Speedometer issues

[Kool-aid-82]

Originally Posted by dap930

I hope this not your problem, but there is a disk in the transmission with 8 magnets on it. If one or more of these magnets come loose (hopefully goes to the bottom of the transmission) then the speedometer will not read correctly. The speedometer is looking for 8 pulses per tire revolution. Disconnect the puck sensor wires in the tunnel and check with VOM to see if you get 8 pulses per tire revolution with the back of the car jacked up. If there are less than 8 pulses then the magnet carrier inside the transmission is the problem. It can be accessed by removing the transmission side plate. With fewer than 8 pulses the odometer will still read, but not accurately.

Where do you get the replacement part/wheel?
And how hard is it to replace?

[A horse with no name]

Hi...As mentioned, just use a multi-meter with it set to 'continuity' to check the magnets...You can use either lead on the multi-meter. Each pulse will produce a 'beep'.


PS - Billy, its the puck that is posted in the photo. It's on the side of the transmission and is very easy to replace - One bolt holds it on. It unplugs from the speedo wiring harness which is located under the tin plate on your back seat floor board- 4 screws.

Our host stocks them.

For SC's Electronic Speedometer Sending Unit
Part #: 911-606-209-00-M100

For 911(1985-86) Speedometer Sending Unit, 911 Turbo (1985-88)
Part #: 911-606-209-01-OEM

[yelcab1]

The magnet speedo plate is easily found used. I had one and could not sell it so I threw it out. Getting to it is somewhat of a chore but doable with the gearbox inside the car.

[drdeech]

Reads up to 55 mph max now but going much faster than 55.

[brianlay]

when you are going 20, does it read 20? How about at 40?
You're sure the odometer is correct?

[dap930]

Kool-Aid,

Check out Uwon's post in the 911 Engine building forum where he removed the magnet carrier from the transmission. He found all the magnets in place, so he has some sort of problem with puck (sensor), which is nothing more than a reed switch whose contacts "make" when in presence of a magnetic field.

[rick-l]

counting 8 pulses per revolution sure would answer a lot of questions.