Technical question on gearbox's

[tigger67]

Hi Guys,

Can one of you clever people help me with a problem please?

I have a 911 915 C2 gearbox with the electronic speedo output..fine so far. I also have an aftermarket VDO electronic speedo. However I am experiencing problems calibrating it. I was told that Porsche use an inductive type pick-up. But wiring the speedo for this (I have 6 wiring options to cater for all the different types used by manufacturers) has so far proved unsuccessful. If some kind soul could tell me the type used I can rewire and the rest as they say....is easy !!

Thanks in advance

[Early_S_Man]

Well ... C2s didn't use a 915 transaxle, but rather a G50 ...

If you truly have a 915, the sender for the speedometer wasn't a magnetic pickup in the inductive sense, but rather a magnetic reed switch triggered by eight magnets on a carrier behind the ring gear. You can test it by connecting a multimeter to the two sender leads while the meter is set to a low range Ohms scale ... rotate the output flange on the passenger exactly one revolution and look for eight contact closures measuring less than 1.0 Ohm.

[tigger67]

Thanks for the reply, it is definitly a 915 with the remote cooler...

I didn't think the G50 was used until the 3.6 went in ?
This is a 3.2 Carrera 2 gearbox.........


I have done that and all works fine, my problem is it isn't talking to the speedo

[ChrisBennet]

'86 was the last year of the 915
'87 was the first year of the G50
'90 was the first year of the (964) C2, C4 came out in '89 (in the US at least)

You can see why we might be a little confused.

Sounds like you have a Euro 915 transmission.
-Chris

[tigger67]

Oh ok well that at least means I not going completly ga ga.....thanks for that.

Going back to the previous chaps reply.....if i may.

I can wire for an open collector circuit, interruption oscillator sensor, or inductive. Both 12 and 24 volt.

Do any of these ring a bell with OEM speedo


Thanks again for your help....

[Early_S_Man]

Sounds like an open collector circuit might work ... can you describe the 'interuption oscillator sensor' circuit in any detail? Interrupted ground is what the OEM VDO speedos used from '75 thru '86.

[tigger67]

Thanks for coming back...

Looking at the wiring diagram for the Interruption oscilloator sensor :
There is a 12 volt power feed to the sensor, an earth and a single feed to the speedo

There is also a diagram for a hall-effort sensor, this diagram is identical to the first described, however if the hall effect sensor has an 'open collector' it requires a bridge on two of the connections

Looking again at the diagrams, the induction sensor is the only one that does not have its own power feed, which is why I assumed the info I had was correct as there are only 2 wires coming off the gearbox and both were the same colour..... still !!!