MGB & GT Forum

Recently my overdrive has been "jumping" out of overdrive while driving at speed. The first thing I'll do is check the oil level in the transmission. If the oil level seems sufficient, what would be other aspects to check? I'm not much of a mechanic, but I have a guy in my neighborhood who I can turn to. Thanks in advance, Dean.

Try holding the shifter firmly, and to the right in 4th gear when it goes out. If it re-engages, your 3/4 lockout switch needs either tightening or replacement. Common issue.

Work your way back through the connections (dash-overdrive), top up the oil, check the lockout switch.

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If it could be better, it's as good as broken.

Good advice from Anthony. The overdrive isolation switch sits atop the shift tower, on the left/carburetor side. It is difficult to access, and replacement is made easier by removing the removable cross member in order to lower the gearbox a bit. However before doing that, you can remove the two wires from that switch and install a temporary jumper wire between the two wires, properly insulated of course, to see if the problem goes away. If it does, then you'll know it's the switch that's at fault. Avoid reversing with the overdrive on/off switch in the On position when this jumper wire is in place.

Note that the switch is exactly the same as the backup light switch -- same part number and everything. It simply screws into place. The only provision for adjustment is by adding or removing red fiber washers under the head of the switch, moving it in or out a bit when it is tightened. It is common for the backup light switch to require one washer, and the overdrive switch to require two, but YMMV...

You can test the adjustment of the switch by connecting an ohmmeter across the two terminals of the switch. When the gearstick is in fourth gear, and also third gear if you have an earlier black label gearbox, you should have continuity across the switch. In any other gear the switch should be open...

Dick

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Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

Thanks to the three members who responded to my post regarding the problem with the overdrive. I took the car to my trusty mechanic this afternoon and discovered the problem was caused by a loose connection. Got lucky this time!

Hey Dick,
Can't you alter the blue label shifter to trigger the switch in third and fourth, it is a linkage thing not a limitation of the OD right?

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Michael J. Caputo
'79, '77, '76, '74.5 (rubber dual SU), and '73 owner. Extensive experience in 12v Audio System design and installation. Vendor of Regalia and Promotional Products since 1993. Supplier of Accessories to MOSS. Forum Member since 2009; with a warped sense of humor since birth. Publisher of the annual MGB & GT Calendar, mailed worldwide.

The conversion requires two different changes in the internal shift mechanism to properly reconfigure a fourth-only overdrive to a 3/4 unit. The overdrive unit itself remains unchanged...

Dick

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Errabundi Saepe, Semper Certi
(Often wrong, but always certain)