[Mgs] FW: Overdrive Diagram

PaulHunt73 paulhunt73 at virginmedia.com
Thu Jan 24 01:45:20 MST 2019


The main difference between the earlier black label on all chrome bumper cars and the later blue label ODs on all rubber bumper cars is speedo gearing - the black label is 1280 turns per mile and the blue label is 1000 turns - a very significant different to speedo reading if you use the 'other' one.  This change was made on RHD cars as well and they always had OD on 3rd and 4th.  OD was restricted on North American spec cars only for the 1977 year on as part of emissions reduction, tied in with the 4th gear only vacuum advance system.

But never mind swapping the OD, it beggars belief that they could give it back with it still not working after having it all that time!  You could take it back, but I wouldn't based on previous experience.  You can check for electrical continuity by putting an ammeter in series with the manual switch.  With the ignition on, in 4th gear, and the manual switch on, you should see about an amp flowing.  If not then you will have to check for 12v on each side of the gearbox switch to see where you have 12v and where you don't.  As they had the gearbox out to change the OD it's quite likely they mucked up the connections there.  You can get at the switch by lowering the rear cross-member and going in through the top of the tunnel, exact method depends on which of your cars is involved.  But first peer up from below to check the wire from the solenoid cover is attached via a bullet connector to the sub-harness that goes over the gearbox, and both spades are on the switch, it should be just visible.

If you DO see an amp, then the problem is mechanical or hydraulic.  First thing would be to drop the solenoid and check that the plunger moves into the central position freely when powered, and the little ball bearing is present between plunger and cap http://www.mgb-stuff.org.uk/lhod.htm#5  Check all seals are sound, the one on the plunger is essential to develop pressure.  After that really it needs a high-pressure gauge to measure the pressure.  You could remove the pressure relief valve assembly and check it is complete and correctly assembled http://www.mgb-stuff.org.uk/goc.htm#3, ditto the pump, but you would still need the gauge to see if you have pressure or not.

PaulH.
  ----- Original Message ----- 


  The only difference is that the blue label unit only functions in 4th gear, not 3rd and 4th. Maybe you should try disabling the lockout switch.
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