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Overdrive partial disassembly in-situ?

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Overdrive partial disassembly in-situ?
#1
Samrobmoe Sam Moeller
Cardiff, Cardiff, UK   GBR
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Hi all, I've been experiencing slip under heavy load in 1st and 2nd. I've changed the clutch twice (2nd time more rigorously, including other work) but I think the consensus is that this is an indicator of a slipping inner cone clutch on the Overdrive?

It is a 79 RB, so it has the blue label LH overdrive. I'm wondering if it is possible to remove only the back section of the overdrive unit to inspect the cone clutch, whilst it is still on the car?

Otherwise, is there any other way to check the cone clutch without removing the engine and gearbox?

(I have also done 2 oil changes on the gearbox with the current oil being a specific gear oil made for cars with overdrives - Dynolite Gear 30)

Many thanks!

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Re: Overdrive partial disassembly in-situ?
#2
AbingdonNerd Gold Member Jeffrey Delk
Hartwell, GA, USA   USA
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1950 MG TD
Dick Moritz will most likely chime in on this, and he knows a great deal about these units.


Edit note: I misread your initial post, I thought that you were slipping when in overdrive. My apologies

It sounds like you have low pressure - that might be addressed with a new relief spring and shims and possibly new ball for the seat. Sometimes the balls are also placed in the wrong positions, or an incorrect size is used as a replacement. I would check the pressure first and that can be done easily with the unit in place.


Jeffrey W. Delk
Hartwell. Georgia USA
1950 MGTD #2301,1954 MGTF 1500 #7673,1957 MGA
1964 Alfa Romeo Giulia Spider,1967 MGBGT,1971 MGB Tourer



Edited 1 time(s). Last edit at 2026-04-15 04:35 PM by AbingdonNerd.

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Re: Overdrive partial disassembly in-situ?
#3
ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
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1972 MG MGB
The filter, pump, solenoid and pressure valve assembly can be serviced from below, beyond that the engine/trans will need to come out.

What fluid is in the transmission?

Were it me, I would borrow/buy a hydraulic pressure gauge and check the pressure, electrical etc before disassembly.

The OD lockout switch is a common issue, it can be adjusted from above through the shifter hole in the tunnel.

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Re: Overdrive partial disassembly in-situ?
#4
QUADTEQ Avatar
QUADTEQ Ken Q
Appleton, WI, USA   USA
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1964 MG MGB "Red"
1967 MG MGB MkI "MIDG BE"
In direct drive forward gear, the one-way sprag clutch, not the cone clutch, transmits the torque. If it is slipping, disassembly will be required.

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Re: Overdrive partial disassembly in-situ?
#5
dickmoritz Avatar
dickmoritz Platinum Member Dick Moritz
Philly 'burbs, PA, USA   USA
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Actually in direct drive forward propulsion is provided by both the unidirectional/sprag clutch and the cone clutch via the annulus. Regardless, I agree with others that any internal service of the overdrive requires engine and gearbox out...

Dick


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

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Re: Overdrive partial disassembly in-situ?
#6
Samrobmoe Sam Moeller
Cardiff, Cardiff, UK   GBR
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Thanks all, that's a shame, it'll be annoying to have to remove the engine for a 3rd time in 4 months!


Does it sound like this is the only likely cause? I bought the car for autotest - like Autocross in the US, so it's often being pushed in first or second gear. It seems to slip smoothly (no real judder) but only in low gears. I might try running with the solenoid disconnected, just incase it is somehow partly activating.

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Re: Overdrive partial disassembly in-situ?
#7
Samrobmoe Sam Moeller
Cardiff, Cardiff, UK   GBR
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Sadly, disconnecting the Solenoid did not solve the problem, so looks like it's not anything to do with it partially engaging. Does anyone know a way to conclusively prove it is the overdrive slipping?

I'm partly tempted to just get a non OD gearbox and fit that as I'm worried that with my use case for the car, having the OD as a potential failure point makes it not worth it!

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Re: Overdrive partial disassembly in-situ?
#8
ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
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1972 MG MGB
I've not heard of your situation before, maybe an experienced autocrosser will chime in or ask over on the MG Performance forum https://www.mgexp.com/forum/mg-performance-forum.83/

I think the limited product MGC and V8 MGBs used the same OD transmission which likely generated a lot more torque in the low gears.

In my locale can't give a non-OD transmission away, they usually end up going to salvage after taking a few of the good bits. I would think you could locate one easily in your area.



Edited 1 time(s). Last edit at 2026-04-17 07:42 AM by ClayJ.

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Re: Overdrive partial disassembly in-situ?
#9
Granville58 Avatar
Granville58 Silver Member David McNair
Blacksburg, VA, USA   USA
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1974 MG MGB
Given that all home market cars, starting in '76, came with OD, it should not be hard to find a working OD trans.

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Re: Overdrive partial disassembly in-situ?
#10
Samrobmoe Sam Moeller
Cardiff, Cardiff, UK   GBR
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I had a look at the solenoid again today and it almost seems like there was a bit of grease or gel at the end where the oil flows. I've cleaned it up as best I can.

In the case that there is somehow contamination, how bad of an idea would it be to open the sump plate and spray a lot of brake cleaner onto the cone clutch area?

I can't decide if it would all be able to run back out of the sump, or if I risk contaminating the oil?

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Re: Overdrive partial disassembly in-situ?
#11
Arizona Shorty G M
P, A, Albania   ALB
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The only time I have experienced slip in first and second gears is when I left the OD engaged in error. I have defeated the lock out function on first and second by choice, but then I need to remember to disengage OD below third gear. I know this is not normal, but then I am not normal.

So perhaps your OD is stuck in OD, which would omit the sprague clutch. It is really hard to tell the difference in speeds and acceleration in first and second as between OD and no OD, except for the tendency to slip under power.

I would verify that if things were gummy inside, whether your situation was that the solenoid was stuck in the engaged position rather than the default disengaged position.. Dick knows more about these things than just about anyone. All I can say is that even with a working OD solenoid, having the transmission in OD does not work well in first and second gears because the cone clutch wasn't made to handle the load.

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Re: Overdrive partial disassembly in-situ?
#12
dickmoritz Avatar
dickmoritz Platinum Member Dick Moritz
Philly 'burbs, PA, USA   USA
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I wouldn't spray anything in there, since it might not play well with the rubber o-rings. And if two oil changes didn't help, it's unlikely that spray would help...

Dick



In reply to # 5043330 by Samrobmoe I had a look at the solenoid again today and it almost seems like there was a bit of grease or gel at the end where the oil flows. I've cleaned it up as best I can.

In the case that there is somehow contamination, how bad of an idea would it be to open the sump plate and spray a lot of brake cleaner onto the cone clutch area?

I can't decide if it would all be able to run back out of the sump, or if I risk contaminating the oil?


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

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