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Examining the Magnetic Disk Carrier in Differential

I thought that the engine gurus would be better equipped to help me out with my issue.
Car is long hood '86 running gear based turbo charged hot rod called the "RST" (Evolution of a Carrera RST). It's been a five year build and the last gremlin to sort out is a non working speedometer. After much frustration, I decided to send the speedo and sensor to Palo Alto Speedometer who confirmed that both were in working order. I ran new wires from the speedo to the sensor with no success at speeds approaching 45 kph. Speedo blips when I move a magnet across the sensor.
Only thing left, as I see it, is that the magnetic disk carrier has dropped several magnets. So, before I drop the motor/transaxle yet again, I would like to pull the diff out to examine the disk and hopefully find the missing magnets unchewed, in which case I can hopefully put it back together properly in situ. I just want to get the car certified so I can get plates and drive it for a couple of months before winter sets in. I'll then plan to assess whether a full transaxle rebuild is necessary.
I have never pulled apart a 915 transaxle before so I need some guidance on how a can get at the disk, as well as reassembly. Also what gaskets and seals I should replace (on the assumption that I find the magnets in tact).
Thanks in advance,
Cheers,
Johan
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Old 09-12-2016, 07:25 PM
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Johan
This should be pretty simple. Drain the oil. If it is in the car, remove the axles - at least disconnect them at their transmission side flanges. Remove the bolt from the two flanges, and pull the flanges. Undo all those bolts around the left side of the round side cover. Pull the cover. Lift out the differential, ring gear and all. On the passenger side you will find a round aluminum plate, which should have 8 bar magnets arraigned radially. That's what you expect to find missing some magnets. If they are in there, they most likely are in the diff area somewhere, though it is just possible they could migrate forward into the gear housing.

Get a used good magnet plate from someone, or search the 911 forum for someone who has found substitute magnets. They are just epoxied into place in their slots.

If you have to replace the whole disk, I seem to remember you might have to pull the bearing on that side to get it off. The parts manual doesn't help understand this. If you do have to pull the bearing, be careful, as it is easy to damage the thin metal carrier parts for the rollers. And if you pull it, be sure to put back the shims as they came off. Otherwise there is no adjusting needed.

The round cover plate has a large diameter O ring which seals it. Chances are you won't have to replace this. No other gaskets for this job.

There are seals on each side, but unless they look bad somehow they don't need to be replaced. Of course you could replace them. Installing new ones is tricky, as there is no "ledge" to stop them in just the right place when pressing them in. Note how deep in the old ones are before removing.

Reinstall in reverse order
Old 09-12-2016, 11:10 PM
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Walt,
Many thanks for your explanation. I will tackle the project this weekend and post my findings.
Cheers,
Johan
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Old 09-13-2016, 04:09 AM
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Excellent explanation, Walt. I assume the same would be true for the 930 transmission as well?

Apologies for somewhat off-topic.

Thanks
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Last edited by WERK I; 09-13-2016 at 12:46 PM.. Reason: reason for edit: (I'm an idiot) Removed carrier question
Old 09-13-2016, 10:31 AM
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Quote:
Originally Posted by WERK I View Post
Excellent explanation, Walt. I assume the same would be true for the 930 transmission as well?

Also, when you say; " Undo all those bolts around the left side of the round side cover" are you saying passenger side or driver's side?

Apologies for somewhat off-topic.

Thanks
Good follow-up question. The 915 diff only opens up on the left (drivers) side.
Johan
Old 09-13-2016, 12:56 PM
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Well, thanks. I went out to look at my 930 tranny sitting on the garage floor and had a head slapping moment. The plate is on only one side.

Pretty cool they put the magnet plate on the axle side. Regardless of R&P, the axle speed will always work with the speedo accurately, unless you change the tire diameter.
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Old 09-13-2016, 01:02 PM
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WERK 1, I think we have all been there! LOL

Walt, any idea what the torque value is for the flange bolt? Can't find it in any of my literature.
Johan
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Old 09-14-2016, 07:22 PM
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The 2.7 transmission (I have the spec book for that on my desk) lists the "joint flange" as a M10x1.5 expansion bolt at 26-30 Newton/meters.

Not sure the 3.0 is the same - something tells me it is thinner.

However, I don't think this is particularly critical (may be wrong there), as it buts the splines of the hub up against the end of the female splined hole in the end of the diff output, and the splines do the work, and there can't be many forces putting tension and compression on the joint (unlike, say, a rod bolt).
Old 09-14-2016, 09:36 PM
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Johan,

Walt is correct that the speedo magnet disk is held behind the carrier bearing. So you'll have to pull off the bearing with a suitable puller (VERY carefully like Walt said) to get the magnet disk removed. I suspect you'll have to grind the puller arms of a universal puller to make the hook end of the arm thin enough to get around the back side of the bearing, so you can grab it properly at the edges of the inner bearing race.


The torque specification for the drive flanges in your '86 transmission is listed at 44Nm (32 ft-lbs) for the M10 x 1.25 bolt





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Old 09-16-2016, 10:11 AM
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Also, if you can't source a good used magnetic disk locally? I have one I can send you.

Just last night I acquired a magnetic disk and a bunch of other parts from a friend cleaning out his garage, and wanted all of his leftover 915 parts gone (car sold years ago). Just some last remaining pieces of bad memories from transmission failure due to gearbox being run w/out oil in it.

Here's a thread with other threads linked within, if anybody's interesting in seeing what happens to a transmission with very little oil in it.

Missing shims on a 915 tranny
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Old 09-16-2016, 10:24 AM
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Kevin, many thanks for the diagram and torque value table. I am especially thankful and released that you have a disk available.
I am letting the oil drain over night and will hopefully pull the diff tomorrow so I'll let you know right away.
Johan
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Old 09-16-2016, 07:57 PM
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Well, this is a real bummer! There is nothing wrong with the magnetic disc. All 8 magnets are in tact and are equally magnetized.





I tested the magnets with the sensor and the sensor only closes if it is moved across the magnets with the outside face against or near the magnets, like this...



With the sensor in the correct position, i.e. Inside of sensor facing the disk, then the sensor does not close on the ohm meter,



I'm at a loss!
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Old 09-17-2016, 10:09 AM
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Tested my old and new Porsche sensors plugged into the tunnel. Rubbed the sensors against the magnetic disk and with both sensors the speedo needle moved only when the outside side of the sensors was moved across the magnets. The needle would not move when the inside side (the side with the two prongs) faced the magnets.
I have to guess that if I were able to rotate the disk fast that the needle would moved with either side of the sensor facing the magnet. So I am still baffled why the speedo does not work with the sensor installed in the diff.
Anyway, I have basically given up and reassembled everything. If the speedo miraculously starts working I'm a happy guy. If not, I'll have to develop an external pickup similar to what is used with rally equipment. Basically, glue magnets on the flange and build a bracket to hold the sensor close.
I'll post an update soon.
Cheers,
Johan
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Last edited by Uwon; 09-17-2016 at 07:03 PM..
Old 09-17-2016, 06:40 PM
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Does it behave any differently if you place the sensor at the location and in the orientation it will be in when the dif is in the case and the sensor in its hole? Positive and negative pole alignment? I've never cut a sensor open to see how the switch in it is lined up.

Does it matter if you reverse the leads of your VOM? Shouldn't, I suppose. But at this point more things to try.
Old 09-20-2016, 11:19 PM
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Johan,
"I ran new wires from the speedo to the sensor with no success at speeds approaching 45 kph."

Could you explain this a little further? Are you saying the speedo registers the proper speed up to 45KPH or does it register after 45KPH?
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Old 09-21-2016, 02:01 AM
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Johan,

I looked at the transmission in my '86 Carrera last night. The picture above shows the proper orientation of the sensor when installed in the trans. case.

On my car, I clearly saw the tan area of the sensor facing outward
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Old 09-21-2016, 09:34 AM
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Kevin is correct above; the sensor puc is supposed to be installed with the raised tabs facing outward - pointing away from the diff housing. It does not work if the puck is flipped over (like the last pic in post #12).
Old 09-21-2016, 10:14 AM
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Dave and Kevin, I hope that you are right that the sensor should be installed with the prongs (and crappy looking side exposed) away from the diff.
Amazing that this detail was not picked up earlier in this thread and in my build thread.
I will change the sensors around and report back.
Cheers,
Johan
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Old 09-22-2016, 04:07 AM
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Hallelujah
Speedometer works!
Solution was to reverse the sensor at the differential as per Dave and Kevin's suggestion.
This way speedo does NOT work....



Reversing the sensor so the two prongs face out works...



BTW the retaining click is not Porsche standard issue.
Thanks to everyone who chimed in. Car is back on the road and running well.
Cheers,
Johan
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Old 09-24-2016, 07:43 PM
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Disk

Quote:
Originally Posted by KTL View Post
Also, if you can't source a good used magnetic disk locally? I have one I can send you.

Just last night I acquired a magnetic disk and a bunch of other parts from a friend cleaning out his garage, and wanted all of his leftover 915 parts gone (car sold years ago). Just some last remaining pieces of bad memories from transmission failure due to gearbox being run w/out oil in it.

Here's a thread with other threads linked within, if anybody's interesting in seeing what happens to a transmission with very little oil in it.

Missing shims on a 915 tranny
KTL - I am looking for a 915 Magnetic disk if you still have an extra one.
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Old 09-24-2016, 09:44 PM
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