Hi all

im trying to help a friend with his XJ12 Series 3. HE with TH400 gearbox.
Mileage is fairly high, 350+ kkm. Starts, runs and pulls fairly well, but it changes up at around 4000 with WOT and I don’t think it will do kick down at any RPM.
Can the vacuum governor get stuck in ‘vacuum mode’, making the box think you’re using light throttle all the time ?
I have not checked oil level or if oil is burnt, but I don’t suppose that could cause this ?
We have checked that throttles opens fully and tested another set of ignition coils. Injectors has just been tested (and hoses changed). Spark plugs are new. The B bank fuel pressure regulator has been changed, all without any impact on the gear change RPM.

 

Leo
I would suspect the modulator and/or its vac tube to the A bank manifold rear spigot.

A good test is to put the gearstick in 1st gear and then do a flat to the floor start. It should change up to second on its own at about 5,500 revs. If it does not and still will not go above 4400 revs, certainly a gearbox fault of some sort.
If it does rev to 5500 then I would be almost sure it is a modulator or governor fault.

 

If you have no kick down, first check the micro switch on the throttle cable at the capstan; there should be 12v on one of the wires, and on the other when the cable is pushed into the capstan against spring tension. If that is working correctly, the kick down solenoid on the transmission valve body is most likely failed (very common).
If the kick down isn’t working correctly, you will get 4000 rpm up shifts.
When you access the valve body solenoid, change the filter. Pain to do, as the exhaust and transmission mount have to be removed. Probably plan on the rubber bushing and spring seat while you have it apart, if it is like the XJ-S.

 

Two good suggestions, thanks.
Agree about the gearbox filter change, I did it recently on my XJ-S, and it’s hard not to get a bit irritated by the mount being in the way. I guess it must be some safety afterthought, cannot have been like that from the start ?

 

It is important to have the kick down solenoid functional if you are using wide open throttle shifts; it works by increasing line pressure, which increases the clutch hold in addition to delaying the up shift. Although I don’t know how necessary with the power level of the V12…..

 

I don’t see any safety benefit from that mount design, my guess is that the car was designed for the Borg Warner trans and when they went to the TH400 they cobbled it together to keep as many of the existing parts as they could. For its unusual design, it does work well as far as not transferring NVH. I think the spring idea was something they had used on prior vehicles with success.