Speedo stopped working

[gas guzzler]

Car was due in for it's MOT today so took it out last night to give it a blap to clear the system and the speedo stopped working, and hasn't come back. Stopped working for a few minutes a couple of weeks back but came back on whilst I was driving. Any ideas on likely causes, is there gearbox connection that causes problems or is it more likely to be the instrument pod? This car is driving me crazy, soon as I sort one problem another one pops up.
Cheers Andy

[Southernboy]

I have had that happen to me a couple of times recently. But it comes back on after the car has been turned off for 15 minutes...

[colb]

Check out the speedo clock using the self test to see if the actual clock works, link to how t0o do this here
http://www.zroadster.net/forum/viewtopi ... st#p191422

If the speedo passes this test then perhaps its the actual speedo sensor or dirty or loose connection.

Colb

[pingu]

Or dirty ABS sensors.

[Robert T]

A few have had all the gauges go off, but the odo and lights still work, including myself. If you turn the car off and take the key out for a period of time, it all comes back on again. It appears that the cluster detects a fault and shuts down the ADC for the gauges to protect them. The time to reset my be down to the immobiliser. The problem is so intermittent it has proved impossible to find the root cause so far.

Cheers R.

[peter2b]

You dont need a working speedo for a mot, i got through 4 mot's on my kit car without a speedo working i just used my sat nav when driving here and in france
peter2b

[bertiejaffa]

If you find theres nothing in front of you and lots of things in your rear view mirror then you're probably going too fast... (sorry doesn't help your question I know but...)

[pingu]

If you find theres nothing in front of you and lots of things in your rear view mirror then you're probably going too fast... (sorry doesn't help your question I know but...)

or you are towing a caravan

[pingu]

You dont need a working speedo for a mot, i got through 4 mot's on my kit car without a speedo working i just used my sat nav when driving here and in france
peter2b

http://www.motinfo.gov.uk/htdocs/m4s06000701.htm

If you have an stringent tester you may not get away with it as it needs to be "clearly inoperative" to fail. What he doesn't know...etc...etc

[bertiejaffa]

If you find theres nothing in front of you and lots of things in your rear view mirror then you're probably going too fast... (sorry doesn't help your question I know but...)

or you are towing a caravan

More often than I care to admit.... do you need a new drive?

[Mike Fishwick]

The speedometer has noting to do with the ABS sensors - the sensor is on the back of the final drive - get under the car and make sure it is clean and secure before you start suspencting the instruments. Also remove the sensor and make sure its magnet is not coated in metallic sludge, as this can shield the pickup.

[pingu]

The speedometer has noting to do with the ABS sensors - the sensor is on the back of the final drive - get under the car and make sure it is clean and secure before you start suspencting the instruments. Also remove the sensor and make sure its magnet is not coated in metallic sludge, as this can shield the pickup.

I had my vehicle's rear wheels in the air and the speedo worked until it got to 30mph. At 30mph, the speedo stopped. It wouldn't start working until the car was back on the ground and the front wheels were rotated at the same speed as the rears. As the only sensor on the wheels is the ABS sensor. I suspect that there is some feedback going on to tell the car that there is a mega burn-out going on and to shut down the speedo.

[stevov]

If you find theres nothing in front of you and lots of things in your rear view mirror then you're probably going too fast... (sorry doesn't help your question I know but...)

Sorry but I have to disagree. If there are lots of things in your rear view mirror you're going to slow

[Mike Fishwick]

'As the only sensor on the wheels is the ABS sensor. I suspect that there is some feedback going on to tell the car that there is a mega burn-out going on and to shut down the speedo.'

Yes - of course there are no other sensors on the wheels other than those for the ABS/AST - I clearly stated that the speedometer sensor is on the rear of the final drive . . .

Life is a lot easier if we avoid making up complex explanations to suit our ideas, and work from the facts! While your car's rear end was jacked up you could have easily seen the speedometer sensor. It is connected to the car's wiring by a plug, which is supposed to be secured by a pair of plastic 'springs' with barbs on their inner ends - but these often lose their springiness and make allow the connectors to make poor contact. Corrosion is another probable factor.

[Southernboy]

It's the unit with the blue highlights....

[BladeRunner919]

While your car's rear end was jacked up you could have easily seen the speedometer sensor. It is connected to the car's wiring by a plug, which is supposed to be secured by a pair of plastic 'springs' with barbs on their inner ends - but these often lose their springiness and make allow the connectors to make poor contact. Corrosion is another probable factor.

So, are you saying that Pingu's speedo drive lost it's springiness while jacked up, but then regained it when back off the jacks?

[gas guzzler]

Thanks for the replies, a lot of useful information there. Not having a workshop manual I'm relying on the forum and you tube so cheers for the help
Andy

[pingu]

While your car's rear end was jacked up you could have easily seen the speedometer sensor. It is connected to the car's wiring by a plug, which is supposed to be secured by a pair of plastic 'springs' with barbs on their inner ends - but these often lose their springiness and make allow the connectors to make poor contact. Corrosion is another probable factor.

So, are you saying that Pingu's speedo drive lost it's springiness while jacked up, but then regained it when back off the jacks?

In reality, none of us know, because none of us have reverse engineered the IC inside the instrument cluster. The speedo signal AND the ABS signal go into the instrument cluster where the signal(s) are made to force a needle round a clock.

Logic would say that Mike is right, but the logic used by cars that work with bits and bytes instead of bits of string use binary logic (1 + 1 = 10, not 2 ).

I still stand by my assumption that the ABS is connected to the speedo circuit in some way, as it's the only explanation I can think of. But if I've magically found a way to return the springiness to the clip, I'll get down the patent office in the morning .

[Southernboy]

.......too late - I've already been to the patent office... they tossed my patent out... they said using a small blade to pry the spring clip wider didn't constitute a patentable concept - any monkey can do it.
All the small connectors on these cars use the same terminal connector system... and it's a simple process to remove the individual clips (pins) from those tiny little connectors, clean them up, and pry the "spring" apart a tad before re-fitting....

[Mike Fishwick]

There is a powerful force out there called coincidence - really! After spending my career on quite complex ships I am a firm believer in it. As an example, I have twice flushed the loo, whereupon all three main engines stopped and the lights went out. Knowing that there was no connection between the toilet and the engines, I looked for non-sanitary causes and always found them.

The theory that 'The speedo signal AND the ABS signal go into the instrument cluster where the signal(s) are made to force a needle round a clock' is not confirmed by the BMW wiring diagram, which clearly shows that the only connection between the instrument cluster and the Slip Control Module are the warning lamps for the ABS and Traction Control systems. There is therfore no connection between the speedometer and the wheel sensors

The speedometer is a (relatively) simple circuit which counts the negative-going pulses from the sensor on the back of the final drive, which are generated at a rate of 4712hz per km, and converts their frequency into a proportional DC voltage to drive the speedometer.

The moral of the tale is - as I suggested - do not invent a theory and then try to prove it, but consider the symptoms, find out how the system works, and then start using logical diagnosis - anything else is a waste of time, effort, and imagination.

[gas guzzler]

Well had a look today, cleaned the sensor on the diff, wasn't that, did the instrument check and all dials were working so took the pod out, guess what, the smaller connector block (black one) wasn't clicked all the way in. Now working so will re book for MOT to see what else is wrong.
Cheers Andy