You are right, the two larger switches are the reverse light switch and the
overdrive enable switch. I don't remember which is which for sure, but it's
easy enough to check with a test light or ohmmeter. Or, you can look up
under the shifter assembly and watch to see which of the selector rods
activates which switch.
By the way, the switch that enables overdrive is active in 3rd or 4th gear.
With practice, you can actually shift 6 times. While 3rd gear OD is pretty
close to 4th non-OD, you can accelerate a bit more in 3rd OD than a regular
MGB that has just shifted into 4th. Then you shift to 4th non, and finally
4th over. No harm done to the tranny - I actually visited with one of the
Rallye drivers from Brown & Gammons who modify their OD trannys to operate
in 2nd, 3rd & 4th. Not recommended for 1st gear though - the engine torque
can damage the OD clutch in 1st gear, according to B&G. But I've been
driving my '79 (upgraded 4 cyl engine) pretty hard using OD in 3rd & 4th for
about 40,000 miles now, and no problems experienced.
A word of caution - don't get the wiring on the two switches mixed up - you
will have reverse lights in 3rd & 4th, and you will enable overdrive in
reverse. Doesn't take much speed backing up and the overdrive can engage.
The OD clutch has a uni-directional mechanism that will destroy the OD if
engaged in reverse.
I'm not sure about the other switch you mention. None of the overdrives
I've worked on have had that, but I did see an OD tranny for sale at a swap
meet that had a third switch - it was more like a small micro switch than
the two larger switches.
The wire coming from the underside of the OD unit is the power to the OD
solenoid.
Installation wiring (Late model MGB with overdrive selector switch on the
shift knob):
Locate a source of switched 12v (on when key is on) under the bonnet, right
side inner fender. This power is routed down the underside of the car
(follow the reverse light wires) and connected to one terminal on the
3rd/4th gear enable switch. From the other terminal on this switch, a wire
goes to the shift knob OD selector switch. From the 2nd terminal on the
selector switch, connect to the solenoid wire coming from the bottom of the
OD unit. It's recommended that you put an in-line fuse in this path at the
point where you connect to the switched 12v under the bonnet.
In the case of an earlier model MGB with the OD selector switch on the wiper
stalk, the wiring is already there under the bonnet. You should find a
yellow wire with nothing connected. Check this for 12V when the key is on
and the wiper/OD selector stalk is moved into the OD position. Run a wire
from this point (yellow if you want to maintain the color code) down to one
terminal on the 3rd/4th gear enable switch, and from the second terminal to
the OD solenoid. Again, it's recommended that you put an in-line fuse where
you connect to the yellow wire.
Hope this helps a bit, and good luck! Enjoy your overdrive!
Wayne Kube
Plano, TX
'63 MGB
'79 MGB (soon to have Rover 3.9 EFI)
----- Original Message -----
From: .
To: mgb-v8@autox.team.net
Sent: Wednesday, May 19, 2004 9:02 AM
Subject: trans. switches
am working on switching transmissions in my B.
i have a 1973 trans. with overdrive. trying to identify the electrical
switches on it.
if standing behind the trans. facing forward, there are two identical
switches on the top front of the housing. i know one is the reverse
light switch and the other possibly the lockout for 4th gear. which is
which.
there is another switch further back on the housing on the top right
side. not sure what this one is.
there is a single wire coming from the underside of the overdrive unit
which i assume is the switch for the overdrive.
i am sure the lockout and overdrive switch need to be hooked up in line
so i need to know the sequence.
can't seem to figure out these from my reference material.
any help would be appreciated. thanks
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