which you refer.

Based on all that, the left-side switch (which I suppose is currently a TCSA swicth) will now become my overdrive switch...

And I can just find a nice place to mount the switch that would have been in the shift knob, or do as Gerald suggests and make a new wooden knob (nice suggestion by the way).

I hope I got all that correct!

Here's to a fun winter project.
David
David Steverson

If you presently have TCSA (a solenoid in the vacuum pipe from inlet manifold and distributor) and want to keep its functionality then you will have to run a pair of wires up to the manual switch from the left-hand gearbox switch.

One side of the gearbox switch should have a yellow/purple or white/brown wire. This should be live whenever the ignition is on and is the input to the gearbox switch.

The other side of the gearbox switch, which currently should have a yellow or yellow/red wire on it, is the gearbox switch output going to the TCSA solenoid. One wire to your manual switch goes with this wire. The other wire coming back from the manual switch goes to the overdrive solenoid.

If you don't have or want TCSA you can insert your manual switch in a more convenient location in the engine compartment. You should have a white coming off the inertia switch going to a 4-way bullet connector. This should have one white wire going into the rear harness for the fuel pump, and a white/brown going into the gearbox harness for the TCSA/OD. The manual switch should be inserted where the *gearbox* white/brown joins the connector. At the gearbox the white/brown (which may have becomne a yellow/purple) goes to either side of the gearbox switch and the other side of the gearbox switch goes to the OD solenoid.

Whichever method you use it would be a good idea to fuse the OD, and the fuel pump, as this is another circuit which has been known to short and burn harnesses. If you insert an in-line between the inertia switch and the 4-way bullet conenctor mentioned above you will be fusing both circuits. If you want to fuse them independantly then insert one where the white from the rear harness joins the 4-way, and another where the white/brown from the gearbox harness joins the 4-way. A standard 17-amp rated 35amp blow cylindrical type is best as you have (or should have) two spares in the fusebox. There is nothing to be gained by using lower rated fuses, and it just means you have to carry more spares, same as if you use blade-type instead of cylindrical.

One thing to find out is when the new gearbox switch oeprates i.e. 4th only or 3rd and 4th. If the latter and you have and keep TCSA this will also operate in 3rd and 4th (together with the OD) rather than 4th only as before. Clausager says the TCSA was added to prevent 'surging' in lower gears, you will have to wait and see if this happes to you :o)
Paul Hunt

OD in 3rd is okay, btw.

I mounted my switch in the spot where the dash light dimmer knob used to be. Connected the wires on the two sides of the rheostat together, such that my dash lights are always at full brightness, no big loss there, and that gave me a very nice location for a switch with no new holes drilled. This proved reliable and convenient.

I tried with and without TCSA. Losing TCSA is not tragic, but keeping it is not difficult. I kept the TCSA. Paul described how to do this far better than I could hope to.

Sam

This thread was discussed between 11/11/2008 and 16/11/2008

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