Well, I can share a couple more observations, but these are all related to Dodge trucks. Those that use computer-controlled transmissions, (which we know you have too because they can store diagnostic fault codes), there are two speed sensors that are always bolted to the outside and are very easy to replace. They are simple magnetic pickup sensors that give very little trouble. One common problem is metal chips or filings sticking to the magnet and reducing its signal.
The first speed sensor is an "input" speed sensor and watches the input shaft speed, which is kind of like watching engine speed. The second one, the "output" speed sensor watches drive shaft speed. Chrysler puts that in the back of the transmission so its the same with or without a transfer case, but the operation would be the same if it was in the transfer case.
The computer knows which gear it has selected, and based on input speed, it knows how fast the output speed had better be reading. If there is a discrepancy, the output speed will be too low. That can only happen when there is slippage between the clutch plates in one of the clutch packs. The system will default to "limp mode" and stay in second gear to allow you to drive slowly to a repair shop without needing a tow truck. A fault code related to "gear ratio error" will be set in memory because the wrong relationship between the two speed signals is being received. If one speed signal is weak, intermittent, or missing, that will set a different code spelling that out.
Transmission codes are generally in response to something that could have an adverse effect on emissions, in this case from excessive fuel consumption, so the Check Engine light must be turned on. Even though those codes remain in memory when the ignition switch is turned off, you can make some valuable observations by when the light turns on the next time. If it comes on right away when the engine is started, the codes are not going to be related to slippage in a clutch pack because they haven't even engaged yet. Those codes will be related to an electrical problem with a sensor or solenoid, ... Anything that is self-tested all the time by the computer. On Chrysler transmissions, all of those things are mounted on the outside and can be replaced rather easily.
If the Check Engine light turns on after the vehicle starts moving but hasn't up-shifted yet, you can expect the fault to be with a missing speed sensor signal although the sensor is okay electrically. That's a somewhat rare failure and is typically due to those metal chips or filings stuck to the magnet.
If the light turns on during or right after a shift, it is almost always due to slippage in a clutch pack. Chrysler had a lot of trouble with that at first that was caused by normal but not excessive clutch plate wear. Replacing the Transmission Computer or installing updated software that was more tolerant of the increased normal slippage as a clutch pack applied solved most of those problems. There were a lot of other problems in the early years related to "tolerance buildup" on the assembly line, but that's a topic for another day.
Those Chrysler transmissions also had a reputation for having a high failure rate. That is true but some of the problem was due to the intended design of the entire system. All automatic transmissions have stacks of clutch plates that must have fluid pressure applied to them to make them stick together to engage. The fluid passages are designed to make that occur very slowly, as in a fraction of a second. Think of gradually releasing a clutch pedal on a manual transmission car vs. Sliding your foot off to the side and letting it snap up. Gradual clutch pack engagement cushions the engagement to a comfortable level. As those clutch plates wear over time, it takes more and more fluid volume applied to those plates before they fully engage. We felt that as sloppy shifts, unlike the nice crisp, solid shifts it had when it was new, and in severe cases it caused "engine runaway" where the engine speed would momentarily pick up a noticeable amount until that clutch pack engaged. This wear occurred over 100,000 or more miles and happened so gradually that we became accustomed to it and ignored it. That is until the day there was so much wear that it always slipped and probably overheated. The fix was to rebuild it, but we had months or years of warning that wear was taking place.
Those transmissions were all hydraulically-controlled strictly by load and road speed. Chrysler developed the first computer-controlled transmission for their front-wheel-drive vehicles in 1989 to do with a computer the things computers were never needed for before, but that allowed them to build into the software the ability to update those shift schedules. The computer knows how much fluid it takes to apply each of the four clutch packs. Where we could observe the effects of the clutch plate wear in the past, this system continually updates the shift characteristics to maintain the "as-new" feel all the time. For example, if there is excessive wear in the overdrive clutch pack, the computer will apply that clutch at the appropriate time, but it will wait just a little longer before it releases the third gear clutch pack. By the time it finally releases third gear, overdrive is solidly engaged and we don't feel the slippage. That masks the wear taking place that we used to be aware of, until the day comes when it can't update any further. THAT'S when noticeable slippage occurs between shifts and the system defaults to limp mode. We go from "shifting fine yesterday" to "sticking in second gear" today. No warning this was about to occur. The same normal wear has been taking place all along but we weren't aware of it.
Experienced mechanics have two things to identify that slippage is due to normal wear. One is by reading the "clutch volume index", (CVI) on a scanner, and the other is with a test drive.
The clutch volume index is a set of four numbers indicating the volume of fluid it takes to apply each clutch pack. Those numbers reset to factory programmed values when the battery is disconnected or the computer is replaced. It can take a couple of miles and up to a dozen shift cycles to relearn that information. Until then it might shift too harshly or sluggishly. Those numbers can also be used to determine the life expectancy of that transmission.
If the mechanic performs a test drive and sees the system go to limp mode when it shifts into third gear, he can reset it by turning the ignition switch off and restarting the engine, then, before it is time to shift into that gear again, he will accelerate quickly to cause that shift to be delayed, then let off the gas. That will allow the shift to occur with no load on the transmission and little likelihood of slippage. If he can continue driving normally after that, clutch plate wear is the suspected cause. If slippage is still detected after that clutch has had plenty of time to lock up, broken parts or cracked or hardened rubber seals can be expected to be found. Both of those are always addressed with a normal rebuild. Major rebuilders like Jasper and others will disassemble and rebuild a whole series of like transmissions at once. For efficiency, everyone is using the same tools, new parts, and procedures. They don't have time to figure out what caused each transmission's failure; they just build them to like new.
When your local mechanic or transmission shop rebuilds a transmission, they typically do just one at a time and unless it's a real common failure with common causes, they are going to pay very close attention to the CVI and test drive observations. In the case of slippage that I described, if they do not see evidence of excessive clutch plate wear, they know they had better find hardened and cracked rubber seals, and if they don't find those, smart mechanics will keep looking for the cause of the failure rather than just putting it back together with some new parts. If you don't KNOW the cause of the failure, you have no way of knowing you ELIMINATED the cause of the failure.
Okay, getting back to your transmission, as it was explained to me many years ago, GM and Ford thought so much of Chrysler's design, they wanted to copy it. GM wanted to buy the rights to use it in their cars, but they wanted to "wait until the bugs were worked out". As of the mid to late '90s they were still waiting so they designed their own computer-controlled transmissions. Ford did the same thing, and they both had all the same problems Chrysler had, and they had to go through the same learning process with the same failures and customer dissatisfaction. By that time Chrysler had a real bad reputation for their transmissions but they had also developed a lot of improved and redesigned parts to address most of the problems.
As for replacing your transfer case to get a new output speed sensor, there are a few different ways of looking at that. GM gives very little regard to repairing their vehicles after they leave the assembly line. I've written five-page articles, (I know that's hard to believe), on their very poor business practices and vehicle designs. It is common to find solenoids and sensors stuffed inside transmissions and transfer cases. Usually those assemblies must be removed and taken apart to replace those sensors. Repair expense is very high, but those sensors tend to have a relatively low failure rate. When a new transmission or transfer case is needed, those sensors and solenoids will already be in them.
When solenoids and sensors are on the outside, as with the Chrysler parts, you will often get new ones with a rebuilt assembly, but that is done to insure the assembly will work properly the first time. To save money, some discount rebuilders will not include them, but then the mechanic runs the risk of unknowingly installing a defective part, or installing it incorrectly. A solenoid gasket could leak. A sensor could be cross-threaded. Things like that. Most rebuilders want to do everything possible to insure they have a satisfied customer and aren't going to skimp on the cost of a few parts.
Regardless whether those sensors and solenoids are inside or outside, it is not standard practice to replace an entire transmission or transfer case just to get a new one. All of those parts are available separately. I would never agree to replace the transfer case just to get a new speed sensor and "see" if that solves other problems. We pay mechanics for their experience and training to be able to diagnose defective parts as efficiently and inexpensively as possible. The exception, in your case, is that a different problem is already going to be addressed, and since a new output speed sensor sounds like it will be included, lets wait and find out what symptoms remain after the transfer case is replaced.
I really hope a major transmission repair isn't needed, but I can offer another observation. You listed fault codes suggesting clutch pack slippage in three gears. If there was a speed sensor problem, the codes should be related to an electrical problem with that sensor or slippage-related, (gear ratio) codes should be set for first and fourth gears too. To clarify those clutch packs, there is not one for each gear. I'm familiar with the "low / reverse" clutch, "2 / 4" clutch, and the "overdrive" and "underdrive" clutches. Multiple clutch packs in various combinations are applied for the various gears. Based on your gear ratio error codes for three different gears, and the slippage you originally observed, my expectation is there is normal wear in one clutch pack that is used for all three of those gears. I'd be quite happy to learn I'm wrong and the repairs are not going to be expensive, but if you find out I'm right, and this is just normal wear, feel free to think I'm a genius.
Saturday, January 14th, 2012 AT 3:39 AM