Guys
One of the reasons that the TH400 in the V12 pre facelift gets such a bad rep is because the kickdown is so lazy. This is as a result of the kickdown switch mechanism on the throttle system being very hit and miss, and even when working as it should, very uncertain in operation.
The OEM kickdown mechanism consists of a microswich in the throttle cable, actuated in a very poor manner. To activate the kickdown microswitch, the throttle pedal has to be floored, and then push a bit more against a noticeable extra resistance. This secondary push does NOT open the throttle more (the capstan is already open fully when the pedal is floored); what is does do is to move the OUTER of the cable against a spring, and by so doing cause a small pip to come into a small gap and push the microswitch arm. The switch then sends 12v to the gearbox kickdown solenoid that kicks the 'box down.
Now all this adds up to a litany of uncertain operation, delayed action, the need to have 100% WOT to get a lower gear, and a fairly useless kickdown that as a consequence never gets used. In turn the sprung activation mechanism corrodes and jams, and everyone says the TH400 is a poor gearbox that masks the performance of the car -
UNTIL NOW.
What follows is NOT my invention, Grant has done it, he advised me on my effort, and he believes that Doug has too. What I set out below is therefore to give a bit of much needed publicity to this mod, which costs next to nothing apart from 1 dollar/pound/euro microswitch.
The requirement is to find a better more positive way to activate the kickdown microswitch. This is provided by the capstan. In USA spec cars there is already a microswitch on the rear of the capstan, which is the failsafe in the vacuum operated full throttle enrichment system. In UK spec cars there is no failsafe switch, so this is the place i decided to move it to. If you have a switch there, just piggyback another on on top.
I had to make a bracket to hold it, about 2 inches by 3/4 out of 6mm steel. To engage the microswitch arm properly on the correct bit of the capstan the bracket needs to be about 9mm thick, so I added a spacer. It was fixed to the capstan using the rear RHS capstan fixing:


The blue line shows the flat spot already on the OEM capstan. This allows the microswitch fitted to USA models to be in the off position except at WOT. The red line shows where I filed the edge to extend this flatspot about 10mm in length to ensure that, at closed throttle, the microswitch was not held open. At the RH end f the blue line you can see where it gets wider to activate the switch as the capstan turns near to WOT.


Green line = 2inches, Blue line = 3/4 inch, red line = 9 mm, purple line = secured by 2mm stainless bolts, blue ring = bracket bolted using capstan fixing


The blue box shows the OEM kickdown mechanism with the OEM microswitch bolted underneath it

Two green and green/white loom wires with female Lucar connectors just have to be pulled off the OEM switch and with a short extra bit of loom connected to the new one.

The filed extra flat spot to ensure the switch remained closed at closed throttle

General view of the completed mod, the switch is adjusted so it closes when the stop ear on the capstan (below the throttle rod in the photo) is about 1/2 inch from its WOT stop position (the screw adjustable item sticking up just 'above' the microswitch in the photo):


Now to how it performs!
Well it TRANSFORMS the car's feel. For example, before, when following a car doing say 55 mph, a bit of a prod on the throttle does not produce startling acceleration, certainly not what 300BHP should. Well now, push the throttle down a bit and BANG you are out and round the car in astonishingly quick time, doing very decent speeds you are not accustomed to! This absolutely transforms the car's responsiveness, to the extent that, if I had not known about the change and had just got in the car, I would have thought a new modern gearbox, or some other huge engine power mod had been done to it. I promise that I do not exaggerate! get out in the garage and do this fix during the cold winter months boys and girls. You will be amazed.

 

Good job. Yes, I did the same mod....one of three kickdown mods I've tried over the years and the most satisfactory of them all.

Really does change the driving experience!

Cheers
DD

 

I think given the expense and difficulty of a manual swap, this is a great option for people who don't want to drop $5000-$7000. If I wasn't in the middle of one (and not driving the car cause of all the salt on the road), I'd be setting this up this weekend.

 

The kickdown mod, plus a mild shift kit, can really wake things up. On my XJS I added a 2500rpm stall converter as well.....which helps with off the line acceleration. And the gearshift gate is easily modified to allow much easier manual shifting. So, yes, it can make enough difference to make (at least some) people re-think the need for a manual trans conversion.

Add a 3.31 diff, low restriction exhaust, and SuperEnhnaced ECU....and you've got a pretty decent performer


Cheers
DD

 

Oh, no doubt, especially with the cost and the rarity of Getrag 265s and 290s these days. I didn't find the car to be slow or unresponsive, I just kind of wanted a manual car. That being said, I'm really looking forward to seeing how much pep the five speed, S.E. ECU, large-bore throttle kit, and a low backpressure exhaust put in the old girl's step. I suspect it'll be plenty, although the idea of swapping the camshaft is currently floating around my head.

 

I have the AJ6 kit and it makes a huge difference. I think you will find the car will be really good, especially with the manual box, because of the TQ losses goign straught to the rear wheels! I would not bother with a camshaft change though, as unless you are racing the car and can do substantial higher-lift valve work together with a pre HE head, it will do nothing and may well make the car slower and less driveable.

 

Greg. Great job with the post. I'm wondering. I have a 1988 Australian spec which as far as I know is the same as UK speck. I see you're is a 1985.
I have the microswitch on the capstan on a specific height bracket ( looks factory to me) which engages in the flat area as you have shown as blue in your first photo, and I thought it was the EFI fuel enrichment microswitch. I also have the microswitch on the throttle cable spring under the bracket but it does nothing on pressing to WOT. Ie it doesn't have a 'pip' that comes out to make the microswitch arm. I'm I missing something ? Thanks

 

Lachy,

Yes, that micro we have is the EFI Open Loop Switch.

I attached another Micro on top, longer screws, then fiddled and farted with a "clip" to attach the top switch arm to the lower switch arm, so when the factory switch activated, that "clip" allowed the top switch to switch, and apply kicdown, SIMPLE.

Your cable switch is possibly knackered, and/or the cable fiasco at that area is seized, so it no longer slides when really mashed. All mine were seized, lack of really HARD driving to make that nonsense work being the culprit.

 

The capstan switch is the full throttle enrichment switch. There are two switches wired in parallel, the other one is a vacuum operated switch on the end of the A bank inlet manifold. The purpose of this circuit is to ensure the engine does not go lean at WOT. As the HE runs very lean and also at high compression, leanness is a danger at WOT.


This is the vac operated valve situated on the cabin end of A bank manifold. It is wired in parallel with the capstan switch, so if either makes contact, the enrichment occurs. Yours might be a different colour.

The reason you think the OEM setup has no pip is because the mechanism is jammed - normal situation! So as Uk cars, for example, have just the vac switch for the enrichment, you have two options:

1. Just rely on the vac switch (just check it is functioning as it should, that is: the switch closes at LOW manifold vacuum = throttles fairly wide open; switch OPEN at high manifold vacuum = throttles fairly closed) and re-purpose the capstan switch for the kickdown; or
2. Place a second microswitch on top of the first one on the capstan, which is what Grant did, I think. My opinion is that the vac switch on its own is fine, ALL UK Lucas ignition cars just have this one switch, and just connect the kickdown wires to that switch.

 

Using the enrichment switch for the kickdown is a viable choice. I ran my XJS that way for years. And my Series III as well, but only for a short time. IMO, it makes the kickdown a bit too aggressive....which I apparently liked many years ago but now, in my wizened state, find a bit annoying . The flat on the capstan is such that the switch operates at about 75% throttle.

The flat that I conjured up for the KD switch gives operation at something more like 90%, which I find to be about right.

Cheers
DD

 

Perfectly understandable.

Cheers
DD

 

Doug
Interesting point. I had to estimate my position for the microswitch, and only having had one test drive it is possible I might find it coming on too early. In which case I have two options (being as I have no capstan enrichment switch on-point to worry about):
Either I can file the capstan a touch, or less drastically
I can slot the bracket fixing hole a touch to move the microswitch arm further away from the capstan on-point, so the throttle is further open when it hits to arm of the switch.

Once I have more experience of the change, I might also smarten up the bracket etc. We will see on further use; but I still have a silly grin on my face from the change it has made!

 

Nice one Greg grant and Doug

 

Like Lachy, I too have the Australian spec car with the two switches. I'd like to keep the standard fuel loop switch. The kickdown switch activates at +100% WOT (so not stuck) but we all know that is next to useless. Does anyone think it would be viable to make the kickdown switch adjustable by elongating the "bump" hole in the cable holder and doing the same for the switch holes so that the switch can be slid towards the cable end and activated earlier? Wouldn't think the adjustment would have to be very much. That way you could adjust the kickdown to where it suits you. Not sure my shaking hands would be up to it but maybe worth a try.

Cheers, Ian.

 

Thanks guys. I'll check why it's not working first. Very interesting though how it all works. I think I'd better get it driving first and see then what adjustments and changes I can make. Early days as the car is still in resto mode but I love all the tweaks to get the best of of things. Cheers

 

I would be careful, the throttle cable is NLA, as far as I know, so a ****-up could be a problem. The difficulty with your idea is that the throttle has to be wide open to get the kickdown mechanism to work at all. Until it is, the spring-loaded part (which has to move to reveal the pip) will not move at all, regardless of where the hole and pip are. It is only when the capstan is fully open against its stop, and the throttle cable thus fully taught, that the extra push on the pedal can operate the spring-loaded mechanism to reveal the pip.

Now what might be possible, subject to the electrical gurus on here's OK, is to use the capstan microswitch for TWO purposes:

1. to operate the fuel enrichment as normal, and
2. to operate the kickdown.

I believe, as long as both the input 12v live wiires for each circuit are on the same terminal of the switch, and the outputs also both on the other terminal, it would do both circuits. The only question would be the current carrying capacity of the switch, but I would be very surprised if that was more than about an amp.

 

Forget it. I misunderstood how the mechanism works and any adjustment would have to go the opposite direction which is not practical.

 

I like this idea of matching electrically the enrichment to kickdown if it can be done. The piggy back (stacking of switches on the capstan) seems to be the same idea but without having to manually find a way to connect the small thin microswitch arms together so they work as one. Be good to know what the sparkies and gurus here think.

 

Good stuff going on here!

I have a S3 XJ12 that would benefit from these mods. It has the 3.31 diff already.
I hadn't thought about modifying the shifter, but I have always thought the orginal gates were clumsy.
Mr Bywater is slowing down a little I think, I'm not sure if I should send an ECU.

Thanks,

Rob

 

Hi Greg

Seeing as the 'Micro Switch' seems to Make or Break the Electrical Circuit to the 'Kick-down' at a Certain Throttle Opening

Would it not be possible or even more desirable, to do away with the 'Micro Switch' idea altogether and instead 're-purpose' the part of the Steering Column Stalk Lever that is used for Flashing the Headlights, as a sort of a 'Paddle Shifter' to operate the 'Kick-down' at any time decided by the driver, who might base that decision on the reading of the 'Tacho' at the time

Giving 'Total Flexibility' as to when to operate the 'Kickdown' and for how long

Or even do so using one of those ON/OFF Foot Switches (operated by the Left Foot) like I think they used to use back in the day for dipping the Headlights

Alex