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Overdrive
#1
174gt scott croxford
sheffield village, OH, USA   USA
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Have a new rebuilt black tag in my 74 BGT.
It will down shift out of OD if under a load (sometimes) and the last trip revealed at high speed/70 mph it would not even engage. Slow down to 60 and it worked fine.
What's up?

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Re: Overdrive
#2
gooser Avatar
gooser Drake Myers
Danville Va, USA   USA
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rebuilt by whom? sounds like you've got a bit of gunk under the piston in your solenoid.

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Re: Overdrive
#3
mbarjbar Avatar
mbarjbar Michael Barrera
Warner Springs, CA, USA   USA
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1967 MG MGB GT
1971 MG MGB "GIGI"
Start here:

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Re: Overdrive
#4
stvclif1 Avatar
stvclif1 Steve Clifton
Las Vegas, NV, USA   USA
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1972 MG MGB "Annette"
Correct fluid type? You'll get a lot of opinions- but a lot of people use 30wt. non detergent motor oil. The detergent in conventional oils reportedly will foam up in the OD.

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Re: Overdrive
#5
Davew70mgb Dave Windisch
Solon, IA, USA   USA
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1970 MG MGB
The John Twist video is a good thing to see and get some really good info on the problem. But it may be something as simple as low oil level in the trans. If the level is low, there's not enough oil to pump into the system to activate it. Also frothing due to the low level might cause the higher speed problems as well because it pumps air with the oil.

Might be a good easy place to start checking and go from there.

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Re: Overdrive
#6
ErnieY Avatar
ErnieY Ernie Y
Albatera, Alicante, Spain   ESP
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1976 MG MGB GT
Leaving aside the fact that down shifting under load is not really good practice your symptoms are contradictory.

On the one hand you say it won't release and on the other that it won't engage at 70 but will at 60 (which on it's own makes no sense) but there is no fault which I can think of which would give both those symptoms at the same time.

If it were me I'd get a pressure gauge on it because until you know how much pressure it's it's producing, and how that's reacting to being engaged and disengaged, you're prodding in the dark.

'Gunk' under the solenoid piston would effectively prevent engagement and I'm not sure how knowing who rebuilt it is going to further the diagnosis unless of course Drake is suggesting that nobody but Quantum Mechanics can be trusted to do the job properly ?

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Re: Overdrive
#7
ohlord Avatar
ohlord Platinum Member Rob C
A tiny Island off the coast of Washington State, N.W., USA   USA
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1957 Land Rover Series I "EYEYIYI"
1957 Land Rover Series I "OVRLND"
1971 MG MGB
1971 MG MGB "Bedouin 2"    & more
Drain,Clean,service,seal,fill properly.
It will work again.


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Home of Project "INCARN8'


1957 Series 1 Land Rover electric VEHICLE CONVERSION

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Re: Overdrive
#8
ErnieY Avatar
ErnieY Ernie Y
Albatera, Alicante, Spain   ESP
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1976 MG MGB GT
Maybe !

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Re: Overdrive
#9
joron Avatar
joron Silver Member Michel Joron
Roxboro, QC, Canada   CAN
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1977 MG MGB "Victoria"
2018 Jaguar XE "Meghan"
Drain, fill the gearbox with ATF. On the jacks (be very careful) just run each gear slowly. ATF will splash the internal of the gearbox and the overdrive everywhere it will dissolve the gunk for at least 10 to 15 minutes or a slow ride around the neighborhood few times.

Stop the engine, flush the ATF, let it drain for at least 1 hour, fill the gearbox with the normal oil for the gearbox. I used 20w50 GTX Castrol.


1977 MGB Red 'Victoria' and 2018 Jaguar XE 'Meghan'
All Classics:
Married, 2 children, MG MGB May to December, Jaguar the rest of the year.

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Re: Overdrive
#10
ErnieY Avatar
ErnieY Ernie Y
Albatera, Alicante, Spain   ESP
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1976 MG MGB GT
I've tried the ATF 'trick' with an OD which was reluctant to release and even drove the car a hundred miles or so with it but it made not the slightest difference.

Not saying don't try it, by all means do, but equally don't get your hopes up too high.

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Re: Overdrive
#11
joron Avatar
joron Silver Member Michel Joron
Roxboro, QC, Canada   CAN
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1977 MG MGB "Victoria"
2018 Jaguar XE "Meghan"
The last thing is maybe the valve relief needs to be cleaned or the slump filter where are the magnets to clean up as well.

The valve relief is behind the big bolt on top of slump filter where are the magnets need to unscrew and there you will see few o-rings, a valve with a screening/filter after with a spring then a ball-bearing.


1977 MGB Red 'Victoria' and 2018 Jaguar XE 'Meghan'
All Classics:
Married, 2 children, MG MGB May to December, Jaguar the rest of the year.

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