Speedometer issues

[drdeech]

I am having issues with my speedometer on my '78 911SC. I had the speedometer checked out at North Hollywood Speedometer and it checked out fine. I replaced the electronic sensor and still have the same issue, maximum MPH 30. Could the new sensor be bad, any thoughts are welcome. Thanks Stephen

[DRACO A5OG]

Is your sensor a magentic puck mounted on the gear box?

[69porsche]

Originally Posted by drdeech

I am having issues with my speedometer on my '78 911SC. I had the speedometer checked out at North Hollywood Speedometer and it checked out fine. I replaced the electronic sensor and still have the same issue, maximum MPH 30. Could the new sensor be bad, any thoughts are welcome. Thanks Stephen

when in doubt tap the glass! and see if anything happens!

[drdeech]

Yes, it's the magnetic puck.

[DRACO A5OG]

Make sure it is in the correct orientation ( post a pic if you can), to test remove and pass a magnet over it and ask a helper to watch the speedo to see if it moves.

Also check the connection in the tunnel is fully seated

[drdeech]

Regarding the tunnel connections, does it make a difference as to which wire connects to which post? I connected it as it was previously assuming that was correct.

[brianlay]

Is the odometer correct? Connection polarity doesn't matter.. The sensor is just a switch

[drdeech]

Odometer is correct.

[Bob Kontak]

Originally Posted by brianlay

Is the odometer correct? Connection polarity doesn't matter.. The sensor is just a switch

There is a green and brown wire from puck to speedo. If they are changeable go for it just for fun. Even change on the back of the speedo?

Believe sensor is just a magnet. Metal thingies from the trans pass over it creating a pulse. You can't fry anything reversing wires. Is this possible?

Continuity test from trans to speedo is advisable. Also low DC voltage test at puck may be a good way to see if there are blips but I don't know how to do this without starting engine and putting it in gear and even then not sure if that would work.

[DRACO A5OG]

It doesn't matter +1, use a pick tool or something rigid and skinny to push the connectors in factory rectangular shield.

[DRACO A5OG]

Hey BK, You back

[Bob Kontak]

Originally Posted by DRACO A5OG

Hey BK, You back

Yep. One week ban for being a stinker in a post. Roger rightfully said take a break, come back when you cool down. Get off my lawn for a week. I thought that was perfect.

Missed you guys.

[brianlay]

If the odometer is correct that tells us the pickup/wiring/magnets on the ring gear are all ok. I know NH said the speedo is ok but it sure seems like that's where the issue is.
Does the car have cruise control? If so unplug the connector to the control unit under the dash. The pick also feeds that unit and maybe it has a problem...although i've never heard of that. Easy to try though.
Otherwise it points to the speedo. Is the needle possibly sticking?

[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.

[drdeech]

Here's a picture of the sensor. I plan on removing it cleaning everything and re-installing it.

[brianlay]

After already replacing it? As I said before it's not the sensor.
Is the speedo accurate up to 30?

[drdeech]

I was told by NHS, the speedometer was relatively accurate maybe off a couple of MPH at 50 MPH. The picture is post removal and re-installed.

[brianlay]

How does it behave in your car? at 10 mph does it read 10? 25 reads 25?
And then stops going any higher at 30?
Is there at least a solid 12.5V feeding the speedo?

[rick-l]

Originally Posted by brianlay

After already replacing it? As I said before it's not the sensor.

The magnets on the ring gear are not part of the sensor.

It sure is simple to rotate the wheel and look for 8 contact closures (when one of the magnets goes by the reed switch) per revolution.

[drdeech]

The unit reads 40 MPH max and I know I'm going a lot faster than 40. Stephen