Speedometer sensor mystery (pic)

[djdawson2]

When I bought my '89 930, I quickly learned on my 1000 mile drive home that if I exceeded 90mph, the speedometer would start to drop-bounce to a lower reading. Lower the speed just a little, and all would return to normal.

Fast forward to an annoying little blue puddle on the ground after every drive. Based on its location and appearance, I determine the leak is from the axle flange seal on the transmission. I replace it.

After I finish, I start the car, put it in 5th at idle (still off the ground on the lift), and let it run for a bit. Raising it back up... low and behold it is starting to leak again. Since I had cleaned things up fairly well for the seal replacement, I can now clearly see that the leak is coming from where the speedo sensor mounts.

Hmm... a connection... it leaks and gives off funny readings. I decide to remove it, as I'm thinking perhaps it is cracked. I remove it to find this:





What the heck? 3 copper crush washers and no o-ring? Pretty obvious why it is leaking trans oil. I remove the washers, install a new o-ring... test again. Now the speedo doesn't work at all. I remove it and try one washer... nothing. I try 2... now getting a bouncing needle. Back to 3, and all is well until 90mph.

Has anyone heard of this before? I hate to spend $75 for a new one if this isn't the problem.

Thanks,
Dave

[A930Rocket]

Do you have any other pix? From what I recall, after you remove the 10mm head bolt and spring, the sending unit with pops out. What's left in the trans stays in the trans.

[djdawson2]

Originally Posted by A930Rocket

Do you have any other pix? From what I recall, after you remove the 10mm head bolt and spring, the sending unit with pops out. What's left in the trans stays in the trans.

There is just a hole on the trans case where the sensor goes. The pic above is the sensor removed from the hole... showing the 3 crush washers that were used to space it further out.

Normally when you remove the bolt/spring, the sensor still takes a little work to get out, as the o-ring should hold it firmly in the housing.

[Speedy Squirrel]

There is a disk inside the trans with 8 magnets in it. When a magnet goes by the sensor, a reed switch (a type of small mechanical switch) closes. If you measure the resistance of the sensor when there is no magnet adjacent to it, the resistance should show open circuit. If you then rotate the wheel (passenger or right side) until a magnet is adjacent to it, it should show a very low resistance (<100 ohms).

I have never heard of the magnets getting too weak (that's why they are called premanent magnets!), but I suppose anything is possible. This is hard to diagnose without a known good sensor, unfortunately.