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XC90 Alarm Siren Missing

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Old May 14th, 2020, 18:25   #1
Father Ted
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Default XC90 Alarm Siren Missing

I'm going to throw this to all to see what comes back. I think I know what I'm going to need to do, but want to rule out all possibilities first.
The symptoms:
Random false alarms from alarm system. Using the reduced guard button had no effect ( I first presumed that it would be movement sensors).
I put my laptop on it today and the error message ( well, one of them! ) is Alarm Siren Missing.
So, I thought, OH no! Battery!. But I remembered that the first thing I did when I got the car 3 years ago was to replace the battery in the siren module.
Still, I went ahead, removed and opened up the siren and the new battery is in fine (physical) condition.
There is some marking on the tracks from when the previous battery failed, but these have not deteriorated further and look to be electrically ok.

Before I write off the siren, are there any other known issues which could cause this issue?
I seem to be getting a lot of issues one after the other at the moment....

Anybody selling a siren?
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Old May 14th, 2020, 18:53   #2
Father Ted
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Another thing which springs to mind: Does the alarm siren form part of the CAN Bus?
There are three wires to the siren. Without checking, I'm presuming Gnd, Trigger +ve and something else, charge maybe?
If it is just voltage and not data, then I could replicate this easily. But, if it was this easy, somebody else would have done it, so what am I missing?

Does anyone have an electrical schematic for the alarm wiring?

Think I'll have to remove it again tomorrow and start taking some measurements.
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Old May 14th, 2020, 20:59   #3
Father Ted
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Well, Got my VIDA back on it and have answered one of my questions (still learning how to use VIDA). The connections seem to be Gnd. +ve and Serial data. Why data???????? Really! Just switch voltage.

Anyhow, that kills my chances of coming up with an alternative. It does, though, give me pointers for testing. At least I know which wire goes where....
the data goes to pin 21 on UEM ( inside mirror: who the hell designs these things?!?!??!?!?!?? )

Hoping for a dodgy connection.....not expecting it though, if I'm honest. Still, in fitting the new radio, I had the covers off the A posts, so maybe I disturbed something.
Fingers crossed!
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Old May 15th, 2020, 23:25   #4
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OK. Well, to keep this thread ticking along for other people who might come looking:
I read the error code and I had UEM-0004, Siren communication fault.
I removed the siren: I won't go into detail here as there are plenty of threads showing how to do this.
Then I removed the interior mirror which contains the UEM.I tested with a multi meter from pin 21 (Bottom row, 2nd from right as you look at plug) to middle pin on siren. This was good.
So then I checked Pin 1 on siren to ground and Pin 3 to +ve. All wires were good.
I kind of suspected this would be the case, but I hoped.....

Anyway, this pointed at the fault being the siren module itself.
So I cut the case open and found that there was some corrosion on the board. The new battery hadn't failed, but the residual "juice" from the original battery had continued to eat the board since I replaced the battery (2 years ago). My fault, I should have cleaned the board!
Attached is a photo of the board before I started work on it. It doesn't look too bad. In fact, I was concerned that it was a chip failure.
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File Type: jpg Original PCB.jpg (345.9 KB, 36 views)
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Old May 15th, 2020, 23:35   #5
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SO, I got to work.
First thing I did was to remove the battery connector. I was no longer using this as I'd soldered my replacement battery directly to the rear of the PCB.
I could then see more verdigris and signs of corrosion.
I used neat white vinegar and a small paint brush to clean the area and let the vinegar soak into it for a while then wiped it off and repeated a few times, being very careful not to disturb the components. I then used water to flush the vinegar away. I then made sure everything was thoroughly dry.
Once the board was cleaned up more I could see a lot more damage. One of the tracks which had been hidden beneath the battery connector was totally eaten through.
I Googled for images of the board to try to work out where the track went to.
Credit to Cybertronix on Facebook as they have images of an intact board and bare PCBs which I was able to zoom into to find what I was looking for. I've used two of their images below, so here is a link to his page (credit where credit is due!). https://www.facebook.com/Cybertronix...7742197669587/

The missing track went the long way around to just cross over to the chip, so I soldered a link in directly on to the chip. This was very delicate as everything is so small. I couldn't solder a link in the normal manner, so I soldered a leg of a resistor to the peg on the chip, I then soldered the other leg from my sacrificial resistor to a component on the good part of the track. I then soldered the two legs together and trimmed. I had to do it this way as bending the link over would have pulled it off the board as the solder points are so small.

The pictures below show, in the following order, the extent of the damage once board was cleaned up. The repair (from a couple of different angles) and the reference images from the Cybertronix page which I used as a map to trace the missing track.
Attached Images
File Type: jpg First Stage Repair.jpg (377.5 KB, 36 views)
File Type: jpg Repair Highlight.jpg (388.8 KB, 36 views)
File Type: jpg Repair 2.jpg (204.4 KB, 34 views)
File Type: jpg Bare PCBs.jpg (329.9 KB, 36 views)
File Type: jpg Good PCB.jpg (248.0 KB, 34 views)
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Last edited by Father Ted; May 15th, 2020 at 23:39.
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Old May 15th, 2020, 23:41   #6
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I'll know better in a few days' time, but I refitted the board in a temporary location and without the piezo sounder and armed the system. It has been armed for 4 hours' now and has not activated ( I checked the LED flash rate ).....so it's looking positive so far.
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Old May 16th, 2020, 08:20   #7
oilit
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wow that's dedication for you !! well done
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Old May 19th, 2020, 22:36   #8
Father Ted
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Quote:
Originally Posted by Father Ted View Post
I'll know better in a few days' time, but I refitted the board in a temporary location and without the piezo sounder and armed the system. It has been armed for 4 hours' now and has not activated ( I checked the LED flash rate ).....so it's looking positive so far.
As a final note.
The siren was fully re assembled and fitted to the car on Sunday. It has been armed continuously since as I haven't used the car. No more false alarms, so it appears that my repair has worked.
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