I have a 71 xj6 with bw12 tranny. Just rebuilt it completely with new modulator. Everything is fine it has all gears and reverse ( i rebuild trannys for a living but first time on a 71 bw12) i have this issue where it goes from second to third and if i dont give it more throttle it will jump back yo 2nd and back to 3rd. Up and down amd up and down unless i give it more or less throttle… Anyone had this issue? I know about band adjustment and vacuum modulator adjustment but im hoping someone can specifically say i should adjust either one more or less to save me spending days trying to find the sweet spot… Btw the problem was there before the rebuild and im really confident theres no issue in the tranny.
Greetings from South Africa
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The BW12s shifting is a function of manifold vacuum (modulator), road speed (governor) and engine rpms (fluid pump).
The valve body ‘compares’ the pressures resulting from these components and selects the appropriate gear by moving the spring loaded spool valves in the valve body.
What were the pressure and vacuum readings when you finished the modulator adjustments? Ie, you got it to specs - and what did you do with the valve body during the rebuild…?
If so; one possible problem with your shifting is the governor - improper feedback pressure to the valve body. The other is an undetected fault in the valve body itself. With either fault; you cannot adjust to a ‘sweet spot’ - the fault must be rectified
All that said; all automatic boxes are programmed to upshift as quickly as possible - depending on pedal position. And to downshift when the driver indicates that he/she wants power by stepping on the pedal.
So have you checked that the shift points are to specs…?
Frank
xj6 85 Sov Europe (UK/NZ)
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Thank you for your reply here is what ive done… On the valve body i only cleaned and inapected all parts… Cleaned out cylinders with ho ing tools and checked all piatons and springs. The governer i checked for smooth movement of the valve. Pressures and vacuum i checked accorsing to my service manual. It feels as if the throttle is too sensitive ie if i let off a bit on the throttle it will stay in 3rd but if i accelerate just a little in 3rd it kicks down to 2nd and back up to 3rd. The shift points i corrected to spec using the vacuum modulator and at the same time the shifting has become smoother but the kickdown and upshift is still there not really sure where else to look. Valve bodies are very rare for this car where i live. Ive now adjusted the bands to spec as well with no effect
Hi Devon. The model 12 also has a solenoid operated kick down valve whose switch is activated by the throttle linkage. You should check that it’s adjusted and operating correctly and when it’s supposed to.
~Mike
Thats actually a very good idea thank you i did see it but didnt think it was affecting this… I did play around with throttle cable adjustment but perhaps i should disconnect that switch amd see what happens… Will report back thanks mike
Also regarding the solenoid itself, it has an o ring around it’s nose where it locks into the valve body that deteriorates… I guess you would’ve replaced that when it was apart?
More importantly, the solenoid locks into the valve body by a tiny, 3/8" long, thin wire bar that is trapped between the cover and main valve body. Can easily fall out when you open it. IIRC it doesn’t show in the exploded view in the S1 XJ manual and isn’t mentioned in the text. If this is missing or you didn’t notice when pulling the valve body apart it’s possible that the solenoid isn’t locked in place which would be a severe leak in the kickdown system.
The way it locks is the solenoid nose has a flat on one side that fits past that little bar protruding into it’s hole, then you push and turn to lock it in place. When pulling it apart you should’ve had to push and turn it to remove it from the valve body. If not maybe a PO had it apart and didn’t put it back correctly.
~Mike
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Are you testing at ‘idle’ speeds, Devon…?
If so, have also tested at, say, 20 mph - to verify downshift versus pedal movement there? The box is likely very sensitive to pedal movement at very low speeds - it doesn’t quite know what to do. I’m not entirely convinced if you actually have box or a perception problem…
If you have the same sensitive reaction to pedal at all speeds; something is definitely wrong; the faster you drive the more pedal is required for downshifting. In principle; auto boxes are biased towards earliest possible upshifts with steady pedal…
You have done everything according to the book - and in principle the box should perform to factory specs. However, even if clean, the governor may not perform to specs. And the spring length and tension in the valve body makes the characteristics of the shift pattern. But as your shift points are to specs it is likely OK…
I don’t like your honing of the valve body; the spool valves are a deliberate tight fit to avoid leaks. Wear/clearances in the valve body will cause internal leaks - though the symptoms are not clear cut…
Frank
xj6 85 Sov Europe (UK/NZ)
Hi frank. Thank you for your response again. Let me answer one by one. It has the same problem at all speeds the problem though is not that it kicks back but that it kicks back and up so it jumps between second and third the whole time till you change the pedal position like it doenst know where it wants to be… Ive tried it at all speeds. Perhaps my wording was incorrect with regards to the honing tools. Im not honing the cylyinders they are more like a bottle brush type tool made specifically for valve body bores and i use them on all the valve bodies i do. Also the problem was there before the rebuild so i dont think that was the cause… I will look into the governer again and maybe try get another one somewhere ill also look at the valve body again. I dont think i need to take out the transmission again as it drives perfectly and smoothly and has all its gears the shifting up and down is my only issue. I must also add that the customer sais this issue arose immediately after he had the engine rebuilt thats why i was thinking about the throttle cable and kickdown switch etc. I really appreciate everyones help i promise as soon as its resolved i will post it here to help the next person.
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Ie, the box reacts ‘instantly’ as described at the touch of pedal throughout, Devon…
Never met a symptom like that - and the solution, when found, would be very interesting indeed! The only thing directly involved with pedal movement is manifold vacuum and the modulator. Principally; lowering vacuum causes downshift/delayed upshifting - but both engine rpms and governor pressures modifies reaction…
The GM400 also uses the same items for shifting - but there is no mention there either of remedial actions for symptoms you describe. And I cannot see that you have done anything wrong - nor that an engine rebuild should influence the box beyond necessary ‘trimming’ of the modulator…
I trust that when you adjusted the modulator that there were normal reactions, pressure variations. The d… ‘12’ is virtually bullet proof…
Frank
xj6 85 Sov Europe (UK/NZ)
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Even though not an expert here, but I think you are in permanent kickdown mode.
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My friend you hit it on the head… The kick wn switch has 2 circuits normally open amd normally closed… Whoever did the engine connected it in normally closed so it was giving permanent signal to the solenoid… Thank you everyone for your input i was just sitting down to report back when i saw this comment… I found it by disconecting the switch all together and the problem was gone but i just had no kickdown… Whipped out the old multimeter and found that there are 2 circuits across the 3 pins…so releived thanks everyone
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Thanks to Aristides, Devon - and your promise to enlighten us is fulfilled…
The box should still downshift with pedal movement - it just hasn’t got the forced downshift with the pedal floored…
Frank
xj6 85 Sov Europe (UK/NZ)
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