Guest article by 'Yuri'. Hosted by permission of the author.

NOTE: This article is intended to provide you with information only! If you have any doubts about what you are doing then STOP and get help from someone having experience doing this type of modification. As with all modifications be aware you are undertaking this at your own risk! Neither the author or owner of this site will be held responsible if you screw this up.



Article begins here:

Ever since my AW4 swap, I've wanted to take advantage of it being electrically shifted, and modify the way it functions to suit my needs. The two ideas I wanted were a manual-shift (paddle shift), with the ability to hold it in any gear, and a (user-friendly) torque converter lock. I'm still working on a design for the shifter. But the TC lock has been designed, installed, and tested to my liking. Follow along as I try to explain.

What is it? For those unfamiliar with auto transmissions, I'll try to explain as simple as i can how it works and what it is I'm trying to do. First of all, a torque converter is the link between an automatic transmission and the engine. It uses hydraulic force created by a rotating impeller in order to apply pressure to different clutch packs inside the transmission. Think of it like an airplanes propeller... it can sit there on the runway rotating without the plane moving forward. The pilot increases engine speed, which turns the prop faster, and air pulls the plane forward. The torque converter works the same way, but this 'forward potential' is how it applys pressure to the clutch packs (just like your foot applys pressure to the brake pedal). When the engine speed gets high enough, the first set of clutches engages and your Jeep starts to move forward in first gear. The engine RPM drops. When the engine speed increases again, the next set of clutches engages, which shifts it into second, and so on.

The problem with this design is that the engine always needs to be spinning faster than the input of the tranny, simply because the impeller (prop) needs to move the hydraulic fluid (air) to work. This 'slip' causes excess heat to build-up. This is where the TC lock comes into play. Essentially, the TC lock does just as the name implys... It locks the engine to the tranny input, thus creating a 1-1 relationship. The engine and tranny can then turn at the same RPM, thus reducing heat. It also saves a little fuel milage because the engine doesn't need to run as high RPMs to provide the same input speed to the tranny. Most trannies accomplish this automatically by using a combination of fluid passages and/or electrically-driven solenoids (as is the case with the AW4). You can see this driving down the road. When you get going a good speed in your highest gear, the tranny will drop a couple hundred RPMs. This is the TC lock functioning. It's almost like gaining another gear.


In our World: Four wheelers have figured out that when you add larger tires, the factory specs just don't cut-it anymore. You can change axle gearing to compensate for it, but there are so many other factors effecting the way it drives, like wind resistance, etc. that the vehicle isn't going to drive like the factory designed. But... that's why we modify things! To IMPROVE them! If we could lock this torque converter manually, we could improve how OUR Jeep works to suit US. If we force-lock it a little earlier on the highway, it will stop searching for the right gear. If we force-lock it while we're going down-hill, we can use the engine to help regulate our speed.


My goals: I could have just used a switch to turn on/off the TC lock, but what fun would that have been? Besides, that wouldn't have functioned like I wanted. My goals for the project were to keep the TC lock user-friendly, since the wife drives my Jeep quite often, and I've got two future drivers coming in a couple years. So I needed this to turn on/off without much effort, yet still be easy enough for me to use on the whim. I had many ideas on how to do this, but after much investigation, most had drawbacks of one kind or another.

I had two reasons why I wanted a TC lock. The first was to help out on the highway, possibly saving a little gas, but mostly just to keep it from downshifting at the slightest incline/headwind. The 'overdrive gear-hunting' has been one of my pet peaves in the past, that I'd like to get rid of. The other reason is for a psudo compression braking effect while out wheelin'. Brakes work very hard on an auto transmission in 4low. So any help I can give them will be extremely beneficial. Many people warn to not use the TC lock for this purpose because it wasn't designed to handle such high force. Truthfully, I'm not planning to use this very often. But I still wanted this functionality built-in. And I figure, if this transmission can stand-up behind Tundra V8's and high-horsepower supercharged Celica V6's, than it should be able to handle some abuse.

Here's how it looks on paper, and some explanation on what's going on:

Remember when I said "user-friendly"? Well that's where I had a hard time figuring things out. For the road, I figured that a good way to operate the TC lock would be a push-button latching switch. Hit it once, and it turns on, twice and it's off. This is done using a bank of relays and a momentary switch (R1 through R5). But what happens if I have to slow down quickly and forget to turn it off, or worse yet PANIC STOP? After a little thinking, I liked the way a cruise control system solves this problem... by disabling it with the brakes. Brilliant! This is Relay # R6 in my diagram. This relay has a constant ground on pin 86 until the brake pedal is pushed, which opens the circuit.

But not quite yet... this would not suit it's compression braking aspect, as I would probably be on the brakes if the hill was steep enough to warrent compression braking. And that's the part that stumped me for quite some time. What to use as a trigger for when I DON'T want the brakes to shut the TC lock off. This is when I figured that I'd only be using the compression braking while in 4wd. So I used the 4wd light as a trigger to disable the brake relay. This is R7 in my diagram. The instrument cluster 4WD indicator (mine says part time) is illuminated using a ground. When this ground is on pin 86, the relay closes between pin 30 and 87. This closure takes R6 out of the circuit.

Relay R8 is used a little differently than would normally be seen (it's kinda reverse, running either from pins 87 or 87a and out pin 30). This relay serves a pretty important purpose. It's job is to switch between using the factory TCM and My TC lock. Basically, in it's normally closed state the relay acts like a solid wire from the TCM to the TC lock solenoid, using pins 87a and 30. When the TC lock turns on, that connection opens up and closes pin 30 with 87. This supplys 12v to solenoid C in the transmission, bypassing the tranny computer in the process.

Relay number 9 is simply a polarity converter. This relay allows me to use the LED on the switch as an indicator for when the TC is locked. When you first flick the switch, the light stays off. It remains off until you push the TC lock push-button on the shifter, and only illuminates when the TC lock is engaged, as opposed to 'when the switch is on'... know what I mean? It does this by switching the ground.

Now I know what you're saying... 9 relays????? Holy cow! But, it's not bad at all if you have a decent pick-n-pull nearby. I pulled the perfect relay box from a ZJ. The relay box was located behind the glove box. It holds 6 of the smaller relays, three normal relays, and one of something else. While at the junkyard, search some of the other vehicles for spare relays. It's not too hard to take the box apart and re-use alot of the pins. All you have to do is pull all the relays out, then from the back side, unclip the yellow plastic pieces and push them out. Then from the front, you can see how the pins are held in the box. Use a pick tool or jewelers screwdriver to unclip the pins. These are the same pins found in the other fuse/relay box under the hood. It's also the same exact pins in your TJ's fuse/breaker box as well as most other chrysler vehicles. So if you need more pins, just go get 'em out of something else. I mounted my new relay panel behind the steering wheel knee-knocker.

And the conclusion: I like the way this thing works! The only issue right now is that I now need to get moving on the 'manual-shifting' project to take full advantage of the TC lock. I can not hold it in gear yet, therefor the tranny can still shift even with the TC locked. This give pretty hard shifts, like dumping the clutch on a manual tranny. If you have the TC locked in 4low, it really jerks (try not to do this very much!). Other than that, the thing functions exactly like I wanted! It really calmed-down the gear-hunting on the highway. I haven't tried it yet off-road except for a small gravel road near my house, but it worked as designed there too. In fact, the 'indicator light' part of the project is an update that isn't quite finished yet because I didn't have the correct switch on-hand. The rest works great tho.


Here is a link to the original thread posted on J.U. - Be sure to check it out for any feedback or follow up questions to the author.


The12Volt.Com - A great automotive wiring tech site with TONS of help in wiring up your vehicle mods. Visit the12volt.com for your automotive wiring tech answers!


If you have pics of your own repairs or can suggest other methods - please contribute your ideas (and pictures) to this article!

Revised on: November 08, 2006









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