How do I get the tachometer and speedometer working in a 1990 TPI car after an LS/T56 swap?

We recommend a Dakota Digital SGI-100BT between the transmission and the chassis wiring. The F-body T56 VSS puts out 17 pulses per driveshaft revolution, and the factory speedometer needs 4,000 (or 2,000) pulses per mile; Application 3 in the SGI-100BT manual covers this setup. On 1990 TPI cars the ECM itself was the VSS buffer, so feed the converted signal into pin D (grey wire) at connector C207. For the tach, we've seen a direct connection to the back of the cluster fail until the SGI-100BT was changed from 4-cylinder input to 8-cylinder output.

Check this first:

• Whether your speedometer expects 4,000 or 2,000 pulses per mile
• The grey wire at C207 pin D on your car
• The SGI-100BT's tach input/output settings

Based on what we've seen work across our own builds, our friends' builds and the wider LS community, reviewed by our lead tuner (20 years of LS tuning). Every combination is different, so use at your own risk and check our safe tuning limits before any wide-open pass.