Bonnet alarm sensor not working

Bob123 · Post by **Bob123** » Thu Jun 01, 2023 8:01 am

I'm wondering if I could have some advice on diagnosing why the bonnet alarm sensor on my car is not working?

I bought the sensor recently off a forum member and have just fitted it. The previous original sensor was missing and the cable had been secured to the bulkhead.

I've tested it twice now, firstly with the bonnet open and the sensor button depressed (to make it think that the bonnet was closed), I locked the car with the central locking button on my key while doing this and counted down to roughly 1.5 minutes, then released the bonnet sensor button. No alarm sounded. Secondly I sat in my car, locked the car with the central locking button on my key, counted down about a minute then pulled the bonnet release lever, the bonnet popped up and no alarm sounded. I know that the car's alarm works as I forgot to unlock the car with the central locking button on my key a few days ago and the alarm sounded when I started jacking my car up.

I'd be grateful if anyone could suggest things that I could check to see why it's not working. Could it be that the bonnet sensor has been disabled in the software, a fuse pulled, maybe wiring disconnected in the white box in the engine bay (seems to contain wiring and maybe soleniods) where the lead goes to etc?

smorris_12 · Post by **smorris_12** » Thu Jun 01, 2023 8:22 am

It's only a switch and, years ago, there was a tendency for them to die without anything noticing. Put a test meter on it and check its operation.

1536Z4 · Post by **1536Z4** » Thu Jun 01, 2023 8:26 am

Bob123 wrote: ↑Thu Jun 01, 2023 8:01 am Could it be that the bonnet sensor has been disabled in the software, a fuse pulled, maybe wiring disconnected in the white box in the engine bay

Cant help you with any diagnostics but if you do a google search for Z4 forum bonnet sensor that will show you posts about the issues with the sensor and people just removing it as I did .

Bob123 · Post by **Bob123** » Thu Jun 01, 2023 8:28 am

Thanks for the info smorris_12, unfortunately I do not have a multimeter. I think that I really aught to get one.

Bob123 · Post by **Bob123** » Thu Jun 01, 2023 8:30 am

Thanks 1536Z4, I may remove it but for now I'll try to get it to work as I bought it off a forum member and I would hope that I was sold a working sensor.

enuff_zed · Post by **enuff_zed** » Thu Jun 01, 2023 2:24 pm

Bob123 wrote: ↑Thu Jun 01, 2023 8:30 am Thanks 1536Z4, I may remove it but for now I'll try to get it to work as I bought it off a forum member and I would hope that I was sold a working sensor.

Indeed you were! Checked before removal.
However if it has failed since removal then you can have a refund of course.

Bob123 · Post by **Bob123** » Thu Jun 01, 2023 2:26 pm

Cheers enuff_zed, I've ordered a (long overdue) multimeter and I will test that it is responding to the pressing of the switch.

enuff_zed · Post by **enuff_zed** » Thu Jun 01, 2023 2:28 pm

Bob123 wrote: ↑Thu Jun 01, 2023 2:26 pm Cheers enuff_zed, I've ordered a (long overdue) multimeter and I will test that it is responding to the pressing of the switch.

Ok, just taking a break on the way back from my hols. Let me know how it goes.
In the meantime I will test the one on my old 4-pot and if that works I’ll set it aside for you.

Bob123 · Post by **Bob123** » Mon Jun 05, 2023 9:12 pm

My new multimeter has arrived and I've read the instructions and watched a few YouTube videos on how to use it. It seems that I need to use the Continuity test and touch two contact points while holding down the bonnet button to test that there is a flow of current.

Here's a couple of photos.

: Z4 bonnet alarm sensor (1).JPG (78.38 KiB) Viewed 337 times

: Z4 bonnet alarm sensor (2).JPG (165.13 KiB) Viewed 337 times

I've tried this initially with no luck, however there are three pins inside the sensor and they are labelled 1, 2 and 3. I've tried different pin combinations, but this is a bit random and I may have missed a combination etc.

I'd be grateful if anyone could advise me on how to test it.

Thanks.

enuff_zed · Post by **enuff_zed** » Mon Jun 05, 2023 10:01 pm

Another thought came to mind. Anyone near you with a Zed that you could swap switches with as a diagnostic check?
If it does turn out the switch is duff I'll send another.

Bob123 · Post by **Bob123** » Tue Jun 06, 2023 8:12 am

enuff_zed wrote: ↑Mon Jun 05, 2023 10:01 pm Another thought came to mind. Anyone near you with a Zed that you could swap switches with as a diagnostic check?
If it does turn out the switch is duff I'll send another.

That's a good suggestions enuff_zed (and thanks), I'll see if I can find someone who would be prepared to carry out the test.

If anyone on here could advise me on how to test the sensor (there are three pins) using my new multimeter I'd really appreciate it. Thanks.

smorris_12 · Post by **smorris_12** » Tue Jun 06, 2023 9:53 am

I'll try and find time to squint at the wiring diagrams later and see what's what. With the pins there ought to be a common and then one pin that gives continuity when pressed and one that gives continuity when released[1]

[1] I'm now worried BMW have found a way of building in complicated electronic circuitry instead of a nice simple microswitch.

Bob123 · Post by **Bob123** » Tue Jun 06, 2023 10:03 am

Thanks smorris_12, that'd be a great help.

smorris_12 · Post by **smorris_12** » Tue Jun 06, 2023 4:25 pm

There is a line in the WDS text that says "If, after arming, one or several input signals are not in rest position (e.g. door open) the status LED will flash for 10 seconds." Oddly enough, I noticed mine doing this last night which makes me think my bonnet switch is probably U/S too. So I've just popped out there and stuck a meter on it. Turns out it's fine.

Well, it is a simple switch. It's press to break (counter-intuitive as logically it'd be press-to-make so the alarm would protects against wires being cut) and it simply bridges or breaks between pins 1 and 2. Pin 3 is connected to pin 2.

So, with the meter on continuity or its lowest resistance setting, see what it does when you touch the probes together. It should read almost 0 ohms and possibly squeak at you. Then put the probes on pins 1 and 2 (or 1 and 3) at the same time (good game, good game) and it should do the same thing when the switch is in the relaxed position and stop when you press it.

Bob123 · Post by **Bob123** » Tue Jun 06, 2023 5:45 pm

smorris_12 wrote: ↑Tue Jun 06, 2023 4:25 pm There is a line in the WDS text that says "If, after arming, one or several input signals are not in rest position (e.g. door open) the status LED will flash for 10 seconds." Oddly enough, I noticed mine doing this last night which makes me think my bonnet switch is probably U/S too. So I've just popped out there and stuck a meter on it. Turns out it's fine.

Well, it is a simple switch. It's press to break (counter-intuitive as logically it'd be press-to-make so the alarm would protects against wires being cut) and it simply bridges or breaks between pins 1 and 2. Pin 3 is connected to pin 2.

So, with the meter on continuity or its lowest resistance setting, see what it does when you touch the probes together. It should read almost 0 ohms and possibly squeak at you. Then put the probes on pins 1 and 2 (or 1 and 3) at the same time (good game, good game) and it should do the same thing when the switch is in the relaxed position and stop when you press it.

Thanks for your help with this, really appreciate it. I'll do that and check. Is the WDS a resource that has all the wiring info for Z4's in it? When you say 'the status LED will flash for 10 seconds.', do you mean the light on the underside of my rear view mirror?

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