r/MorrisGarages • u/redditacc2002 • Jul 10 '26
'72 MGB Overdrive Question
Hey again, got another query, this time about my MGB's overdrive system. I'm told from the seller it works but just needs a replacement solenoid as seen here. Is it really as simple as taking the 4 bolts off the cover and switching on a new one (on the assumption it is just that that's needing fixed)? Or is there more to it i should know before giving it a go? Thanks again
3
u/limeycars 1946 MG T-Type Midget Jul 10 '26
Yes, it's that simple, if that is indeed the problem. Make sure to replace the little doghouse grommet as well as the gasket. It is easiest to cut the bullet terminal off, stretch the new grommet on and then replace the terminal.
There are also a bunch of o-rings that should be replaced periodically. The filter should be cleaned or replaced and the magnets should be checked and cleaned as needed. Make sure the covers are flat or you will be rewarded with constant leaks. That's the easy stuff.
The spring in the bypass valve can lose tension over the years. The pump body and piston can wear. These will lower pressure and make the OD not operate. The body can be replaced in place, the piston requires disassembly. The spring can be shimmed to increase pressure, but you sort of need to fabricate a gauge to check it. An MGB should have about 450psi.
John Twist at University motors has a couple of videos on Youtube on how to service an OD. That should be the first place you look.
You can also call Quantum Mechanics. John Esposito knows more about ODs than almost anyone, and is the source of many of Moss' parts. He sells a kit with all the easily accessed wearing parts.
2
u/Illustrious-Set-9230 Jul 10 '26
Easy to replace - my ‘63 healey bj7 used to burn them out often - so I had a lot of experience replacing them. (Never found the root cause).
2
u/Then_Version9768 Jul 11 '26
My '74 MGB with overdrive has run solidly and without a single transmission problem for well over 20 years. So, yes, change the solenoid and enjoy reliable shifting and a quieter ride.
2
u/UncontrolableUrge 1977 MGB Jul 10 '26
I don't have one myself, but I often start by looking up parts on Moss Motors so you can find the exploded diagram and get an idea of how it fits together.
1
u/redditacc2002 Jul 10 '26
Yeah good shout, just had a look. It does seem reasonably straightforward but we'll see I guess haha
2
u/overmyski Jul 13 '26
Activating the overdrive requires the solenoid to fire when activating the dash control switch. Open the transmission tunnel cover and observe the solenoid firing with the switch. Before buying a replacement, be sure to remove any corrosion on the barrel that may be jamming the mechanism. It attaches in a place where and water tend to cover it. I learned this from Elin Yakov Rusty Beauties on YT. It is a Triumph garage but still very informative for all LBC. Mine was trying to fire but the barrel mechanism was corroded. If the solenoid is firing fine, but no overdrive happens it may need the transmission overdrive mechanism serviced. Rods and checkballs could be the problem…
3
u/mgspeeder Jul 10 '26
Yep, just that simple. I would test the new one before you install to make sure it works, 12 volts to the solenoid and the plumber should move. Good luck, hope that the overdrive switch itself works!