I have a 1968 S type with the BW35 Auto box.
2015 the gearbox was fully rebuilt and has run flawlessly until a couple of days ago. I was pulling away from a junction up a very steep incline when the gearbox started slipping. I was in drive and the car was trying to pull forward but kept hopping in first and took several attempts to get enough momentum to pull up the hill. Once going it was fine again. No problems at any other time. Accelerates as it should on the flat. Kick down in all gears is fine. I have checked the fluids when the car is hot and running in "P" and they are fine. I have possibly done 5000 miles since the rebuild and reading the 420 service manual which covers the BW model 8 gearbox it suggests that the front and rear bands should be adjusted after a 1000 miles from new (rebuild) which was not done or 21000 normal driving. (Only just read this is the 420 service manual)
Is this slipping a band adjustment required?
If so is this the front or rear band that is effecting first gear?
The service manual states front band requires removal of the sump but rear can be adjusted from the outside.

If it is a sump off then I may as well hand it back to my gearbox man to do for me and get a full gearbox service at the same time.

 

Cass you are correct the pan must come off but you also need a special tool to properly set the torque. If you have someone who can service the unit I would let them. If you have been slipping the fluid should probably require changing at the time. If you get a chance you might want to grab a copy of this publication--it seems to have a little more information than the 420 service manual.

 

Thanks and noted. I have just found a copy of this and have ordered it so should be with me by the end of the week. The special tool you refer to I think is a 6.3mm bar used to measure the tension gap on the internal band adjuster but according to the 420 manual I have read the second band can be adjusted from the outside so no need to strip the sump off. Just need to know which band controls the first gear part of the box.
Lastly as you have all these things and the knowledge. My 1967/68 S type came with the BW35 auto box. Prior to this all the Auto S types had the DG250. I have three service manuals for the S Type (blue book) but all of them cover the DG250 and don't mention the BW35. The 420 has the Model 8 gearbox so not the same. You have suggested the Model 35 service manual for the 240 and 340 which I have ordered but was there a service manual for the 1967/68 S type that had information about the BW35 that was fitted for the last year?

 

Cass go here and take a look https://jcna.com/shopimages/tools/Ja...950s-1960s.pdf. I am sure the tool you mention is CBW 34 or a .25 in go no go. The tool really needed to do a proper job is CBW 548-2 and CBW 548-2A. Due to restrictions it is almost impossible to do a proper job w/o them. Looking at the manual it is necessary to know what selection you have made to determine the fault. We do not know of a manual for the 35 that addresses the 'S' directly. There is one bulletin that addresses a fault in production for the 35 (Bulletin FF21) but it also addresses all 35 car uses.

 

Thanks for the info and the link. Looks like the car is going to the gearbox shop.

 

And remember not to run Dexron family fluid in it. It was designed to run on Ford M2C33 F or G spec fluid for quick lock up. Dexron II etc. has too much friction modifier in it and allows excessive slip & subsequent overheating & failure.

Volvo has it correct in all their publications on the 35.