Help needed - no shift before WOT

Hello guys, me again. Still in the process of the revival of my 1988 Jaguar XJ12 with TH400 transmission, with several delays due life/work/family.

But need your guidance on a transmission issue. Car was running ok, for no reason stopped shifting from 1st to 2st (use to shift around 3000 rpm’s on low/mid-throttle). Have P, D and R, but the car now only shift from 1st to 2nd at WOT, @5800 rpms’s.

Checked vacuum at manifold, replaced vacuum hose, checked vacuum at the modulator, replaced the modulator itself twice, disconnected kick-down at the cable and at the vacuum sensor, now disconnected kick-down wire at the transmission. New transmission fluid in the correct level, new (3 months old) filter. Differential and rear suspension completely redone recently.

Any ideas? Should be the Governor (considering it shifts properly at 5800 rpms)? Is it possible to replace the governor without dropping the transmission (no access role from the cabin in my car).

Any recommendation is very much welcome. Thank you very much again for the support, Giuseppe.

Hi Giuseppe,

with people talking about Italian tune-ups you seem to give a fine example;-) Must be quite a show cruising down the entire neighborhood in first and then, upon getting on the highway, revving out the old Lady …

I don’t know much about auto boxes, but some of the framework condition would clearly help the pros around to get on the right track: When did the problem occur first? Do you remember any circumstances accompanying the occurrence, e.g. work in the neighborhood of the gearbox, engine, ignition etc. Any history about the gearbox? When did it get its last fluid change? How about fluid level? Did you check it? The correct procedure may be a bit awkward (it certainly is for BW65, which I know) .

How does the gearbox behave apart from the shift 1-2? How about R and shift 2-3? Is that normal again? - Revving first to 5800 RPM in a 2.87 ratio XJ should get you pretty fast - maybe the car then goes directly from 1-3? What happens after the first shifting through the gears? How is downshift 2-1 - very high or rather normal (i.e. without throttle not far from standstill)?

Good luck


75 XJ6L 4.2 auto (UK spec)

Probably the governor; fairly common failure point.

Google “TH400 no upshift” and you’ll read tons of advice


1 Like

Disconnecting the kickdown cable is likely counterproductive, Guiseppe - but since that has no effect; reconnect. What was the actual vacuum reading at the modulator? We can agree that 3 different modulators cannot have the same fault…?

Disconnect the electric plug for the detent solenoid and test drive - it is possible that an electric fault tricks the box into believing it is in kick-down mode. If this disconnection rectifies the problem; it may be a faulty detent switch and/or an eletric malfunction…

The governor is of course a possible suspect, but will usually interfere with shift points to a lesser degree than full on max rpms. Testing the governor involves checking fluid pressures - but I don’t recall how the governor itself is accessed for eventual remedial actions…

However, I also noticed that you referred to ‘upshifts at light throttle’ to be ‘3000 rpms’, which is plain wrong. At light throttle the 1/2 upshift should be at 10 - 13 mph and 2/3 at 12 - 20 mph - seemless. The governor only modifies shiftpoints by road speed, the shift at ‘5800 rpms’ is forced on by other factors than road speed - it denotes that the box is in kick-down mode. In which case the governor data is disregarded, and while such high rpms upshift is then ‘normal’; it does not mean the governor is functioning as it should…

The problem may also lie in the valve body, of course - but generally with abnormal shift points; pressure readings must be used to clarify issues…

xj6 85 Sov Europe (UK/NZ)

Your post was really really helpfull, helped me narrow down some issues.

The vacuum at the solenoid is 15.4 which is the best i could achieve in the manifold as well (not so different from my other cars, I’m at 3000feet altitute). Reason why I also tried a double red stripe modulator for low vaccum, with no sucess (that one and 2 jaguar spec original ones). The valve from the vacuum modulator moves freely in and out, good spring tension, but I can’t remove it by hand, seem to be some kind of lock on it to avoid this jumping out of the position.

I’ll check transmission pressure, I have to buy a gauge for that.

Regarding the deterrent switch, i will check - actually i believed that the kickdown solenoid was the same thing as the deterent reason why i disconnected on all 3 points. So, where the deterent connection ahould be disconnected?

Thanks for the support

Thanks Doug,
There’s quite a lot of information on that. I will chase some of Franks recommendations and report back.

Actually excited to share some repairs done that are making the car closer to a reliable machine.

Thanks and best

The kickdown solenoid is activated by the kickdown cable operating a switch to power the solenoid, which activates kick-down function. There should be an electric connector "somewhere’…?

The pressure ranges are specified for various tests, but I cannot find any adjustments to rectify errors - only references to possible component faults. Are there any visible means of modulator adjustment at all? While the altitude/low manifold pressure may play a part - it should not really interfere with the box to the extent described…

It seems possible to access the governor in-car by removing the cover over it (where??) - but there are no description as to what to look for. The box should upshift, at light throttle, at some 1000 rpms - given the specified 1/2 upshift at 10 -15 mph road speed - but whether the governor or other factors are involved in your original ‘3000 rpms’ is uncertain. It is specified that locked in ‘1’ the box will upshift at 6200 rpms, near enough to your 5800 - but what that means is somewhat uncertain…:slight_smile:

I’m raining on your parade - my experience is the box fitted to the xk engines…:frowning:

xj6 85 Sov Europe (UK/NZ)

Dear all, thanks for all the support on this. Just to close this discussion, actually was more than one issue. I had a worn governor as Doug pointed, this was the cause of no-shift. The shift points are lower now, just replaced the modulator with a dual red stripe type. Full oil change and filter. So, it’s shifting, but not as smooth and silence as I imaged it could be, maybe I just had unrealistic expectations on the TH400 (my daily drive has a ZF8HP that raised the bar in terms of comfort, maybe I am the issue :).

Well, the TH400 was introduced in 1964 so it’s a bit of a dinosaur.

The shifting should be quite smooth, though. It might be a matter of adjusting the modulator a bit. Still, perhaps not a smooth as a modern transmission.

Through its entire life the 400 suffered a noisy first gear due to the design of the planetary gears.


Second that, Dough - or his expectations are too exacting…:slight_smile:

xj6 85 Sov Europe (UK/NZ)