Overdrive only in 4th gear

[Johnw60]

Hi,

Finally got the overdrive gearbox in my spitfire and it’s great, works a treat, the overdrive works instantly as well. But only in 4th gear., I have wired this up from fused power to the inhibitor, then through the gearstick switch to solenoid, good earth as well.

When checking with a meter, ignition on, 3rd and 4th gear gear when selected in both in overdrive gives 12 volts at solenoid. So you would think everything is fine, I’ve checked this a few times, the only power the solenoid gets is if 3 or 4th gear is selected then flick the switch in and our. Power goes in and out at solenoid.

If the ignition is on, flick the switch and you hear the inhibitor tick, all good and well.

Pop the tunnel on, take it a test drive and, only overdrive in 4th works. It’s instant.so not like a sticky solenoid. I’ve ran till it’s warm 20 miles. Trying but same  thing only 4th. Funny thing is. Just got back home, switched engine off, put into 4th, flick the switch and you hear the solenoid work. Try it in 3rd and you only get the tick from inhibitor, will have to take the tunnel of to check again for power,  but done this twice already., all wiring is neat and not catching anywhere, have you any idea at all. It’s got me bambozaled, the oil level in gearbox is ok as well.

thanks

john

 

[68vitesse]

Famous last words, must be electrical, if it works in forth but not in third overdrive should be ok. Inhibitor switches can be a bit iffy.

Regards

Paul

[RogerH]

Hi John,

firstly the inhibitor switch should not tick - that sound is probably from the OD relay 

I don't know how many inhibit switches Spitfires have but whatever controls 3rd gear needs adjusting. They do wear.

Roger

 

[Johnw60]

Thanks .

I’ve ordered a new inhibitor switch, so I’ll try this. On the spit fire there isn’t a od relay. I thought about the switch, but was thinking why 3rd, only. 
hopefully it will work with the new switch.

will post my findings

john

[Pete Lewis]

when in 3rd hold the gear stick to left and right  , sounds like the cam is not fully prompting the switch  and this is just simple adjustment 

i take it this is single rail Box + Jtype  ??

pete

[Johnw60]

Hi Pete,

yes it’s single rail and j type, I’ve just had these overhauled. I have just converted from non overdrive,

I’ll try and get back our to try this, do you mean the cam under the inhibitor switch and how can this be adjusted, or can I just screw the switch in further

thanks

john

[RogerH]

Use a thinner washer on the inhib switch

 

Roger

[Pete Lewis]

rogers is on the ball.  its normally a simple card/paperish washer under the switch if you remove it use a smear of sealer on the threads 

Pete

 

[Johnw60]

That’s a great idea, will hopefully post the results next week, wont have a chance till next weekend now with work

im sure this will fix the problem

thanks

john

[Paul H]

I had a similar issue with my vitesse mk2 , my issue was that when in overdrive 4 it slipped out of OD . Pete suggested when it slipped out of OD hold the gear leaver on 4th and see if OD worked . It did and identified the inhibitor switch issue . In my case it needed a packing plate made out of scrap alloy . Sorted 

Paul 

[Johnw60]

Hi Paul thanks, I definitely think the problem is related to the inhibitor switch with all the advice, be next weekend when I can get the tunnel of and have a look, fingers crossed

thanks

john

[Johnw60]

Hi had time this morning, fitted a new inhibitor switch and yip that’s exactly what it was, thanks for all the advice it’s much appreciated

john