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Author Topic: Where does speed signal come from?  (Read 3682 times)

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VR4Dave

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Where does speed signal come from?
« on: 28 October 2010, 17:20:03 »
Hi, got a weird problem. Thought I needed to replace my servotronic relay for power steering as sometimes i get full assistance and sometimes limited, I know it's not a pump problem.

However i noticed today that speed dependant volume is not working nor are any of the speed dependant calcs for economy, trip distance etc.

Speedo, abs and traction control all work fine, any ideas?

Dave

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Kevin Wood

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Re: Where does speed signal come from?
« Reply #1 on: 28 October 2010, 17:47:58 »
They should all be on the same signal from the ABS/TC ECU. I wonder if there's a break in the connection somewhere?

Kevin
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TheBoy

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Re: Where does speed signal come from?
« Reply #2 on: 28 October 2010, 18:53:53 »
does the speedo die at any point?
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VR4Dave

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Re: Where does speed signal come from?
« Reply #3 on: 28 October 2010, 19:15:05 »
does the speedo die at any point?

No, speedo is fine even during the low steering assistance episodes.

ECU was replaced in Jan this year by previous owner (I have the bill for £1150 ouch!)
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TheBoy

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Re: Where does speed signal come from?
« Reply #4 on: 28 October 2010, 20:00:35 »
Meter time then!
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VR4Dave

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Re: Where does speed signal come from?
« Reply #5 on: 28 October 2010, 20:43:22 »
Haven't got a clue where to start with this, I'm ok with spannering but not good with car electrics.

Where is the abs ecu? I'll at least check everything's physically solid.
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Kevin Wood

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Re: Where does speed signal come from?
« Reply #6 on: 29 October 2010, 09:36:58 »
Quote
Haven't got a clue where to start with this, I'm ok with spannering but not good with car electrics.

Where is the abs ecu? I'll at least check everything's physically solid.

If the signal's reliably getting to the dash, I'd say the ABS ECU end is fine and the problem is under the dash, as that's where the servotronic box is.

Kevin
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Kevin Wood

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Re: Where does speed signal come from?
« Reply #7 on: 29 October 2010, 09:38:06 »
On another tack, do you have the original stereo fitted, or aftermarket? Just wonder if the stereo is loading the signal.

Kevin
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Dave DND

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Re: Where does speed signal come from?
« Reply #8 on: 29 October 2010, 09:56:50 »
If prodding around for a speed pulse signal, an old fasioned analogue meter is often easier to use than a digital one for this purpose, as the pulse can be so brief that a digital meter may not react in time.

Set to 12V, and push the car forwards or backwards a few feet whilst monitoring the speed pulse wire, and you will see the analogue meter swing quite clearly.

Each model of car is different, but "generally" speaking a speed pulse signal will be between 5V and 8V and will pulse every couple of meters of distance travelled.
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Kevin Wood

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Re: Where does speed signal come from?
« Reply #9 on: 29 October 2010, 10:18:34 »
Agreed.

The signal wire is Blue with a Red tracer on an Omega, if I'm not mistaken.

Most systems that receive the signal can report it in their live data if you can get it to a tech 2. Still doesn't tell you where the break is, though, because there's no telling how the wire is routed.

Continuity check between the systems is probably the best way.

Kevin
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Kevin Wood

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Re: Where does speed signal come from?
« Reply #10 on: 29 October 2010, 10:27:59 »
Signal is routed to the following systems, as far as I can see (if fitted):

Source: ABS ECU

Engine ECU
Instrument Panel
Climate Control
MID/TID/CID/GID
Radio / Nav system
Servotronic ECU
Headlamp Levelling
Suspension self-levelling
Sunroof

Wiring to the headlamp levelling ECU (in the driver's side front wheel arch) does sometimes chafe through. Might be worth a look.

Kevin
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Andy B

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Re: Where does speed signal come from?
« Reply #11 on: 29 October 2010, 12:30:34 »
Quote
....

The signal wire is Blue with a Red tracer on an Omega, if I'm not mistaken. .......

It definitely was in a Senator  :y
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TheBoy

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Re: Where does speed signal come from?
« Reply #12 on: 29 October 2010, 17:40:37 »
Its around 25,000 pulses per km, so 25 pulses per m (about 2/3rds of a wheel revolution) so a digital meter will defo struggle as Dave DND says.


Kevin Wood - I have seen a failed ABS, yet speedo worked (nothing else did - MID, cruise, SDVC on radio etc). Can't explain that. Didn't make sense to me at the time, and I promise I hadn't been drinking ;D
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VR4Dave

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Re: Where does speed signal come from?
« Reply #13 on: 30 October 2010, 08:25:59 »
Thanks for all the info so far. Speedo is original, cruise control works fine, satnav a bit hit and miss - it knows where I am to start a journey and calculates a route but then doesn't recognise that I've started moving.

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Kevin Wood

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Re: Where does speed signal come from?
« Reply #14 on: 30 October 2010, 11:46:55 »
Quote
Its around 25,000 pulses per km, so 25 pulses per m (about 2/3rds of a wheel revolution) so a digital meter will defo struggle as Dave DND says.


Kevin Wood - I have seen a failed ABS, yet speedo worked (nothing else did - MID, cruise, SDVC on radio etc). Can't explain that. Didn't make sense to me at the time, and I promise I hadn't been drinking ;D

I'm guessing the different devices have different thresholds as to where they see a transition of the signal, so if the signal were becoming weak due to a failing connection in the ABS ECU I can see why it might affect some items before others.

Would be good to get a scope on it perhaps? :-/

(he said, forgetting that "normal" people don't have 7 of them in their loft).

Kevin
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