MGB & GT Forum
Overdrive 1972 MGB
Posted by slat grill
slat grill
Dean Bowman
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Topic Creator (OP)
Sep 28, 2016 07:37 PM
Joined 14 years ago
51 Posts
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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.
mgbanthony
Anthony Henderson
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Sep 28, 2016 07:40 PM
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Joined 18 years ago
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James.Spencer
James Spencer
Taunton, Somerset, UK
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Sep 28, 2016 07:43 PM
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dickmoritz
Dick Moritz
Philly 'burbs, PA, USA
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Sep 29, 2016 01:35 AM
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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
Errabundi Saepe, Semper Certi
(Often wrong, but always certain)
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
Errabundi Saepe, Semper Certi
(Often wrong, but always certain)
slat grill
Dean Bowman
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Topic Creator (OP)
Sep 29, 2016 07:48 PM
Joined 14 years ago
51 Posts
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Oct 1, 2016 12:54 AM
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Joined 14 years ago
19,079 Posts
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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?
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.
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?
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.
Member Services:
Please send photos for next year's calendar to photos25@mgbcalendar.com. If you need help with your Weber downdraft carburetor, feel free to call me at 978-249-5760, USA, Central Time!
dickmoritz
Dick Moritz
Philly 'burbs, PA, USA
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Oct 1, 2016 09:02 AM
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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
Errabundi Saepe, Semper Certi
(Often wrong, but always certain)
Dick
In reply to # 3365242 by spikemichael
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?
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?
Errabundi Saepe, Semper Certi
(Often wrong, but always certain)
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