Erratic Speedometer

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lraxv
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Erratic Speedometer

Post by lraxv » 08 Apr 2018, 13:04

Hello,

I've done a quick search through the forum and I can't find anything specific on this, so any help would be very much appreciated.

My '06 Dispatch 2.0HDI has developed a fault that affects the speedometer and, seemingly, the ECU.

Within the forst few minutes of driving, the speedometer either starts reading lower than it should or, usually, drops to zero and stays there, although sometimes it comes back. When the speedo is at zero, though, the engine obviously develops some sot of fuelling problem, as it starts surging at low vehicle speed.

My thoughts are that this is a fault with the vehicle speed sensor in the gearbox - I had this changed by a local garage, but the problem is unchanged. The garage have suggested this is an internal gearbox fault which, given the cost of removing and stripping the gearbox to investigate (or even the work involved in fitting a replacement gearbox) writes the van off...

I read on a different forum that vehicle speed is also read from the ABS sensors - this seemed unlikely to me but, when I investigated, my van has ABS sensors and reluctor rings, even though I'm pretty sure it doesn't have ABS. I'm in the process of removing the sensors to clean them and the reluctor rings in case this makes any difference - I don't think it will.

Can anybody offer any advice? Failure in the gearbox speed sensor drive just doesn't seem very likely to me - a wiring fault seems far more likely to be the problem, but I can't find a wiring diagram for the van that shows the speed sensor and the ECU together. If anybody can point me to one, I'd be very grateful.

Thanks for any help :)

lraxv
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Re: Erratic Speedometer

Post by lraxv » 09 Apr 2018, 07:34

Hello,

yesterday, then, I figured out which wire is which of the three that go to the speed sensor on the gearbox, connected an oscilloscope to the signal feed and went for a drive - when the speedo cut out, the signal went to 0V, so the problem's definitely there somewhere.

As the signal went from working fine to completely dead in an instant, I suspect the 12V feed to the sensor is being cut off or there's an intermittent ground-fault. With no wiring diagrams, I can't tell where the sensor power comes from, but I'm going to try replacing it with a switched 12V feed and ground from somewhere else and see what effect that has.

If anybody has a wiring diagram for a 2005/6 2.0HDI Dispatch, I'd be very grateful if they could let me have a copy.

Thanks.

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Re: Erratic Speedometer

Post by Hell Razor5543 » 09 Apr 2018, 09:36

If there are any other electrical issues it could well be worth checking the engine/gearbox - bodyshell earth straps. Michel recently had a problem with his C4GP in that it would not start (and this was not long after a long drive from Berkshire to Cambridgeshire and back). It turned out that, after the clutch had been recently changed (prior to his ownership) the earth strap had been incorrectly connected. It took the AA man less than 1/2 hour to resolve.

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Re: Erratic Speedometer

Post by GiveMeABreak » 09 Apr 2018, 11:34

lraxv wrote:
09 Apr 2018, 07:34
If anybody has a wiring diagram for a 2005/6 2.0HDI Dispatch, I'd be very grateful if they could let me have a copy.
Send me a PM with your VIN and I might be able to get the relevant section for you.

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Re: Erratic Speedometer

Post by lraxv » 09 Apr 2018, 17:43

Hell Razor5543 wrote:
09 Apr 2018, 09:36
If there are any other electrical issues it could well be worth checking the engine/gearbox - bodyshell earth straps. Michel recently had a problem with his C4GP in that it would not start (and this was not long after a long drive from Berkshire to Cambridgeshire and back). It turned out that, after the clutch had been recently changed (prior to his ownership) the earth strap had been incorrectly connected. It took the AA man less than 1/2 hour to resolve.


Thanks for that - I reckon it's probably an Earth problem...car faults so often are!

The speed sensor has its own earth feed, as it's plastic and effectively isolated from the gearbox, but I reckon there must be a fault in the loom somewhere. Given the choice between stripping the loom to find the fault and just bypassing the connection with an alternative earth, I'll go for the easy option :)

Thanks for your help :)

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Re: Erratic Speedometer

Post by GiveMeABreak » 09 Apr 2018, 19:31

Right so these diagrams are split in 2 so you can follow the second diagram on from the first one.

Click the Diagrams to Zoom
Dispatch Speed Sensor A.PNG
Dispatch Speed Sensor B.PNG
CodeDescription
1620is the Speed Sensor
1320is the Engine ECU
BH28 is the Passenger Compartment 28-fuse box
4000is the Instrument Panel Central Electronic Module
4630is the Vehicle Speed Indicator
0004is the Instrument Panel
Bear in mind the diagram is intended to show options for the different engine variants - so in your case, it will be the lower left one identified as DW10 - however, they simply interchange with the wires actually shown on the 'EW10' engine next to it

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Re: Erratic Speedometer

Post by lraxv » 09 Apr 2018, 20:52

Hi Marc,

thanks very much for that.

I've connected a new earth to the sensor, so I'll see what difference that makes when I drive it tomorrow.

I see the 12V supply from the passenger-side fuse-box feeds the sensor and the instrument cluster - do you know if port 20 is the power to component 4000? If 4000 lost power, presumably I'd notice something on the dashboard, like the odometer going blank or something? If that's the case, I think I can fairly well narrow it down to a bad earth.

I'll let you know if I've fixed it tomorrow :)

Thanks :)

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Re: Erratic Speedometer

Post by GiveMeABreak » 09 Apr 2018, 21:08

The diagram just shows the connection into the instrument display for the speed sensor only - there are lots of other components going into the instrument cluster, so this probably only shows the relevant connection.

Earths are always a good place to start though 👍

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Re: Erratic Speedometer

Post by lraxv » 10 Apr 2018, 17:38

Hello,

unfortunately, changing the earth hasn't fixed it :(

I also changed the supply to the sensor for a switched 12V from a redundant fuse in the engine-bay fuse-box (according to my handbook), but that didn't help, either.

The only faults I can think of now are either the signal line being grounded somewhere intermittently (although it can't be a dead short, as I fused the new supply to the sensor and it hasn't blown that - it's only a 5A fuse), or the drive to the sensor inside the gearbox really has failed in some sort of intermittent way.

The wiring diagrams Marc has kindly supplied show the speed sensor connecting to the ECU at terminal K2 - can anybody shed some light on where abouts on the ECU that is, please? I think my next attempt will be fitting a new line from the sensor to the ECU, although I'll need to figure out where the spur for the speedometer comes off to make sure it's still connected...or just put a new connection in to that as well...which would be a pain, to be honest :(

I still find it hard to believe that the gearbox has failed (without some sort of outward indication, like horrible grinding noises or vibration etc), but I don't suppose you, Marc, or anybody else has access to a diagram showing how the speed sensor is driven inside the gearbox? Not that I'm going to go opening it up to try to fix it, but I believe there's an intermediate gear that comes out of the gearbox with the sensor when you remove it and I wonder if that has worn out or come loose or something...

Thanks for your help :)

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Re: Erratic Speedometer

Post by GiveMeABreak » 10 Apr 2018, 20:01

Here's the Tachometer control on the gearbox pinion for your engine:
Dispatch Gearbox Tach.PNG

lraxv
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Re: Erratic Speedometer

Post by lraxv » 10 Apr 2018, 20:21

Hi Marc,

thanks for that - I suppose it depends how gear 3 is driven, but I can't see from the diagram how the sensor drive could just stop...hopefully it's still an electrical fault.

Thanks for your help :)

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Re: Erratic Speedometer

Post by GiveMeABreak » 10 Apr 2018, 20:29

Some of these issues can be down to the instrument cluster - if you are getting correct mileage increases, I doubt it will be the sensor, but the dash assembly possibly.

There's a place here that does repairs on these - might be worth giving them a call to see of the issue is something they recognise.
http://www.clusterrepairsuk.co.uk/repai ... speedo-mk1

If you are not getting the correct mileage increment, then it is likely the sensor - here's a video on a 206 - but will be similar no doubt:

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Re: Erratic Speedometer

Post by white exec » 10 Apr 2018, 20:55

Speed/distance sensors are known to fail, and it would be worth removing the sensor to make sure it's being driven, and capable of output when spun. Odometer behaviour is certainly the clue. Cruise control (if fitted) also depends on this sensor for correct opetation.

lraxv
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Re: Erratic Speedometer

Post by lraxv » 11 Apr 2018, 10:54

Hello chaps,

thanks very much for our input on this.

I'm fairly certain it's either the wiring to the sensor or the mechanical drive itself - my reasoning is as follows:

> When the speedometer drops to zero, the engine seems to experience fueling problems, so I'm assuming the ECU is also not receiving a feed from the sensor.

> When the speedo is at zero, the odometer also stops incrementing, so it seems the signal to both has failed, rather than this being a fault with the speedometer.

> When the speedo is at zero, when I turn off the ignition, the needle drops slightly, indicating to me that the speedo is powered and is registering zero, rather than however it's actuated has failed and the needle is resting at its stop.

> I've driven around with a oscilloscope attached to the signal feed from the sensor; when the speedo drops to zero and the engine goes into lumpy running, the signal drops to 0V; I've measured a square-wave output of around 100Hz at about 50mph if I recall correctly.

> Although I didn't fit it, I recently had a new speed sensor fitted to the gearbox by a local garage; I'm getting the same fault, so I'm happy it's not the sensor.



I suppose it could be that the dash assembly is grounding the signal line, which is, in turn, grounding the signal to the ECU, but that seems a bit unlikely...

Do you have a diagram showing which port is which on the ECU that you could let me have? If I connected the sensor directly to pin K2 on the ECU, that would discount the signal line and the speedo as being the fault.

Also, I've been wondering about the state of the gearbox oil; the old sensor had pretty black-looking stuff inside it - I've got an old outboard motor that relies on the conductivity of the gearbox oil to activate the forward/reverse gear - does the quality of the oil in this gearbox affect the speed sensor at all?


Thanks for all your help with this- I think I might be flogging a dead horse, but I appreciate the info you're giving me :)

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Re: Erratic Speedometer

Post by GiveMeABreak » 11 Apr 2018, 11:46

Ok, so here's the wiring for the Engine ECU, showing K2 connection (wire 6739) from the top of the ECU, located in the 48 pin brown plug.

Diagram in 2 parts again with overlap:
Dispatch Engine ECU A.PNG
Dispatch Engine ECU B.PNG