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overdrive on a 1961 humber hawk
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jiminox



Joined: 12 Dec 2011
Posts: 5

PostPosted: Tue Dec 13, 2011 8:40 am    Post subject: overdrive on a 1961 humber hawk Reply with quote

Hi, does anyone know why after driving my humber hawk for a few miles the overdrive stops working. thanks jim.
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victor 101



Joined: 03 Apr 2009
Posts: 446
Location: East Yorkshire

PostPosted: Tue Dec 13, 2011 5:22 pm    Post subject: Reply with quote

Have you checked the gearbox oil level?
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Phil - Nottingham



Joined: 01 Jan 2008
Posts: 1252
Location: Nottingham

PostPosted: Tue Dec 13, 2011 7:26 pm    Post subject: Reply with quote

also the filter and/or elecric connections and relays/switches
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jiminox



Joined: 12 Dec 2011
Posts: 5

PostPosted: Wed Dec 28, 2011 5:09 pm    Post subject: Reply with quote

The gearbox oil level is fine, and so are the connections. i have not checked the filter. the over drive will stay engaged if i leave it switched on until i turn off the ignition, then it will not engage again until it has got cold. thanks for your replies. also my manual says it needs 20w 20 oil, but i cannot get hold of that grade.is there a good equivalent oil i can use . cheers jim.
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MVPeters



Joined: 28 Aug 2008
Posts: 822
Location: Northern MA, USA

PostPosted: Wed Dec 28, 2011 7:46 pm    Post subject: Reply with quote

On the face of it, this sounds like low oil pressure in the O/D unit. Hard to test & harder to fix.

Fresh oil - & 20W/30 or even 20W/50 should be fine - might be a help.

If there's a Lucas 6RA relay in there somewhere, put a new one in! This was a common MGB failure point, though not really connected with temperature.
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Jim.Walker



Joined: 27 Dec 2008
Posts: 1229
Location: Chesterfield

PostPosted: Thu Dec 29, 2011 2:32 pm    Post subject: Reply with quote

The operating solenoid will have two coils (assuming it is not Laycock). One engages the overdrive and then cuts out leaving a holding coil to keep it engaged.
Your symptoms seem to suggest a heat related electrical resistance somwhere wich prevents the engaging coil to work until things cool down. Perhaps the operating relay or the operating coil in the solenoid. But first check the operating lever adjustment to ensure it is moving through the correct arc of travel. Usually done with a dowel (twist drill shank) in the on and off position.
Jim.
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richardlw



Joined: 08 Jun 2009
Posts: 138

PostPosted: Mon Jan 02, 2012 2:26 am    Post subject: Reply with quote

Reference to a 20W-20 oil is a reference in motor oil SAE chart, where we should be looking at the gear oil SAE chart. a 20W-20 would be a 75W-80 gear oil, and should be a GL-3 or GL-4 for most transmissions, especially synchronized gear boxes.

You should find several brands with a 75W-80 GL-4 or GL-3 on the shelf.
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jiminox



Joined: 12 Dec 2011
Posts: 5

PostPosted: Tue Feb 28, 2012 11:51 pm    Post subject: overdrive Reply with quote

thanks for all your feedback people. i am going to try them all and let you know the outcome. much appreciated. jim.
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