Jump to content

Fitting a GKN overdrive to a Defender

Recommended Posts

The following guide is the fitting of an Overdrive to a 300TDi 110 CSW, owned by DrGoon. You can use the 51 pictures in this thread as an addition to the instructions supplied with the unit, or as fitting instrutions on their own.

What you get for your money - the main O/D - there are three wiring looms, a speed sensor, gearknob, gaskets, control unit, bolts, clips, etc - in fact everything you need apart from 600-700ml AFT and some gasket sealant. The supplied cable ties (zip ties), are a bit weedy and break very easily. I used larger ones.


Remove the centre seat (or cubby box). There should be a flat plate retained by 4 x screws.


With the plate removed the end of the gearbox/transfer case can be accessed.


The round plate and bearing carrier have to be removed, so carefully clean around the gasket joint to prevent dirt getting inside the transfer case.


The 5x13mm head bolts and 1x15mm stud have to be removed. Also not in the instructions - the handbrake cable is in the way. In order to more easily fit the first part of the O/D - the cable needs to be tied out of the way. Also, the exhaust mounting bracket has to be removed for the same reason.


The handbrake cable is tied to the prop. I did think about just tucking it behind the handbrake drum, but it would mean that the cable would be kinked over for quite a while and may get damaged.


The exhaust mounting bracket has a single 13mm head bolt on the top and then the rubber that attaches it to the exhaust pipe.


With the bracket removed, there's significantly more room.


Remove the bolts and studs. It's quite common for this area to leak, so the cover may have been removed in the past and sealant used in addition to the gasket, so be prepared to have to fight to remove it. Take care not to cause damage to the alloy components.


With the end cap off, the bearing carrier is next to come off. Note that if you have an LT77 gearbox, there will be two m8 countersunk screws to remove. With this gearbox (R380), there are no additional fixings.


The bearing carrier - this, the end cap, and the bolts/stud, will not now be needed, so put them to one side.


The inside of the transfer case can now be seen. The exposed gear has to be removed. You may have to wiggle the prop with the gearbox in gear and apply some leverage to remove the gear, but it should come out fairly easily.



The input gear assembly, you will not be using this either, so put it to one side.


Clean the gasket face, the instructions recommend the use of gasket sealant as well, so stick the smaller of the two supplied gaskets in place. Note the holes are not uniform, so it'll only go on the one way.


Now to fit the first part of the O/D. Make sure the assembly is fitted as seen in this picture. There's an internal splined sleeve that can't be seen in this picture. There are two sets of plines that have to line up, and this makes if quite awkward. You could fit the assembly one piece at a time - otherwise get someone to turn a propshaft.


The fixings are supplied - 6xcountersunk socket head (allen) bolts. Torque them progressively until all are tight.


Note that the flange will be 3mm away from the transfer case body, and this is normal - due to the wave washer between the collar and the new input gear. As the bolts are tightened, the washer will compress and the joint will close.




The second gasket has to be used with sealant, so stick it in place.

The handbrake cable will now prevent you from fitting the main body of the O/D, so tuck it behind the drum. Once the O/D is bolted in place, put the cable back in it's original position as much as possible.


The main body of the overdrive - a second film of sealant.


The O/D fitted and torqued with the bolts suppllied.


View from underneath. The red cap is protecting the electrical connections for the solenoid.


You can now fill the unit with ATF oil. This is a filler/level arrangement, 600-700ml.


I replaced the exhaust bracket, and it was almost touching the body of the O/D, so I had to remove it, bend the bar downwards, and then re-fit it.



Breather next:-

I decided to run the supplied hose along the passenger side of the gearbox - less things to catch it on.

You could do it without removing the stick gaiter etc, but this is the way I did it.



The filter is flimsy plastic, be careful not to break it when fitting the hose or doing up the clips.



Tied to an air-con hose under the bonnet for now. The hose can be placed elsewhere or extended up a snorkel.


Now for the electrics:-

First of all decide where you want to fit the control unit. I opted to go for it being inside the fuse box somehow.

It could go inside the battery box, or anywhere within the length of the wiring. Bear in mind it's position if you go driving through deep water.


Remove the gearknob - you won't need it again, so put it to one side. The new gearknob incorporates a switch to operate the O/D, and an indicator light. Undo the 1mm allen grub screw and remove the top. Also locate the the wiring loom for the knob.


Cut the shrink wrap to the right length, slide the wire up inside it, then screw the main body of the gearknob on and thread the wire through the offset hole in the base of it.


Be careful how you route the wire.


Second part of the loom coming up through the foam insulation.


Connect it up. ( I think this should be a waterproof connection, but it's just an ordinary 3-pin plug).


Take up any excessive slack in the wire (don't just yank on it). a small amount of slack is good, and there's room for it inside the gearknob.

The wire in position and cable tied to prevet it rubbing through.


Speed sensor fitting. There's a plastic plug that has to be removed from the O/D


Use a 22mm spanner and a small amount of thread sealant. (NOT a washer) to seal it. The plug from the main wiring loom is water proof and fits on just one way.


Wires to the solenoid can be either way round.


If you locate the earth wire up here, it keeps the solenoid wires nice and tidy against the O/D body and cable tie the loom to the side of the speed sensor.


Now to find a way to get the wiring out from the O/D to the controls. Back of the seat box has a recess that can be used for this purpose.


A slight alteration to the corner of the panel. Then wrap the wiring with a small amount of insulation tape to protect it.


Nice and tidy.


The instructions advise not to cut the wiring loom, but I see no reason not to. depending on where you fit the control unit, there os miles too much wire. As long as you are competent enough, I see no reason not to. I had to lose the excess wire by wrapping it around the gearlever housing, then cut a small amount of the carpet away for the ontrol unit to fit snugly.

Now you see it ..............


Now you don't.......


I had to temporarily attach the wires to the battery, so there are three wires here. With the power supply connected to an ignition controlled source, there would be two. Cubby box in this vehicle, but if there was a centre seat - the wires would be totally concealed.


Turn the gearknob to a position you like and then secure it with the grub screws (1mm and 3mm), once you are satisfied that all is ok, play a hair dryer over the shrink wrap to tidy the gearstick up and hold the cable in place.


That's it really, not too bad, but making it a tidy job makes it awkward. The instructions that come with the unit are good, but a bit disjointed, and a few things I feel should be added - the handbrake cable and exhaust bracket being a case in point, but then different vehicles would probably make this pointless. You also have to replace the oil after 1500kms and then more lengthy periods.

Les. :)

  • Like 1
Link to comment
Share on other sites

  • western changed the title to Fitting a GKN overdrive to a Defender

  • Create New...

Important Information

We use cookies to ensure you get the best experience. By using our website you agree to our Cookie Policy