ON-BOARD DIAGNOSTICS 

A few days ago I was driving behind a string of cars through Liesbeek Parkway when I was startled by several drivers repeatedly  hooting at an Audi A4 driving in front of them to get out of the way or change lane. The Audi A4 that was holding up the traffic had a Guateng registration plate and  my first impression was that its occupants got lost at the spaghetti junction fly-over, not knowing which off ramp to take.

However when these impatient hooting drivers finally overtook the Audi A4 and I got to drive behind it;  I then realized that the Audi A4 was in fact in Limp Mode. Its driver was attempting to get to the shoulder of the road from the centre lane and the traffic just wasn't easing up.

In my opinion, these impatient driver just weren't aware that when a vehicle goes into Limp Mode it cannot go any faster than it's already going even though its driver was flooring the accelerator pedal. And I may add that ignorance about Limp Mode is no excuse, because its been around since 1996.

Vehicle Delivery Services Salvaged Limp Mode car

DRIVER COURTESY

Driver courtesy is very important to bolster safe driving conditions for everyone but an education regarding Limp Mode would be considered far more important. When a car suddenly reduces speed after driving at normal speed, it could be one of several reasons; among which could be steering vibration due to a puncture. Or the vehicle ran out of fuel. Or the engine may have switched off due to a dead battery caused by either alternator issues or snapped fan belt. 

Or the engine may be overheating, or the driver heard a disturbing noise coming from the engine. Or it could be an electrical fault like a faulty fuel pump or an ignition system failure like a defective engine or transmission control unit. Or the driver could have fell ill behind the wheel, to mention but a few of the myriad of reasons why a vehicle could stall or it may have entered in Limp Mode. 

AUTOMATIC TRANSMISSIONS

The engines of cars with automatic transmissions can sometimes switch off mid travel for some obscure reason. The only option the driver has, is to pull off the road, bring the car to a halt, place it in park or neutral to restart the engine. I know of someone who shifted his automatic transmission into neutral when the engine cut out mid travel, restarted the car and shifted it back into drive. This caused his car to almost come to a stand-still instantly causing the wheels to screech as the engines inhibited the transmission.  This type of action can and probably will damage the transmission subject to the gearing system inside and should never be done. Unfortunately no On-board diagnostics makes provision to prevent this.

When an engine cuts out at say 100kph both the power steering and vacuum boosted brakes stops working, thereby making steering difficult and the braking inefficient. It is therefore best to pull off onto  the shoulder of the road and check what the problem is an remedy it before continuing on your journey.

ON-BOARD DIAGNOSTICS

On-Board Diagnostics does a pretty decent job of protecting the engine and transmission against damage by limiting  acceleration, keeping the engine revs to a maximum of 2000 (RPM) and speed to about 45kph - aka Limp Mode. When Limp Mode is enabled, it may lock an automatic transmission  in low gear and even disable both heating and air conditioning. Yet keeping the engine running so that it can be driven to a repair shop.

However, as clever as an ECU is,  it doesn't do anything to alert the driver of the car that follows close behind. Considering tail lights, brake light, reverse lights and  indicators represents a language used by vehicle drivers for those  following behind, to indicate their driving intentions. This light language that's been around for the better part of the automotive industry's existence yet it still haven't come-up with an appropriate and safe warning sign/method for Limp Mode. 

LIMP MODE INDICATOR

To remedy this, car manufacturers could include flashing hazard lights or perhaps fit an LED Display with a scrolling message along the the rear window as an alert to tell the driver following behind that the car in front of it has gone into Limp Mode. Alternatively, electronics savvy car owners can fit their own aftermarket hack by identifying the switching output of the appropriate automotive  High Side Switch (HSS) responsible for protection and diagnostics inside the ECU when Limp Mode is enabled.

Automotive industry High-Side Switches /Drivers - Integrated Circuit 

This may not be as easy as it may seem or sounds because of the myriad of automotive chip manufactures, each pushing their our integrated circuits (ICs) running custom/propriety software, among which are Infineon Technologies AG,   Robert Bosch, Qualcomm, Renesas Electronics Corporation (Intersil), NXP Semiconductors,  STMicroelectronics, Texas Instruments, Intel and Microchip Technology Inc, etc.

However, most of these manufacturers produce Power Switches and incorporate Open Load Detection in their design so that they can perform open-circuit diagnosis on loads, such wiper motors, fans, head lamps, fuel pump, mirrors, actuators in general and LED lights while  the load is enabled or disabled. Open load diagnosis is probably the most important function of the software driven High-Side Switch (HSS) and Low-Side Switch when wired in a specific configuration which allows for currents from 5mA to more than 10A to be accurately detected.   

As such able to generate a hardware signal (Limp Mode signal) that can directly control the hardware without the participation of the microprocessor in the ECU. This output can be used to as either a digital High of Low (using CMOS inverter) to drive a LED display that flashes LIMP MODE..... LIMP MODE..... LIMP MODE.....