Yes it is strange the box has some sort of hydraulic speed sensor, it uses hydraulic pressure to determind what speed the vehicle is travaling at and thus the change point, it seems that when a seal starts to leak pressure is reduced to certain parts of the box due to the leak, which causes all sorts of problems. It will hold gears longer when accelerating, but it requires substancial throttle movement to hold of the change up.

D

Hi Dean, it is the governor connected to the output shaft that regulates the pressure to the shift valves dependent on engine speed, this pressure is balanced by the thottle valve which is positioned by either the throttle cable or vacuum modulator dependent on fit determining shift point.

thanks

D

Hi Dieselman,
The engine revs on a 2.1 auto. are far higher than you mentioned at the stated roadspeeds, giving rise to the familiar buzz bomb effect. Very irritating around the legal limits of 30 and 40 mph as discussed on another thread recently.

Robertmnorton, is there any way of altering the parameters causing the changepoints to create a bigger difference between the roadspeed at which it changes up and the roadspeed at which it changes back down?

Robert.

Hi Dieselman,

Adjusting the cable doesnt really do anything, from what i can gather, it tells the box what sort of load is on it and thus what gear it should be in, it would not affect its light throttling gear selection points.
I think to give the box a custom setup you would need some sort of hydraulic tester to tune the valveblock itself.

D

@robertmnorton;

I will get him to try the Vacuum suggestion But I think it wont be until Thursday/Friday before he does...


@Dieselman I am going to get him to find out which box he has... What is the easy way of telling if it is a HP18?

Just to add to this..

At times the car will go 1st, 2nd, 3rd gear within 20/30 yards at other times it will try and pull off in 3rd

I will let you know... thanks again

Hi Robert, not that you can do external to the g/box.The governor output pressure is determined by the engine speed ,being driven directly by the output shaft and regulated by a centrifugal fly-weight governor.So adjustment to the fly-weight governor would be required to change the output pressure, a very precise arrangement.Also the throttle valve as biased by this pressure and the load compensator (either the vacuum modulator or throttle quadrant stepdown cable) operates an hydraulic time control valve which times the overlap engagement of the various clutches and assisst operation of the sequence valves,these prevent the early or unwanted upshift, and is the reason a large throttle movement is needed to hold off the change up during aggressive acceleration.The hp20 primarily uses electronic input shaft/output shaft sensors for determining relative speeds for valve operation ,but the interactive inputs to the ecu (pedal rate/position, snow/sport switch etc) provide modification to the valve operating programme allowing adaptive change up points,it also allows torque back-off for smoother changes.

Forgot to mention, the snow/sport/eco switch mounted in front of the hydro-active switch gives away the hp20 fit, but as previously posted, the spec sheets show it was never fitted to any xm other than the V6.