Inertial Switch + pictures - Alfa Romeo Forum
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Inertial Switch + pictures

I've got the inertial switch intervened message. I've had it a couple of times in the past. First time it was preceded by a discharged battery and just decided to fix itself after I was fiddling with the switch on the battery and the one under the seat. I never knew what was wrong. Second time, battery was fine but I had slid the passenger seat fully forward. I moved the switch under the passenger seat slightly and all was good (it was loose) This time I took the seat partly out so I could see properly. The inertial switch was again loose - it was getting caught by a wire under the seat which was dragging it out place when the seat was adjusted. It is now upright, ion place and not getting caught but still no change.

The switch on the battery does not seem to have popped out, at least pressing on it doesn't do anything. The earth points are both good. As you can see from the images, I have 12.7 volts before the switch but only 1.4 V after (key not in ignition) The third image shows there is a small voltage from the wire that leads to the box, not sure what this does!

Given that it is stuck on my drive and it is not time urgent, I'd like to try to trouble shoot this myself, with all your help

Are the voltages I'm seeing normal? Should there be 12V after the box?
If there should be 12V after the box/switch thing, can I test start the car by bypassing it or can I only test with a new box (I don't want to create new problems or error messages unnecessarily)
If it is normal and the problem lies in the switch under the seat, how can I test it? Again, is there a way of bypassing it or can it only be tested with a new switch?

However, could it be that the problem is elsewhere? before it happened this time, the battery discharged overnight down to nearly 3V. Does this sound like an ECU problem instead?

Many thanks in advance for any advice.

Edit: I've got a spare fully battery which I have tried as well but no change. Unless both batteries are duff I think it is something more complex.
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File Type: jpg 12V.jpg (80.8 KB, 39 views)
File Type: jpg IMAG0012 (1).jpg (69.6 KB, 30 views)
File Type: jpg IMAG0006.jpg (86.6 KB, 34 views)
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(Post Link) post #2 of 7 Old 18-09-16 Thread Starter
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I've now found out that the wire in the last picture comes from the inertia switch and disconnects the power to the starter cable. If it has been tripped it needs resetting by pressing the yellow button.

If it has actually been tripped by the inertial switch, shouldn't there be 0 volts at the starter cable rather than 1.4V? Does this indicate that the battery control unit is failing? I can't get current to the starter cable by unplugging the inertia switch wire and pressing the yellow button; it doesn't appear to have popped out. Should there be any power at all in the wire from the inertial switch (bottom photo, 1st post)

Big ask, but would someone with a working Brera or 159 measure the voltage at the inertial switch wire and starter cable and see what is normal? Thanks in advance.
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File Type: png Battery disconnection.png (62.4 KB, 36 views)

Last edited by Rewe; 18-09-16 at 17:22.
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(Post Link) post #3 of 7 Old 18-09-16 Thread Starter
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I may be going around in circles now!

If I'm reading this wiring diagram correctly, the inertial switch (I050 near the bottom of the image) is earthed via the wire at the bottom of the image (NZ). It normally has no connection to the battery cut off switch (B099) near the top of the image. However, if the inertial switch is triggered, it now means the battery cut off switch, which has a permanent live from the battery, now has a path to earth and energises, whilst at the same time the fuel pump loses it's path to earth and stops?

Does this mean that if I unplug the wire from the inertial switch to the battery cut off (third image in the first post) AND the battery cut off switch is working, there should be 12V at the starter motor cable? I can't work out why there is 1.4V in this wire when unconnected though - is this to do with the dash warning message?

I don't know what to make of the inertial switch being loose rather than fixed in place. Clearly someone else has been trying to chase the same problem!
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Last edited by Rewe; 18-09-16 at 17:27.
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(Post Link) post #4 of 7 Old 02-10-16 Thread Starter
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Just remembered, I haven't updated this. Problem solved!

It was in fact the battery cut off switch. I got a second hand one from our friends at Autolusso.

With the new switch in (negative battery terminal off first of course!) I had exactly the same voltage across both sides of the switch, car started normally and with a quick time, date and window reset (drivers window fully down hold for 10 seconds, passenger window fully down hold for 10 seconds, passenger window fully up hold for 10 seconds, drivers window fully up hold for 10 seconds) all was good.

So what was wrong with the old one? It looked fine from the outside and didn't appear to need to be reset so it was off with the cover to have a look. The cover is held in place with some goo which can be cut through with a stanley knife and it just lifts off. The problem with mine was that it was only loosely making contact. With the cover off I could disconnect it and connect it properly but pushing only where the yellow switch touches I could only get a partial contact. I'll keep this in the boot with the cover not fixed back as a spare. It is not obvious in the pics but I have highlighted where it switches. You can feel when it is properly seated and when it is only partially caught.

So what have I learned?

If you get the Inertial Switch Intervened message you can check the battery switch with a multimeter. In the first post, the voltage should be identical on both sides of the switch

If your battery discharges unexpectedly check this switch. It isolates the power to the starter motor but also the current from the alternator.

In an emergency if the yellow reset button is not working you can take the switch off, pop the cover off and make sure it is properly switched. I expect you could also bypass the switch altogether if needed.

It can cause intermittent problems for a while. Pressing the switch obviously was allowing enough of a contact to be made to allow the car to start for a while but the underlying problem remained. It happened to me 3 times in the 10 months I've had the Brera and as both the inertial switch under the seat was not properly fastened and one of the nuts on the battery cut off switch was loose, someone has clearly been chasing this problem for a while. Leaving the inertial switch under the seat loose is not a good idea!

I'll keep monitoring to double check that it is solved permanently but I'm pretty confident that it was all to do with the partial contact in the battery cut off switch.
Attached Images
File Type: jpg Batt open.jpg (58.2 KB, 23 views)
File Type: jpg Batt 2.jpg (74.4 KB, 26 views)
File Type: jpg Switch.jpg (101.1 KB, 26 views)
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So after coming out to a very dead Alfa this morning thought I'd whip out the battery and put the new one in I bought last week only to notice I don't seem to have a battery cut off switch on my car...what age did they start putting them on, or is mine just missing?

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Originally Posted by Karl_Shad86 View Post
So after coming out to a very dead Alfa this morning thought I'd whip out the battery and put the new one in I bought last week only to notice I don't seem to have a battery cut off switch on my car...what age did they start putting them on, or is mine just missing?

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Mine is a 2006 made 2.4 Brera and it has one.
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(Post Link) post #7 of 7 Old 25-02-17 Thread Starter
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Mine is 2007. I can confirm that 6 months on the 'new' switch is still working perfectly. No more battery problems and no more random warning messages ( the one about the steering lock was the most common)
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