Speedo sensor - replace?

[andy_harries]

Hi all

Been searching through the archive as the needle on my speedo can be quite erratic across the whole of the speed range  - can't see any pattern, except on a recent track day it gave up the ghost above around 90mph running down a straight.

I've found the sensor, checked the gap and had a poke around with the wiring (issues that seem to have been the case according to previous posts).

The sensor is very well held in place, the gap seems to be 1mm and the two nuts holding it are very tight so I don't think that it has moved at all.  The wiring seems to be in good condition as well.

What should I look at next?  Making the gap smalle than 1mm (other posts have suggested 0.8mm have fixed issues before).  Change the speedo sensor itself?  Or look at a potential issue with the speedo itself?  The rev counter is rock solid.

Pic of the sensor on the rear drivers wheel hub (car is a 2016 Roadsport):

[John Vine]

The first thing to do is to check that the LED at the end of the sensor (where the cable enters) illuminates for every tooth of the toothed reluctor ring.  Raise the rear of the car onto axle stands, turn on the ignition and turn the O/S wheel slowly.  As each tooth of the reluctor ring passes the sensor, the LED should light up.  Check for a full 360 deg. If the LED fails to light up for any tooth, reduce the air gap slightly and repeat until all teeth light up the sensor.

Next, if all that works OK, apply the earthing mod.  This involves making a new sensor earth much closer to the sensor.  CC Workshop Notice WN572 explains what to do.  If you need a copy of this, send me a private mail with your email address.

These two steps together fix most speedo problems.

JV

 

[Brian27]

Mine failed just the same when the car was about 2 years old. Mine was worse in hot weather. I did the earth mod as John suggested, but had to order a new sensor from Caterham which fixed it.

[John Vine]

Andy, YHM.

JV

[gulf 1]

See post below re wheel sensor. 

[andy_harries]

Thanks to all that commented. 

A 360 manual spin of the wheel showed that the sensor wasn't picking up consistently, so I backed it off to 1.5mm gap which I think has done the trick.  A little drive around shows that its rock solid now, albeit reading consistently 6 mph below the reading from a GPS speedo app on my phone (measured from 10mph to around 65mph).

The earth mod is next on my list, just to avoid any issues in the future.

 

 

[John Vine]

Good news, Andy!

The earth mod is really only needed if you're suffering from other symptoms such as a flickering needle whenever you brake or use the indicators.  In fact, a recent car such as yours may already have had the mod applied at the factory.  Check the earth wire (black) running from the sensor.  If it disappears into the loom, the mod won't have been applied, but if it terminates at a point on the chassis behind the rear seat panel, it's already been done.

It's very straightforward to calibrate your speedo to make it much more accurate.  If you'd like to embark on this, just say, and I'll explain what to do.

JV

[andy_harries]

Using the diagram in the notice, the mod definitely hasn't been done yet as the earth wire does go off into the loom.

I'm going to take this small victory and not push my luck, besides the brakes need bleeding and my tube is too small!

[Gibbbo]

My 2005 sigma engined road sport has the same problems with the speedometer  needle moving erratically on occasions, advice on the problem would be great and also I would like to know how to calibrate the speedometer 

regards Gibbo 

[Jonathan Kay]

"My 2005 sigma engined road sport has the same problems with the speedometer  needle moving erratically on occasions, advice on the problem would be great and also I would like to know how to calibrate the speedometer"

I suggest starting with the Speedometer Guide. That covers adjusting the sensor gap and the calibration. 

But we still have the experts in case they're needed...  : - )

Jonathan

[ScottR400D]

I've found that the best way to set the sensor is to start at 1mm then try a tenth or two below and above that, checking how it is each time. 

When I had my diff issues a couple of years ago I had  the same issues which I hadn't seen since building the car. I followed the above process and it's now fine up to an indicated 125mph or so. 

There is a newer sensor of course which is reputedly easier to set but I still have my original 2015 one.

That's assuming it's not the more recent issue of moving toothed rings.  

[Geoff Brown]

It works ! The gap is always a fluctuating measurement. I think that anything from 1mm to 1.5mm is fair game if the speedo needle becomes erratic or just fails at a high speed.

But one of the most important things is cleanliness. The head of the sensor should be free of road muck & the use of an old tooth brush on the reluctor ring crenellations once in a while goes a long way to keeping the system working well.

[Jonathan Kay]

"But one of the most important things is cleanliness. The head of the sensor should be free of road muck & the use of an old tooth brush on the reluctor ring crenellations once in a while goes a long way to keeping the system working well."

And don't foget the:

: - )

Jonathan

[Geoff Brown]

Actually the abrasive quality would clean the gaps out nicely & give out that nice clean fresh breath smell !!