140 M46 Overdrive Gearbox into 1974 140 wiring

Hi All,

I am planning through fitting an M46 4 speed +overdrive gearbox to a 1974 140 Volvo.

I've got the mechanical side of things worked out and am now thinking through the electronic control of the overdrive. Does anyone know how overdrive control is usually wired into a 1974 140 for the M41 and if this will work with a M46?

Many thanks, Ess.



  • 1971_144GL1971_144GL @1971_144GL Launceston, Tasmania

    I'm looking at the same. I have two 140s with factory overdrive , 71 and 73, and they have an extra toggle like an indicator switch on the steering column. A little awkward to operate. I think these are available from cvi,. Then perhaps modify the steering column shroud with a new slot. The factory system does not have a relay from memory. But I think that they make sense to protect the circuit.

    Or the other option is to keep the m46 selection system and relay? Just wire in the donor harness and relay. Given that the m40 selector doesn't fit the m46.

  • Thanks for the insight @1971_144GL

    Another lever on the column does sound a bit fiddly. Interesting they went without a relay.

    I might dig up some early 240 greenbooks and replicate that setup.

  • carnut222Greg S @carnut222 Daylesford VIC

    To my knowledge, the early cars (pre-240 and early 240 with slide switch on gear lever) didn't have a relay (but they were fused to protect the circuit). The latching relay was introduced when the slide switch was replaced with the momentary button switch. The benefit is that when you shifted out of 4th gear while the OD was on, it cancelled OD and you had to re-engage OD after shifting back into 4th gear. Early cars of course you had to either remember to switch the OD off, or when you go back into 4th gear the OD came back on (sometimes abruptly or surprisingly in my experience). There was somebody who made a late 240-style gear lever and knob with momentary switch for the later 1800s...my dad has that on his 2 1800s (an E and an ES) and it's a lot more convenient than the steering column stalk. You might be able to replicate that using 240 parts. Not really an option on the 120s or early 140s with the long gear lever though. Another option would be to use a stalk on the column but with a momentary action instead of toggle switch...and wire up using the latching relay. If you're good with relays and 12v electrical systems, it's not too hard to come up with a latching set-up even with a couple standard relays, or use the designated relay from a later manual car with OD.

    Hope that helps a bit! :)

  • 1971_144GL1971_144GL @1971_144GL Launceston, Tasmania

    For interest, this is the setup on the right from my 71 144 gl with factory overdrive.

    I'm thinking about m46 in a 145 and a 164 - using the later turbo model overdrive unit for the 164. That suggestion of making up a selector to look factory is interesting. I can see how it could work even on the long gearshift given enough time.

  • carnut222Greg S @carnut222 Daylesford VIC

    Nice! Yeah, you could definitely come up with something for a long-stick car as well...I think the retro-fit one my dad has in his 1800s is a machined tube which is of course easier being shorter. With the long stick models, just means longer wires and of course you want to have some way to keep it from rotating so the wires don't twist in the shaft. IIRC the long stick on my 145 seems to rotate pretty easily, whereas I think the 240s have a pin so the gear lever can't rotate. Even my ES gear lever can rotate, so it must just be the rubber bush inside the shaft that offers resistance from rotation.

  • There's the original 1974 wiring diagram. The overdrive components are at 81 to 85.

    They didn't use a relay in the '74s, they used a SPST switch which hooked up as @1971_144GL and @carnut222 have described.

    Some M46s use a momentary switch on the gear lever, which will require the use of a latching relay. The slide on / slide off type switch type on other M46s can use an ordinary relay - it's a good idea to add one anyway so you're not drawing all that current required to operate the solenoid on the transmission through the little contacts in the switch.

  • Make sure the inhibitor switch is wired & working correctly with the slide type switch. If you don't & decide overdrive in all gears could be fun it's dangerously easy to leave overdrive engaged and slap it into reverse which destroys the one way clutch.

  • There hasn’t been any mention of what bellhousing to use in a M46 conversion to suit an upright 140 engine.

    Can someone refresh my memory.

  • Many thanks everyone for the discussion and sharing the common traps.

    I've now got the gearbox set up for an on-off type switch positioned on the gear lever with plans to convert to a momentary switch with latching relay arrangement in the future. This seemed like the simplest method to be able to drive/test the car as I'm considering changing diff ratio also. I made sure to carefully check the inhibitor switch on the gearbox and it only allows current flow to the overdrive solenoid while 4th gear is selected and the overdrive switch is in the on position.

    @Anthony based on the wealth of discussion here, a B20 to M46 mechanical guide could be its own thread! If someone has a source of the factory B20 to M45/6 bellhousings I'm sure there would be a lot of Australian interest.

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