Just wondering whether whatever prevents kickdown at high road speed might be involved here. I presume it’s some kind of governor; could it jam in the " above the speed kickdown can occur" position, whilst letting the 'box perform correctly in other ways? As Paul says, mechanical, rather than electrical is more likely, especially as the solenoid tested out OK on the bench. Anyone know how that governor works?
The governor, working correctly, gives continous road speed info to the gearbox, Kevin - which is required for shift points. It’s just a pressure regulator; attached to the output axle it varies its pressure output according to road speed. As pressures operate valves in the valve body to change gears according to rpms, gas pedal position and road speed; a stuck governor will throw shift points out of spec…
Auto boxes are programmed to shift in response to driver demands; flooring the pedal means that the driver wants max power, and the only way to get power is to increase rpms - ie, downshifting. Boxes are sort of tuned to rapidly upshift with light pedal, for economy, and to delay upshifts for power - proportional delay with gas pedal position. Ideally, boxes should be ‘tuned’ to the characteristics of the engine and car - weight and assumed driving style - rpms versus power and torque curves. Crudely, the box will not shift if rpms in the present gear is as good as it gets…
Example; the BW66 should not downshift 3/2 at speeds above some 75 mph, and 3/1 above 37 mph. A sensible driver on a manual car will not downshift for more power if the rpms, downshifted, is outside the power curve of the engine. And auto boxes were generally made to imitate ‘sensible’ drivers…
Whether such limits, whatever they are on the BW12, is overridden by the kick-down function, I don’t know. But refusal to downshift by kick-down is not necessarily a fault indication. It must be assessed by the driver if the kick-down refusal is ‘sensible’ at the speed/load while driving…
As far as I can see; the BW12 kickdown acts mechanically on a valve - but whether separate and/or meant to override all other valves is arguable…
Frank
xj6 85 Sov Europe (UK/NZ)
Thanks for the explanation Frank.
The kickdown feature will not operate at any speed, although the switch is proved to activate and send 12v to the solenoid, which, in turn, can be heard to operate. My line of thinking is, given that all else seems to be in order, perhaps the box won’t kickdown because it thinks it is running above the kickdown range, ie above 80mph, or whatever speed it won’t kickdown above, something controlled by the governor. I ran this suggestion by the auto guy, but he is absolutely adamant that the problem is with the solenoid; if the governor was at fault there would be other symptoms. I must defer to his expertise, even if against my instincts. I’ll try and source a solenoid, and if that doesn’t fix it, who knows?
The solenoid should drop pressure, until it’s powered.
There is a balance between the pump/ input shaft speed, governor/ output shaft speed and throttle position:
The pump produces pressure depending on engine speed, and has a valve that ensures the pressure never exceeds a maximum value.
The throttle valve bleeds off pressure: the higher the torque (throttle position) the higher the transmission pressure, this means harder shifts and higher shift speeds/ downshifts.
Kickdown bleeds off some pressure until activated, which is equal to extra throttle.
(Other BW transmissions have a cam that pushes the valve closed, and that cam simply has an extra bump for kickdown).
The governor also manipulates the pressure, probably bleeding off pressure as the output shaft speed rises, limiting the maximum shift speeds.
The valve body was designed to keep the engine in the rpm range no matter what, so if the kickdown and throttle are bleeding off pressure the box is supposed to keep the engine below redline.
With shift points to spec the governor is fine, if the throttle position also influences shift speeds it should be a kickdown fault.
If the box otherwise downshifts with pedal movement, or indeed the shift point are to specs, Kevin - the governor is functioning as it should, and is innocent…
And I have a problem, calling on all listers; when i choose ‘reply’ or use ‘quote’ - the reply window covers the text of the original post. What have I done wrong?? And how to rectify??
Frank
xj6 85 Siov Europe (UK/NZ)
Do you have a blue bar at the bottom of the screen? Close that, otherwise I don’t know. Is it on all threads and always including the original post of the thread in which you are replying - or always the same original post?
Same thing happens to me, I thought it was normal.
I have windows 7, which may be the problem, Davld? Seems they changed something - I can read and sort of reply,but the set-up is haywire…
Frank
Xj6 85 Sov Europe (UK/NZ)
| OldJagNut KevinKeereweer
October 2 |
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Frank_Andersen:
And I have a problem, calling on all listers; when i choose ‘reply’ or use ‘quote’ - the reply window covers the text of the original post.
Same thing happens to me, I thought it was normal.
As I told David - it is likely change of something by administration, Kevin - and likely permanent for windows 7 users 
Frank
Oh, I misread. While replying you can’t read the text you’re replying to.
No way to resize? I’m sure that’ll be fixed.
Just picked up the car. The tech showed me a Type 12 valve body with manual kickdown arrangement ( rod, rather than solenoid) and explained the difference and how the Jag system works. He is adamant that the rest of the system is good to go and the solenoid must be the issue. I’ve bought the car home to attend to a loose rear wheel bearing; this will buy me time while I decide whether or not to prevaricate on the issue. 
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Unfortunately, David - it seems to be an update by geeks, and I, and likely Kevin, are some of the victims. Haven’t heard from the Administrators, but it is likely permanent and irreversible. Whether applicable; ‘if it ain’t broke, don’t fix it’ doesn’t seem a priority for the forums…?
Frank
I agree, also not a fan of the new look - way too complicated.
It’s a complaint for #site-feedback
David
In order to rule out a switch or wiring fault the only other test you can do is rig up a temporary feed via the cabin direct to the solenoid terminal at the gearbox. Road test and have a passenger make the feed connection at the appropriate road speed and throttle position. If that doesn’t make it work it has to be an issue inside the box. Frankie.
Thanks Frankie,
Yes I had considered that, and think it must be my next move. I also might try a direct 12v feed to the solenoid and see it the noise is any different to when operated by the switch.
Thanks.
FWIW mine gives only a soft click when energised - only just audible from drivers seat with window open and engine off, and needs a sharp jab on the throttle for the kickdown to actuate when driving; a slow progressive depression of the pedal won’t activate it. Good luck.
That’s interesting; I would have thought that once the switch operates it would kickdown, perhaps there is some interaction with the modulator\vacuum?
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No idea, but worth a try - possibly mine is slightly out of adjustment or my legs are just too short 
Frankie
Just move the engine back! Did some more diagnosis today; resistance of solenoid = 7.9 ohms. I = E/R , 12 divided by 7.9 equals 1.52 A. With my meter in series on 20A scale, meter read off scale, so in excess of 20 A, if meter is reading correctly, not always guaranteed. Solenoid clicks faintly even with direct 12V from the battery, so it is looking like a solenoid issue. Problem now is, no-one seems to have one, not SNGB nor the local DLOC spares club. Might just need to do a bit more procrastinating, not really a major issue at this point; still have manual intermediate selection. Thanks for all the suggestions, folks.