Good day everybody and, first of all, happy new year!

i have been reading the forum for a while, finding it very useful and interesting. Last time was reading about de M46 transmission because I have a slipping issue with it.

I am in the process to restore a 245 DL with B21A engine and M46 transmission. It has no wiring or switch to activate the OD. I just rebuilded engine and clutch (including fork and cable) and sometimes traction “disconnects” or slips. Is there any way to cancel or nullify the overdrive system? If I take the solenoid out what will happen?

As I live in Argentina and Volvo is not very common, there is no chance to get another M46 around and it is frustrating to have a new engine and not been able to drive


tahnks in advance and best regards!

 

I doubt that by eliminating the overdrive you will cure the slippage. Sounds like the way you have it now you have in effect no overdrive as you need the electrical part to activate it. You slipping issue lies elsewhere, probably with the clutch installation/adjustment.

 

The clu5ch is new and the cable is adjusted correctlu, that is why I think the problem is the transmission itself, but I have no idea how the overdrive and the transmission are “connected” so My doubt is if there is any way to eliminate the overdrive and keep only the four manual gear transmission working

 

the OD is actually an underdrive (reduction gear), on the output of the transmission. when its powered, it reverts to 1:1, which is 'overdrive'. the OD relay that controls this is wired so you can only enable the OD when you are in 4th gear.

unless its making crunchy sounds, slippage is almost certainly the clutch. I remember once replacing the clutch and transmission on a vehicle (non-volvo) just to have the input shaft seal on the used replacement tranny fail and leak gear oil all over the new clutch, whadda mess. yes, I learned, you should ALWAYS replace the seals when you swap a used transmission, including the engine output shaft seal as well as both transmission seals.

 

I don't believe they are connected in a way where the overdrive unengaged would cause the transmission to slip. The M46 is basically a 4 speed with the electric OD added as an afterthought, just another gear that slides in there and changes the final ratio. The OD unit engages or it doesn't, how would that cause a tranny to slip? What you have now is exactly that, a 4 speed with no working OD!

I have had my share of M46 trannies and never had, or heard, of this. The OD engages or it doesn't, the OD pops out and there is the only problem I have ever heard with these units when they are failing.

 

the OD is actually a Laycock J unit, same guts as were used on late model TR-6's and other such british sports cars. it has a planetary reduction gear, and has a hydraulically actuated conical clutch that locks the annular (ring) gear for "OD". even if this clutch is slipping, you'd still be in regular underdrive mode, there is no state where it won't drive you forward.

btw, its VERY important that you NEVER engage OD while in reverse or you'll destroy things. The TR-6 would actually allow OD use in 2nd, 3rd, and 4th, but on the Volvo, its only enabled in 4th as the B230F is quite a bit more powerful than the old 2.5L 6 cyl that Triumph used. When in 4th, you can enable/disable the OD without touching the clutch, as long as you back off a bit on the throttle so you're not pumping out too much torque.

 

When the one way clutch in the OD unit fails - it will slip when trying to accelerate in lower gears. The cone clutch is not strong enough to power the car in the lower gears so a one way clutch is used - I have replaced several for the slippage problem (many, many years ago.) Here's a picture of a destroyed one. It's a row of fat needle bearings - but the inner race is not flat, there is a "ramp" for each needle that forces each to lock against the outer race. You can see the ramps in the second photo.

 

Thanks for the replys! I see there are different points in this matter, unfortunately for me, what I understand from hoonk is that a bad OD could cause slippage and there is no way to blank it and just keep the four gears.

 

Well, I stand corrected then and will defer to your hands on experience even though I still don't understand how it would work as described....