View Full Version : Fairey Overdrive question..

12-13-2006, 10:29 AM
Earlier this year I had a Fairey overdrive unit installed in my '73 Series III. The guys who installed (who I trust on things automotive very much!) told me to only use the overdrive in 4th gear...meaning I've been shifting in and out of the overdrive when I come to a light on the highway. Do you have to take the truck out of overdrive to shift back up through the gears or can you leave it in. I know this may be a dumb question...but I suppose the only way not to be dumb anymore is to ask!


12-13-2006, 11:33 AM
According to some you should only use it freely in 3rd and 4th, as in the lower gears it may put too much torque on the bearings in the overdrive. I have used mine in 3rd and 4th in the past, and occasionally forget it is engaged and use it in 1st. No damage to date. I don't know if anyone really has data (or reliable unbiased first hand experience) on driving technique vs longevity of the overdrive. The fact the operation manual makes no comment about not driving it in reverse, 1st, 2nd, 3rd or 4th suggests it is not that big a deal, but the units do wear out after a while. Also, the operating instructions state the ratios in low range, and 4th low range carries a lot of torque.

12-13-2006, 07:01 PM
I had a Fairey Overdrive in my 109 and used it only in 3rd and 4th. I had it set up so that I couldn't put it in second without it either not going into second or it would kick out of overdrive (I would love to say that I used all sorts of bells, whistles, plenums and fancy actuators to make this happen but in reality it was just that the two gear levers would hit each other)

Though I am sure that using the overdrive in 1st or 2nd puts additional stresses on the unit I think that the big reason is wear and tear. Why use it and wear it out in gears (and at speeds) that you really don't need it in? If every once in a while you come to a light on a highway I don't think that you will do the unit any harm if you just leave it in overdrive. But if you leave it in overdrive while you do all your errands around town then you are using the overdrive for no real reason.

DEFINITELY keep an eye on the oil level!

12-14-2006, 09:23 AM
There are many things that will kill bearings but torque isn't one of them. Oil starvation is. The things that will suffer are splines on the shafts or potentially the gear teeth although this is less likely. The effect is to wear them at an accellerated rate which will manifest itself as a bit of a 'clonk', the traditional whine and increased backlash in the drivetrain. Eventually this will lead to the 'bang' as they let go.

The thing is that 1st and 2nd are used for accelerating the vehicle from a stand still or very low speed and therefore require the most torque and therefore the highest loading on the spline/gear teeth. 3rd is used at a higher speed/less torque and 4th is your cruising gear. Unless you have something a bit more powerful thanthe average 2.25 you will find it preferable not to use it in 1st and 2nd anyway as the higher gear ratio will slow your acceleration even further.

As stated above everyone forgets to disengage overdrive at one time or another and it very rarely has a dramatic effect. Doing this as a habit will dramatically shorten the life of your unit.

Its good advice to stick to 3rd and 4th.


12-15-2006, 01:37 PM
Now I'm in the know...sounds like I'll stick with my current usage of the overdrive!


12-17-2006, 06:55 PM
i recently destroyed a rear axle an 3rd mbr playing in the dirt. i had to crawl home in 4wd, only the front axle working, and stuck in low range. 20 mph was top end in 4th. so i engaged the fairy od to hurry my retreat to the garage. the whine was pretty horrific and only increased my speed to 30 mph. i nix that in about 30 seconds and return to my slow humiliating crawl home.

12-17-2006, 08:13 PM
Couldn't you have dropped the rear shaft and engaged 4 HI by pushing down the yellow knob?

12-17-2006, 09:20 PM
i didn't think of removing the properller shaft. i did try to get it back into 4 high without sucess. i figure it was due to the rear axle break. once i got back into town i played with forward and reverse and got it out of low. tomorrow i put in a mutant range rover classic/series iia third mbr and can take it out to break something else

Jeff Aronson
12-18-2006, 05:59 PM
I have the same overdrive as you in my '66 II-A, and that overdrive is now 7-8 years old with a lot of miles on it.

I use mine routinely to help with accelaration in 3rd and 4th gear. You'll find that shifting from 3rd to 3rd overdrive lets you get up to 55 mph entering a highway. It's also helpful for keeping some speed on a long, long hill. I will sometimes shift to 4th and then 4th overdrive to help keep revs up.

I've forgotten at times that I'm in overdrive at traffic lights or turns, but I've never heard untoward noises. It would be differernt if you're just dumping the clutch at these times.

Keep the hypoid level checked and replaced often and the overdrive will last for a long time.