BW8 Transmission pressure too low

I have transplanted a S2 engine into my Saloon and re-fitted the original BW8, which was working fine.

While it was out, I replaced the leaking trans selector seal, which required removing valve body

Now the Transmission pressure as measured with a gauge is too low.

This means the bands will not engage without slipping, and is undriveable.

measured pressures are less than half what they should be 25psi at idle should be 50

50-70 if I rev it hard, should be 72 at 1250 and 150 at stall

fluid is full and overfull…I am at a loss, as it should idle at 50psi according to the manual. (so kickdown cable cannot be the cause, tried complete range of adjustment

the only thing I can think is maybe I did not push one of the pipes back in hard enough, or there is some assembly or other issue with the valve body springs…or a sudden internal seal leak.

this will require removal of exhaust and pan.

I have never heard of auto trans requiring bleeding, but ready to try anything at this stage

any advice, experience or comments welcome

Well, either you have some internal leak but at least on my spare bw65 the tubes are relatively tight, forgot something? some particle blocking valves, something clogged even though you didn’t do too much to the transmission - or do you have too much fluid in there, foaming it up? You said it’s overfull? :slightly_smiling_face:

You might already have this from your manual but if not, assuming everything is adjusted correctly (fluid level, selector, idle):

Oil tubes missing or broken, or
valve body screws missing or not correctly tightened, or
primary valve sticking.

This is from the S1 XJ6 manual for troubleshooting low idle pressure.

Adjusting the kick-down cable length (increasing length) had no effect, Tony…?

If indeed the transmission worked properly before the valve body removal - you just have to go back in there to see if you have indeed botched something. If you did nothing to the valve body itself, it is unlikely that it is the cause - though, of course, it does not take much dirt to clog up the works…or muff up at pipe connection…

Don’t like your ‘fluid is full or overfull’ and the question arises; did you use the appropriate procedure to verify level - ‘overfull’ is as bad as ‘too low’. There is no transmission bleeding - it’s ‘self-bleeding’…

Best bet is a missed reconnection when you replaced the valve body - trusting you refilled with correct fluid at the correct level. As the box worked before means that the engine driven fluid pump works, delivering correct pressures. Which is then modified inside the valve body as required for valve operation and band actuator operation - but pump function is essential for proper operation…

And not driving the car before pressure is rectified is right and proper…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

thx for the replies gents.

consulted with a grizzled autotrans pro, he agrees with me by far the most likely issue is some assembly fault in the “pressuure regulator body” which is a sub-assembly to valve body.

kickdown adjustment make NO difference, pressure is less than half it should be at ALL RPM, idle being the giveaway…on the good side, means oil pump drive is not sheared (can happen if TC fitted improperly, or there would be NO pressure.

auto trans do not need bleeding, and if too full will just spit out breather (according to pro)
When u fill a fully empty autotrans it will not take anywhere near its full amount until fluid has been well & truly pumped thru torque convertor, valves, servos etc.

I have waterrlogged old spare BW8 I dismantled some time ago, for possible spares, but mainly in preparation for what I thought might be an OH, turned out it was a very minor problem,I took many digipics, including valve body dismantle.

Its possible to assemble with some part in the wrong direction, including the few valves in the pressure regulator.

so off with exhaust manifolds, drop TX pan, valve bodies and pipes, keep fingers crossed

I might just have a tiny rest before I do though, this jobs been fighting me all the way

the xplanted engine seems to be running very nice

Let’s hope you find the trouble instantly when the patient is opened up, Tony…:slight_smile:

It’s disappointing that a simple operation turned complicated, but the ‘fiddle factor’ is always lurking - at least you have the wherewithal, most likely unneeded, to tackle.deeper problems…

xj6 85 Sov Europe (UK/NZ)

Makes much sense to me. Not good, but could be worse. busted pump drive!!!

In the late 60’s I acquired a decent 57 Ford 1/2 ton short bed
V8 powered pickup. Got a good price as it had a flaw. although it’s auto box worked, it leaked mightily. Not a fan of auto trucks, but the price was right, so it came home.

Two speed auto, air cooled by fins on the converter. Not the best of ideas. A BW percussor of later more complex units.

I got a "soft kit’ and took it down. Not all that bad. Did not mess with the valve body. Someone cautioned me on that.
I did swap in ling for it’s one band. Seals not hard to install fore and aft. Cleaned it and it’s surroundings. I prefer to work on clean things!!!

Got it back and filled with fresh fluid. No go, unless well “goosed”, then it would drive… I thought, “this ain’t going to work”.

So, like Tony, off to a pro. Found a small shop not too far
away. Sympathetic younger guy put it up on lift. whoosh, pan down. A few whacks with wrenches. On with the pan. added fluid. Dropped it and backed off. Scratched some gravel in front of the shop. "You did ok, almost. I tightened the band, added some line pressure by adjustment, your good to go. Fluid plus about an hour shop rate. Quite reasonable.

One issue down. Then, came the dreaded valve rattle. A weakness of that engine. Another tale for another day.


some joy at last, it would seem, had to open it up TWICE, (my fault did not pull enough parts 1st time, problem caused by me anyway)

The problem seems to have been I did not sufficiently torque down the 2 bolts that hold the Pressure Regulator body to the trans case…these are machined surfaces with matching oil galleys, so any tiny gap allows pressure to bypass the galleys

these 2 bolts are hidden underneath pipes, springs and valves, and the correct order and method of assembly is not easy for someone with limited experience of transmission internals…still not a good enough excuse. I noticed they did not feel very tight upon removal, and that clued me up for later when i noticed the machined ports in the old trans case

trans pressure gone from 20psi at idle to ~50 (correct) was 30 at 1250, now about 80, goes to over 150 with throttle (was 60). Vehicle fully engages transmission forward and reverse (didnt before), not fully road- tested yet

To severely complicate matters, this vehicle (420G) has had a S2 XJ engine xplant, (due to oil consumption of original), IFS rebuild, and trans opened up to fix badly leaking selector seal, all at the same time.

The throttle and kickdown adaptor brackets are incomplete prototypes (they are close to right though)

made diagnosis frustrating

I can say for sure the leaking selector seal is a common problem, frequently mis-diagnosed as a leaking pan gasket…its almost impossible to do with trans in car, as the selector shaft must be withdrawn thru the outside of the case, and body sheetmetal gets completely in the way, also selector ball&spring is a mongrel upside down.

I could not ignore this leak with trans out, (even though I wanted too)

managed to do all this in less than 4hrs this time, much better than R&R engine and replace a perfectly good (and original) trans for no good reason.

I have to pull exhaust manifolds to clear pan, as have 3.8 pipes fitted instead of 420G, when my 420G pipes were rotted, I did not know the pipes i obtained are likely from a 3.8MKX, until later study of parts manual shows different part number, and cant drop pan direct… can drop manis in 30min now)

both valve bodies were also completely removed, dismantled, cleaned and examined

would NEVER have been able to sort this without my dismantled bw8 experience…it was by looking at the old case, i could see the machined surface and ports. This was a last 1% hope…had nearly convinced myself the Torque Convertor dog clutch had failed to mate with the front oil pump. (destroys them)

Doing this work grubbing on the ground on your back with trans few inches from yr face isnt much fun

very necessary to have my previously handmade from Ebay parts, 0-230psi pressure gauge, cost me $30, (most expensive part was 2m of hydraulic hose). This will allow me to correctly set the trans pressure kickdown cable, which should be 72.5psi at 1250rpm, with each turn of the adjuster mechanism equal ~9psi)

1 Like

Beautiful work, Tony - just beautiful!

xj6 85 Sov Europe (UK/NZ)

good and bad, feel mighty relieved…it needs adapted kickdown bracket and linkage remade, as pressure is somewhat too high (as measured)…gears grab like a train now.

During my re-assembly, even could see exactly how this took place, as its hard to refit regulator body without wiggling parts, but in fact, one bolt only that is under the pipes MUST be torqued before fitting any other parts, and the pipes jammed in…no if or buts

I re-aligned tie-rods today, (after IFS rebuild) and am not far off road test…works good in driveway, which is steep

2 things of note…if you can drop down the engine and trans ~90mm, the selector shaft can be slid out, still not easy at all…is a horror job… would apply to XJ/other saloons

also these BW trans can be completely rebuilt, piece by piece by removing and adding each part in reverse from underneath without removing engine…will add link later

The closest thing to a BW8 is a 1954 Flight-o-matic, I think they may be identical

(jumping in from the E-Type forum).

Yes, Tony, good work!

I “trolled” with interest as my E has a BW8 (now fully rebuilt by a shop) and for the life of me, even after reading the shop manual, I still have a hard time with the pressure adjustment. I am in the throes of rebuilding the engine (yet again- the trans did well, but that was done by a pro, and the engine not so much so, as I did it myself). After many years of suffering without a 3rd to 2nd kickdown I am hoping to get it, this time. Do you have that downshift?

Sorry to commandeer the thread.

Glad to hear that the engine is doing well!



Good informations, Tony…

xj6 85 Sov Europe (UK/NZ)

no worries, i read about your issues in researching my trans last time.
I am surprised there isnt more on the E-types cause I thought many would have bw8?

it did not kickdown properly, and had major issues slipping, but the “major issues” were all down to the cable bracket misaligned!

i had made a pressure gauge and stripped down an old bw8, but never got around to hooking up the gauge. or attemptng adjusting the kickdown, once I had sorted the other stuff

thats a mistake, as i found out, a baseline should be taken, and the manual states its very important.

the kickdown mechanism/controls are simple, so whatever causes the issue must not be complex.
Upon WOT, a lever (attached to the cable) pushes a control valve all the way in, and that interacts with another valve.

here is a diagram that helpfully names the parts, (unlike the Jag manual)…they ARE identical

note the part “downshift spring” if the kickdown wont work, that may mean internal pressure is too low, due to various reasons including adjustment, or internal seal wear.

OR, fit a different (weaker?) spring here, or possibly others associated

it does not seem like any other parts are involved.

consultation with a Ford FMX specialist would be preferred, or other trans expert, as issues with these
would be better known, and shift kits available…I believe some FMX had vacuum kickdown like bw12.

I can post links to several sites that give a great deal of info on working on these trans if others are interested

Hi Tony,

It seems that I came across this trans some many years ago, but at the time did not do anything but drive the vehicle, as I was a youngster and it did not need anything. Anything at all. Am not sure if I ever even put fluid init the couple of years of (hard) driving it endured, it being my first car, a Checker Marathon.

I am convinced that the much-lighter-than-original flex-plate I am going to install will do me a world of good once in place after the upcoming engine work (am dropping the engine/trans as unit). The original drive-plate is a monster of metal, more like a flywheel, really, that was superseded by something more like what the rest of the world uses in front of the torque converter, weighing much, much less. I probably could have just gone with a later stock XJ series flex-plate, but opted for an aftermarket I found in England, made for the E, and I only want to do this once.

I have also rigged up a pressure gauge for when the time comes and the car is back together and will see what is happening inside the big box of oil soaked spinny stuff. I had a hard time of it, even after replacing the cable, cable-end lock tab, and making sure all was mechanically in place and moving properly and still no kickdown. My pressure readings were high, if I recall, before the rebuild (did not get a chance to read, after, yet) at best, and could only lower enough to keep the durn thing from exploding. My shifts were a wee bit beyond “crisp” and all upshift was good and downshift upon decel so I left well enough alone.

I am intrigued by the “L” for "Lockup and want to learn how to use this feature. I have read about it-
do you know when it would be worthwhile to engage? I initially thot it was for “Low” like starting off and staying in first gear, but that is not really the case, from what I can tell, as supposedly, it can be engaged at freeway speeds, for some purpose.

Thanks for sharing info and insights! Yeah, I am surprised more E-Type info isn’t available or relayed as there must be a lot of the early E’s with the BW8!

69 2+2

If you carefully read the FSM on hydraulic control and selector Lockup does the following.
(the valve is physically locked into position)

if started in L…will hold 1st

if started in D1, will start in 1st and shift both up & down 1-2-3 (downshift suspect)

if L selected when in D1 will drop down to the appropriate gear based on speed. (supposedly)

In D2, will not allow 1st and car starts in 2nd (should still kickdown 3-2 supposedly)

I tend to use D1, and manual shift to L for a lower gear

the pressure adjustment is very sensitive, and there is a lot of data in the FSM

tons of info about these trans can be found here

the BW8 case is 9 & 7/8" long, that is important

the FMX case is 10" long, and there is vast info on rebuilding them.

The BW12 can be fitted, and I am fairly sure the E-type would have had them standard from ~1970,
the S1 XJ manual covers them both

Hi Tony,

I do the same, with the D1, from a stop, not much D2 action and L when I want to just keep it in 1st. It is the "[quote=“awg, post:15, topic:352844”]
if L selected when in D1 will drop down to the appropriate gear based on speed. (supposedly)
[/quote]’ that woories me to attempt when going any sort of speed!

Thanks for the FSM reading material, I am going to look into pressure adjustment hints.

If I didn’t have so much invested in this box I would either go to a BW12 or a 5 speed manual setup!

Thanks again,

69 2+2

yeah its a bit scary…here you go…spells it out

I have a V12 5speed, nice, would also like to fit an A340 behind 4.2 (4speed Toyota or Volvo box)