In the last picture, to the left of the sensor, is a bolt head - that is holding down a small, bent rod that is the "hold down" for the sensor. So you remove the bolt and the small rod and then the sensor puck just lifts out. There are no visible magnets behind the sensor (or on the sensor puck), but it would still be worthwhile to remove it and clean the surfaces since there is so much grease/oil buildup on there.

If the odometer is working then the sensor is ok and if it's accurate then all the magnets are intact.
My guess is the speedo circuit has developed a problem. If you could borrow another to swap in you could prove it, otherwise you could just send it out for analysis/repair.
I've had good luck with North Hollywood Speedo.

That white wire connected to one of the sensor wires is definitely not original, but if the speedo/odo was working with it connected then it's probably ok. maybe it was a replacement for the brown (ground) wire that should be there.

don't waste time removing the sensor...nothing to see behind it anyway and we already know it's working.

Ok, a few things I tried with no success. I disengaged the white wire from the speedometer and nothing changed. Also did the test to check the magnets the sensor is supposed to read. This involved raising the right rear wheel, putting an ohm meter across the two brown/red wires that attach to the speedo, and counting the number of opens/closes as the wheel is rotated 360 degrees. My understanding was that I was to get 8 readings. I counted four. I am not sure if going from open to some reading and back to open constitutes one reading or two. If it's two, then I'm ok. But if its one then I'm missing 4 magnets which I find hard to believe. Can some one confirm that I'm supposed to get four resistance reading when turning one wheel 360 degrees?

Unlikely to lose 4 magnets at once. More likely a measuring error. You did have the left wheel on the ground, correct?
I asked before if the odometer is accurate. If it is then the magnets are OK.
The sensor is just a switch that closes as each magnet passes it.
I still think your problem is with the internal circuitry in the speedo, especially if the odometer is working correctly. They both use the same sensor.

Yes I had the left wheel on the ground. I also checked the odometer today against a GPS and so happens it is also reading lower mileage than it should. A 31 mile trip only registered as about 20. I never really checked the accuracy of the odometer before , assuming that since it was functioning it was fine. (So can't say if the odometer inaccuracy is new or old, but Ithe speedometer problem is definitely new). This makes me suspicious of the magnets or the sensor, but still find it hard to believe that I lost four magnets at the same time, and that they happen to be spaced out fairly evenly because I get readings at evenly spaced angles of the wheel rotation.

Speedo only goes to 60 kms
 

hi guys...yeah i have the same problem with my speedo it only goes to 60 kms and the needle dosnt bounce!... it is solid all the time.I thought it was my speedo gauge at first so i took it into a European instruments specialist locally here in Australia to recalibrate the gauge.They said the gauge was good and working properly!.They also said that the magnets in the gearbox may have fallen out of their places inside the gearbox and they said you can drain the oil out of the gearbox and the magnets will come out with the oil!.

has anyone experience this! and if so how do you get the magnets back in their right places?

Maybe do this! just drain your oil and see if any magnets come out!..if so then you know its the magnets thats causing the problem!...cheers

I had asked these questions in another thread. Responses were: The magnets would NOT come out with the trans fluid. (makes sense of course, magnet would be attached to something, not just floating around). The magnets ride in a circular plate, which can be replaced by opening the differential. Can be done without removing engine, but I imagine it won't be a piece of cake. I think the whole plate would need to be replaced unless you can find the actual magnets and glue them back on or something.

Anyway, I don't think there is an easy solution for both of us. I was hesitant to send the speedo out because I had heard from others like you that it wasn't the real problem.

Jwakil,

I agree your latest finding does point back toward the magnets/sensor. Would you say the odometer (20/31=.64) error correlates with the speedometer error (maxing out @ 60)? Driving at 100mph would read 64?
When I lost a single magnet several years ago the errors matched almost exactly.

I do agree that losing 4 magnets at the same time is really not plausible. It's easier to imagine a sensor failure mode that makes it less sensitive, but even that is a simply a theory.
When you do the sensor/magnet check, is it always the same ones that appear to be missing and are they evenly spaced, every 45 degrees?

Options that I can see-
-Bite the bullet and replace the sensor, although I'm usually not a fan of replacing parts without knowing if it's bad.
-Devise a method to test whether the magnets are really missing. I'm thinking that if you remove the sensor you might be able to use another magnet or a small compass? to detect them.
-Drain the transmission and remove the diff cover to inspect.

My vote would be the second option if you've got the time.

Your second option sounds like an ingenious plan, but I'm not sure how much other metal is around that area which could confuse the reading. When I did the test with the ohmmeter, it seemed that the readings were pretty regular and evenly spaced (I did about 3 times) As mentioned before, what surprised me was that I was expecting brief blips but got readings that lasted a good 20+ degrees of wheel rotation approximately 90 degrees apart. And I was rotating the wheel fairly slowly. While I didn't write down exact degrees and readings, based on memory I got something like below:
0-60deg no read
60-90 ~70 ohm
90-150 no read
150-180 ~70 ohm
180-240 no read
240-270 ~70 ohm
270-330 no read
330-360 ~70 ohm
I have to mention that there was one time where I left it in a read location for a minute or two as I was doing something else and all of a sudden it stopped reading. As I started to rotate again to what I believed was the next magnet it started reading again. I don't know if this suggests something weird with the sensor or my voltmeter has such a slow response that I need to move the wheel super slow.

while searching further i found this.
http://forums.pelicanparts.com/porsche-911-technical-forum/503635-g50-speedometer-sensor-problem-2.html

There are some who insist they have a four magnet version! And there are discussions of different sensor versions that were sensitive to positioning.

Use this custom Google search tool to see more results
https://cse.google.com/cse/publicurl?cx=014238775269149376701:ojolaky8hjg

Regarding your readings I don't understand why you're seeing 70 ohms...it should be 0. This is a switch after all. You are connected to the wires as they come into the tunnel?
I use the continuity (beep) tester on my DVM. You could use a test light connected between +12v and ground...might make it easier to see.

There are reports of sensors becoming erratic/sluggish.

Good luck searching/testing!

Thanks for that great link. Quite a bit of information there, some of which I haven't digested. I'm thinking I might have the 4 magnet version. The only thing that is confusing is that they are able to see the magnets through the sensor hole. I originally thought that, but when I took the sensor out it looked like it was just a blind hole. (Or maybe that was part of the rotating plate? I'll have to take it out again and rotate to make sure). As to 70 ohm vs 0? I assume that could just be the resistance in the wires up to the speedo, or the switch itself could be set to 70ohm. Anyway, its much lower than an open for sure.

you have a 930? what year? Those transmissions could be different.
The 70 ohms bothers me. did you try measuring right where the wires cone into the tunnel? Just open the access panel in front of the rear seats and you should see a connector.

81' 930, with the four speed transmission. I imagined the speedo mechanics should be the same as a non-turbo. I measured the 70 ohms at the wire connections at the speedo itself, which is why I was thinking the resistance in that length of wire could account for most of that 70. Trying to figure out how to disconnect that weird connector in the tunnel without pulling the wires from their crimps was puzzling me so I just read across the red/brown wires that attach to the speedo.

The sensor is a reed switch. Contacts close as each magnet passes and opens in absence of the magnet. Simple.

You can force needle nose pliers into the plastic connector and grab the crimped part of the bayonet connectors to unplug them. At the moment I can't remember which side comes out but I seem to recall it's the chassis side wiring which comes out of the plastic connector. The plastic connector has a slight rubber quality to it, won't split when you force the needle nose pliers into it.

Should see 8 magnets for one revolution of the wheel.

I have a magnet wheel here which has lost strength in a few of the magnets and won't trigger the sensor, which may be an alternate to magnets which have fallen out.

Ok, so where are the infamous magnets, getting annoyed...
 

I'm getting frustrated now. I removed the sensor from the trans again and even rotated the wheel, and all I'm seeing is a blind hole, no plate moving with magnets or anything. I've read the other posts discussing how to view and clean the magnets. What's up with mine? Are there different versions. How does my sensor detect the magnets through what appears to be a solid stationary plate (see pic).

http://forums.pelicanparts.com/uploa...1452398729.jpg

^^^

In the 930 gearboxes (the 4-speed, not the G50 - I don't know about those), there is a magnet carrier disc that is attached to the outside of the differential housing (the actual differential body - not the transaxle case it's installed in) and the disc has eight magnets (not 4, 6 or any other variation - they all have 8) - the sensor pics up the "signal" from the magnets through the metal of the transmission case.

In order to actually get to the magnet carrier, you would have to remove the differential from the case (or at the very least, remove the stub axle and differential cover - a lot of work, in other words).

Page 126, item #15 shows what I'm talking about.

http://www.porsche.com/all/media/pdf/originalparts/usa/911_USA_87_89_KATALOG.pdf

If the magnets are present, and the sensor is working, you will get an ohm reading of zero when a magnet is not in position and an ohm reading of less than 100 when one of them is in position. This requires that you have a known good sensor, however. I don't know how to just test the sensor or how to test for magnets if the sensor is bad.

My understanding was that you should be able to view & clean the magnets through the hole in which the sensor is installed. Clearly I cannot see anything by removing the sensor. Does the hole access only apply to certain types of differentials?

That might be the case with other 911 gearboxes, but I am not familiar with any other than the pre-1989 930, so don't know the answer for sure, and it doesn't apply to your situation anyway since you have a 930 gearbox - those are all the same, with regard to the speedometer sensor and magnets, and are as I described (no outside access to the magnets).

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.