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Author: Subject: How to - fix or create Speedo input
miskit

posted on 21/6/18 at 05:50 PM Reply With Quote
How to - fix or create Speedo input

Thought this might be useful info as it was simple and painless.

For my Sandrail I used an MX5 Mk 2.5 engine with a VW Bus gearbox and the back end of a Porsche 944. The Mx5 needs a speed input and I wanted to use the instruments anyway. The original speed sensor was a VR sensor take off from the gearbox - that I am not using. So I bought a Dakota Digital SGI-5E and re-instated an ABS sensor on the Porsche arms.

The MX5 instrument panel wants to see 8000 pulses per mile - all the tech details here
ABS Ring has 45 teeth so 45 pulses per revolution.
Put my tyre size into a Tyre calculator to get the revolutions per mile, in my case 651
651 x 45 = 29,295 pulses per mile from the ABS. To get the 8000ppm I need I just need to make the correction factor 0.273

Wiring was simple, one wire is an earth and one is a signal. Signal wire goes into the Dakota, comes out modified by the number you set and into the instruments. Programming the unit was easy and it all works giving an accurate speed on the MX5 speedo.

You can adjust it on the fly as well if for example you are using a sensor on a propshaft instead. It seems to be a really flexible unit and can output either a raw sine wave or a processed 5v square wave that a lot of after market speedo's expect to see. I bought mine via the ebay global shipping program and paid $13 for UK shipment + VAT + import duty. = 89 delivered

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