Four speed box with overdrive, how do I wire it?

[crazygore]

I have a 3 litre essex in my mk3, just got my hands on a four speed box with overdrive, which came out of a transit recovery truck or similar! Can anyone tell me how to wire it? all I have is the three wires to/from the switch on the knob, one of these goes to a solenoid? at the rear, the other two are in a plug hanging loose, there are two switches/senders in front of the selector plate on the left, I have no idea what should go to these!
Hoping someone can help me,Thanks, Keith.

[FredTransit]

Don't you need the prop and crossmember? What gearbox do you have in at the mo? I think the OD is the same basic box as the 4 speed, with the OD unit on the side.

[crazygore]

I have the crossmember, I will have to get the prop sorted, thats not a problem, the gearbox looks the same, just has the o/d unit instead of the tapered end casing! do you have a wiring diagram for the o/d switch etc?
Regards, Keith.

[FredTransit]

Got to say electrics are not my strong point, but do a search cos I seem to remember somebody on here posting a wiring diagram.

[crazygore]

I still can't find any info on my overdrive problem! there must be someone out there that has the answer???

[V184]

You used to be able to buy a sub loom from Fords which just plugged in.

[jonathan050975]

I am a scimitar expert and thus know a fair bit about overdrives.

Overdrives are very easy to wire up. Switch on dash or gear lever has a live ignition supply via a fuse. Output of switch goes to an inhibitor switch on gear linkages so that overdrive will only come on in 3rd and 4th gear, output of inhibitor goes to the overdrive solenoid.

DO NOT USE THE OVERDRIVE WITHOUT THE INHIBITOR, the torque in lower gears will break the overdrive out very very quickly and if it is accidentally engaged in reverse the overdrive will be instantly destroyed!

Jonathan

[crazygore]

Thanks for that Jonathan,sounds very straight foreward, I guess that the two switches in the side are the inhibiters for 1st&2nd gear on one, and reverse on the other, I will take a look this week one evening, Tanks again for the help, Keith.

[Rolo]

Output of switch goes to an inhibitor switch on gear linkages so that overdrive will only come on in 3rd and 4th gear, output of inhibitor goes to the overdrive solenoid.

Am I right in assuming this overdrive solenoid takes power only while it's engaged and it reverts to 1:1 ratio when power is removed?

DO NOT USE THE OVERDRIVE WITHOUT THE INHIBITOR, the torque in lower gears will break the overdrive out very very quickly and if it is accidentally engaged in reverse the overdrive will be instantly destroyed!

Jonathan

Would the same thing apply when mounting that overdrive the other way around and using it as an underdrive?

My latest plan was to get rid of my T5 gear lever to clear the floor and getting three more speeds in the process by swapping it for a wide ratio top loader (2.78:1, 1.93:1, 1.36:1 and 1:1) in series with another top loader with a 3.82:1 first gear used as underdrive; that would give me a total of eight gears, with the first four gears multiplied through that 3.82:1 second gearbox to give me two crawler gears - normally unusable but very handy in mud or sand (final ratios 32.7:1, 22.7:1) then the usable first at 15.96:1 (very close to my current 15.5:1 first gear), then 11.76:1,
at which stage the solenoid in the underdrive should be switched off while moving the gear lever to the "first" position to get 8.56:1, and then 5.94, 4.18, and 3.08.

And no more overdrives anywhere which I believe should be some advantage particularly regarding propeller balance and vibrations.

3.08:1 is the diff ratio I intended to have at this stage although a 2.88:1 diff is still in the cards instead of my current 4.63:1 diff ratio which with the T5 overdrive becomes 3.6:1.

Why shouldn't this work and what else could I use other than a second top loader gearbox, which some people claim is indestructible, as an underdrive?

[jonathan050975]

overdrive activates when power goes to solenoid, when power is cut it goes back to 1:1.

An overdrive cannot be used as an underdrive (at least not without major re-engineering) they use a one way bearing and depends on hydraulic pressure moving a friction plate and thus it will only work one way. This is why an overdrive will not work in reverse also.

I believe lorries used to have a 'TWIN SPLITTER' which is an overdrive/underdrive combined, however I have never seen or used one of these.

On an essex engine many scimitar owners convert to the Ford Type 9 5 speed gearbox from a sierra, not sure of this can be used on a transit and bellhousings are rare (fitted to certain 2.3 sierra V6's I think) or expensive in the case of aftermarket alloy ones.

[Steve4x4]

Hi, I think the switches work on 3rd and 4th gear so the overdrive is switched on only in these gears if you see what I mean.
Some cars fitted with overdrive have relays on them, probably to save the switch on the gear lever. I will have a look through my manuals to see if I can find a wiring diagram although if you put a few wires, small battery and a test light you should be able to work it out. On my Triumph I fitted an extra switch to second gear so you get a severn speeder although it doesn't do the overdrive much good.

[Steve4x4]

Here is the diagram from a Mk2 manual, although they actually make things seem more complicated.

[crazygore]

Thanks for the info/wiring diagram, sorted it this week, very straight forward all working fine, its transformed the old girl! with a bit of luck I may be able to get into the 20 to the gallon club now! not that that is important,but I will keep up with those sprinters!

[madmark]

your engine must have not been a factory job then as overdrive was standard fit with the 3ltr on a mk3!