2007 Charger wouldn't get up to speed [Archive] - Dodge Charger Forums

: 2007 Charger wouldn't get up to speed


SteelBlueMoparman
09-02-2012, 08:43 PM
My 2007 SXT Charger 3.5 had a couple of episodes where it just would not get up to speed. It got to about 50mph and that's was all it would do. The accelerator was all the way to the floor. We pulled over and turned it off and restarted it. It was fine, then a couple of weeks later same thing happened. I took it to my mechanic and he said because it did not register a check engine light it didn't record a code. He suspects a TPS is probably going bad. I switched to Shell fuel and haven't had any more problems...Anyone else had this problem or can you offer any suggestions.

Ddaddy
09-02-2012, 08:53 PM
Your car went into limp mode. It happened because the PCM had input readings from one or more of the various sensors that confused it. It went into limp mode to protect itself. This usually happens because of something in the transmission (TCM module) that it doesn't like.

Ddaddy
09-02-2012, 09:01 PM
Here is and explanation of Limp Mode...



'Limp Mode' or 'Fail Code' Conditions



Nearly every system in your automobile is controlled by onboard computers these days, especially your engine and transmission functions.

'Fail Code' conditions, or 'Limp Mode', happens when the vehicle computer recognizes a problem in it's logic. When an expected signal value from a sensor is sent to the computer and is not within the computer's programmed specifications, 'secondary' programs are activated by the computer to strive to protect the transmission from any damage the improper sensor signal might cause to occur, be signaling, or contribute to.

In other words, the computer is always expecting certain signal values from certain sensors i.e. the temperature sensor, the speed sensor, the throttle position sensor or MAP sensor, etc. As long as these signals are what it would normally expect for the current operating conditions and are normal based on all the other signals it is receiving from other sensors, it acts normally and accordingly.

If the computer, all of a sudden, receives some crazy signal from one of the sensors that is out of the normal range expected from this sensor, it is programmed to go into 'emergency' or 'secondary' measures.

These emergency measures vary depending on the severity of the defective signal. All of this is preprogrammed into the computer's logic by the manufacturer. The manufacturer has decided that as long as a certain parameter of a particular signal is sent from a sensor to the computer, all is well. The manufacturer decided that if this signal is higher than their highest parameter or lower than their lowest parameter, something is wrong with that sensor and the computer should make someone aware of the situation and take action to try to 'save' the vehicle systems or powertrain.

What type of action does the vehicle's computer take?

Well, perhaps the computer will simply cause the 'check engine' light to come on. The signal variation wasn't severe or critical enough to indicate any mechanical failures but the vehicle's operator is made aware that he or she should have the vehicle checked out electronically to see if a minor sensor has broken down or is starting to deteriorate and send the odd irratic signal. This type of condition is commonly referred to as a 'soft code'. Normal functions are not affected but if the repair is not made, performance or fuel efficiencies might suffer. Perhaps the sensor only malfunctioned one time and all other times was fine. This might be an early warning of a sensor that is beginning to fail or maybe it's a matter as simple as a loose connector or connection.

But sometimes, the signal needed to perform all operations normally is so far out of specification that the computer has no choice but to go into a more critical 'survival' mode. With vehicle transmissions, when the computer detects an obvious, dangerous signal value of this sort it will cause the internal tranny-fluid-line-pressure to default to 'high pressure' (in order to protect clutches and bands). The computer also turns off the transmission's electronic shift solenoids which in turn causes the unit to default to a single gear only (usually second or third). All normal signals to vary and control line pressure are overridden and everything defaults to 'full on' so a hazardous 'slipping' condition within the clutch pack cannot occur easily. This theoretically (and practically) is so that the vehicle's driver can get the damaged vehicle to the next town for repairs.
This condition is commonly called 'Limp Mode' for this reason --- rather than being stranded, you're able to limp to the next town in either second or third gear, with full tranny line pressure being applied, so that the clutch guts won't slip on your trip in and your vehicle will move along slowly but steadily to a service center.

The vehicle's computer would immediately sense that it has lost contact with the transmission and would set the codes and send 'limp mode' signals to the tranny. But because the harness is severed between the computer and the transmission, no computer signals will reach the transmission. These sent signals, however, would have had the identical affect on the transmission as what taking away supplied power to the shift and line pressure solenoids has as in the case of a transmission harness being detached or cut. Due to the engineered voltage strategies of the solenoids, the transmission simply defaults to a single gear and line pressure defaults to high, all in order to 'limp' you home.

A Throttle Position Sensor (TPS) that improperly sends a reading that it is wide open when in fact it is physically closed would be detected by the computer as being illogical when it compared this reading with the vehicle speed sensor that perhaps is showing very slow vehicle speed, at the time. The signal in this case, by itself, can't be considered wrong but when put against all the other sensor signals of the system might not make sense and cause a computer concern. The computer, at this point, unable to 'trust' the collection of signals because together they are not making sense in it's logic, will simply go to limp mode in the transmission, as well, to protect it.

In your electronic transmission, many important functions are controlled by the computer. Shift timing, sequence, feel, line pressure are controlled. The information from the vehicle speed sensor affects fuel injection, fuel mixture, ABS, transmission operation, etc. Load information of your engine is commonly taken primarily from the TPS (throttle position sensor) or the MAP sensor (manifold absolute pressure). This controls transmission shifting and downshifting when stepping on the gas or climbing hills.




Whatever the issue was that caused the computer to become confused may or may not be a real problem. Folks have experienced transient "limp mode" conditions because of oddities in temperature, humidity, connectors vibrating loose a little on rough roads, slick spots on the road, sensors starting to fail, debris in the TB, etc. Many have experienced it once or twice only to find it never happens again. There are a thousand things that can cause limp mode that aren't serious just as there are a thousand that are serious.

If it doesn't happen again or can't be replicated by you or the dealer, I would just chalk it up to a weird circumstance. If it's a real problem, it will return and you will be able to diagnose it when you can replicate the circumstance of the failure.


.

SteelBlueMoparman
09-03-2012, 06:38 AM
Thanks Ddaddy, I'll print this and take it to my mechanic. He can probably diagnois it after reading this or at least know where to begin looking for a problem.

06Daytona
09-05-2012, 11:13 AM
I thought the car wouldn't go over around 20mph in limp mode?

Jonny
09-05-2012, 12:01 PM
I thought the car wouldn't go over around 20mph in limp mode?

Won't get past second gear. That's around 55mph, give or take, and screaming like hell.