A604/4*TE Chrysler FWD 4-speed Automatic Transaxle Information

The A604 is the Achilles heel of these cars and everyone knows it. They break under stock conditions, and they break under moderate engine mods. In some cases they explode, and it has less to do with power and more to do with design. When properly built, the trans can handle a decent amount of power input, but it is important to keep up with maintenance and get the build done by a reputable shop. We'll outline some of the major problems in dealing with the A604 and the ways to counteract or alleviate those problems all together.

The first and most frequent killer of ANY transmission is heat. When you are putting power to the transmission, clutches create a ton of heat. This in turn breaks down the transmission fluid, causing accelerated wear on the clutches, burning out the seals, and setting you up for a big bill. Transmission fluid generally starts to break down at about 240F degrees - that is when the fluid starts to get thick and varnishes. This clogs and sticks in passages or clutches and prevents parts like the solenoid pack from doing a good job of shifting gears. At around 265F degrees or so, you get to the threshold of the seals. They can only take so much heat and at this temp they start to get rigid thus creating internal leaks. Internal leaks cause loss of line pressure, and when you have poor line pressure holding a gear is harder. The clutches aren't pressed to the steel plates hard enough and they wear faster - you'll notice your fluid turning brown or potentially black if it's severe. At 300F degrees your fluid is so bad; the clutches start to slip more often. This refuse can cause clogged filters and clogged ports on the shift solenoid. Heat starts to build even more and once you hit 315F degrees your seals burn out, the clutches burned out, and you build up a nice concentration of carbon filled gritty fluid. How do you prevent this? An aftermarket inlne trans fluid cooler will help. The stock system has a cooler built into the radiator, but it is small and is highly inefficient. Most people pick up the popular line of B&M Super Coolers. They work great and you can run them in tandem with the stock cooler for even more efficiency. Truth is any 6-pass cooler will work, and do the job perfect. There is such a thing as TOO BIG when it comes to trans coolers, however. If you run a 10-pass cooler in tandem with a stock cooler and leave for work on a cold morning, you'll notice your car is holding second gear a lot longer than it usually does. This is the trans going through a warm up mode trying to get the fluid to optimum temps for operation. It's not bad for the trans per say, but it's annoying if you hit the gas and it sticks in gear and then pounds into third gear. The temps you want to stay around are 185F-200F degrees. Those are optimal temps for good life of the trans and better power transfer.

The second most frequent problem is the differential idler shaft. I myself have been victim to this little chunk of hatred, and it almost cost me a car. The idler shaft is a round chunk of metal a little over ½ inch in thickness. This shaft sits inside the idler gears which in turn rotate the spider gears in the differential. This shaft is held in by a pin (sometimes called a "diff pin") which from the factory is pressed into place, and then the surrounding differential carrier material is pinched over it to attempt a lock on the pin. The shaft itself does exactly as it says and remains idle holding the gears in alignment in the carrier. There are many ways for this part to fail, a lot has to do with oiling and leads us back to the cooling. The shaft sits static, while the spider gears turn the idler gears which rest on this shaft. The speed as you can imagine is high and if no oiling is poor the heat builds and those gears weld themselves to this shaft. This causes catastrophic failure and generally leads to the shaft snapping and going through the case of the transmission. Heat is probably a part of nearly all the failures in the transmission. Doing one wheel burnouts in parking lots causes heat and also forces the gears and shaft into an uneven balance. When you are spinning that one wheel, the gears can come out of their alignment and start to bend and grind on that shaft. This creates shrapnel and in turn goes parading through your trans clogging and cutting seals as it goes. If this doesn't kill the differential, the retaining pin will. This little piece of metal is responsible for holding that shaft and prevents it from going out of carrier and from rotating with the idler shaft. If this pin were to come out, you guessed it. If the shaft seizes on the idler gears, it will tear that pin off and then the party begins. How do you fix ALL of this? There is no bulletproof way to do it in the A604. Companies like Quaife or Kaaz manufacture differentials that are nearly indestructible and if they do blow they'll replace it. I myself have never seen one fail, so if you have that available in your model trans I envy you. Now, for us poor A604 owners with no Quaife option, there are a few alternatives.

• You can have the retaining pin welded into the carrier, this is OK, but it won't solve most of your problems.
• You can have the idler shaft straps installed on the carrier to keep the pin from damaging the case when it comes flying out. These little things hold the pin from the outside.
• You can install a limited slip differential. I don't really promote or like versions like the Phantom Grip limited slip. These use metal to metal contact to evenly distribute power (or attempt to) to both wheels. They claim to be torque sensitive and put power evenly under acceleration or cornering. I'm not sure how a spring and metal block can sense a biased torque condition, but hey, I'm no genius either. If that doesn't deter you, think about what metal to metal contact does. The other alternative to the Phantom Grip is to install a clutch-style limited slip. These use clutch material instead of metal plates to lock the gears into an even output. Instead of springs, they use a series of washers and shims to properly set the tension on the gears so you can set for your power needs. With a unit like this, all parts of the differential are modified to promote oiling of the gears and the shaft and items like the retaining pin are upgraded to stronger materials.

Now we get into the internal parts of the transmission, the guts. Say your trans is going or you plan on putting some decent power through there and want to be safe. Time to rebuild with some beefy parts, but don't know where to start or what to look for. I'll lay out the parts I look for in a performance build, and also explain why I chose these parts.

• Raybestos Carbon Kevlar Clutch Kit: These are a much better alternative than the stock asbestos material inside your transmission from the factory. Kevlar lasts longer, grabs better, and both of those lead to great power transfer and longer life of the trans. You can get these at just about ANY rebuilder, the A604 is a very common rebuild trans, so places like Cotmans, Amco, or Bob's Local Trans store should have plenty in stock. These usually come with everything you need for a rebuild if the trans hasn't had any hard part damage to the planetary gears or other parts.
• New Steels: Steel plates are what sit between your clutches in the clutch hubs. This is what the clutch locks up against and these take a beating. Most rebuilders will mic the plates for flatness, heat marks, or check for them to be out of round. If you are spending the money to have it torn apart anyways, spend the money for the new plates and save yourself the trouble.
• Front Pump Modification: This isn't needed, but if you are putting some decent power down you need all the line pressure the computer will let you have. Most rebuilders know enough to modify the front pump to raise the pressure. Too much pressure though and you'll experience shifting problems and your trans will go into limp mode. Most rebuilders in the know will understand what you are asking for; it is a standard practice even on stock transmissions. Newer versions of the 4*TE use VLP - variable line pressure - so this may not be needed at all depending on the model you have.
• Shift Improvement Kit: This isn't going to be like the shift kits you see for the TH350's or the like. The TCM prevents any insanely high pressure from damaging the trans, therefore you aren't going to get tire chirping shifts. This basically shortens the shift times between gears and also raises pressure enough to get the clutches to grab harder and faster. This prevents wear on the clutches and also makes acceleration a lot quicker. If you can't feel when you are shifting into gear, you have problems and your clutches are riding together too long before grabbing. This is bad. With the shift kit, opt for the aluminum accumulator pistons for the valve body. Stock is ABS plastic and with heat and time they can crack.
• Upgraded Solenoid Pack: If you have ever listened to your car when you stop, or shifted into gear you've heard it. CLIK CLIK CLIK CLIK! This is also referred to as "the mechanical fart." The fast sound is your shift pack routing fluid to parts of the transmission for gear selection. In late 2003 a new pack was released that is nearly silent even with the sound shield off. In fact, these are designed to run without this cover.
• High Stall Torque Converter: Torque converters transfer and convert the power from the engine into power to the trans via the input shaft. The stall of the converter is judged by how many RPM's are needed to build up the fluid pressure enough to make the converter transfer power to the transmission. Stock is around 1800-2000RPM. By raising the stall, you can brake torque the car and push the RPM closer to the engine's powerband. So, if you get a 2600 stall converter, you can now stall the converter until 2600RPM and get a better launch along with more acceleration.
• There are other internal mods that are offered in converters that increase the torque multiplication (example - 1lb of torque comes into the TC, 3lbs comes out to the transmission.) Other parts include needle thrust bearings which decrease friction.

Now that you have your new trans, you have to take care of it. Maintaining a built trans is the same as maintaining your stock trans. Only use Mopar-approved trans fluid. Change your filter every 30-40K and make sure to get the correct year and model for your car. I use the dealer for this because the OEM replacement works best. While you are there ask for Mopar Anti-Foam. This little bottle gets poured into your transmission and prevents foaming of the fluid. Bubbles compress and you have bad line pressure; this eliminates bubbles. Caring for your transmission takes only a few minutes each time and saves you money in the long run. If you take care of it, it'll take care of you.