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Question for OD Experts - OD Dropping Out When - Updated Sep-10

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ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
1972 MG MGB
Afternoon All,
Sunday before last I was about 45 minutes into a 65+mph run in afternoon heat near 94deg F. The OD started dropping out, probably dropped out 6-8 times over 20 minutes at speed. That evening returning home at 6pm and much cooler, no dropouts. Checked level of Castrol 20w50 in transmission, it was full to side fill hole.

Yesterday afternoon after 45 minutes of mixed driving in 89deg F afternoon heat, OD dropped out 3 times. Returned to shop and hooked up pressure gauge, per shop manual at 30 mph I'm getting 250psi of pressure, at 50mph I'm getting 280psi, definitely well below the 400-420 I should be getting at 30mph per manual. It does generate the 250psi quickly and without hesitation.

This OD transmission was professionally rebuild ten years and 60k miles ago, never had any issues with this transmission in that period. Engaging/disengaging OD has always been very positive, and continues to be except for the dropouts. I have two years on this filter/oil change. I've inspected all electrical and everything seems to be in order.

With the low pressure reading, I assume I'm loosing pressure somewhere (o-rings, valve balls, springs). Before pulling the transmission what parts should I service to correct this? Is there any particular order? (solenoid assembly, pump check-valve, by-pass valve,...)

Thanks in advance!



Edited 1 time(s). Last edit at 2017-09-10 08:48 PM by ClayJ.

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mgbanthony Avatar
mgbanthony Platinum Member Anthony Henderson
Gananoque, Ontario, Canada   CAN
1962 MG MGB
1962 MG MGB
1970 MG MGB GT
1974 MG MGB    & more
Try tightening the 3-4 lockout switch, or removing one of the fibre washers on it. It can cause what you are experiencing, particularly in significant heat, if it is not screwed in far enough.

ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
1972 MG MGB
Anthony,
That was my first thought but not sure that would account for the 250psi diagnostic reading.

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dickmoritz Avatar
dickmoritz Gold Member Dick Moritz
Philly 'burbs, PA, USA   USA
Clay, you can replace the various o-rings that are accessible with the gearbox in situ, which include those for the solenoid (3), pressure valve (2), pump body (1), and pump plug (1) but, frankly, your pressure loss with hot thin oil is, most likely, oil bypassing the large o-rings on the two operating pistons, and the only way to access them is removal of the gearbox and disassembly of the overdrive unit...

Dick



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

ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
1972 MG MGB
Afternoon Dick,
Many thanks for your reply, probably not too surprising with 60k on the unit. Other than o-rings, main filter/gasket and solenoid gasket, any other obvious parts while I'm ordering?

I will probably take a stab at the o-rings to see if I'm lucky (I'm never lucky).

dickmoritz Avatar
dickmoritz Gold Member Dick Moritz
Philly 'burbs, PA, USA   USA
No other obvious, routine parts are accessible with the gearbox in situ, and, frankly, none internally either. You just have to get inside these boxes to assess the three ball bearings, the condition of the lining on the cone clutch, the presence and integrity of the bush in the main case, and the operation of the unidirectional clutch. Occasionally, but rarely, you'll find the axles worn on the planetary gears, and every once in a while you'll find that one of the little external circlips that retain the thrust rods has escaped.

Of course there are always the anomalies, like the box I built a couple of days ago. Customer complained that the gearbox he had purchased and installed "slipped" in forward gears and would not generate more than about 250 psi, far below the spec of 400-420. Turns out the previous owner must have "rebuilt" the gearbox, lost the solenoid check ball, and inadvertently replaced it with the ball specified for the non-return valve. Only a few thousandths difference in diameter, but enough to cause his malfunction. An overhaul including the correct check ball and the unit is now working just dandy...

Dick

In reply to # 3586027 by ClayJ Afternoon Dick,
Many thanks for your reply, probably not too surprising with 60k on the unit. Other than o-rings, main filter/gasket and solenoid gasket, any other obvious parts while I'm ordering?

I will probably take a stab at the o-rings to see if I'm lucky (I'm never lucky).



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

ohlord Avatar
ohlord Gold Member Rob C
North of Seattle, N.W., USA   USA
1957 Land Rover Series I "EYEYIYI"
1971 MG MGB
1971 MG MGB "Bedouin 2"
Has it been serviced? Maintenance intervals observed? Level checked?20w 50 conventional
Drain,flush,replace filters,screen,clean magnets,replace O rings
2years/24000 miles



"I'm a long way gone down this wild road I'm on
It's gonna take me where I'm bound
It's a long way around"



"These are the days that must happen to you"

RD2 Radar/ Electronic Warfare Technician
Vietnam 1969-1972

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ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
1972 MG MGB
Hi Rob,
OD last serviced about 2 years and 12k miles ago. Drained & replaced Castrol GTX 20w50, replaced filter. No flush, no o-rings replaced.

Unit was fully rebuild 10 years/60k miles ago. No issues prior to this.

ohlord Avatar
ohlord Gold Member Rob C
North of Seattle, N.W., USA   USA
1957 Land Rover Series I "EYEYIYI"
1971 MG MGB
1971 MG MGB "Bedouin 2"
Start with the basics.
Check fluid level
Easiest cause of intermittent drop out especially with the symptoms.



"I'm a long way gone down this wild road I'm on
It's gonna take me where I'm bound
It's a long way around"



"These are the days that must happen to you"

RD2 Radar/ Electronic Warfare Technician
Vietnam 1969-1972

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ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
1972 MG MGB
Fluid is at proper level, electric checks appear good.

Ordered all the o-rings, filter and solenoid gasket for Saturday.

ohlord Avatar
ohlord Gold Member Rob C
North of Seattle, N.W., USA   USA
1957 Land Rover Series I "EYEYIYI"
1971 MG MGB
1971 MG MGB "Bedouin 2"
Drain oil, fill with dex3 ATF, drive till the weekend engaging od frequently, drain,service,a thin thin bead of Toyota FIPG103 or similar around the gasket on both sides and light snuggling up will prevent leaks.



"I'm a long way gone down this wild road I'm on
It's gonna take me where I'm bound
It's a long way around"



"These are the days that must happen to you"

RD2 Radar/ Electronic Warfare Technician
Vietnam 1969-1972

Boris67MGB Richard Boris
Kings Park. NY 11754, USA   USA
1967 MG MGB
Last visit to Quantumechanica Ltd with a few LH types & a D type for test & rebuild; John is setting the LH's to 500 psi and 590 psi for D types. On the next visit will discuss his rationale.

ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
1972 MG MGB
Interesting info. Thanks Richard.

gooser Avatar
gooser Drake Myers
Danville Va, USA   USA
your oil could be foaming when hot. try replacing it with 30w non detergent oil.

gooser Avatar
gooser Drake Myers
Danville Va, USA   USA
i can only assume that if ernie felt the need to pm me that he also pm'd clay to express his opinion. i stand with what i said. the guy who runs quantum who regularly rebuilds overdrives only recommends 30w non detergent oil because multi grade oils can cause foaming problems in SOME overdrives. i do not know if these additives that we use in the usa are the same as the ones that are used in france where ernie lives nor do i know if the additives of today are the same ones that were used 40 years ago when these cars were new. my suggestion is a cheap and easy way to try and resolve the problem. in the past it has worked for me.

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