From kc0ahz@juno.com Wed Mar 29 20:40:28 2000 Hi John! Dave Kamp here, on Remote Control... Saw the posts, so had'ta send a refresher- Post it if you think it's appropriate! Hey Guys! Take another look at your Factory Service Manual. You'll see the following: --TCU watches Throttle Position Sensor (a power demand signal) --TCU watches Speed Sensor (on the TRANSMISSION's output shaft) (a ground-speed signal...) --TCU watches state of the BRAKE PEDAL (to know when to unlock the Torque Converter (TC)) --TCU watches state of the shift-lever position. --TCU operates Solenoid 1, Solenoid 2, and Solenoid 3. *SOL 1 and 2 select ratio *SOL 3 locks and unlocks the TCU. Very simple. Note that the throttle position sensor signal is used not only to determine upshift and downshift points, it's also used to determine when to lock and unlock the TCU. Under light demand, the TCU will lock up, to save gas. Under moderate and heavy demand, the TCU will UNLOCK the TC, for power. At IDLE, the TCU will unlock the TC, to keep you from killing the engine (and the Torque Converter). The TC watches brake pedal for same reason- if you're stepping on the brake, a locked-up TC could end up fighting the brakes. Also note that the Speed Sensor signal is taken from the transmission's output shaft- this is BEFORE the Transfer Case... so when in HIGH, the speed sensor sees ground speed. When in low, the speed sensor sees a FRACTION of ground speed. (--Just in case you've been considering suing me, it's right there in the text. ;-) >From a blank-stare perspective, it's not obvious, but look at this: Since the TCU is comparing throttle position to that sensor, your shift points WILL be different... and if you did a mathematical comparison between HI and LO range shift points, you'll probably observe them to be different... not because of anything that the TCU is doing differently- it's because your throttle-demand in HIGH range will be higher than when in LO range. Takes more torque to accellerate in HI than LO. Now read the explanation of the Throttle Valve's function (again, right there in the text)... it modulates pressure... it doesn't control the SHIFT POINT, but it does control SHIFT PRESSURE... the side effect of changing shift pressure is a change in how QUICKLY brake-band servo pressure rises. And it's been a while since I last had hands in mine, and am not able to verify right now, but I believe I recall the TV cable biases BACKWARDS in relation to pressure- meaning, pressure is relieved at high-throttle, rather than raised... but beware- high throttle is not necessarily the same thing as HIGH RPM! Some of you guys could be observing high shift-points simply because the combined TV setting and RPM are such that servo shift pressure only loads the accumulators so quickly. I've noticed that my (TCU-less) XJ has a nominal upshift delay of about 1/2 to 3/4 second under normal conditions. The downshift delay, however, is almost instantanious. I recall noticing that the TV cable adjustment affects the rapidity of upshifts, however, I don't recall it ever affecting downshifts. Of course, no adjustments to the TV cable had any effect on what RPM my XJ shifts at- since there's no TCU, the shift-point RPM is controlled only by my switch. Same goes for the TC lock- it's on a switch. Totally manual, totally hillbilly, but it does work... when neither an '88 nor '93 TCU will. BTW... I haven't had any problems winding her up to 5 grand... but I wouldn't advise you leaving it there for hours on end... and you should make sure it's warmed up before spinning it that fast. If you happen to be running an HO motor, you'll notice that it's powerband is very stout up there. I actually built a 250ci Chevy inline (dimensionally very similar to the 4.0) for my boat using the 5krpm as an assumed WOT point, and it's happy as a clam to do it for essentially all day... and hasn't had a bit of problems (other than vibrating a few bolts loose) in over 1500 hours. I wouldn't recommend running your XJ 4.0 under 5krpm WOT conditions for periods longer than about 14 seconds, though... because in most cases, you'll be well over the national speed limit. In all remaining cases, you're stuck and in dire need of a winch. ;-} DK :-) ________________________________________________________________ YOU'RE PAYING TOO MUCH FOR THE INTERNET! Juno now offers FREE Internet Access! Try it today - there's no risk! For your FREE software, visit: http://dl.www.juno.com/get/tagj.