Active Bonnet fun and games

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Apr 22, 2021
South UK
S212 pre facelift. Lots of previous Mercs
A year ago a fox ran out in front of my pre facelift S212 [ E350 AMG sport] and despite there being no obvious signs of collision the back edge of the bonnet shot up due to the Active Bonnet pedestrian protection system.

Despite trying to follow the recommendation in the manual to open the bonnet and push down I found the heavily sprung actuators wouldn't reset. After reading a whole host of conflicting information on various forums [ The bonnet should go back to its normal position by opening and closing - it didnt. It is part of the airbag SRS system - no, it seems to be separate. The bonnet should go back to it's normal by pushing down hard at the back both sides - the manual advises expressly against it and it didnt work anyway. The actuators are pyrotechnic so have to be replaced every time- no they appear to be electro mechanical so should be able to be reset and reused. ]

Anyway, I ended up buying a 2nd hand pair of actuators at the time, refitted with no issues.

Now what do they say about lightning not striking twice ? It just happened again at the weekend, this time it was caused by a dog off it's lead running out in front of me. I braked heavily and the dog hit the front of the car but ran off hopefully unharmed. This time it had dislodged the number plate and put a slight crack in the lower front grille as the only signs of any harm to the car but the proper nuisance though was the Active Bonnet pedestrian protection system released again, raising the rear of the bonnet by a few inches. Now as a safety feature to pedestrians it may have some use but it is of no benefit at all with dogs or foxes which are no where near the height of the bonnet and the system is just a damn nuisance.

Rather than going straight to new actuators this time I decided to use the alternative solid hinge mounts. These brackets are present on all W212/ S212s and are the standard way of mounting the bonnet on every car that does not have the Active Bonnet system. . Im not sure if the active bonnet system may be standard on all W212s/ W212s in the UK or maybe only on some spec cars. There are certainly a lot of 212s around without it though it seems.

The hinges on a W212 bonnet (at least the pre facelift) are made with an alternative set of threaded holes that allow the hinges to be bolted straight to a solid mounting on the car, dispensing with the actuators.

With one or two helpers undo the 2x 13mm bolts each side that hold the bottom of the hinges to the actuators and lift the bonnet clear. Remove the 2x 13mm nuts that hold each actuator in place and lift up to access the electrical connector. This can be separated by careful use of a very small screwdriver and long nose pliers to carefully release the grey plastic lock tab, the plug can then be pulled out of the socket. Remove the actuators. Right alongside where the actuator is you will see an long, low up stand. This is the alternative mounting bracket for the hinge but wont be immediately apparent as it has a plastic cover. The cover is very easy to remove, just prises out with no special tools. You now have access to the proper solid mounting bracket. With your helpers, bring the bonnet back and bolt the hinges on. This time you will pass the bolts through the brackets into the alternative, threaded holes in the hinges. Adjust bonnet for a good fit.

Incidentally I have now managed to physically re compress the actuators whilst they are off the car. Though the manual says this can be done whilst they are fitted I didnt have any joy with this despite several tries with several people helping. I found it can be done off the car quite easily though with getting an actuator in a normal DIY bench vice and using a large G clamp. Just be careful when doing this as the springs are quite strong so keep fingers clear.

Starting the car with the actuators removed and the bonnet now solidly mounted brought up an Active Bonnet malfunction text warning in the disp[lay - though no SRS light.
This warning was not present before the actuators were removed so it can only be due to the system not seeing connectivity or resistance across the actuators' electrical connections (now they are not present). The good thing is I can now at least drive the car with the bonnet sitting normally.

When time allows later I'll investigate if the warning message stops if I temporarily reconnect the actuators. and if so then decide if I will take the bonnet off again to refit the actuators or leave the hinges solidly mounted and find a work around.

Does anyone know if this active bonnet system was present on all UK 212s or was it optional/ only fitted to certain spec cars ?
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Does anyone know if this active bonnet system was present on all UK 212s or was it optional/ only fitted to certain spec cars ?
AFAIK, it was fitted to all UK-spec 212's except the AMG E63 variants.
My guess is that to complete the job, you'll need to have the Active Bonnet option SCN coded-out of the Datacard (i.e. deleted).

This will allow the front SAM to download the modified configuration from MB and then your car will think it no longer has the Active Bonnet option, and no warning will be raised.

If I am correct, then any MB specialist with genuine STAR can do this, however, understandably some might be reluctant to disable a factory-fitted safety feature.

The other related issue is that you should really declare the modification to your insurer - and this might not go down well if they understand that you have disabled a safety feature that could save them a pretty penny in the event of a collision with a pedestrian - though my experience with declaring modifications to insurers is that the person you are talking to usually has no idea what you are talking about, they just make a note of what you say, the only question I was asked is whether 'it increases engine power', so there's a good chance that you'll get away with declaring it.

Best of Luck either way!
Another thought - perhaps for others in your situation - given that the car in question is now 10 years old (or thereabouts), it is possible that the undesirable behaviour described was actually fixed in later firmware revisions - i.e. that the latest software can differentiate between a dog a human etc - so possibly worth trying a software update to all the relevant systems before disabling this feature? A long shot, I agree.
The resistance across the actuators is 1.6 Ohms so resistors of this value should stop the warning coming up so i will try this out of curiosity. However now I have managed to physically reset the actuators I will probably refit them once i can organise 2 people to help me remove and refit the bonnet again.
The resistance across the actuators is 1.6 Ohms so resistors of this value should stop the warning coming up so i will try this out of curiosity.
Any resistors so used will likely be sacrificial if the system is triggered again, of course ;)
However now I have managed to physically reset the actuators I will probably refit them
That sounds like the best course of action.
Success with the 1.6 Ohm resistors= no Active Bonnet warning message.

You are quite right in that the resistors would need to be replaced if the system did fire again but they cost less than 50 pence each and can be replaced in seconds.

To recap:
The vague instruction in the owner's manual to reset the actuators by lifting the bonnet straight upwards hardly ever works, even with the force of several people
The actuators can usually be reset for reuse by removing them from the car and compressing them with a large G clamp
Alternative solid mounting points are already present with no need for additional parts should anyone choose to remove the actuators and delete the Active Bonnet feature
A pair of 1.6 ohm resistors will avoid the Active Bonnet Malfunction warning if the actuators are not present
Assuming the voltage applied is 12volts, then each resistor will dissipate 90 watts when activated.
Enough to self destruct but a considerable amount of heat if applied for more than a second or so.

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