Security alarm malfunctions on a Daihatsu Mila. The root cause of it. L275S ダイハツ・ミラ セキュリティー誤動作とその原因

日記 #練習用
L275S ミラの盗難警報が誤動作してお困りの方が多く、近くのショップ
でも、それでお困りでした。店主様からSOSをもらい、その根本原因を
調べたので、日本語でサマリーを先に書いておきます。アメリカの友達たち
向けに私が書いているブログは、英語で書く必用があり、そうしていますが、
ミラは国内専用車種なので、日本語で書かないといけないと指摘されて
しまいました。

問題の根源は、リアゲートの検出スイッチ周りに水分が入り込むと、
電流リークが起きることでした。

何故リークするかは、接点のベリリウム銅と亜鉛メッキのスイッチフレーム
の間でリークが起きるからですが、異種金属の組み合わせによる
ガルバニック反応で電池ができ、800mVもの電圧が発生することが、誤動作
を助長します。スイッチに付けられたクッション性のゴムは水分を吸着し易く
これが２つの異種金属の間で電解質として働き、電池を形成するということ
までをダイハツのエンジニアは考えられなかったのが、市場不良多発に
つながったとも言えます。

The owner of the car below was upset due to the fact that the problem
was happened at a middle of night. It happened all of the sudden when
no one was there.

The problem was the malfunction of the security system and it
was not duplicated after that for months. But it happened again just
once recently and it was impossible to duplicate the malfunction.

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I was requested to help by a local car shop owner to solve the problem.
He has been trying to duplicate the problem and find the reason why
the malfunction was caused. He has needed to fix it finding the root
cause of it since the complain from his customer was very serious.
But his efforts so far have been not successful. Whatever he tried,
the result was just a vain. The occurrence was very seldom and could
not be duplicated. Then he decided to call me.

As he and I checked the Internet, there were so many posts about
the problem but we couldn't find any definitive answers or solutions.
You can see how there are so many malfunction problems in the fields.
Some owners were quite angry with the problem. Judging from the
announcement by the manufacturer listed at the bottom with *,
they too are struggling. Every post below is in Japanese since the
car is only sold in Japan. Use the Google Translation if you need to
understand the contents.

ダイハツミラ盗難警報装置誤作動とディーラー対応
http://spiritualworld.cocolog-nifty.com/blog/2013/01/2013110-ff10.html

車の防犯ブザーが鳴るのを解除したいのですが、....
http://gizport.jp/gp/question/1792270/?id=20045823

L275S ミラのセキュリティアラーム
https://oshiete.goo.ne.jp/qa/9781751.html

L275Sミラカスタムのセキュリティー誤作動。
https://minkara.carview.co.jp/userid/1033221/blog/41039837/

純正セキュリティー　誤作動　L275S　ミラ
https://blogs.yahoo.co.jp/nikocyanndai/56325201.html

ダイハツのミラですが、先程いきなりセキュリティアラ...
https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1158084331

大至急!ダイハツのミラに乗っています。数日前から...
https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q11185197756

家の中にいたら、駐車場に置いてある車のカーアラー...
https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q10177687820

* キーフリーシステム装着車をご使用のお客様へ
https://www.daihatsu.co.jp/service/carlife/carlife_1.htm

As I read above and hear from the shop owner, the malfunction tends to
happens when it's badly raining of after that. The rain water may be related
and doing something, I imagined.

There are 5 switches for the detection. Each door/gate has it's own switch.
One or more of them may be malfunctioning or the loom to those may be
leaking or the MPX body computer may be malfunctioning.

As I checked the internal circuits of the MPX body computer where the
security circuit was built in, I found that the sensing line B5 is pulled up
to 5.0V DC line via a diode with a 680 ohms series resistor. This pin is
the input pin for 5 switches. All those 5 are paralleled and connected
to here. I must say the line is rather high impedanced. High here means
that the current draw at the line is sensitive. A slight leakage may cause
the problem. Actually pouring small amount of water to the rear gate
switch causes the malfunction and  the problem is duplicated easily.

The switch is covered with a cushion rubber as is shown in the photos
below. It absorbs the water and the insulation between the switch
contact plate and the body became poor. In addition to that, the
material used for the contact is the beryllium copper and this makes
a battery with the base material zinc plated steel when the cushion
rubber is wet. As I check the voltage, it shows around 800mV.

Come to think of it, the beryllium copper's galvanic voltage is around
-0.2V and the zinc plated steel's -1.0V. It is natural these 2 metals
make the voltage difference of 800mV. The voltage affects the
sensitivity too. You can find galvanic voltages here.
http://l-36.com/corrosion.php

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After the black cushion rubber is removed.
イメージ 3
The black one absorbs water very well and this makes a current bridge
between the contact and the zinc plated steel frame.

I don't say this is a design fault but it is not a good idea to use a
cushion rubber just above the switch for the rear gate.
The designer's intention must be to protect the switch contacts
against dusts and other foreign objects falling from above but
he should have to think about the side effects such as a moisture.
It sure causes the current leaking problem between the contact
and the body easily when the rubber is wet. In addition to that,
there makes a 800mV battery due to the Galvanic series and
this accelerates the problem.  I was easily duplicated problem
just poring water there. If I were him, I wouldn't design that way.
I'd use a moisture free switch such as a lead switch there to
prevent the malfunction of the security system especially when
rained or the humidity is high enough to have water condensations.
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