My speedo is freaking out!

[casurfer911]

When i am slowing down to under 5 mph my speedo will randomly shoot up to about 80 mph and then immediatley go back down, it does the same thing upon acceleration. Doesn't do it every time but its still a lot. Any ideas?

[Anton CH.]

Last time I asked, someone said that the plug on the diff might be dirty with oil. Luckily for me, this sort of problem went away on it's own.

[Kalevera]

It's likely speed sensor related. As Anton intimated, the wiring around the diff can corrode over time and cause erroneous speedo readings. Here's what I'd do if I were you:

1) Jack up the back of the car and inspect the wiring harness (two leads...going into the rear of the diff on my 535). They'll likely be fine, especially since you've got a cali car.

2) Order a new speed sensor from BMA (if it's never been switched and you have > 100k, it probably needs it anyway). Replace it (or have someone else do it if working in tight spaces for long periods of time is an issue). Service the diff (change the fluid with some Redline), if it hasn't been done in a while, while you're at it.

When I bought my car and got it back on the road, the speedo was erratic. It would work sometimes, other times it wouldn't. There's nothing between the gauge and the sensor, and it's unlikely that the gauge will fail, so replacing the sensor will probably solve the issue completely.

best, whit