Speedo Earth Modification

[malcolm1sim]

Am I being silly?

My 2009 R300 has covered some 35000 miles and for most of these miles the speedo has been rock steady. Last couple of thousand and the wobbly needle has struck. Only at higher speeds though.

Research on here has lead me to the earth modification.

My question is, as it has been working well for such a long time could it be that the existing earth is failing and just needs cleaning etc

If that is a possibility, I should like to try it first.

So second question is, where is that earth?

Thanks

[paul richards]

It can't do any harm to modify it. 

2009 car. I'm not sure when they moved from gearbox drive to driveshaft sender. 

[Geoff Brown]

Had to take my R400Ds back end to pieces at 13,000 miles for Diff maintenance so the speed sensor had to be removed along with all the other gubbins.

The sensor was carefully stored in a plastic bag taped to the rear bulkhead but still connected.

During the test drive post work the speedo dropped out at 60mph + & the correct gap of 1mm was checked twice. So I committed to the earth mod being the cheapest route & possibly the cure. But no !

Ordered & fitted new sensor. All OK.

One suspects that the sensor at £57+ is still a cheap piece of s**t so is prone to failure however much care is taken.

[BigCol]

I have never measured my sensor's gap...  with the rear jacked-up and the offside wheel off, turn the ignition on and rotate the hub...  if the gap's right, the end of the sensor will light...

That said, I do need to adjust it most years!  

[Jonathan Kay]

1. As Paul says: gearbox or wheel sensor?
2. If wheel do you have the Workshop Notice for the modification? That shows the connector and the earth entering the loom but not where it emerges.

Jonathan

[NeilCSR]

If it's a wheel sensor there are other possibly critical factors that influence the speedo.

I put a new sensor in but that made little difference.

Positioning, rather than gap is vital.  On mine the hole in the brake caliper meant that the sensor was only half-covering the serrated wheel.  Created a bracket out of sturdy stainless sheet that lined the sensor up correctly, and gapped it to 1mm.  

Also did the earth mod. All this helped and meant that the speedo still worked up to about 100 (instead of 60 or so).

Then took it all apart again, repositioned and gapped it, but also properly cleaned the serrated wheel, brushed off all corrosion and coated the tops with a bit of POR15.

Still get problems, but well into three figures.

Turning the wheel to see if the sensor lights is fine to see if it works at all, but it's the capacity to operate at speed (under load) which is the issue.

As Geoff says 'it's a cheap pice of s**t' and I'd guess that some work OK, others work for a while, and some hardly at all.  Buy ten, maybe three or four will work forever, the rest will start to show signs of failure at some point.

Neil

[ChrisC]

I don't blame the sensor, but it's location.  Why is it bounced around near the rear wheel, when it could have as easily been on the over end of the drive shaft near the diff, not exposed to as many elements and wire fatigue.   

[East Kent AR]

Hi Malcolm, for it to have lasted that long is a massive achievement! My car is 5 years old and is on it's 6th sensor - although not cheap to buy from CC (£67 incl p&p for the last one) it is clearly a cheap part that is not in the ideal position given its fragility.

The guys above have covered the adjustment piece - i can send you the workshop notice from CC if you need it?

Best, Garry

[malcolm1sim]

Thanks all.

Defo wheel sensor and the light comes on for each tooth.
​Looks like I have some playing to do over the winter. I also
have some concerns over the connection to the speedo unit
​itself as touching that sometimes causes wobble even at quite
​low speeds. Might try a small cardboard insert to tighten it
​up a bit.

I do not have a copy of CC workshop notice and I think it may
​be useful, JK and Gary. JK first perhaps as he posted first. Do 
​you need my personal email for that?

Anyway many thanks to both.

Cheers for now.

Malcolm
 

[John Vine]

I do not have a copy of CC workshop notice and I think it may
​be useful, JK and Gary. JK first perhaps as he posted first. Do 
​you need my personal email for that?

If you're still looking for this WN (#572), send me a PM with your email address.

The purpose of the WN is to establish an earth much closer to the sensor, rather than rely on the existing earth wire (which disappears into the loom and emerges who knows where?).  The idea is to reduce the interference picked up by the standard earth wire, which tends to confuse the signal emitted by the sensor.

JV

[malcolm1sim]

Attn JV,

Email sent to your private email address. It was
already in my system.

Thanks again.

Malcolm

[dave_h34]

Could I have a copy of the workshop notice please?

[John Vine]

Malcolm, Dave - WN572 on its way.

JV

[AndrewB]

Reading the tech page for the sensor it is described as being robust and suitable for fast response.  You can buy the sensor from Farnell although you will have to put an econoseal connector on it which you can get from polevolt.   

http://uk.farnell.com/schneider-electric/xs108b3nal2/inductive-sensor-2-5mm-12-24vdc/dp/2455529

[John Vine]

That's a useful link - thanks.

So, it's not a Hall Effect sensor after all, but generates its own magnetic field.

JV

[NeilCSR]

Hi Andrew,

Thanks for pointing this out.  One question - NPN or PNP?  I'm ignorant about the difference between the two, other than there will be a difference and only one sort will work.  The link goes to NPN so I'm assuming that's the one?

Having the sensor with naked leads will allow a protective sheath to be added before wiring up the connector as the wire is very exposed in quite a 'wild' area.  

Neil

[AndrewB]

Hi Neil,

The weblink is the same part number as the one that is on my car at the moment.    When I installed mine I split some washer tube and wrapped it over the sensor cable from teh sensor to the connector.  A few tie wraps hold the washer tube on and onto the car.  I can't recall who gave me that gem of advice.  

Andrew

[tomwood]

Hi John,

sorry to be a pain but could I also trouble you for the WN please? My speedo has also died completely recently and with MOT due at end of the month I will be trying various things to try and bring it back to life!

Thanks also to Andrew. £35 is a little more palatable than £60!

Thanks,

Tom

[John Vine]

Tom, can you send me a message with your email address?

JV

[NeilCSR]

Many thanks, that's really helpful.

Neil

[Rogercsr7]

This is a fortuitous thread as I travel through France to Portofino with a defunct speedo (at 80 read 30). Been fine for years until disembarked the St Malo ferry, now depending on Satnav to avoid speeding fines. About to have a reccy in the hotel car park, having read this (and other ) threads, think I'm opening a can of worms.

Would some kind sole send me the earth mod WN please, can see me getting stuck in on return to Blighty.

Thanks, Roger

[John Vine]

Roger - please send me a private message with your email address, and I'll forward the WN (plus some tips etc about debugging speedo faults, if you'd like them).

JV

[Rogercsr7]

Thanks John, will do a little later. In beautiful Rocamadour, unfortunately both our emails are playing up for some reason. Hopefully sorted later

Roger

[makingff]

There have been issues whereby the speed sensor causes a fuse to blow which disables the car. The only solution being to disconnect the sensor before replacing the fuse (can't remember which fuse but when it happened on a new 420 just outside the Eurotunnel on the French side, it was definitely the last one we pulled out!).

Having made a bracket to reposition my sensor, I always intended to put a fuse in line at the sensor end so that, if the sensor broke, it wouldn't disable the car. Never got round to doing it though...

[East Kent AR]

Its fuse 4 on my car (2011 Duratec). Roadside fix is disconnect sensor (or simply cut and isolate the cables to it as the thing is toast anyway once its blown the fuse and killed the car!), replace fuse and use satnav as speedo!

Having done this a few times now I can get going again in about 15 mins