Last weekend saw the arrival of some sunshine over Morecambe Bay, which got me keen to progress work on the Stag. The first task I decided was to look at the problem with the BW35 auto gearbox. I got it as as a recon unit a few years ago but sometime later it lost 3rd gear. It was then rebuilt which worked for a bit but has now started to fall out of Drive when setting off, then slamming back in again until eventually it shifts to 2nd gear! Not very dignified and highly embarrassing as it tends to spin the wheels each time it drops back into gear.
I suspected it was because I'd got the wrong oil in the gearbox. A lot of this work was done many years ago, before the current age of forums which are an invaluable source of information. At the time I was unaware that there were different types of ATF, so I wasn't sure what sort of mixture I had in the gearbox. I'd already done one oil change when I was sorting out the leaks from the dipstick/filler tube but after this the gearbox seemed no better. However, I'd read that it can take 2-3 changes to get most of the old stuff out as a lot sits in the torque converter. So I ordered the correct type of oil and changed it again.
What a difference that made! The first time I put it in gear it seemed a lot more positive and after a few runs up and down the drive it hadn't dropped out of gear. So a road test was called for, a quick blast round the bypass and the gearbox was performing well. Gear changes were positive with much less slipping than I was used to.
Unfortunately the Rover V8 engine didn't seem to be performing quite so well. It wouldn't accelerate well with a lot of popping and banging through the exhaust so I returned home to investigate. Figuring on a timing issue I set up my timing light, only to find it had failed. Not to be deterred I decided I couldn't make it much worse doing it by ear and set it to a point where the engine seemed to run best. After another road test the popping had dissappeared but it didn't seem to be running on all 8 cylinders, so back home for more investigation.
Deciding to remove the spark plugs to see what they looked like I found one that was very loose (cylinder 7). On removing them all I found that they were all very black (so running rich) apart from one (cylinder 8) which looked normal! The one normal plug had me confused as the car only has 2 carbs so I wouldn't expect one to be significantly different to the others on that bank. After cleaning and re-gapping the plugs I had a think about it and realised that the inlet tract for cylinder 8 has the vacuum take-off for the brake servo. I'd taken this off when re-filling the gearbox because the pipe gets in the way of the filler tube. When I'd removed it I found that this connection to the inlet manifold was very loose, so I guess this was drawing additional air into this cylinder to weaken the rich mixture.
So after another road test and everything seemed to run better, the gearbox was performing well, the engine was a lot smoother and overall performance was much improved. So a big step forward. I still have the timing to sort out once I replace my timing light and need a crash course in adjusting the mixture.
Whilst researching the problems with the gearbox I came across this workshop manual for the Borg Warner 35 Automatic Gearbox:
A link to a list of the individual scanned pages:
http://www.anugraha.org.uk/rover/bw35
Alternatively, download the whole manual as a zip file from here (nearly 8MB):
http://www.anugraha.org.uk/rover/bw35/bw35man.zip
The second thing I found is a document from Castrol which describes different types of automatic transmission fluid and why Dexron type fluid shouldn't be used in the BW35 gearbox.
http://www.aussiefrogs.com/forum/attachment.php?attachmentid=29405&d=1219058696
(Links directly to a pdf file)
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