M46 4th Gear Switch Issues

My 86 Volvo's overdrive was working fine, and after letting it sit for the winter I have noticed an issue with the overdrive unit.

When I am cruising on the freeway it will not engage 5th, but if I push it all the way to the right, I can get it to engage. However, as soon as I let it go, it shifts out of fifth.

My suspicion is the 4th gear switch. Where is the darn thing located, and is there a fix for it?

Thanks for the help!

The switch is on top of the transmission. You could bypass it and have an 8 speed, just don't engage O/D in reverse or it will break.

Thanks! Do I have to drop the transmission to reach it, or can I access it by reaching up around. How would one go about bypassing it?

chrisjbrooks said:

Thanks! Do I have to drop the transmission to reach it, or can I access it by reaching up around. How would one go about bypassing it?

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Just snip the bullet connector off the end and put on a ring terminal, ground it anywhere.

Just dont' cut the wires for the reverse light switch. That switch has 2 wires going to it...don't confuse the two. I'm 99% sure that the wire for the 4th gear switch is red.

Also, just because you can engage the OD in any gear doesn't mean you should.

OD units are known to break in the lower gears. my first 240 when i was young, i routed the wiring to the rear defroster switch to engage 5th lol

Thanks for the help, guys. I will get under the car in the morning and try to sort it out. Bryant - I was thinking of doing something like that!

wires are suspect, your car is from the years with biodegradable insulation

chrisjbrooks said:

Thanks for the help, guys. I will get under the car in the morning and try to sort it out. Bryant - I was thinking of doing something like that!

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there are tons of options available for you to do.

bypassing the 4th gear switch is gonna be the priority, then go from there! lol

I bypassed the 4th gear switch, its was pretty easy. On my car I followed the relay wires down to the 2 wire (Red & Yellow) connector. Put Yellow to Yellow for the solenoid, and Red to Red for ground. Red is the 4th Gear Switch.

JamesShinfo92 said:

I bypassed the 4th gear switch, its was pretty easy. On my car I followed the relay wires down to the 2 wire (Red & Yellow) connector. Put Yellow to Yellow for the solenoid, and Red to Red for ground. Red is the 4th Gear Switch.

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So its always in overdrive? Do you lose acceleration? Also, I was under the assumption that this was a bad idea because you do not want to engage reverse if its in overdrive?

I think he means he bypassed the switch so he can engage O/D in any gear. He probably doesn't drive around with it engaged.

More info about the 4th gear bypass/8 speed M-46 mod:

The hydraulic pressure in the M46 O/D unit gets up to about 400 psi when its operating. The pump is driven off of the tailshaft which turns faster as the car's speed increases. Its actually pretty hard to generate enough hydraulic pressure in 1st and Reverse because the road speed is so low. So unless you have brand new seals or something its unlikely that you would be able to engage O/D in Reverse, or 1st gear.

3rd with the O/D engaged is going to give you nearly the same gear ratio as 4th, which is 1:1. I did the math once and it came out to .98:1 or something so close its not going to matter.

The biggest benefit I saw to having the switch bypassed was in 2nd gear. Because the O/D functions like an automatic transmission, you don't need the clutch to shift into it. Depending on rear end gearing and tire size, this might mean you can do 0-60 with only one clutched shift. Which would knock some time off of you're 0-60, if you're into that kind of thing. I did it with a 3.54:1 rear end and 225/55R16s.

No I never have o/d on in any gear other than 4th. Its still operated by the switch on the shift knob. I had it on in 1st by mistake one time and started to burn the clutch lol I don't recommend that.