The overdrive unit on M46 transmissions is basically a reduction gear, similar to a two speed rear axle on heavy trucks, and with modification it can be engaged in any gear, not just fourth. This can give you an 8 speed transmission, kind of, not really.
There aren't a lot of benefits to this mod, and there is one potentially catastrophic failure that can happen because of it. But if you're careful you can avoid problems and have a little bit of fun. The biggest advantage this mod gives you is that you have one more thing to do inside the car while you're driving, but its not something that demands attention if you don't want to mess with it. Basically, its something to play with and make yourself smile.
The mod is simple. There is a 1-wire switch on the transmission that grounds out when fourth gear is engaged. Remove the wire from the switch and ground it permanently.
*Beware* The overdrive unit has a ONE WAY CLUTCH inside of it and it will break if engaged in reverse. It is unlikely that you will be able to get overdrive to engage while you're driving in reverse, but if it does happen the overdrive will be ruined. Also you are going to put more stress on the overdrive if you use it in the lower gears. It will have to handle more torque and has a higher chance of breaking and will probably wear faster.
The benefit is being able to engage the overdrive unit (theoretically) in First, Second, and Third gear, as well as the normal function.
Let's look at what that will actually do for you though. And before that let's look at some if the inner workings of the overdrive.
The overdrive works off of hydraulic pressure. There is a tiny pump inside it that is driven off the output shaft of the transmission. That means it only makes pressure when the wheels of the car are turning. The system needs upwards of 300 psi to operate, so the car has to be moving at a decent speed to make the overdrive work. The specific speed will depend on the condition of your car, how worn the pump and seals are, the viscosity of your transmission fluid, etc.
On an older car it is unlikely that you will be able to get the overdrive to engage in first gear or reverse.
So we are down to 7 speeds and don't have to worry about the reverse catastrophy as much.
The other bad news is third gear. Third gear with the overdrive gives you a nearly 1:1 ratio. The same as fourth. There is a slight difference, but it is imperceptible driving the car.
Down to 6 speeds.
Second gear is where we will see the only real mechanical benefit. Second with O/D engaged splits the second and third gear ratios nicely. You should be able to make enough pump pressure by the time you're into second gear too, unless there is something seriously wrong with your car.
So you get a 2.5 gear. That makes it a 6 speed. 1, 2, 2.5, 3, 4, "5"
Here's where I go off the rails a little:
Since the O/D functions kind of like an automatic, you don't really need to use the clutch to engage it. I'm not saying it will last a long time driving like that, but its possible. I have to remind myself not to be lazy and to use the clutch to engage O/D just driving around town. With the right rear tire size and rear end gear, and the O/D mod, its possible to get to 60mph with only one clutched shift. I used 225/55R16s on a 3.54 rear end to do it in my car.
I never took it farther than that, but my plan was to mod the electrical part of the system to make it work faster using a better switch (maybe an early slider style switch) and possibly eliminating the relay.
Ultimately it would be time and money better spent just swapping in a T5, but there are some weird things you can do to have fun with an M46.
There aren't a lot of benefits to this mod, and there is one potentially catastrophic failure that can happen because of it. But if you're careful you can avoid problems and have a little bit of fun. The biggest advantage this mod gives you is that you have one more thing to do inside the car while you're driving, but its not something that demands attention if you don't want to mess with it. Basically, its something to play with and make yourself smile.
The mod is simple. There is a 1-wire switch on the transmission that grounds out when fourth gear is engaged. Remove the wire from the switch and ground it permanently.
*Beware* The overdrive unit has a ONE WAY CLUTCH inside of it and it will break if engaged in reverse. It is unlikely that you will be able to get overdrive to engage while you're driving in reverse, but if it does happen the overdrive will be ruined. Also you are going to put more stress on the overdrive if you use it in the lower gears. It will have to handle more torque and has a higher chance of breaking and will probably wear faster.
The benefit is being able to engage the overdrive unit (theoretically) in First, Second, and Third gear, as well as the normal function.
Let's look at what that will actually do for you though. And before that let's look at some if the inner workings of the overdrive.
The overdrive works off of hydraulic pressure. There is a tiny pump inside it that is driven off the output shaft of the transmission. That means it only makes pressure when the wheels of the car are turning. The system needs upwards of 300 psi to operate, so the car has to be moving at a decent speed to make the overdrive work. The specific speed will depend on the condition of your car, how worn the pump and seals are, the viscosity of your transmission fluid, etc.
On an older car it is unlikely that you will be able to get the overdrive to engage in first gear or reverse.
So we are down to 7 speeds and don't have to worry about the reverse catastrophy as much.
The other bad news is third gear. Third gear with the overdrive gives you a nearly 1:1 ratio. The same as fourth. There is a slight difference, but it is imperceptible driving the car.
Down to 6 speeds.
Second gear is where we will see the only real mechanical benefit. Second with O/D engaged splits the second and third gear ratios nicely. You should be able to make enough pump pressure by the time you're into second gear too, unless there is something seriously wrong with your car.
So you get a 2.5 gear. That makes it a 6 speed. 1, 2, 2.5, 3, 4, "5"
Here's where I go off the rails a little:
Since the O/D functions kind of like an automatic, you don't really need to use the clutch to engage it. I'm not saying it will last a long time driving like that, but its possible. I have to remind myself not to be lazy and to use the clutch to engage O/D just driving around town. With the right rear tire size and rear end gear, and the O/D mod, its possible to get to 60mph with only one clutched shift. I used 225/55R16s on a 3.54 rear end to do it in my car.
I never took it farther than that, but my plan was to mod the electrical part of the system to make it work faster using a better switch (maybe an early slider style switch) and possibly eliminating the relay.
Ultimately it would be time and money better spent just swapping in a T5, but there are some weird things you can do to have fun with an M46.
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