My speedometer still reads with sensor disconnected!?

[jwakil]

Originally Posted by Jim2

Not sure where you got the idea that access could be had via the sensor. The folks who speak of cleaning the magnet ring had to do it via draining the transaxle, removing the axles and stub flanges, removing the side plate off the transaxle. The diff can then be lifted out and has the ring attached to it.

What mystifies me is I can't imagine anything building up on the magnet ring. This thing spins at axle speed (couple thousand RPM at highway speed) and those small magnets are so weak any small metallic particles will get slung off, not to mention the constant torrent of splash off the crown/pinion.

You seem purplexed how the sensor works through the metal case. The transaxle case is aluminum which is non-ferrous, ie non magnetic. The magnetic field from the sensor ring magnets projects past the aluminum housing to the sensor.

You might see if an independant porsche service shop in your area has a sensor laying about that they might loan for a deposit and a case of beer. 10 minutes to install it and do a test. There might be one on the pelican classifieds too, can't see it being more than 20-30$$ used.

Good luck and hope you find the issue.

It would be nice to borrow a sensor to confirm for sure. There are plenty of postings that talk about cleaning the magnets through the hole. In fact, one was referenced in this very thread, from brianlay, i.e. http://forums.pelicanparts.com/porsche-911-technical-forum/503635-g50-speedometer-sensor-problem.html. My conclusion is that the transaxle case and sensor of the 930 and perhaps 915 are different than the G50.

[Jim2]

I had a look at that thread and unfortunately that is a different model of transaxle than what is in your car. Your transaxle designated "930" changed to "G50" model in 1989 for the turbo cars, and for the 911 a sensor like yours was used in the transaxle designated "915" up to 1987 when it changed to "G50".

[Ronnie's.930]

Originally Posted by jwakil

There are plenty of postings that talk about cleaning the magnets through the hole. In fact, one was referenced in this very thread, from brianlay, i.e. http://forums.pelicanparts.com/porsche-911-technical-forum/503635-g50-speedometer-sensor-problem.html. My conclusion is that the transaxle case and sensor of the 930 and perhaps 915 are different than the G50.

Correct, none of the info in that linked thread applies to 4-speed, 930 gearboxes (like Jim noted).

[jwakil]

Originally Posted by Ronnie's.930

Correct, none of the info in that linked thread applies to 4-speed, 930 gearboxes (like Jim noted).

Well, that finally clarifies things in my mind. However, bad news as far as a quick fix in my case. My only hope would be to try a different sensor or cable as suggested. My remaining question is whether there are truly four and eight magnet disks and do I need to worry about getting the correct version of the sensor if I have the four magnet disk.

[Ronnie's.930]

Originally Posted by jwakil

Well, that finally clarifies things in my mind. However, bad news as far as a quick fix in my case. My only hope would be to try a different sensor or cable as suggested. My remaining question is whether there are truly four and eight magnet disks and do I need to worry about getting the correct version of the sensor if I have the four magnet disk.

Like I wrote in post #37, all of the 930 4-speed gearboxes have 8 magnets.

If you go back to that post and look at the parts manual link I provided, you will see that there is only 1 part number option for the magnet carrier (the other two part numbers are for the G50 transmission), and only one part number for the 930 sensor, as well. I also checked the parts manuals for model years 1977-1985 (the link I posted is for 1986-1989) = exact same part number for the magnet carrier and sensor for all years 1977-1988 930 gearbox.

[jwakil]

Thanks, I guess the four readings per revolution my test showed is probably due to the sensor or cable, as one of the other thread suggests.

[brianlay]

That picture of your sensor shows that someone has tried to seal up the cover?
I think you need to try a new sensor.

[ewave]

The reason you are only getting four pulses with one rotation of the wheel is because of the differential unit!

With one wheel stationary, you have to rotate the other wheel two full rotations for the equivalent of both wheels rotating one complete revolution.

Here are three links to ponder...

http://forums.pelicanparts.com/porsche-911-technical-forum/372410-sewer-backs-up-speedo-smokes.html

http://forums.pelicanparts.com/porsche-911-technical-forum/247776-carrera-vdo-speedo-sucks.html

http://forums.pelicanparts.com/porsche-911-technical-forum/485078-vdo-really-really-really-sucks-930-turbo-speedometer.html

[Jim2]

Just so you have an idea of what the magnet ring looks like here is a picture posted by another member:

Originally Posted by DeutschMark

[jwakil]

Originally Posted by brianlay

That picture of your sensor shows that someone has tried to seal up the cover?
I think you need to try a new sensor.

No, that was my doing the first time I tried to remove the sensor. I stuck a screw driver in the slot between the outer ring and inner protrusion, and tried to pry it out, breaking off a piece of the plastic molding that insulates the wire connections (stupid move). I don't think I did any harm and glued some epoxy over it. It behaved the same before and after.

[jwakil]

Originally Posted by ewave

The reason you are only getting four pulses with one rotation of the wheel is because of the differential unit!

With one wheel stationary, you have to rotate the other wheel two full rotations for the equivalent of both wheels rotating one complete revolution.

Here are three links to ponder...

http://forums.pelicanparts.com/porsche-911-technical-forum/372410-sewer-backs-up-speedo-smokes.html

http://forums.pelicanparts.com/porsche-911-technical-forum/247776-carrera-vdo-speedo-sucks.html

http://forums.pelicanparts.com/porsche-911-technical-forum/485078-vdo-really-really-really-sucks-930-turbo-speedometer.html

I thought about that at one point but disregarded it because none of the other postings I had read even mention that, so I thought no way!

[ewave]

Once again. There is nothing wrong with your sensor and the number of pulses you counted. The problem is in the speedo. it's probably a failing capacitor.

I can't believe I am on the only one in this thread who understands what happens when you keep one wheel stationary when you have a differential unit involved.

[jwakil]

Ok, focusing more on the speedometer itself, I took it apart to see if I could maybe just try replacing the capacitor and see what happens. It looks like more of a delicate operation than I could handle. I assume the capacitor is the gold cylinder, with 47/16 written on it. The soldering of the pins into the board without damaging the circuitry on the back makes me hesitant to do it myself. Which brings me to ask if anyone knows anyone near Houston who can test/rebuild these speedos.

[jwakil]

Well, today I took the speedo to a specialty shop to get it tested and they said it is working fine. So back to square one. I guess I have to take the plunge and buy a new sensor to eliminate that as well.

[ewave]

Just because the speedo works on a test bench doesn't prove it will work correctly in your car. Did they vary the Power Supply Voltage up to 16 Volts when testing it?