20 replies to this topic

[CruzKing]

Can anyone explain "limp mode" if there is any such thing.

 

 Automatics or six speeds?

Causes?

 When does it happen?

 Does it damage anything?

How to prevent it?

 

Thanks CK.

[CruzKing]

I just found this:

 any other ideas or comments welcomed.

 

It's only on the automatics. The TCM (Transmission Control Module) will put the transmission into that mode to protect it from damage. It's called limp mode because it still allows you to get home or to the shop, stuck in 2nd gear and with an rpm limit so it won't be fast or fun. Often times the car will show a code (Check Engine Light) to indicate the problem and can be restarted to get out of limp mode (if no actual damage).

Typically, the TCM prevents the driver from harming the transmission and takes inputs to the stick or paddles as mere suggestions, but there are ways to still get into limp mode. With my Charger it was when I had TC/ESP on and had 'excessive' wheel slip (like in the rain). The TCM detects an abnormal condition that it interprets as potentially damaging and shuts down the fun. That's one reason why I always disable the electronic nannies - to prevent the TCM from overreacting.
But you can also have mechanical issues such as the transmission needing service or having any kind of failure. In that case, limp mode is your friend by helping prevent further damage and alerting you to the problem.

ETA: Found it in the manual:

Transmission Limp Mode Transmission function is monitored electronically for abnormal conditions. If a condition is detected that could result in transmission damage, Transmission Limp Home Mode is activated. In this mode, the transmission remains in the current gear until the vehicle is brought to a stop. After the vehicle has stopped, the transmission will remain in second gear regardless of which forward gear is selected. PARK, REVERSE, and NEUTRAL will continue to operate. Limp Home Mode allows the vehicle to be driven to an authorized dealer for service without damaging the transmission.
In the event of a momentary problem, the transmission can be reset to regain all forward gears by performing the following steps:
1. Stop the vehicle.
2. Shift the transmission into PARK.
3. Turn the engine OFF.
4. Wait approximately 10 seconds.
5. Restart the engine.
6. Shift into the desired gear range. If the problem is no longer detected, the transmission will return to normal operation.
NOTE: Even if the transmission can be reset, we recommend that you visit your authorized dealer at your earliest possible convenience. Your authorized dealer has diagnostic equipment to determine if the problem could recur.
If the transmission cannot be reset, authorized dealer service is required.

Another cause: I believe I remember that while the car cannot be revved over 4,000 rpm in Neutral to protect the transmission, some people have managed to try so hard that they ended up in limp mode.

__________________

[Mopar muscle 73]

I've had my car go into limp mode a few times for a few different reasons.

 

1) crank sensor came loose.

 

2) issues with throttle body. (code p2173 I think it was)

 

3) transmission issue. Car wouldn't accept any shifts.

 

Had the crank sensor replaced. Tune revisions for the throttle body seemed to help. No idea what the hell happened on the transmission.

Resetting the code on the TB fixed that issue while it occurred. and a simple restart of the car, reset the transmission.

[StevoHemi]

Greg... started this one a few days ago. Dunno if you have read it.

 

http://modernmoparfo...-hurt-the-nag1/

 

Oversized TB's can be a culprit if not tuned properly.

 

Stevo

[CruzKing]

I don't have a oversized throttle body. This is the first time that it has happened, only change was the converter install.

 

 it happened on my first run last night, and then again on the fourth run. I quit after that since I never experienced that before and didn't know if I was damaging something.  Funny thing the two "limp mode" runs started out to be my best.

 

OH on the last run I looked down and saw the symbol for Electronic throttle control was on. It did feel like I let off the throttle (but I didn't) and the car was in second gear. I did a 17.419 1/4.

[srt5939]

It's what happens when the Viagra wears off

[CruzKing]

get with the times john, Cialis is the new Viagra.

 

  seriously with the electric throttle control light coming on is there a way to avoid this at the track. 

[ChargerRob]

You also get limp mode if you are using your temp spare, at least I did.

[70Barracuda]

I don't have a oversized throttle body. This is the first time that it has happened, only change was the converter install.

 

 it happened on my first run last night, and then again on the fourth run. I quit after that since I never experienced that before and didn't know if I was damaging something.  Funny thing the two "limp mode" runs started out to be my best.

 

OH on the last run I looked down and saw the symbol for Electronic throttle control was on. It did feel like I let off the throttle (but I didn't) and the car was in second gear. I did a 17.419 1/4.

 

What size drag radials (if any) are you running, when I ran 275/40/17's I had it happen numberous times.  Sometimes I could get away with the shorter tire for the whole track outing but other times it would dump me into limp mode every run.  My solution was to go to 275/50/17's which more closely matches our stock tire height, no problems since then.

[CruzKing]

70Barracuda, on 19 Jun 2014 - 6:21 PM, said:

What size drag radials (if any) are you running, when I ran 275/40/17's I had it happen numberous times.  Sometimes I could get away with the shorter tire for the whole track outing but other times it would dump me into limp mode every run.  My solution was to go to 275/50/17's which more closely matches our stock tire height, no problems since then.

running 275/50/17 , and have running them over a year with no problems.

[KeavDog]

When you had the tc installed did they change the rev limiter at all?  I had issues when I was messing with the shift points.

I went back to 6200 and never had another issue.

Others will know better but if the shift points are higher than the transmission rev limit it can go into limp mode - something like that.

[ostmike]

You will hit TCM limp mode if the engine speed exceeds 6750 rpm's (unless you have a custom TCM). It's a very common problem for modified cars that rev quickly and have raised rev limits.

[lakersfan7272]

PCS

[1FST4DR]

 

Oversized TB's can be a culprit if not tuned properly.

 

Stevo

 

Different type of 'Limp mode'  One doesn't have to do with the other..  Just an FYI 

[CruzKing]

used the predator to read codes and got this:

 

P2299 break pedal position / accelerator pedal position.   

 

 any ideas what this might mean??  

 

 thanks for all your responses, CK.

[70Barracuda]

used the predator to read codes and got this:

 

P2299 break pedal position / accelerator pedal position.   

 

 any ideas what this might mean??  

 

 thanks for all your responses, CK.

 

I have no idea, however it almost sounds like you went into limp mode for doing a power brake burnout to heat your drag radials.  Did you have TC off when you did your burnout ?  not that it should necessarily matter but some of these automatics do seem to be finicky about certain things.

[KeavDog]

Or a wheel speed sensor?

[CruzKing]

automotive electronic throttle control systems

 

Note that you will see this abbreviation:  LH = shorthand for "limp-home"

 

Ok, your next question:  What is "limp-home"?

 

It's the "return spring" in the throttle body and its effect on linearity:

 

An electronic throttle is a low-power dc servo drive which positions the throttle plate.   Its application in modern automotive engines leads to improvements in vehicle drivability, fuel economy, and emissions. Transmission friction and the return spring "limp-home" nonlinearity significantly affect the electronic throttle bodies performance.

This is an insidious failure as it may not provide any symptoms until there is total failure. All cars having a TPS have what is known as a 'limp-home-mode'. When the car goes into the limp-home-mode it is because the accelerator and engine control computer and the throttle are not talking to each other in a way that they can understand. The engine control computer shuts down the signal to the throttle position motor and a set of springs in the throttle set it to a fast idle, fast enough to get the transmission in gear but not so fast that driving may be dangerous.

Oh, it even gets hairier if you really want to dig deeper:

 

A dynamic friction adaptive is developed in order to adequately capture the observed characteristics of the presliding-displacement and breakaway effects.  The linear part of electronic throttle process is also analyzed and identified.  A nonlinear control strategy is proposed, consisting of a proportional-integral-derivative (PID) controller and a feedback compensator for friction and limp-home effects. The PID controller parameters are analytically optimized (adaptive's) according to the damping  optimum criterion.

Also note that ESP has direct effect on the vehicle's "drive-by-wire" control when it sees and unstable situation.  Varying degrees of throttle "over-ride or containment" may be set when ESP takes over.

 

Now if you have any further questions about drive-by-wire systems, I highly recommend that you can go to the IEEE site below and continue your research there:

 

An electronic throttle control strategy including compensation of friction and limp-home effects

 

Dodge Challenger Basic ETC system and components;

 

Now, let's look at how the ETC system basically works in our Dodge Challengers. We have some basic hardware that are part of the Electronic Throttle Control System;

 

          1.  Accelerator Pedal Position Sensor

          2.  PCM - Powertrain control module (engine computer) which contains the  proportional-integral-derivative (PID) controller
                   and a feedback compensator for friction and limp-home effects

          3.  Throttle Body Servo Control

 

          4.  ETC Fault Indicator Light on Instrument Panel

 

So, in short, the PCM monitors the position of the accelerator and provides the proper throttle body position to control air flow into the intake plenum and sets the pulse width for the fuel injectors.

 

Note that the PCM also accepts inputs from ESP and other devices to trim or alter the accelerator pedal signal for vehicle safety and control.

 

The Accelerator Sensor and the Throttle Body Control have additional feedback sensors to monitor the primary position sensors to detect errors or faults.  This is why it is very important that if you have and ETC fault light on your instrument panel, that you take this very seriously and get the vehicle in for service and diagnostic testing as soon as possible.

Here is an electrical sample diagram of the ETC pedal sensor (accelerator) and note dual isolated 5VDC power supplies and grounds feeding the two separate pedal position sensors.  This pedal assembly and wiring is tied directly into the PCM as two isolated inputs. 

 

The redundancy is for self check and real time ETC diagnostics.

 

 

Accelerator Pedal Position Sensor

 

 

 

ETC Throttle Body Control

 

 

Electronic Throttle Control

 

Definition:

Electronic Throttle Control (ETC), sometimes called Drive-By-Wire (DBW), is a technology that replaces the conventional cable/linkage connection between a vehicle's accelerator pedal and the engine's throttle body.

 

Almost all modern automobiles with mechanical linkage use a throttle position sensor to determine throttle plate opening once the linkage has set it. The most common arrangement is a Bowden wire mechanism.

 

ETC, instead, determines necessary throttle opening using various input sensors (accelerator pedal position, engine RPM, engine vacuum (load), vehicle speed), and then positions the throttle via an actuator driven by a small, high-torque DC motor.

 

The main benefit of ETC is much quicker and accurate throttle control that helps the powertrain and emissions management systems deal with quickly changing conditions (ambient temperature, atmospheric pressure and accessory loads) that are normally outside the driver’s scope of control. In addition, this arrangement allows near seamless interaction between the motors, engines, controllers and drivelines of hybrid vehicles as well as motor/generators in electric vehicles (EV)s.

ETC facilitates the integration of features such as cruise control, traction control, stability control, and precrash systems and others that require torque management, since the throttle can be moved irrespective of the position of the driver's accelerator pedal. ETC provides only a very limited benefit in areas such as air-fuel ratio control, exhaust emissions and fuel consumption reduction, working in concert with other technologies such as gasoline direct injection.

A criticism of the very early ETC implementations was that they were "overruling" driver decisions. Nowadays, the vast majority of drivers have no idea how much intervention is happening. Much of the engineering involved with drive-by-wire technologies including ETC deals with failure and fault management. Most ETC systems have sensor and controller redundancy, even as complex as independent microprocessors with independently written software within a control module whose calculations are compared to check for possible errors and faults.

Failure modes

Before drive by wire technology was introduced, if a throttle stuck open a driver could generally put a toe under the accelerator and lift up.

Occasionally after servicing or repair, the wire or cable between the accelerator and throttle would not be correctly reinstalled causing sudden acceleration.

However, with the ETC, the movement is all done by electronic controls moving an electric motor. But just moving the throttle by sending a signal to the motor is an open loop condition and leads to poor control. Most if not all current ETC systems have a closed loop system whereby the ECU tells the throttle to open a certain amount according to an algorithm based on the geometry of the throttle. Then, if due to dirt build up in the throttle bore or a damaged TPS a signal is sent from the TPS to the ECU, the ECU can make appropriate adjustments to compensate, though it might result in surging, hesitation or uneven idle.

There are two primary types of throttle position sensors: a potentiometer or a Hall Effect sensor (magnetic device). The potentiometer is a satisfactory way for non-critical applications such as volume control on a radio, but as it has a wiper contact rubbing against a resistance element, and dirt and wear between the wiper and the resistor can cause erratic readings. The more reliable solution is the magnetic coupling that makes no physical contact, so will never be subject to failing by wear.

This is an insidious failure as it may not provide any symptoms until there is total failure. All cars having a TPS have what is known as a 'limp-home-mode'. When the car goes into the limp-home-mode it is because the accelerator and engine control computer and the throttle are not talking to each other in a way that they can understand. The engine control computer shuts down the signal to the throttle position motor and a set of springs in the throttle set it to a fast idle, fast enough to get the transmission in gear but not so fast that driving may be dangerous.

Recently, ETC has been suspected by some to be responsible for some incidents of unintended acceleration in Toyota and Lexus vehicles. This is fiercely disputed by Toyota, which blames unintended acceleration on owners, weather mats, and most recently defective gas pedals (outsource production).  Federal Highway Safety Commission finds no electronic failures as root cause of unintended acceleration. Here is NHTSA Federal Commission Report.

Current Patent;

Pedal position rate-based electronic throttle progression

 

United States Patent 6915779
 

An engine control system in a vehicle including an internal combustion engine, an electronic throttle controlling air flow to the internal combustion engine, a controller controlling the position of the electronic throttle, an accelerator pedal having an accelerator pedal sensor that generates a signal to the controller, and where the controller computes a rate of change for the accelerator pedal

Now i'm more confused than ever:

 

 

 

Ok for me this is a big WTF.   so I have the electronic throttle control  error on the evic,  read the code as posted above..(break peddle/ accelerator  peddle, locations etc)  nice I still don't know WTF caused the limp mode.

 too bad speddy didn't have an auto I would have watched the video and been done with this.

[Jerseyboy]

If you press the gas then press the brake at the same time you can get that fault. You been doing some two foot driving lately?

J

[CruzKing]

J, yes(pressing break then gas) burnouts and while staging. been doing that for 5 years and never had that issue before.