1eliminator1 Posted June 30, 2011 Share Posted June 30, 2011 out a new master cylinder and booster out of a 99 cherokee in the mj. and my brother disconnected the switch at that top of the brake pedal, i don't know why but now after we put it all back together my truck drive at about 2000 rpm in overdrive. it was around 1600 before we did anything, any suggestions? Link to comment Share on other sites More sharing options...
kro10000 Posted June 30, 2011 Share Posted June 30, 2011 The lockup clutch isn't engaging. It uses the brake switch to tell it to disconnect when you're braking. Link to comment Share on other sites More sharing options...
1eliminator1 Posted June 30, 2011 Author Share Posted June 30, 2011 so should i just check the switch to make sure it is working properly? Link to comment Share on other sites More sharing options...
88 Beast Posted June 30, 2011 Share Posted June 30, 2011 I'd be interested in this too. My '88' 4.0/AW4/3.55 ratio/30X9.5X15 tires, has always run at around 2000 rpm @55 ( accurate mph, ask the cop :D )since I've had it (6 years ). Always thought that was normal but if it should be lower I've got some work to do ( as usual it seems ). If so, should help with the MPG. I'm getting 18 mpg now. Link to comment Share on other sites More sharing options...
jimoshel Posted June 30, 2011 Share Posted June 30, 2011 Got a similar problem, kind of. I have a 1994 ZJ, 4.0. RE42 auto. Sluggish on take off, not shifting down. When I manually shift it's great. At 75mph the tach is reading 2400rpm. My '93ZJ, similar engine, tranny, and axle ratio reads 1600rpm at 75. I've checked everything I can think of and what the book suggests. Any ideas? :help: Link to comment Share on other sites More sharing options...
kro10000 Posted July 1, 2011 Share Posted July 1, 2011 Well I don't know off hand what RPM the autos run on the highway so I can't say that for sure. Â But, I'll try to explain this better, there's a lockup clutch on the torque converter that gives you near zero slip while your're on the highway. That's why you will notice a 5th rpm drop when accelerating. To prevent damage to the clutch it is only supposed to be utilized while you are cruising, as rapid accel/decel would place a lot of load onto the clutch. So when you accelerate on the highway the clutch disengages or when you brake it will disengage. And it uses the brake pedal switch to know to disengage when you are braking. Â I'm not entirely familiar with the signals to the TCU from the brake pedal switch, but it would make sense to have a failsafe to keep the clutch from engaging if there is no signal from the pedal switch. Â Hopefully there is someone here with a little more insight into the subject, but 2k on the highway does seem a bit high. With a stickshift I was running about 1400 Â so should i just check the switch to make sure it is working properly? Â Yes I would definitely check the switch, I cannot think of any other explanation that would be directly caused by the master/booster swap. Link to comment Share on other sites More sharing options...
88 Beast Posted July 1, 2011 Share Posted July 1, 2011 Nobody out there running a similar set as mine that could tell me what RPM the're running at 55. It might help me know if I have a tranny issue :dunno: Link to comment Share on other sites More sharing options...
xjrev10 Posted July 1, 2011 Share Posted July 1, 2011 FWIW, all of my 4.0/AW4/242 or 231 XJs have ran 2100 rpms at 70 MPH with stock tires. Link to comment Share on other sites More sharing options...
1eliminator1 Posted July 2, 2011 Author Share Posted July 2, 2011 alright. yeah when I purchased my mj 4.0/aw4 it was stock with about 114,000 miles on it. it was running about 16 hundred at 55 Link to comment Share on other sites More sharing options...
88 Beast Posted July 2, 2011 Share Posted July 2, 2011 Thanks for the input. Now to find some t/s info on lock up clutch. Link to comment Share on other sites More sharing options...
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