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Everything you ever wanted to know about the AW4
- Thread starter kastein
- Start date
lawsoncl
NAXJA Forum User
- Location
- North Idaho
If it's a Renix Jeep, check the resistor in the wiring harness near the TCU. That's what normally pulls up the voltage on the brake switch sense line. The switch grounds that wire when the brake pedal is pushed. If the resistor is broken then the TCU thinks you're constantly on the brakes and doesn't lock the t/c.
O-Gauge Steamer
NAXJA Forum User
- Location
- Colorado Springs, Colorado
Just an FYI for those who might be wondering "why the 1k Ohm Resistor" that kastein is saying is needed. And, to be sure, it is needed.
The 1000 Ohm resistor will act as a current limiter on the ciruit so that it becomes impossible to dump the total battery current into the circuit. Well, the current woud run until something fried... This is a standard practice, so do not eliminate this detail.
A common practice in Electronics is to have a digital input "pulled up" by a 1k resistor and the active component that "triggers" the circuit will pull that input line to ground. The pull up resistor prevents excessive current from flowing. The reason to "pull up" an input is to guarantee it's state i.e high. If allowed to float, noise on the data line can create, if you will, confusion.
For a similar reason the Jeep has power to all of the devices that the PCM is going to control. The PCM supplies the grounds for those circuits. There are many compelling reasons for doing this most of which just sound like technobabble to the uninitiate. There are switching speed and current limiting advantages to this form of Logic.
The 1000 Ohm resistor will act as a current limiter on the ciruit so that it becomes impossible to dump the total battery current into the circuit. Well, the current woud run until something fried... This is a standard practice, so do not eliminate this detail.
A common practice in Electronics is to have a digital input "pulled up" by a 1k resistor and the active component that "triggers" the circuit will pull that input line to ground. The pull up resistor prevents excessive current from flowing. The reason to "pull up" an input is to guarantee it's state i.e high. If allowed to float, noise on the data line can create, if you will, confusion.
For a similar reason the Jeep has power to all of the devices that the PCM is going to control. The PCM supplies the grounds for those circuits. There are many compelling reasons for doing this most of which just sound like technobabble to the uninitiate. There are switching speed and current limiting advantages to this form of Logic.
Last edited by a moderator:
lawsoncl
NAXJA Forum User
- Location
- North Idaho
Yes, definitely don't replace the resistor with the short. A replacement resistor should be under a buck at Radio Shack. Or I'll even mail you one if you really need it.
I always wondered why they didn't just stick the resistor inside the TCU (I suspect there is a pull-down resister inside the TCU). Or why the TCU wants the TPS voltage opposite of what the engine computer wants. I think it's a symptom of AMC Jeep adopting parts from other vehicles and then doing bandaids like this pullup resistor and the dual output TPS sensor to get things working. Chrysler to their credit started eliminating these oddities and producing decent shop manuals once they took over.
I always wondered why they didn't just stick the resistor inside the TCU (I suspect there is a pull-down resister inside the TCU). Or why the TCU wants the TPS voltage opposite of what the engine computer wants. I think it's a symptom of AMC Jeep adopting parts from other vehicles and then doing bandaids like this pullup resistor and the dual output TPS sensor to get things working. Chrysler to their credit started eliminating these oddities and producing decent shop manuals once they took over.
- Location
- Rainy side of Washington
The pull-up isn't really a current limiter, so much as a way of making the product cheaper... you could use an SPDT brake pedal switch but it wouldn't be as cheap or reliable. From the factory (on 96 and later, with some earlier covered by a TSB) the pull-up is actually well under 100 ohms, since it is a solenoid that also happens to pull the brake signal wire high as a convenient side effect. You could wire 12 volts direct to the TCU without a single problem, it's internally current limited, but the brake switch would have other opinions, which is why you'd need to use an SPDT one with connections to both ground and 12V instead.
Chrysler did do a pretty good job of un-rats-nest-ing things once they started changing the design to fit their lineup. There are still some stupid features left but they got rid of the worst offenders, like that TPS issue you mentioned. My bet is that the spec for the TPS signal in the RENIX days read "0-5V analog" and the Renault/Bendix engineers understood it one way, while the Aisin Warner engineers (note the Aisin Warner logo on the TCU label and PCB) understood it the other way. After everyone got their sub-assemblies designed, debugged, and fully tested, they tried to put them together and discovered the mistake, and decided to bandaid it by effectively using two sensors in one package instead of messing around with all the code again and risking a bug.
Chrysler did do a pretty good job of un-rats-nest-ing things once they started changing the design to fit their lineup. There are still some stupid features left but they got rid of the worst offenders, like that TPS issue you mentioned. My bet is that the spec for the TPS signal in the RENIX days read "0-5V analog" and the Renault/Bendix engineers understood it one way, while the Aisin Warner engineers (note the Aisin Warner logo on the TCU label and PCB) understood it the other way. After everyone got their sub-assemblies designed, debugged, and fully tested, they tried to put them together and discovered the mistake, and decided to bandaid it by effectively using two sensors in one package instead of messing around with all the code again and risking a bug.
stewie
NAXJA Forum User
Ok, so in referance to swapping an AW4 in for a Manual on an XJ, I haven't heard much talk about swapping in the TCU and its wiring. Nor of the need to swap in the brake pedal from the donor.
lawsoncl
NAXJA Forum User
- Location
- North Idaho
You don't really need to swap the brake pedal if you don't want to. The auto brake pedal is just wider. On the 89 it's still the same gap between the brake and accel pedals.
Going manual -> auto, yeah you need to install a TCU and wiring, and splice in a few wires.
- Location
- Rainy side of Washington
Agreed. 97 and later it's going to be a bit more of a pain iirc but it's all doable. Your harness won't be as pretty as a stock one I'm sure. If you care about not being able to start in all gears, it may be difficult to wire the NSS in to handle that, if not, you can just ignore it and remember not to turn the key to start unless it's in the right gear.
lawsoncl
NAXJA Forum User
- Location
- North Idaho
My harness looks nice, but then again I'm anal about making things look nice and I pulled all the TCU related wiring from the junkyard donor.
The NSS and the tranny had their own wiring harnesses to the TCU, so that's pretty easy from a transplant standpoint. Renix era TPS had its own wires, although tied up in the engine harness. Batt power, ign power, ground, brake pedal sensor, power/comfort switch, and reverse from the nss and maybe something else I'm missing. I seem to recall it was only 7 wires I had to splice when i did mine. I didn't bother with starter relay wiring or diagnostic wires.
Some very wiring skills and a pinout of the connectors is all you really need. Admittedly, though this is a challenge for many shade tree mechanics. Some guys can rebuild an engine or a tranny, but hand them a meter and they get a little lost.
child9
NAXJA Forum User
- Location
- Austin, TX
Pssshhht. You give most guys an auto tranny and say "fix it" and they will look at you like you are crazy, never-mind the electronics and signal flow. Swap it out? Sure, Joe can do that. I think people call it a slush box because how it works is "magic". 
Kudos for being anal about your wiring. We need more people like that.
Kudos for being anal about your wiring. We need more people like that.
- Location
- Rainy side of Washington
The funny thing is - controlling an AW4 with an aftermarket electronic shifter is easier than doing a full install of all the factory TCU electronics. Go figure :laugh2:
Ives2210
NAXJA Forum User
- Location
- Pensacola, FL
I am getting one the "surplus" AW4's from Chrysler and I got a, possibly goofy, question. Is there any new AW4 break-in procedures?
I was just going to fill it, start it, let it sit, go through all the gear selections (pausing for a minute), then try to not to be too heavy on the gas for a couple hunderd miles.
Sorry, but I also PM'd this to you.
I was just going to fill it, start it, let it sit, go through all the gear selections (pausing for a minute), then try to not to be too heavy on the gas for a couple hunderd miles.
Sorry, but I also PM'd this to you.
X E Ryder
NAXJA Forum User
- Location
- Northern California
Avoiding Downshift To 1st Gear
Hey what a great write-up thanks!
When going down the steep road off the mountain I would love to use the 1-2 setting to engine brake in 2nd, but watch out if you go under about 24mph - it downshifts Hard into 1st gear and I've about spun out from this once!
Based on your write-up, it appears if you have the gear selector in 1-2 and power +12v to Solenoid 2, you could keep it from downshifting into 1st.
Is this correct? Or will the TCU have a cow and throw a CEL?
Thanks!
Hey what a great write-up thanks!
When going down the steep road off the mountain I would love to use the 1-2 setting to engine brake in 2nd, but watch out if you go under about 24mph - it downshifts Hard into 1st gear and I've about spun out from this once!
Based on your write-up, it appears if you have the gear selector in 1-2 and power +12v to Solenoid 2, you could keep it from downshifting into 1st.
Is this correct? Or will the TCU have a cow and throw a CEL?
Thanks!
- Location
- Rainy side of Washington
Ives - replied in your other thread. I'm honestly not sure (never had a new AW4 to break in), but I'd probably end up doing what you described, then changing the fluid a few thousand miles later to flush out any metallic debris leftover from finish machining + breakin.
Ryder - for your '91? You could unplug the TCU entirely and it won't really even notice, the '97 and earlier are pretty dumb and the '96 and earlier are REALLY dumb. Even if you do get a CEL on a 91 it'll go away as soon as you disconnect and reconnect the battery.
With the shifter in 1-2 you want power to S1 for 1st and power to S1 and S2 for 2nd. I doubt powering or not powering S1 with the shifter in 1-2 will make a difference though as it selects the 1-2 gear pair vs the 3-OD gear pair (i.e. S1 being off = you are going to end up in 3 or OD) which is forced hydraulically by the manual shift valve with the shifter in 1-2.
Basically, solenoid 1 selects between the 1-2 pair and the 3-OD pair while solenoid 2 selects 1 or 2 from the 1-2 pair, and 3 or OD in the 3-OD pair.
Tech nerd sidetrack:
From the shift table you may also note another interesting feature - it's set up so that only one solenoid drive value changes from each gear to the next, which allows for more controlled shifting. For instance if this was not the case, here's an example:
Assume:
first S1 off S2 off
second S1 off S2 on
third S1 on S2 off
OD S1 on S2 on
Note: this is NOT how the AW4 is set up - anyone who's reading this for info on the AW4, don't get confused!
See that both solenoids flip values from second to third...
Now imagine that the solenoids are a bit sticky and you go to shift from second to third right at the redline in second. Three things could happen:
* solenoid 1 is stickier than solenoid 2, and turns on a little slower, while solenoid 2 turns off immediately. So for a second, you have S1 off and S2 off and the transmission blips into 1st when you were already at redline in second. NOT GOOD!
* solenoid 2 is stickier than solenoid 1, and turns off a little slower, while solenoid 1 turns on immediately. So for a second, you have S1 on and S2 on and the transmission blips into OD before going into third like it was supposed to. Clutches slip more than they have to, the engine slows down then speeds up again, things get worn, you don't accelerate as much as you wanted to. Not going to blow anything up, but not ideal.
* the solenoids switch at exactly the same speed and you go smoothly into 3rd.
The setup with only one solenoid changing state at a time is used for this reason - it's impossible to have those kinds of flaky issues, all that changes is you change gears a bit slower if the solenoids get sticky. It's known as "gray codes" and is frequently used for sensors and actuators for this reason.
Ryder - for your '91? You could unplug the TCU entirely and it won't really even notice, the '97 and earlier are pretty dumb and the '96 and earlier are REALLY dumb. Even if you do get a CEL on a 91 it'll go away as soon as you disconnect and reconnect the battery.
With the shifter in 1-2 you want power to S1 for 1st and power to S1 and S2 for 2nd. I doubt powering or not powering S1 with the shifter in 1-2 will make a difference though as it selects the 1-2 gear pair vs the 3-OD gear pair (i.e. S1 being off = you are going to end up in 3 or OD) which is forced hydraulically by the manual shift valve with the shifter in 1-2.
Basically, solenoid 1 selects between the 1-2 pair and the 3-OD pair while solenoid 2 selects 1 or 2 from the 1-2 pair, and 3 or OD in the 3-OD pair.
Tech nerd sidetrack:
From the shift table you may also note another interesting feature - it's set up so that only one solenoid drive value changes from each gear to the next, which allows for more controlled shifting. For instance if this was not the case, here's an example:
Assume:
first S1 off S2 off
second S1 off S2 on
third S1 on S2 off
OD S1 on S2 on
Note: this is NOT how the AW4 is set up - anyone who's reading this for info on the AW4, don't get confused!
See that both solenoids flip values from second to third...
Now imagine that the solenoids are a bit sticky and you go to shift from second to third right at the redline in second. Three things could happen:
* solenoid 1 is stickier than solenoid 2, and turns on a little slower, while solenoid 2 turns off immediately. So for a second, you have S1 off and S2 off and the transmission blips into 1st when you were already at redline in second. NOT GOOD!
* solenoid 2 is stickier than solenoid 1, and turns off a little slower, while solenoid 1 turns on immediately. So for a second, you have S1 on and S2 on and the transmission blips into OD before going into third like it was supposed to. Clutches slip more than they have to, the engine slows down then speeds up again, things get worn, you don't accelerate as much as you wanted to. Not going to blow anything up, but not ideal.
* the solenoids switch at exactly the same speed and you go smoothly into 3rd.
The setup with only one solenoid changing state at a time is used for this reason - it's impossible to have those kinds of flaky issues, all that changes is you change gears a bit slower if the solenoids get sticky. It's known as "gray codes" and is frequently used for sensors and actuators for this reason.
Boss Cherokee
NAXJA Forum User
- Location
- Riverside Calif.
Kastein MJ/XJ. What is the operating temp on a AW4. I don't really over heat too often but once or twice in 5-6 years only pulling a grade or mountains. I have a Deraile cooler with the electric fan attached I was going to install by pass the radiator but I don't want the trans running cold especially in the winter (2 months or so). I run 4000 to 8000+ elevation in summer and in winter. Last time she spewed I was getting back to camp on the pavement but was running 8 pounds of tire pressure for about 20 minute drive and it pegged the temp gauge. Should I by pass rad. cooler and just run Deraile or run it thru the cooler then rad? I like running the a/c when I can!
- Location
- Rainy side of Washington
honestly no idea.
After blowing up my second AW4 due to my heavy right foot I have done a trans cooler and torque converter delete and added a third pedal.
I'd rather run the trans cold than too hot any day though.
After blowing up my second AW4 due to my heavy right foot I have done a trans cooler and torque converter delete and added a third pedal.
I'd rather run the trans cold than too hot any day though.
O-Gauge Steamer
NAXJA Forum User
- Location
- Colorado Springs, Colorado
Here is how I have my aux transmission cooler plumbed in.
Transmission to fan forced cooler to the radiator to the transmission.
In my case, the cooler is mounted on top of the factory front skid and the fan is on a Thermoswitch with a relay. On at 175F off at 140F, I believe... It is a Derale thermoswitch at any rate.
A note here. A fan assistd cooler absolutely will not fit on top of the factory skid provided you still have the factory sway bar. I run a Currie Antirock and the skid is dropped 3" down from stock. I did this for two reasons. Firstly, what good is the skid once the Heep is lifted... Adn secondly, I needed to room for the cooler and it's fan. Will be mounting an engine oil cooler there someday as well. It is also worth noting that the cooler did fit with the factory sway in place once the skid was dropped 3 inches. The cooler is 3/8" up off of the skid and uses a puller fan. No holes in the skid...
If I lived where it never snowed... I would have no issues at all deleting the radiator connection.
If one has to choose between a "cold" automatic and a hot one, the choice is clear. As long as the transmision comes up to it's minimum operating temperature, all will be well.
Heat is the enemy. According to the factory service manual, the operating range for the AW4 is 125-176 degrees.
Read more: http://www.jpmagazine.com/techartic...ling_tips_and_tricks/index.html#ixzz1yuOds2xq
Transmission to fan forced cooler to the radiator to the transmission.
In my case, the cooler is mounted on top of the factory front skid and the fan is on a Thermoswitch with a relay. On at 175F off at 140F, I believe... It is a Derale thermoswitch at any rate.
A note here. A fan assistd cooler absolutely will not fit on top of the factory skid provided you still have the factory sway bar. I run a Currie Antirock and the skid is dropped 3" down from stock. I did this for two reasons. Firstly, what good is the skid once the Heep is lifted... Adn secondly, I needed to room for the cooler and it's fan. Will be mounting an engine oil cooler there someday as well. It is also worth noting that the cooler did fit with the factory sway in place once the skid was dropped 3 inches. The cooler is 3/8" up off of the skid and uses a puller fan. No holes in the skid...
If I lived where it never snowed... I would have no issues at all deleting the radiator connection.
If one has to choose between a "cold" automatic and a hot one, the choice is clear. As long as the transmision comes up to it's minimum operating temperature, all will be well.
Heat is the enemy. According to the factory service manual, the operating range for the AW4 is 125-176 degrees.
Read more: http://www.jpmagazine.com/techartic...ling_tips_and_tricks/index.html#ixzz1yuOds2xq
Boss Cherokee
NAXJA Forum User
- Location
- Riverside Calif.
Thanks mucho. I guess I'll set the thermo switch for the cooler to 175 on & 130 off. I have a cowl raised hood and I'm going to try to mount it on the pass. side to the hood to blow through the hood vents I have. It should work and help draw more air through the engine bay.
O-Gauge Steamer
NAXJA Forum User
- Location
- Colorado Springs, Colorado
Well, IMO, anything to get heat out of the bay and cool air in has to be a good thing!
