Speedo/Odometer Stumped

[Blagave]

Looking for some guidance on speedo/odometer not working. I have read and tried just about everything in the other posts and still nothing.

1980 911sc
Speedo worked prior to re-building the car and then when car was finished nothing worked.
Replaced Speed sensor at Trans
Sent Speedo out 2x to North Hollywood to have rebuild (second) time was because I thought I shorted it out.
I have 12v power and grounds are good. I have all the wires on correct posts back of gauge...Checked numerous times.
I have metered the wires in the tunnel to the gauge and they check out
I have taken jumper wires from sensor directly to gauge no change
I tired the magnet trick on the new sensor and original and needle does not move.

My fear was that I shorted the wires in the tunnel when welding, but by jumping around the wires it still does not work.

Any suggestions would be very much appreciated.

[brianlay]

Please clarify what you mean by the "magnet trick".
The sensor, as you may already know, is just a switch that closes when the magnets embedded in the diff gear pass by.
Connect a continuity tester, like a DVM with beep setting, across the sensor connections in the tunnel.
Jack up the right side rear wheel (with the left on the ground and the transmission in neutral) and you should get eight beeps per wheel revolution.
If that works, move your tester to the sensor wires behind the speedometer and check there.
Nothing in the above test relies on +12V or ground.

[Blagave]

Thanks for test setup Brianlay.
After performing the test, It seems that there is a problem with the magnets inside the trans. I tried both the original sensor and the new one and neither one works while connected to the trans and spinning wheel. When I remove either sensor from the trans and use an external magnet the meter beeps.
I spoke with the company who rebuilt trans and explained and they said the bearings were good so they didn't remove the magnet plate. He said the diff might be magnetized throwing off the sensor.
All I know is he wants me to pull the axles , side plate and diff to inspect. Not real happy right now.
As far as the magnet trick I mentioned, I read that if you take a magnet and tap it on the sensor the needle in the speedo will jump. Basically the same concept as what you outlined.

Thanks again for the trouble shooting help

[brianlay]

After a transmission rebuild one of my magnets was missing. Apparently they are simply glued into the diff gear. Maybe came out in the parts washer?
With one missing, the speedometer/odometer was inaccurate.
Replaced the magnet when the car was down for a top end rebuild. Problem resolved.
My bet is your magnets are missing...I hope they're not floating around in the tranny.

[Blagave]

Embarrassing update.
not wanting to start pulling apart the trans, I went back to my build pictures and noticed the trans sensor picture. I pulled, a donkey move and was installing the wrong side of the sensor against the trans. As soon as I flipped to the other side, the meter started to beep on my tester. Sadly, I didn't think to do that a few weeks back and spent more money and time than need be.if there is a bright side, At least I didn't rip the trans apart

[Walt Fricke]

Well, one of the few things which is sort of reasonable to do with the transmission in the car is to remove the differential. Pull the CVs, remove the right side bolt holding the stub axle in, remove the same on the left side, undo all those 13mm nuts and pull the round cover plate, and lift out the differential/ring gear. This lets you look right at the plate with the magnets. And when it comes time to put it all back together, you don't have to adjust anything (at least if you left the magnet plate in place).

But you don't have to do any of that! Great