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Overdrive jumping in and out


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Hi, my son's GT6 MkIII's overdrive was jumping in and out randomly a while back.  He had let the oil drop a bit in the gearbox, but topped that up and fixed a loose connection to the switch, but none of this stopped it happening.
Any suggestions, please?


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If its got the switch on top of the gearlever then the most common issue is a chaffed or broken wire , the original wires were special flexible silacon insulation wiiires but most have been replaced with normal wires and these tend to chafe through or break pretty frequently.

The other thing to check is the solenoid adjustment as if the solenoid rod isn't moving the correct distance it can also cause a problem.

One other issue I've had is a broken wire inside the rubber solenoid shroud.

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If the stick-top switch wires chafe, they earth to the stcik and blow a fuse.
See the Canley trouble shooting list: http://www.canleyclassics.com/?xhtml=xhtml/infodatabase/overdrivesdtype.html&xsl=infodatabase.xsl

If it recently ran low on oil, I'd think worn cone and 'hairy bits' blocking the filter.


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The majority of MK3s had D Types, The genuine workshop manual I have at home which is dated Oct 73 (1` month before Mk3 production end) certainly only has details of a D Type included but I do believe a few of the very last of line were fitted with J Types although John Thomason's guide to originality states that no MK3s were fitted with J types at all...

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youve topped up the oil and checked the wiring, so what type is is  
Dtype solenoid is on drivers side   J type is passenger side.

to check setting of D type remove square plate in front of the sol.
  energise the sol.   you will see this move the lever , in the lever is a 3/16" hole , you need to adjust/check this hole aligns with a similar hole in the case behind the lever ... normal to use a drill  shank to slip thro' the  hole's

if you have a ammeter check the current draw  10 amps for a few seconds as it pulls in and there is a switch in the end of the sol, which once travelled cuts it down to 0.5amps.
the switch is not serviceable , most are sealed in the back end of the solenoid boot.

just some idea's  

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1078 wrote:
Thanks for the replies, gents.  I does look like a wiring thing, which, fortunately, the lad is retty good at.  better than me that's for sure.  I'll pass it all on to him.


i could be the switch , you can open them up and clean them ! did it with mine and workes great and saves money  ;)
Just look after overdrive electrical test here on the forum  :)

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Clean the filter, check the actuator arm placement, fill with oil.
Disconnect the solenoid, & run a long temporary wire from the solenoid into the passenger compartment.
Run another long temporary wire from the + side of the battery into the passenger compartment.

When driving in 3rd or 4th, have the passenger connect the two wires (or use a temporary switch).
The overdrive should come on, & not cut in & out.
If it does otherwise, either the solenoid is bad or the overdrive will need to come out for overhaul.
If it behaves OK, the problem is somewhere in the electrical circuitry before the solenoid.

Disconnect the wires (or switch off) before ending the test drive (while in 3rd or 4th).
The temporary wires bypass all the other electrical, including the reverse inhibitor switch, so it's essential not to get in reverse with the overdrive activated.

Possible electrical faults besides gearlever switch wiring:  dirty/intermittent contact in the reverse inhibitor switch, relay, or on/off switch, poor connection at some connector, fragile, worn, or rubbed wire somewhere, etc.

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