overdrive, new member

[clartbuster]

Hi, i recently bought my first triumph, its a gt6 and i love it. All good but when i bought it the seller told me the overdrive didnt work cos the cable had slipped off, i took it to the garage where they told me that it doesnt have a cable, its electric, and all the wiring is intact. The guy hadnt worked on many overdrive cars so he called his dad who is 80 and worked on a fair few spitfires but he didnt know either, can anyone give me any clues as to what could be wrong? cheers

[MissSpentYouth]

Hi there, They can be temperamental what age GT6 is yours as there are 2 different types. As a starter you should check that you are getting 12V across the solenoid on the side of the O/D unit. Jack it up and put it in 4th gear, switch the O/D on and also the ignition then locate a metal cylinder on the side of the O/D about 4" long by 2" diameter with 2 wires coming out of it. Check the voltage across the terminals it should be 12V. If it is not then you are in luck as it is easy to trouble shoot the simple electrics if you do get 12V then likely the problem is within the complex hydraulics of the O/D unit. It is worth taking it for a run for about 30 minutes and then try engaging it using the clutch. I had a box once that needed you to use the clutch first time from cold and then it was OK otherwise it wouldn't engage for 30 minutes :-). If there is any errors above then I am sure someone will set us straight as I am not an O/D expert myself and have a ford 5 speed in mine.

[Deleted User]

make sure the oil level is full as this is one of the most comman causes believe it or not for Od not working, get all the tunnel off and get checking the wires etc could be the solanoid sticking so can give that a tap, if your getting 12v to the solanoid stick it in gear with ignition on and flick the OD switch on the gearknob and you should hear the solanoid clicking (i could only hear it if i got rid of the switch  and just joined the 2 wires up, if its clicking then it should be an easy fix!

[Clive]

poor earth on the solenoid can cause problems, as can the inhibitor switches. And if the switch is on the gearknob the wires tend to break/wear through. Problems are usually electrical! Or oil as J5 mentioned above.

NB do not engage od while in reverse!VERY bad. thats why they have inhibitor switches.

It is bound to be gearbox tunnel out, and get it working/test it fully before putting it all back. It is also worth investing in a decent repair manual, or at least a haynes one. The older the better too (seriously, they seem to be getting thinner  and thinner)

[rotoflex]

Maybe he meant "wire" when he said "cable".  Does the speedometer work?

What year is the GT6?  Is the overdrive switch on the steering column or on the gearshift knob?  We can look up a wiring schematic for you.

Like clifty said, you will start this by taking the gearbox tunnel off to get at it & poke at the wiring, etc.  Removing a seat also is pretty much the best way to get at it.

Remove:  passenger seat, the upright thing the radio is in, upholstered thing over the handbrake lever, gearshift knob, carpet over the gearbox, then the gearbox tunnel.  LOOK IT'S FILTHY & DEPRESSING-LOOKING OMG.   But not a dang space shuttle computer or something, it's do-able.  You can easily clean the grease away with anti-grease handwipes like the ones Gojo makes, then you can see what's up.

[Deleted User]

why ake the passenger seat out? i have mx5 seats in which are way bigger and there is no need for removing the seat? just the H frame, carpet and tunnel.

[rotoflex]

It's much easier to move around & prod stuff, plus he won't be lying across the seat & breaking down the seat foams, dropping grease on the seats, etc.  He'll have room to roll around & look at all the doodads on the transmission.

It is much less trauma for the interior pieces.

Anything which makes the job easier makes the repair nicer, plus it will be a nice opportunity to pull the seat (easy) & see what condition the bases are in, refit with stainless bolts, grease the slider to ward off rust, etc.

And you're apparently a dang elf anyway, wriggling around in all the tiny cracks in the car!

[99176]

Easy enough for you jony, but GT6s have a roof !
Take the seat out,for the sake of two extra bolts I
usually take both out.

[clartbuster]

cheers for all the responses, youve given me plenty to be thinking about, the overdrive switch is on the gearstick and the speedo does work but its a bit funny - goes ok til about    30mph then seems really unresponsive after but does slowly drag itself up to higher readings. thanks agaIN FOR ALL YOUR HELP.

[Clive]

sounds like a new speedo cable might be needed too!

[rotoflex]

Do the easy gear oil checks before the harder stuff.  

You can drain & change the gearbox oil from underneath with the car on a lift.  Take it back to your guy & ask him to let you look at it & poke at it while it's on the lift.

The gearbox/OD oil level is set/checked while the car is level, so if you just jack up the front & get under it, one end will be higher than the other & the level will be wrong (overfilled, most likely).

Buy & take the gear oil with you, so it will be the right kind.  Most cars today use GL5, but our transmissions use GL4. You may have to read the backs of a bunch of bottles at an auto supply store to find the right one.
Oil type:  SAE 90 gear oil, GL4-spec
Quantity:  2 3/8 pints (Imperial)/2.85 pints (US)/1.35 litres



The gearbox section is black in the photo, the overdrive is the additional silvery unit behind it connected to the driveshaft.

That photo is of an OD in an earlier GT6 - my Mk3 with US seat buzzers (& perhaps other later Mk3's) have a backup light switch at 6, which the one in the photo does not have.

Remove the oil plug from the gearbox.  Then remove the drain plug from the OD.
Bottom view of OD:


Ask your guy to draw out as much of the remaining old oil with a suction gun stuck in the gearbox oil hole.

At this point, you're supposed to clean the filter screen for the OD.  It is accessed by removing the plate on the side (marked 1 in the photo above).  However, it has a gasket & a rubber washer behind it, & it is possible that when the plate is removied the gasket will tear up & the rubber washer behind may be found to have disintegrated.  So either skip cleaning the filter, or do the right thing & get a replacement gasket & rubber washer from a supplier like Canley's or Rarebits or wherever to be armed with when you remove the filter for cleaning.

After the filter is cleaned, replace it, & replace the OD drain plug.

Pump SAE 90 GL4-spec gear oil into the gearbox oil hole until the level is even with the bottom of the hole.  (That's what the manuals say, it means pump it in, then when it starts running out you say "Oh crap", & you let it run out then put the plug back in).

Do some investigation to see how the OD was installed.  It may hve been originally in the car, or added later.  If added later, there could be weird wiring, like the reverse inhibitor switch scheme was omitted.  



While the car is up on the lift, check for the presence of the OD gearbox switch (shown in Fig. 13).
When the car is down, look for the OD relay on the scuttle near the fusebox (shown in Fig. 12).

If the wiring to the relay does not look like "factory" wiring (same size & appearance as other wires, colors that look similar, etc.), be suspicious that it was added on.  If added on, you cannot be sure that the wiring will follow a factory wiring diagram, & investigation to find out what they did will first be neccessary to solve electrical problems.

If you change the oil & everything works fine, whoopty doo, congratulations.  Make SURE the overdrive does not engage in 1st or 2nd, because that indicates a non-standard hookup which may enable the OD to be engaged in reverse.  The OD should NEVER be engaged in reverse, or expensive damage can result.  If it does engage in 1st or 2nd, turn the OD off & don't use it until the problem is sorted out. to hopefully avert damage.  I'd even recommend disconnecting the solenoid until you have time to putz with it.

You said the OD switch is in your gear lever; the factory put it there on the Mk3's.  Previous GT6's had the OD switch on a stalk on the column.  Is your car a GT6 Mk3?  Is its commission # pre- or post- KE/KF 20,000?  I see different wiring diagrams for RHD Mk3's at that point.

Some folks have a misconception about what should happen when you turn the OD on.  When you turn the OD on, you get an extra gear that is the next higher up, & it feels like shifting from 1 to 2, 2 to 3, or 3 to 4:  it comes on, & the rpms drop while the car stays at the same speed.  It does NOT feel like a rocket has been launched in the back of the car or something, if anything, that is what it feels like when the OD is turned OFF.

You need a shop manual for your GT6.  It will tell you what screws to turn when things break.  Factory shop manuals show up from time to time on ebay, but you can get a decent reprint immediately on CD from the Heritage Motor Centre:
http://www.heritage-motor-centre.co.uk/shop/books-cds-dvds/technical-publications-on-cd-rom/triumph-bm/triumph-gt6-1966-to-1973/index.html

[don cook1]

Rotoflex - bleedin' brilliant!

[clartbuster]

rotoflex you are the man, thanks so much again, its going in on friday to get sorted and im taking all this on a printout. cant wait