In <199508040322.AA239236534@pmoxsrv1.lvld.hp.com>, Werner Haussmann wrote:
>If this has been discussed before, please send me a copy of any helpful
>information. I am new to the list.
>
>I have a 74 1/2 MGBGT with overdrive. After about 10-15 miles, the overdrive
>'automatically' goes into overdrive. It works just fine when first starting
>out. Other symptoms are;
> * When under load it switches back out of overdrive (normal, no OD)
> * When overunning the engine it goes into overdrive in all gears (even
>first)
> * I get stuck in parking spaces because I loose reverse after driving for
> a while.
> * Everything is back to normal after about a half hour of inactivity.
>
>Because it works when first starting out, I hope not to pull the engine. After
>all the gears, clutch, etc should be OK. Don't you think?
>
>The oil level is OK. I use the engine oil, the green British stuff, for
>the transmission. The solenoid is OK.
>
>I sure would like not to have to pull the engine and would appreciate
>any help.
The way the overdrive is engaged is as follows: There is a pump on
the output shaft of the transmission that generates oil pressure.
Normally, the oil pressure is bled off through a relief port. When
you "engage" the overdrive switch, the solenoid blocks the relief port
and the oil pressure rises to 3-400 psi. This activates some pistons
that move the overdrive clutch (against some springs) into the engaged
position.
I would first assure that the problem is not electrical (....could be)
by disconnecting the line to the solenoid (there is a bullet connector
on the wire that you can pull) and doing your usual run. If the o/d
engages, then it is a hydraulic/mechanical problem. My guess is that
there is some crud in the internal hydraulic lines that is fouling the
relief port and after a while the pressure builds up. The flaw in
this hypothesis is that when you stop, this pressure should leak off and
disengage the o/d fairly quickly. The springs that disengage the
clutch are horrendously strong, and for the o/d to remain engaged
for any length of time after stopping beggars the imagination.
In any event, I'd recommend a complete
change of oil in the tranny (the book recommends 30W, no additives)
and you might consider dropping the filter cover (rectangle on the
bottom of the o/d unit) and changing the filter screen. I am a bit
worried because you say you lose reverse for a bit. This would imply
that the o/d (which has a unidirectional clutch) is engaged when you
go into reverse. This can cause a good deal of damage, and I would
strongly urge that you not even think about reverse if you think the
o/d is engaged.
Good luck! A. B. Bonds
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