[Danny1980]

Sometimes I get the open bonnet red signal on the car dash also when the hood is closed.
It happens randomly. I am aware that there it should be a bonnet open/close sensor beneath the driver sides (LHD) front latch. I believe that the sensor may be defective or needs some contact cleaning or lubrication. Any tips regarding that sensor, could not find the specific issue regarding the sensor on other threads, found something similar but was related to hood malfunction which I believe is a different issue. There is aspecific maintenance to be done? I tried to remove the latch but cannot get beneath it...should I detach the hood latch were it divides in two at the Y?
Any help is appreciated.

 

[Windinmyhair]

Is there any 'give' when the hood is shut? It may be that the stops/support buffers need adjustment.
I'm sure we had a similar question a few weeks ago.

 

[Danny1980]

Is there any 'give' when the hood is shut? It may be that the stops/support buffers need adjustment.
I'm sure we had a similar question a few weeks ago.

@Windinmyhair thanks!
I have to recheck that, possibly slightly on one side...should it be tight and not give a little?
Can the spring buffers be adjusted?

 

[DavidD]

Is there any 'give' when the hood is shut? It may be that the stops/support buffers need adjustment.
I'm sure we had a similar question a few weeks ago.

We did indeed!

Problem getting bonnet closed (R172)?

Ever since I acquired my car 6 month ago I experienced problems closing the bonnet. Either it looked shut but audible alarm and light was on or it simply would not shut without me slamming it shut really hard. I realize the bonnet is made from aluminium, but still why is it so hard to close it...

www.slkworld.com

 

[DavidD]

Sometimes I get the open bonnet red signal on the car dash also when the hood is closed.
It happens randomly. I am aware that there it should be a bonnet open/close sensor beneath the driver sides (LHD) front latch. I believe that the sensor may be defective or needs some contact cleaning or lubrication. Any tips regarding that sensor, could not find the specific issue regarding the sensor on other threads, found something similar but was related to hood malfunction which I believe is a different issue. There is aspecific maintenance to be done? I tried to remove the latch but cannot get beneath it...should I detach the hood latch were it divides in two at the Y?
Any help is appreciated. View attachment 592103

There is a micro switch underneath one of the hood catches, difficult to see but if you open the hood and carefully push in the catch you should hear it click it it is working. On my UK car it is the catch on the driver's side. Will put up a picture in a few minutes.

 

[DavidD]

Sometimes I get the open bonnet red signal on the car dash also when the hood is closed.
It happens randomly. I am aware that there it should be a bonnet open/close sensor beneath the driver sides (LHD) front latch. I believe that the sensor may be defective or needs some contact cleaning or lubrication. Any tips regarding that sensor, could not find the specific issue regarding the sensor on other threads, found something similar but was related to hood malfunction which I believe is a different issue. There is aspecific maintenance to be done? I tried to remove the latch but cannot get beneath it...should I detach the hood latch were it divides in two at the Y?
Any help is appreciated. View attachment 592103

The microswitch is just below the coiled spring in this image

 

[Danny1980]

@DavidD Thanks!
I had all the latches and joints lubricated and checked as per the other post already...there is a minor hood movement when closed but it seems reasonable, my doubt was that one of the switches was faulty or dirty so I wanted to try to clean it or clean the connectors, but as I can see possibly the area can be cleaned directly without detaching the all latch....
Could not also understand if that microswitch was mechanical or had an electronic contact...

 

[DavidD]

The microswitch has wires connected to it, it tells the car if the hood is open or closed, if the switch is dirty and/or faulty it will give you the problem that you are experiencing.
I would recommend trying to clean the switch and also check the wiring as much as possible for any damage or shorts. I guess if you remove the 2 bolts it SHOULD lift off but I have never done that!

 

[Avel Du]

If you can avoid dismantling first I'd go that way.

I saw the data sheet for bonnet adjustment years back (171) and was mortified by how many adjustments there were.
The two stops are worth looking at too (mark before adjusting so you can return them to where they started).

Has the car had any bodywork/paintwork done on the front?
I'm just wondering if it has, whether the technician got everything back to normal.

 

[Danny1980]

The microswitch has wires connected to it, it tells the car if the hood is open or closed, if the switch is dirty and/or faulty it will give you the problem that you are experiencing.
I would recommend trying to clean the switch and also check the wiring as much as possible for any damage or shorts. I guess if you remove the 2 bolts it SHOULD lift off but I have never done that!

I removed the two screws that hold the lock in place although I was unable to lift it from the hole, I will try to push it back posteriorly to see if I can access it from the posterior side...

 

[Danny1980]

If you can avoid dismantling first I'd go that way.

I saw the data sheet for bonnet adjustment years back (171) and was mortified by how many adjustments there were.
The two stops are worth looking at too (mark before adjusting so you can return them to where they started).

Has the car had any bodywork/paintwork done on the front?
I'm just wondering if it has, whether the technician got everything back to normal.

Car is as original no work done on it....it has 110k km by the way so something may have loosen up

 

[Danny1980]

@Windinmyhair @Avel Du @DavidD @jbanks15

We might have something here...
By the way I could not understand the thread very well...I might need an engineer....LOL

Bonnet/Hood open issues resolved!

I thought i would share this with the group as this seems to be a common issue on the R172....... after considerable research and a few solid hours practical, testing continuity, feedback, resistance and disassembling i have resolved the issue. The passenger side lock assembly has 2 wires wires...

www.slkworld.com

 

[DavidD]

@Windinmyhair @Avel Du @DavidD @jbanks15

We might have something here...
By the way I could not understand the thread very well...I might need an engineer....LOL

Bonnet/Hood open issues resolved!

I thought i would share this with the group as this seems to be a common issue on the R172....... after considerable research and a few solid hours practical, testing continuity, feedback, resistance and disassembling i have resolved the issue. The passenger side lock assembly has 2 wires wires...

www.slkworld.com

That does sound somewhat complex! Sounds like there are very small tolerances with the locking mechanism and switches, any build up of dirt might cause a problem, you could try a good clean with something like wd40 to remove any build up of dirt. If you do that you will also need to lube it afterwards as wd40 and the like will also remove oil as well as dirt!

 

[Avel Du]

@Jonathan Jackson has not been in nearly 4 years but maybe will get this mention.
I'll convo as well.

 

[Danny1980]

I'll try to clean the area...
By the way I have taken from the web those pics which match with part 320 and 130 from the drawing. This ones are very dirty indeed...

part 110 and 120 in this drawing....

 

[Danny1980]

Some other pics....

 

[Danny1980]

I think I got the issue sorted...
The R172 aluminum bonnet flexes quite a bit compared to other more "heavy" hoods, therefore when closing it engages the central latch, but it might not engage the lateral ones or at least it engages one and not the other...
I tried several times and found that the side which does not engage properly it moves a bit and is easily identifiable, is just a matter of pressing on that side over the latch area so to make it engage.

There is a circuit that closes when the two lateral latches are in the closed position so to give the ECU the "closed hood" signal.

I have seen in a similar thread that someone was slamming the hood harder when the "open hood" signal would come up on dash, to sort this out...I suggest not to slam but just press on the loose lateral anterior bonnet side, in order to resolve the open hood signal, and avoid slamming which may damage latches or the hood components.

@jbanks15 @Avel Du
I believe is SORTED...but will keep posted if it happens again to exclude any other reasons for this problem...

 

[LeonardoDaBenz]

We'll mark this resolved but as you said, if the problem returns, come back to THIS thread (do not create a new one) and we'll update accordingly

 

[Windinmyhair]

From the Owner manual:
Closing the bonnet

Lower the bonnet and let it fall from a height of approximately 20 cm.

Check that the bonnet has engaged properly.
If the bonnet can be raised slightly, it is not properly engaged. Open it again and close it with a little more force.

 

[Danny1980]

From the Owner manual:
Closing the bonnet

Lower the bonnet and let it fall from a height of approximately 20 cm.

Check that the bonnet has engaged properly.
If the bonnet can be raised slightly, it is not properly engaged. Open it again and close it with a little more force.

@Windinmyhair my personal opinion is not to let it fall but to close it and push gently, but is just my personal view...