89 Carrera odd speedo sensor reading! Sorted!
 

Hi all, my first post to the board which from doing a search, may not get answered.

Anyway. I've replaced the gearbox speedo sensor on my '89 carrera.
I've got a good working speedo head but only works when I reach around 40mph.
I've metered the sensor on the box and it's not giving a reading until said speed, around 40mph.
At this speedo the needle rapidly rises and starts reading the correct speed.
Any ideas, or is the sensor playing up?
I've had the sensor back out and metered it while passnig a magnet over it and it seems to work fine. I'm well confused!

Thanks for looking.

Here is lots of info on Pooshey speedos.

Speedometer thread

Have you noticed if when you are under 40 mph (no speedo) if the odo works?

Do you have any electronic test equipment?

edit: oops I reread your question and you say the sensor doesn't not work (no pulses) under 40 mph, not the speedometer....nevermind I have no clue.

lol, no probs. yeah i've metered the sensor and doesn't seem to work below 40ish, although will meter ok against a magnet out of the car.

Out of the transaxle, next to a magnet ... ISN'T how the sensor is supposed to be tested!

If it fails the test properly seated in the transaxle ... with the axle turned over by hand, well, then you have your test results!

It may not be the haynes/Bentley manual way, but it's how the sensor works.

The sensor is postioned next to a rotating magnet(there's a few on a diff plate). Every time the manet passes the sensor, the internals of the sensor make a contact, creating a continous curuit.
Just like a cycle computer. Magent on the wheel. Sensor on the forks

So, I've tested it in the car/gearbox. I get no continuity until 40ish when it starts to read correct. But out of the car I get continuity whenever I pass a magnet over it.
So why is it reading only when the rotation quickens?

The sensor is supposed to make'n'break some number of times per rotation, think it might be 8(?), so that it provides that many pulses to the speedo.

So are you saying that turning the tranny over slowly - e.g. by hand, or rolling the car - doesn't make and break inside the sensor unless/until it's spinning above 40 MPH? Offhand, I can't think of a failure mode that would fit those symptoms.

I have, on the other hand, owned a speedo head that wouldn't indicate speeds below 20 MPH or so on cold wet days until the cabin warmed up...

Yeah thats it. If I drive slowly, I don't get a reading at all. Thats whats so odd.
A soon as you get going, it'll start reading upto speed.
I haven't tried it by hand, but I can't see it making any difference. Slow driving and turning it slowly by hand.
I haven't a clue!
I've been told that the sensor could be duff from the factory, so I'm pickig up another tomorrow.
If this fails, then it's another speedo head. I bought this one in working order, and it's the same as the last one. I've bridged the green/brown wires to the speedo head and the needle flickers as it should.

Yup all sorted.
Even though the sender was working above 40, and it would give a reading on the bench with a magnet to pulse the sensor it wouldn't for whatever reason read below 40ish.

New part fitted(bearing in mind the other was brand new too) and all is well ;)