How do I get the tachometer and speedometer working in a 1990 TPI car after an LS/T56 swap?
- The Auto Meter 9117 wires inline on the coil-positive 12V supply wire and converts the sensed current into a 12-volt tach signal output.
- The Gen III LS PCM derives engine RPM from the crankshaft position sensor signal and sends that RPM signal out to drive the tachometer. (applies to Gen III LS PCM; LS-series)
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Confirm against the instructions for your exact parts before wiring. More: Tach and speedometer signals in our tuning guide.
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 shop tuner (over 20 years of LS and LT tuning experience). Every combination is different, so use at your own risk and check our safe tuning limits before any wide-open pass.
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Every option we know of
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| Maker / product | Part # | Fits | Price | Where | Status / notes |
|---|---|---|---|---|---|
| HP Tuners VCM Suite with 2-bar SD OS | MPVI2+/MPVI3/MPVI4 | Gen III PCMs | about $1000+ new | hptuners.com | Current production Write Entire required after OS change |
| GM / ACDelco 2-bar MAP sensor | varies (e.g. 12223861) | Boosted/VE-tuned applications | about $30-60 | Buy dealers, RockAuto | Current production Must match OS scaling |
| AEM / Innovate Wideband O2 kit | varies | VE tuning datalogging | about $150-250 | Summit, Jegs | Current production Required for accurate VE histograms |
Prices are approximate as of September 2026. We list every option we know of, including small shops and used parts we can't link to. Confirm fitment with the seller.