That's limp mode" that allows you to drive slowly to a repair shop without needing a tow truck. The first thing to do is have the diagnostic fault codes read and recorded. Be careful to not disconnect the battery or let it run dead as doing so will erase those codes, then that valuable information will be lost. The people at most auto parts stores will read the codes for you for free, but many of them can only read codes in the Engine Computer. You may need to find a mechanic with a scanner that can access the Transmission Computer.
This computer-controlled transmission was one of a long line of Chrysler innovations. One advantage is the computer watches the volume of fluid it takes to apply each of the four clutch packs, and as that changes over the transmission's lifetime, it updates the shift points to create longer overlaps between gears. As the clutch plates wear with high mileage, it takes more and more fluid to take up that space and squeeze the stacks of plates to make them lock up solidly. To make up for that wear, it will apply third gear, for example, a little sooner, and wait a little longer to release second gear. The result is you always get a nice crisp shift like when it was new.
The problem with this design is you don't get the customary two to three year warning the transmission was going to be needing a rebuild soon. We used to have mushy shifts with some "engine runaway" where the engine would speed up, as if it was in neutral, before it locked up in the next higher gear. That would get progressively worse over a few years, but you had plenty of notice that wear was taking place.
The computer on your car constantly updates with each up-shift to give you those solid shifts, until it reaches the point where no amount of extra fluid can make one of the clutch packs lock up solidly. That results in slippage which is detected by the computer, and that sets the diagnostic fault code and puts it in limp mode. Most commonly you'll have a fault code related to "gear ratio error". There's a speed sensor for the half shaft, (road speed), and one for the transmission's input shaft, (engine speed). The gear ratios are programmed in the computer, so it knows for a given engine speed, the vehicle should be traveling a specific road speed. That's not true when there's slippage in a clutch pack.
The clue to this type of failure is it occurs during or right after an up-shift. You can often work around it for weeks or months by accelerating harder than normal, to delay the suspect up-shift, then, when it is about to occur, let off the accelerator for a few seconds to reduce engine torque and let the clutch pack lock up tightly, then accelerate normally. Letting off the gas to cause the shift to occur under low load can prevent that slippage, and keep it out of limp mode.
Limp mode can also be caused by an electrical problem, most commonly related to a failing sensor. One of the easiest-to-identify clues here is when it goes directly into limp mode as soon as you shift into drive or reverse. No up-shift has occurred yet, so it can't be due to slippage in a clutch pack. The fault code will direct you to the circuit, but be aware, fault codes never say to replace a part or that one is bad. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. When a part, a sensor, for example, is referenced in a fault code, it is actually the cause of that code about half of the time. First we have to rule out wiring and connector terminal problems, and mechanical problems associated with that part.
You can also have your mechanic read the "clutch volume index", (CVI), on a scanner. That displays four numbers corresponding to the volume of fluid it takes to apply each clutch pack. An experienced transmission specialist can tell how much life is left in that transmission before needing to be rebuilt. That life is usually expressed as a percentage remaining rather than in miles remaining because the miles varies from driving habits and number of shift cycles. No wear takes place when standing still and when cruising on the highway. Clutch plate wear takes place during the normal gradual lock-up during shifts, so the more shifts it goes through, the faster the clutch plates wear.
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Sunday, December 15th, 2019 AT 12:38 PM
