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Fitting an O/D to a Defender

Les Henson

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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. :)

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Very interesting thread Les. Thanks.

Just to follow up on the comment regarding the wiring, I have now plumbed that in to an ignition live feed as I found that, when I used the battery cut off, the red LED came on as the whole truck was then earthing through the gearknob :blink::o

The unit is working reliably now after the inexplicable hiccup on Saturday morning, when it engaged but wouldn't then disengage, even after I had stopped, turned off and started again.

It makes such a difference to noise levels at speed and I am currently running a check to see the extent to which there is a difference in fuel consumption.

An expensive modification, but one that I would wholeheartedly recommend. Thanks also to Devon 4x4 from whom I bought the overdrive. Excellent service - ordered it on the Tuesday before Christmas, to be told that the units were out of stock and on back order. Delivered to my office on Friday of the same week.

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excellent article Les, can you also do the article on changing the seat fabric as well??? as i guess someone just did the drivers side.....

i highly rate this bit of kit, had mine fitted for over 60k miles now and its fantastic, bit of a speed sensor problem at them moment though, only when its wet it seems. will have to take a peak at some point.

Les do you not need to set any preload on the first plate? (or maybe that just with an UD can't remember)

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to go somewhat off topic, how old is that land rover?

seems to be reasonably recent going by the metallic paint and fusebox, yet has the older style of air-con dash. is it an import or something? i thought that style of dash didn't last too long in the uk.

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I will respond to 2 of the posts above in this reply:

Firstly the seat fabric: the drivers seat was replaced about 4 years ago as the foam in the backrest collapsed. I picked up an ex-display Modular seat from the Exmoor Trim stand at Billing in 2002. The fabric on the drivers seat is the 'Techno' cloth I think. I have now swapped the seat base back to the original as the foam in the replacement had collapsed.

Secondly the spec of the vehicle: the truck was built in 1996 and exported new by Conrico to the British High Commission in Cameroon. It was used out there until 2000 and brought back by the woman to whom it was delivered originally. It was then registered on a 'P' plate. She seems to have used it in London for about a year and then traded it in against, I think, a Range Rover. I bought it in 2001 at 28,800km. It now has just over 140,000km on the clock. The dashboard is the old aircon style and does not have the shelf or the front vents. Where the vents should be there are plastic trim pieces. Underneath them the vent holes are not even cut in the bulkhead.

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Silly question.....(Maybe not.)

I've seen a few threads on this now, Western and Les included, and each seems to be fitting GKN to TDi's in various forms.

Is there anything stopping someone fitting Overdrive to a V8 powered Defender ?

I'm assuming there could be a torque issue on a standard V8 3.5, but on 3.5 EFi / 3.9EFi upwards, is there any reason to NOT do this ?

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I think it was slightly longer than that. I dropped the truck off first thing in the morning and by the time he had dropped me off at work, it was probably about 1000hrs that he started getting things sorted. That did include removing the cubby box.

He rang me about 1730hrs to say it was finished.

I think the clock on his camera is a bit out.

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It normally takes me 'too long' to do a job. Taking pictures slows things down a bit, but I tend to be fussy about how I do it as well. A bit of extra care and time taken can make a big difference in my opinion. You can just tuck the wires anywhere, or have a 'that'll do' frame of mind, but neat and tidy is the best way in my opinion. I suppose 6-hours would be a good time to take to do it though.

Les. :)

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Silly question.....(Maybe not.)

I've seen a few threads on this now, Western and Les included, and each seems to be fitting GKN to TDi's in various forms.

Is there anything stopping someone fitting Overdrive to a V8 powered Defender ?

I'm assuming there could be a torque issue on a standard V8 3.5, but on 3.5 EFi / 3.9EFi upwards, is there any reason to NOT do this ?

fitting booklet only says for LR vehicle type Defender 90/110, doesn't mention any engine types at all.

you planning something then :D

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Planning loads of things Ralph..... Just got to find the money, time, prepare for the potential divorce etc. if I continue ! :)

I'm considering putting a 3.9 EFi in the 110.... Then a GKN for those longer journeys.

Other than that, not too much planned....

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Planning loads of things Ralph..... Just got to find the money, time, prepare for the potential divorce :o etc. if I continue ! :)

I'm considering putting a 3.9 EFi in the 110.... Then a GKN for those longer journeys.

Other than that, not too much planned....

B) :i-m_so_happy:

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Father Christmas got my letter, and delivered me a nice new O/D as well. It took me about 7 hours to complete the fitting, in a professional workshop with a car lift, but then it didn't work! An electrical problem - absolutely dead. It took me a only a few minutes with my tester to find the fault - in their supplied harnesses. The in-line fuse holder had been installed with an over generous use of flux, and the terminals that are held by a spring against each end of the fuse both were covered in a thin glaze of excess flux. Just enough to make good insulation! A quick scrape with my knife, and I was in business.

And what a difference an O/D makes. I'm currently running on Michelin Alpin 205.70.16 tyres, instead of my summer tyres of 235.85.16, which reduce my gearing by about 8%, so the 25% drop in engine revs for any given road speed was specially welcome. Much reduced stress for the engine and for the occupants.

As for economy, I can't yet comment. My 300 Tdi 110 with 2 people and fully loaded, including a full length roof rack, gave about 20 mpg, pedal to the metal all the way from Vienna to N. Wales before I fitted the unit. Averge speed, excluding sleep stops was about 58 mph. On the return, after fitting the unit, still with 2 people, but only half loaded and nothing on the rack, it gave about 22 mpg. with an average speed of about 62 mph. I'll continue to monitor consumption over the next couple of thou miles and see if it does save any money. Even if it doesn't, it's still well worth the investment for anybody who does a lot of motorway driving.

The instructions say that the unit can be engaged and disengaged under full power, which might be useful when towing. Yes, it can , but I found that a much smoother change is achieved if you dip the clutch. This must also generate less stresses on components.

Having done the first oil change, I have one word of warning. The change requires removal of the flat sump plate to gain access for cleaning the filter and magnet. Refitting this plate, the instructions require torquing the bolts to 12 - 14 Nm. (8.9 - 10.3 lb.ft.). After fitting all the bolts just finger tight I set my torque wrench to just below 10 lb.ft and promptly stripped the thread of the alloy casing with the first bolt at well below that torque. I checked the value that I had set my wrench at, and that was correct, and then I checked the accuracy of the wrench, by using a spring balance, and it was just about spot on. So, I didn't torque up to the recommended settings, but only to about 5 ft.lbs (about 7Nm.), but to compensate and ensure a leak free unit, I used red Hermatite on the joint. The gasket was still firmly stuck to the plate, so I didn't disturb it, and it has not leaked at all. The next oil change isn't due until a further 55,000 km (> 34,000 miles), so it's not a frequent chore.


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Thanks for that info Mike - I will be especially careful when retorquing after the first oil change.

I have now done 600 miles or so with the overdrive installed and I too have found that it is well worth dipping the clutch when switching overdrive out as the bang and jolt even on power cannot be doing the rest of the drivetrain any good.

The first full to full check on mpg has revealed a slight improvement, from about 23mpg to about 24.5, although much of that has been at higher speeds than before when on motorways and dual carriageways.

There has also been more stop-start commuting than usual as I am using the truck everyday at present tp get the mileage up to the oil change point before I go on holiday in a couple of weeks time. The real test will come on the long haul down to the Pyrenees, 4 up and fully laden.

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