Series 3 Advice on Overdrive operation

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ricyoung

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Hi all.

Can anyone advise me on the correct way to use a Fairey overdrive? I have a 1984 series 3 with a 200tdi and Fairey overdrive. Its got standard diffs but I do have 7.50 tyres fitted. The overdrive seems to be in good order and isn't noisy.

My commute to work takes me on about 10 miles of dual carriageway. Normally I use the 4 normal gears to accelerate and then engage overdrive at about 40mph. This seems to work well and the syncro on the overdrive seems happy enough to engage. When I get to my exit junction and start to slow down I usually disengage the overdrive and go back to using the normal gearbox. However as I'm changing down it seems to take a long time for the syncro to engage happily. It doesn't make any noises and there is no crunching as it engages, it just seems to take a while to happen - by which point I'm halfway up the sliproad and I've slowed down quite a bit so I'm having to accelerate again to keep up with traffic. I don't think there is anything wrong with the syncro as it does engage very easily at lower road speeds without any trouble.

My other option is to keep overdrive engaged and simply change down using the main box. However I have read that using 1st and 2nd with overdrive can cause excessive wear on the overdrive.

So, my question is. What is the correct way to use it? Keep it engaged, or use it as a 5th gear?

I appreciate this is probably common knowledge but your advise would be appreciated.

Cheers, Ric
 
From memory Fairey recommended that the overdrive should only be used on 3rd/4th ....However I have always used it where needed as a "Splitter" gear especially when towing .
Doesn't ever seem to have had any adverse effects on any of the ones I have had AND I know several other folks that have done the same .
might be worth checking oil levels in the O/D if you havent already done so .
 
as above,i used to find dropping to 3rd for a bend is too far but just out of overdrive seemed about right , baulk rings in over drive are similar to main box gears arent over big so better without huge torque which is why use in the higher gears was recommended, though i used to use overdrive in all gears with no illl effect
 
They say to use it in 3rd and 4th as the torque being put through it in those gears isn't as violent. There is nothing wrong with using it in all gears just keep control of your right boot.

With a Tdi you might find it better to put defender/discovery/Range Rover classic 10 spline diffs in. This raises the gearing by about 33% which is a bit higher than keeping overdrive engaged all the time.

I found when I had my standard series diffs I could start off in second easily and even in 3rd on slight downhill bits. It's very inefficient. The engine runs out of revs very quickly and is always at high revs which makes fuel economy very poor.

I have a fairey on my Tdi 109 and it is used for cruising I can get to about 60/65mph before the engine starts screaming. Engage overdrive and cruise at 65 relatively quietly and doing about 35mpg.

You might even decide you don't need the overdrive and get about £500 in your pocket by selling it.
 
might give that a try the s3 is the std n/a diesel and has a toro, s1 is 2.ol petrol and has a fairey I don't stay in first or secomd for more than a few seconds even in a modern car and don't start to accelerate noticeably till 3rd

I don't do the traffic lights grand prix in the landies :D
 
honolulujoe said:
I thought the Ashcroft or disco diffs was a bit much for the 2.25 diesel and 2.0 petrol, good excuse for a v8 :D
Click to expand...
Dont see it would be much worse than the disco diffs? Probably better with 2.5 petrol or diesel, V8 or Perkins etc. Seen it with ford v6 as well.

mistericeman said:
Not a bad option IF you can afford it .
Click to expand...

TBH I have no idea how much they are now. When I bought mine twenty years ago, they were quite cheap. You gave them your old transfer case, all yyou paid for was their time to mod the case, and the gear cluster. :)

I dont expect good pairs of diffs are available for peanuts either. Never actually tried to buy any.
 
Turboman said:
Dont see it would be much worse than the disco diffs? Probably better with 2.5 petrol or diesel, V8 or Perkins etc. Seen it with ford v6 as well.



TBH I have no idea how much they are now. When I bought mine twenty years ago, they were quite cheap. You gave them your old transfer case, all yyou paid for was their time to mod the case, and the gear cluster. :)

I dont expect good pairs of diffs are available for peanuts either. Never actually tried to buy any.
Click to expand...

Disco diffs start at anything from £40 a pair upwards to Around £80 a pair from greedy folks
Ashcrofts are around £300 for a H/R Xfer
 
Hi All.

Thanks for the replies. Seems like so long as I'm gentle I wont harm anything too much either way. I agree diffs are the way to go, if I forget and leave the overdrive engaged its so much easier to drive and they both have a similar effect on gearing. The transfer box upgrade is probably a better engineering solution but seeing as the overdrive is fitted it may as well stay. I think I want to keep it anyway as you can split gears as mentioned. A friend of mine is breaking a RRC and I've got dibs on the diffs so that is plan A.

Does anyone have an easy guide on swapping front diffs without dismantling too much of the steering?

I have the green bible, but I'm sure there must be a trick to making it easier.

Cheers, Ric
 
I was lucky with my diffs with some patience on eBay I found a pair of reconditioned units for £55.

Depends on how much you use low range. For me with 7.50's and 3.53 diffs I found first low more than low enough.
But one and put it in the rear and see what you think. If you like it then fit a front if you don't like it then return it to standard diff. The rear diff takes about 30mins from start to finish. The front is a bit more involved.
The speedo will also read incorrectly. But not a difficult fix.

Makes all the gears more useable and a lot better suited to the torque of the Tdi
 
As said paper diff gasket. 9/16th spanners for propshaft bolts/nuts. Same size for diff nuts I believe.

Drain oil
Just undo the 6 drive flange bolts either side of the axle. Pull half shafts out so they are out of the diff but keep them in the casing.

Undo propshaft at diff end and hang it so it's out the way. Undo the diff nuts and give the diff a wiggle. It's very heavy I think they are about 35kg. Then refit is the opposite of fitting but with new gaskets.

Whilst the new diff is out check the backlash and the oil seals.

Just don't engage four wheel drive.

Also make sure you get a pre 1993 diff as they are 10 spline. Also check if it has a filler on it. Some don't have fillers and you can either drill and tap a hole to fit a filler. Or jack up one side of the axle and fill it up from the drive flange end.

Front and rear can be interchanged. So if you find a decent front diff you can fit it in the rear.
 
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