I have done a search but to no avail before someone says it

i can drive along and the speedo readings correctly when i let off the accelerator the reading drops in half but when on the speedo it corrects itself.

I have seen the answer to this somewhere but i can't remember where

Any help appreciated

Maybe some flexing causes the sensor position to alter slightly when under power and on overrun ?

Just a guess

not a bad suggestion but the sensor doesn't have any movement in it.

quote:

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Originally posted by prelude1980
not a bad suggestion but the sensor doesn't have any movement in it.

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my magnets are mounted on the diff flange, the diff is solidly mounted.

I think the problem is electrical, something to to with the voltage but i'm not sure

I really can't see this being electrical. What sort of gap have you got between the sensor and magnets?

about 2mm gap. i can only see electrical because lifting off the accel pedal very lightly it does it. seems to be when engine under load and not this happens

What is the sensor mounted to?
Any photo's showing the install?

Straw grasping .....

Let's say your sensor cable is picking up some interference from the ignition. When under load the ignition is in time with the sensor signal. When you take your foot off, ignition should advance (I think - the timing will change anyway ) and the interference will cancel the sensor signal or at least confuse it.

I would make sure sensor signal is thoroughly screened.

Is this a problem that has suddenly appeared on a car that was working OK before or is it a new installation ?

this is a new install. didn't think of screened cable and interference, but would this still happen if the wires didn't go near the front of the car

I get interference from mopeds in the street and I'm on the third floor

Maybe worth a try then - I'm sure I've seen screening recommended here in previous topics.

Does the speedo start to read correctly as the car's speed has reduced a lot? What happens if you declutch and rev the engine? Have you got the magnet polarity the right way round? I suspect that the sensor to magnet gap is increasing when you take your foot off the gas. I would reduce the sensor gap a little if you feel it's safe.