Extreme Speedo Rescue

From CEM to VEM, from LED to lightbulb and more. If you have an electrical problem, like a broken info-centre, search for answers in this category. This is also the place to be when you expect the problem to be of an electrical nature...

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Re: Extreme Speedo Rescue

Post by Robou » Fri Dec 13, 2019 10:21 pm

When shielding is used for equalizing potential it is not shielding anymore. Shielding should be connected only at the input ground preferably.
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Re: Extreme Speedo Rescue

Post by brinkie » Mon Dec 16, 2019 9:04 am

Jaster wrote:
Mon Dec 09, 2019 10:16 pm
Correct, as far as I know the later cars have a two-pin connector. Not exactly sure when the change occurred but I'd imagine around 1990, which I think is when the wiring issues were overhauled. The way this one is wired looks like they planned to do something with the shielding all the way down to the sensor when they designed it, and then decided not to when they came to build it and just grounded it at the connector instead.
I believe model year 1989 saw the first big change (almost as big as halfway model year 1991 when phase 2 with CEM-III was introduced), the instrument cluster and the wiring were heavily modified. Feel free to ask any question regarding wiring, I have all documents at hand. :)

There can be many reasons to ground a shielding only at one end. For the signal in the instrument cluster, we are looking at a 125 kHz oscillator which is forming a pulse train between 0 and 400 Hz because the oscillator interrupts when a tooth of the gearbox passes the sensor. Inside the speedometer, there is a low pass filter that filters out everything above approx. 450 Hz, equivalent to 240 km/h or 150 mph, speeds above the redline in fifth gear. :lol: I can't remember exactly why (when I was following classes in the theory of alternating currents, they were still building the Volvo 480), but magnetic interference will be cancelled out if the shielding is grounded on one end, the cable is too short to pick up low-frequency radio interference (the low pass filter will eliminate high frequencies anyway) and it will prevent currents through the shielding caused by poor earth connections on either end.

Present cars: 1994 Volvo 480 GT 2.0i Vase Green, 2010 Volvo V70 2.0F Momentum Mystic Silver
Past: 1981 Talbot-Matra Murena 2.2 Rouge Mephisto, 1995 Volvo 480 GT 2.0i Paris Blue

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