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The 87 TBI should acquire its VSS signal from the buffer that connects into the back of the speedometer head. As long as you have a suitable cluster with VSS buffer, Code 24 will not set and the ECM will be able to properly register road speed. If push comes to shove (and IIRC) there's a software switch accessible to a programmer that will toggle VSS diagnostics OFF.Code 24 will set if: • CKT 437 voltage is constant. • Engine speed between 900 and 4400 rpm. • Less than 2% throttle opening. • Low load condition (low air flow). • Not in park or neutral. • All conditions must be met for 3 seconds.These conditions are met during a road load deceleration.