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Alistair Riddell
Yet to post message
Username: ariddell

Post Number: 1
Registered: 1-2013
Posted on Tuesday, 15 January, 2013 - 09:55 am:   Edit PostDelete PostView Post/Check IP

Hi All,

New member here, just joined. I have a 1988 (89MY)ex-UK Turbo R that I recently purchased. It has an issue with the speedo and odometer both being non-functional, yet the cruise control is working fine.

This has been a historical issue on the vehicle looking at the extensive service history documentation it has been provided with, and has had both the speed sender replaced and the speedo unit itself sent for repair in the past when the issue was in the past intermittent.

There is a slight bounce in the speedo needle at motorway speeds, but not much more than this.

Now, reason for my post being having had a read through the info at rrtechnical and looking at the wiring diagrams I would be expecting that if the most common failure, ie the sender was again the issue that this would also stop the cruise control from working, correct?

As such since my cruise works fine I am suspecting that this is actually a wiring fault somewhere between the splice where the feed from the sender splits to the cruise control and speedo feeds, and the speedo earth in the A-pillar. Would this logic seem sound?

I am presuming that the odo is driven internally to the speedo rather than a seperate feed as i can't see anything of that type in the wiring diagrams, so if it is indeed a wiring fault stopping appropriate signal to the speedo then this would indeed cause both parts to become inop.

If this is the case i am hoping it should be reasonably straight forward to trace the wiring to identify the faulty section and replace as needed.

Any thoughts, opinions on my thinking and other suggestions appreciated.

I am entirely new to these vehicles and appreciate the sounding board before diving in.

Regards,

Alistair
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Neville Davies
Experienced User
Username: nev_davies

Post Number: 46
Registered: 9-2006
Posted on Tuesday, 15 January, 2013 - 06:25 pm:   Edit PostDelete PostView Post/Check IP

Alistair.Almost certainly the fault is a little plastic gear within the speedometer itself.There are two different ratios dependent on whether the speedo is calibrated in kilometers or miles per hour.
Nev
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Jan Forrest
Grand Master
Username: got_one

Post Number: 429
Registered: 1-2008
Posted on Tuesday, 15 January, 2013 - 11:12 pm:   Edit PostDelete PostView Post/Check IP

Isn't there an article about this somewhere in the Tee-One Topics? Doesn't it also include a source of a replacement part for it?
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Brian Vogel
Prolific User
Username: guyslp

Post Number: 195
Registered: 6-2009
Posted on Wednesday, 16 January, 2013 - 01:53 am:   Edit PostDelete PostView Post/Check IP

Tee-One Topics Issue 76 makes reference to the speed transducer (and the [then, anyway] lack of a suitable replacement) on page 1061.

Here's an opportunity for someone to do their own "functional equivalent" search if they've got one that's failed. I doubt very much that Crewe made this part or had it made for them. It's a matter of doing the tedious work of finding who else used this part and what the "generic" number is.

As to the dreaded "self-destructing plastic gear" I have a number of repair (possibly part, too) sources for speedometers listed in the RR & Bentley Parts, Repair, Restoration & Other Resources Compilation, but since I live in the U.S. virtually all of these are based here.

Brian, always looking to expand the "International" sections
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Omar M. Shams
Grand Master
Username: omar

Post Number: 313
Registered: 4-2009
Posted on Wednesday, 16 January, 2013 - 03:08 am:   Edit PostDelete PostView Post/Check IP

Dear Alistair,
This thread may be of interest to you.

http://au.rrforums.net/forum/messages/17/11416.html?1315466722

Maybe previous disassemblies of your speedo may have been less than professional and one of the wires has come off or something. Essentially there is a motor in the speedo that drives gears to drive the odometer, trip meter and dial. It is a very simple set up. Dismantle the speedo and look inside. I am sure something will leap at you upon disassembly.

All the best
Omar
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Paul Yorke
Grand Master
Username: paul_yorke

Post Number: 929
Registered: 6-2006
Posted on Wednesday, 16 January, 2013 - 05:51 am:   Edit PostDelete PostView Post/Check IP

Hi, got a feeling that on your year car the speedo gets it's signal from the ABS unit and speed control gets it's from the gearbox transducer on the tail of the box.

If the 'usual' gear has failed, the speedo works but has no odometer.

Check for a pulsed signal at the speedo, if you have one then it is probably internal.
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Alistair Riddell
New User
Username: ariddell

Post Number: 2
Registered: 1-2013
Posted on Wednesday, 16 January, 2013 - 11:40 am:   Edit PostDelete PostView Post/Check IP

Hi,
Thanks all for the responses, much appreciated. I had initially discounted the plastic gear as the cause myself on the expectation that were this the case then the speedo would function but not the odo as mentioned, the slight bounce I get on the speedo at motorway speeds also had suggested that some signal was getting through, just not properly.

How involved a job is it to remove the speedo from the dashboard? Any gotchas to watch out for?

Is there a wiring diagram available showing the ABS -> speedo wiring scenario?

Thanks,

Alistair
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Neville Davies
Experienced User
Username: nev_davies

Post Number: 47
Registered: 9-2006
Posted on Wednesday, 16 January, 2013 - 05:49 pm:   Edit PostDelete PostView Post/Check IP

Alistair. Sorry to mislead must get bigger glasses to read my phone.As Paul says gear will not affect the speed reading.
Nev
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Alistair Riddell
New User
Username: ariddell

Post Number: 3
Registered: 1-2013
Posted on Tuesday, 05 March, 2013 - 09:53 am:   Edit PostDelete PostView Post/Check IP

Well I finally got on to removing the speedo and having a look at resolving this issue. It appeared that I had a rather bizarre wiring modification behind the gauge.

Between the red/green pulse feed and the black earth there was a small electronic component, I'm not quite up on my electronics but this appeared to be some sort of diode or resistor (small brown blob on a length of exposed wire that had been wrapped in tape). This had been soldered in place bridging these two connections for some reason that I can't currently fathom.

Anyway, I decided nothing ventured, nothing gained and snipped this connection and low and behold the speedo returned to life, for just under 2km.

I've then gone to the next stage and taken a new pulse feed from just before the cruise control ecu as my cruise has always worked without issue and run it up to the speedo, along with running a fresh earth to the back of the unit too. This appears after a further 25km or so of driving to have resolved the issue however time will tell.

Would anyone be able to speculate what this modification was intended for? An attempt at some form of suppression would be my guess if the speedo has been intermittent in the past.

Thanks,

Alistair