Speedo not working - transducer ?

[taylormj4]

All,
Garage called to say they have finished the respray of my car....hooray ! but the speedo is no longer working.
It is suspected that the speedo transducer is at fault.
They don't have one and need the car out of the garage.

Is this a difficult job to DIY on ramps due to access. I've changed fuel pipes and the filter in the diff area and know access can be tricky up there in the chassis.

[ukdj]

it is possible, but as you say access isn't the best.

It may be easier to remove the mounting bracket from the diff and replace the sensor

[taylormj4]

So that I can drive the car home from the garage (around 150miles) can anyone tell me what the ratio of revs to mph is in 3rd, 4th and 5th please so I can use the rev gauge as a speedo.

I seem to remember it was something like:

3rd - revs x 1.5
4th - revs x 2.0
5th - revs x 2.5

In each case "revs" is the big black numbers on the gauge, not the actually rpm.

So for example if in 4th with the needle at "30" for 3000 revs, the speed is 30x2=60mph.

In 5th, with needle at "30", speed is 30x2.5=75mph

Cheers.

Edited by taylormj4 on Friday 21st October 18:05

[SwanJack]

Get a speed app on your phone,

Edited by SwanJack on Friday 21st October 19:05

[QBee]

Stick your satnav on the screen is my first thought, whether you need it or not - gives an accurate speed read out.
Second thought is that you are roughly right, but on my car 3000rpm in 5th is nearer 80 mph.

Cannot see why painting the car would stop the speedo working.
Unless something they have done has blown the fuse.
Fuse position varies according to the year of the car.
On some it is 5 from the right, others it is 10 from the left.
Have a look in your handbook

[taylormj4]

QBee said:

Stick your satnav on the screen is my first thought, whether you need it or not - gives an accurate speed read out.
Second thought is that you are roughly right, but on my car 3000rpm in 5th is nearer 80 mph.

Cannot see why painting the car would stop the speedo working.
Unless something they have done has blown the fuse.
Fuse position varies according to the year of the car.
On some it is 5 from the right, others it is 10 from the left.
Have a look in your handbook

Thanks QBee. Sorry should have said it's not the garage's fault. I had the diff changed at another garage recently so its probably gone as a result of being disturbed. It is the original transducer so its done its time.

Haven't got a portable satnav and after a Windows 'update' my phone is knackered other than for phone calls so I'm a bit stuffed on that front.

[QBee]

Have to admit mine failed shortly after I bought the car, so 13 years old.
Try getting it adjusted first - if it is now too far away from the toothed wheel then the speedo won't work, as I am sure you know.
if you live in the country, or near outdoor animals, do be aware that you might have mouse damage to the wire(S)

[N7GTX]

Even though I had my speedo calibrated when rebuilt, it still seemed a bit out. Its just within the 10% accuracy so I bought one of these which I have velcro'd to the dash.

http://www.drivingschoolaids.co.uk/product/747/gps...

They are cheaper if you shop around BTW.

[taylormj4]

QBee said:

Have to admit mine failed shortly after I bought the car, so 13 years old.
Try getting it adjusted first - if it is now too far away from the toothed wheel then the speedo won't work, as I am sure you know.
if you live in the country, or near outdoor animals, do be aware that you might have mouse damage to the wire(S)

On the drive home, the above ratios seemed about right.
I was partially driving using the revs and partially from experience and was impressed with myself when I went past two of those speed awareness signs that tell you your road speed. I was in a 40mph zone each time and got 39 and 38mph ha ha.

Interestingly the speedo still jumps into life at just over 50mph. It has always not worked below 8mph and just assumed that was as designed. Now at 55mph+ if reads steadily and seems about right against the other traffic.
Sounds like the gap is set wrong maybe but could be poor connections somewhere. I'm thinking that the transducer generates a square wave or pulsed signal due to induction with the toothed wheel on the diff. The speedo then counts the number of pulses or the frequency of the signal and displays the speed accordingly. However, when the pulses come closer together (faster speed), the RMS voltage effectively increases, so if there is a bad connection somewhere, it is more likely to cut through that at higher speeds.

[blitzracing]

The transducer is a passive device, and generates a voltage that rises as the diff tooth wheel goes faster. Id suspect the gap between the sensor and the toothed wheel is too large, so the voltage generated is too small to trigger the speedo at lower speeds. Otherwise the sensors get water in them, and the joints inside fail or go high resistance.

[sparkythecat]

When I installed a replacement sensor, I got no speedo reading under 40mph.
I adjusted the sensor gap down to 6 thou' using a feeler gauge and that cured the problem

[taylormj4]

Is it difficult to adjust on axle stands. Remember trying to change the fuel filter was pretty difficult due to access.

[Steve_D]

taylormj4 said:

Is it difficult to adjust on axle stands. Remember trying to change the fuel filter was pretty difficult due to access.

Not easy but no where near as hard as doing the filter plus you don't get petrol in your armpit.

Steve

[taylormj4]

Steve_D said:

Not easy but no where near as hard as doing the filter plus you don't get petrol in your armpit.

Steve

You're not wrong. Just had a go at it tonight and you're right, not too difficult.
The gap was at around 0.8mm.
Sparky (above) said it should be around 6 thou, which appears to be approx 0.15mm but that is flippin' close to the end of the sensor getting mashed by the wheel.
I wound it in to 0.6mm but that's not helped.

Looked in Steve Heath's book but there is no gap given. It just keeps saying make sure you don't fit it too close.

Will try SParky's 6 thou next.

Edited by taylormj4 on Sunday 30th October 18:41

[ukkid35]

If your car uses the M0631 sensor like the later cars then I can suggest an improvement for you.

This sensor is slightly longer and therefore more sensitive, so it doesn't need to be fitted so close which means it lasts longer.



I used to have an advert in the classifieds, but I failed to renew it a while ago

[Sardonicus]

I purchased one of UKKids/Paul's sensors years ago and like he says you dont need (and shouldn't) to run them at silly tight clearances I run mine at about 3mm and have no sync issues whatsoever

[taylormj4]

ukkid35 said:

If your car uses the M0631 sensor like the later cars then I can suggest an improvement for you.

This sensor is slightly longer and therefore more sensitive, so it doesn't need to be fitted so close which means it lasts longer.



I used to have an advert in the classifieds, but I failed to renew it a while ago

Interesting. I do use that sensor. Are they cheaper than the standard ?
Does it work at slower speeds any better. Mine never registered below around 10mph.
Got a link to that part ?

[taylormj4]

Adjusted the gap down to somewhere in the region of 0.15mm-0.2mm and the speedo is working again now at speeds above around 10mph. Happy with that. I think it would work at lower speeds if you brought it in closer but I don't dare reduce it more than that.

[QBee]

Don't - I have never heard of a Chimaera speedo working below 10mph.
At those speeds the only thing working is the druver's OCD

[taylormj4]

QBee said:

Don't - I have never heard of a Chimaera speedo working below 10mph.
At those speeds the only thing working is the druver's OCD

Cheers ha ha.
Pleased I saved myself the £70 for a new one as the garage that had a look at it told me the sensor was damaged and no point adjusting it.