I 'm not sure that the OBC readout can be separately calibrated - it takes the same pulse as the analogue needle. Mine both read 10% fast according to a GPS unit.

 

The digital speed is a nice option that I would love to have, is it possible to retrofit an OBC and just connect the speedsignal? Or do you need to follow the whole p-car diy to get it working at all?

 

Great useful write-up on the OBC retro-install .... that is alot of work.


I check'ed the OBC option when spec'ing the MY98 C2S 6sp manual coupe way back when. At the time the thought was the option was,,, kinda toy-like - but like others here the OBC gets used all the time. My September '97 production 993 w/OBC digital speed display mirrors the analog speed display. The GPS speed is right on also. (3GS iPhone w/Tom Tom app. w/ant. base) Additionally, the "actual" fuel use calc of the OBC is scary close to the ole paper and pen/top of the filler neck method; +/- 2 tenths of a gallon over a full tank use.

 

P-Track,
I just completed installing the OBC into my car, all works OK.
Looking at the diagram you would need to connect more that just the speedo pulse line, as there is a connection to 12volts direct, 12volts via key switch, connection to the ECU reference, connection to the door central locking plus many more, the ones I mentioned I am sure would be needed to get the OBC basic operation.

 

Hi Stewart,
I believe the analogue and digital speedos can not be calibrated sperately, doing some research today. Spoke to VDO.
The OBC speedo is calibrated by internal electronic calibration, programming.
My analogue and OBC speedos are different by 5 klms at 60 klms/hr.
I guess I will have to live with it.

 

Hi
Interesting I just did another check, the Klms speedo and the OBC speedo is 100% the same, but about 6% high compaired to the GPS.

I have a US speedo calibrated in Miles and it is accurate to the GPS.
I imagine the difference in the Klms speedo and the Klms OBC speedo has the inaccuracy built into, so if your slightly over the sped limit you will be OK

 

Hey Neil, my gps indicates that the speedo on my car shows quite a bit higher speed than the gps speed too. 6% sounds about right
Cheers Graeme

 

This is good info!

 

Hi Graeme,
Spoke to VDO and the old ADR with respect to 993's it that the speedo has to read high, but no more than 10%.
New ADR is no more than 10% plus 4 Klms.

Interesting the OBC and analogue speedo is exactly the same, including the accurate off set, of no more than 10%.

Neil