Bringing two vehicles back to consciousness after an accident

June 1, 2018
I was recently called to two different shops working on vehicles that would not start after being involved in accidents. It is not uncommon for this to happen, especially if there has been damage to wiring harnesses or possible blown fuses that would render certain operating systems that are vital to cranking or starting a vehicle inoperable.

I was recently called to two different shops working on vehicles that would not start after being involved in accidents. It is not uncommon for this to happen, especially if there has been damage to wiring harnesses or possible blown fuses that would render certain operating systems that are vital to cranking or starting a vehicle inoperable. There are a lot of manufacturers that put engine control modules in harm’s way up front in the engine compartment near the radiator or even tuck them in a wheel well. But this was not the case in either of these two vehicles. 

Figure 1
Figure 2

The vehicles in question were a 2016 Jeep Renegade that was hit in the rear with tailgate damage (Figure 1) and a 2017 Mercedes GLC 300 that hit in the front and had minor front-end damage (Figure 2). There didn’t seem to be enough damage to prevent either vehicle from starting. There are many times where certain things must be disconnected in order to properly work on the vehicle, but I always make sure I do a full visual inspection just to see that everything is in order. After my visual inspection, I could see that all of the main harness connections and components were restored to their proper fit and there were no signs of wire damage. There were also no warning messages or icons on the instrument panels that would alert me that something was preventing these vehicles from starting.  

I performed a full vehicle scan on each car, checking all Control Modules on board. This is a vital step to do prior to working on a vehicle so you can get a full evaluation of underlying problems with the vehicle that cannot be determined by a visual inspection. Sometimes there may be codes stored in memory that were not accident related and simply caused during the repair procedures to the vehicle. It is equally important to view and document all the codes. Your next step is to clear out the entire vehicle and see what codes return as “Current Codes.” I did find many trouble codes pertaining to wiring and component failures from both vehicles, but the codes that did remain after clearing these had nothing to do with preventing the vehicles from starting.  

Manufacturers today have found a way to prevent you from starting a vehicle after an accident in order to protect the driver from causing further damage to the vehicle or themselves. This is a safety feature in the event that there is fuel leakage or a short circuit in wiring that could lead to a fire in the vehicle. This method of shutdown puts the vehicle in an “Automotive Coma” state so that you won’t be able to leave the scene of the accident even if you’re able to walk away from the vehicle. Let’s take a close look at each scenario. 

Figure 3

The 2016 Jeep Renegade was able to crank but would not start up, and the four-way flashers were on all the time. This Jeep was equipped with a fire protection system that is initiated by the airbag control module if it senses a rear impact that could possibly cause fuel tank damage. Most of the vehicles today are equipped with sensors that can determine lateral, longitudinal and G-Force impact. This helps controllers on board to determine vehicle positioning as well as direction of impact. Since the controllers are all on a Buss Network (Figure 3), they share information among themselves and a message can be sent from the airbag control module to the body control module (BCM) if a rear collision is detected, and the BCM will in turn command the engine control module (ECM) to put the vehicle in fuel cut-off mode (Figure 4).  

Figure 4

Once the fire protection system mode is commanded, the BCM will also activate the four-way flashers, unlock all the doors, turn on the interior lights and deactivate the climatic control ventilation. This is an added feature to get you out of the vehicle safely. One of the ways to deactivate this system would be with the use of a scan tool by accessing a bidirectional command (Figure 5) or simply by performing a manual procedure using the signal switch lever. 

Figure 5

The manual procedure involves placing the signal switch lever in neutral position with key off. You turn the key on, apply right turn signal, apply left turn signal, apply right turn signal on again, apply the left signal on again, place signal switch back in neutral position, turn ignition off and turn the hazard switch off. Once this is done the vehicle will start right up. You need to keep in mind that this should ONLY be done by an automotive professional after the vehicle has been fully inspected for safe operation. So now it makes a lot of sense why the four-way flashers were on all the time with the ignition on. This was to alert you that the fire protection system was activated. 

Figure 6

The 2017 Mercedes GLC 300, on the other hand, did not have the ability to crank at all and the starter was inoperative. This could be very confusing because if the battery is low on most new Mercedes vehicles, the ECM will activate the Voltage Dip Limiter under the hood (Figure 6) and open the battery positive cable connector to prevent the onboard computers from experiencing a low voltage surge that can cause many issues with individual controllers onboard. Mercedes does not want these controllers to lose vital information or stored learned procedures that can be corrupted from a very low voltage supply while cranking an engine with a weak battery. 

Figure 7

There could also be another issue where this vehicle may have a positive pyrotech cable (Figure 7) that could have deployed during an accident to remove full power to the vehicle to prevent severed wiring harness issues that may cause possible fires. There will be limited power with idiot lights coming on during key-on operation, but the vehicle will not crank. 

If the battery is up to par and the pyrotech cable was not deployed then most likely the airbag control module sent a message on the vehicle buss line to the ECM to prevent starter operation. Unlike the Jeep, there is only one method to deactivate this safety mode and that is with the use of a scan tool. You would have to access the bidirectional procedure in the ECM (Figure 8). You will see that the starter operation is blocked by a crash event, and you would simply disable this safety mode by hitting the “F1” button on the scan tool. 

Figure 8

Here again you must be sure that the vehicle is safe to start up prior to attempting to do so. I fully understand that during repairs an individual working on the car wants to move the car around without pushing it. There is a reason why these shutdowns are in place, and it’s always about safety and to prevent further damage to the vehicle. You don’t want to start a vehicle with leaky hydraulic or fuel lines. So, again, it is up to ONLY an automotive professional after he or she has inspected the vehicle for safe operation. 

So as you can see, there are many new systems coming to your shop that make you think there is a problem with a circuit without any warning signs to alert you. The vehicles of today are much more sophisticated, and you can no longer go by old-school tactics and what you used to know. There is now a need to learn all these new strategies as they come along. You will have no choice but to engage yourself with continuous education and invest in information systems to help you tackle these problems that you may have to deal with on a weekly or even daily basis. I hope this article has enhanced what you already know or don’t know about these new safety features.

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