Weird Speedometer problem

[ADCD875]

Hello,
My speedometer acting weird. when the car is accelerating, it functions normally, but when I decelerate (off the gas, without brake) speedometer goes down to 0.

Anyone know why this might happen?

Thanks

91 318 IS 6sp

[john318isau]

A friend's 318is had an intermittent speedometer. The problem turned out to be broken wires to the pickup on the back of the differential. Sometimes they were touching enough, sometimes not.

So maybe pull off the sensor connector on the diff and check thoroughly. In this case they were broken just next to the connector.

[Alpine003]

Like the poster above said, the speed sensor on the diff can be either dirty connections or broken. After checking this, you can inspect your SI board to see if the batteries are leaking or worn.

[ADCD875]

What is the SI board? Speedometer Interface? Behind the speedometer?

Thanks

[john318isau]

SI=Service Indicator (the coloured lights that indicate when your car is due for service)

They use batteries which sometimes leak causing damage to the instruments circuit board. Us 318is owners are usually ok because most have lithium batteries not the leaking NiCads.