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Topic: 90 Cherokee Limited Transmission

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90 Cherokee Limited Transmission

I have a 90 Limited, 4dr,  4.0L.  My wife has a base 88 Cherokee, 4dr w/ a 4.0L.  We both have the same transfer case.  (2WD/PT/FT/4Low)   I am just getting into the Jeep thing and have a few questions.  I searched the forums and didnt see my answer.

My questions is......At 55 MPH I am running about 2000=2200 RPM  and at 65 MPH my wife is running about the same or slightly higher @ 2300-2500.  Any reason why?  With the gear shifter all the way back it says 1-2.  I assume this is for 1st and 2nd gear.  It will not go into second gear in this position.  I have to move it to the 3 position in order to get 2nd and 3rd gear.  Is this normal as well?  My wife's Jeep almost seems like it has another gear, I dont drive it much so I havent counted the shifts.  Mine shifts and drives just fine and seems to have (or shift) 4 times. 

My wife says Im paranoid but it they dont seem the same to me. 

Any ideas/comments are appreciated and thanx in advance.


G

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Two things control AW4 shifting:

1) Shift lever (lever position controls tranny valves mechanically)
2) Transmission Control Unit (TCU)

The shift lever makes a very positive move in control, the only weak point being there is no separate position for 1-2, as they are combined. Position 1-2 holds in 1, provided there is no TCU connected. If you have the TCU connected, you'll get a 1-2 upshift at about 4800rpm. The shift lever does do one thing that the TCU cannot accomplish however - ENFORCING compression braking via transmission's internal overrunning clutches and sprags. But besides this, all other transmission controls are performed by the TCU exclusively.

The TCU monitors throttle position, ground speed, and shift lever position (switches in the console) to select the gears appropriately. It also monitors position of the POWER/COMFORT switch (for those of you who have it) to change upshift and downshift points and TCU lock parameters.

The TCU's outputs are very simple, only three wires. One for Tranny Solenoid 1, one for Tranny Solenoid 2, and one for Tranny Solenoid 3. Solenoids 1 and 2 perform ratio selection as follows:

Solenoid 3 is very simple. Put power to it, and the torque converter will lock.

As far as the torque convertor Lock goes. Solenoid 3 is unique, in that it only controls torque converter lock functions. Being able to lock up a torque converter awards you with improvements in fuel mileage, a cooler-running transmission, less cabin noise from the tranny, AND MOST IMPORTANTLY, it has the ability to multiply torque from the engine.. It's in esance like having one more gear.

Also, The TCU is programmed to force torque convertor lockup when engine RPMS's are above a certain point (not sure exactly what RPM though?), and you're at wide open throttle (WOT). Why? easy, top speed. The AW4's torque convertor has a very high stall speed (translates to high torque multiplication factor, and higher RPM off the line). If you allow 2500rpm of stall, that means the transmission's input shaft speed will be quite a few RPMs lower than crankshaft speed when you're under one "quick launch runs". At WOT, you're going down the highway at 75, and the torque multiplication factor really isn't required, thus you need more tranny RPMs.

And lastly, remember how I said the torque convertor lockup mechanism wouldn't handle full engine torque? Well, when you're screaming down the highway at WOT, engine RPMs are usually well above the engine's peak torque point (by nature) it's not developing peak torque output (although it may be generating peak HP). Locking the torque convertor here doesn't put it in jepoardy, but it does eliminate slippage that may be costing you MPH and/or MPG. (and we all know with big tires and a lift we need all the MPG we can get!).

So with all that, why are your's and your wife's different? Most likely it's due to the fact you have a different TCU controlling your shift points due to the year difference. It could also be the fact that your TPS (throttle position sensor's) are slightly adjusted different since the TCU takes signal from that for shift points.

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ya...

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I feel like I just attended AW4 101.

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90XJkid wrote:

I feel like I just attended AW4 101.




x2.... too bad shift points and clutch engagement isn't decided by my brain in my Jeep.  *sigh*  something to put on the 'someday' list.



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ChevelleSSLS6 wrote:
x2.... too bad shift points and clutch engagement isn't decided by my brain in my Jeep.  *sigh*  something to put on the 'someday' list.

Can be: http://www.greatlakesxj.com/tech/aw4shiftmod.html

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