Speedometer issues
 

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

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

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

Yes, it's the magnetic puck.

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

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.

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

Odometer is correct.

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.

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

Hey BK, You back :D :D :D

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.

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?

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.

Here's a picture of the sensor. I plan on removing it cleaning everything and re-installing it.http://forums.pelicanparts.com/uploa...1472761927.jpg

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

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.

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?

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.

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

Parts?
 

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

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

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.

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

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

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.

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