Why You Might Want One. Without a Vehicle Speed Sensor (VSS), many engines including the Chevrolet Fuel Injected engines we specialize in will not run correctly and will not pass a smog test.
Background on Our VSS Products. JTR Stealth Conversions began offering aftermarket vehicle speed sensors (VSS) for engine swaps in 1991. Our speed sensors are smog legal for engine swaps and have been issued CARB EO #D-371. The VSS tells the Engine Control Module (ECM) how fast the vehicle is going. Most people think the VSS is only used for the lock-up torque converter. But the VSS is also used to control the EGR valve, the charcoal canister purge valve, the electric cooling fans, idle speed, and air/fuel ratio.
The VSS even helps control idle speed when the vehicle is moving, which means that running without one may cause stalling under certain conditions. Needless to say, this can be dangerous. Raising the minimum idle speed with the adjusting screw can eliminate stalling, but the engine will still not run optimally without a VSS.
And without a VSS, the engine computer (ECM) will not get the correct signals to run the engine for best fuel mileage, such as running lean under conditions called "highway mode." With so many functions of the ECM depending on the VSS, it's clear that the VSS needs to be connected and functioning for best operation.
We sell manuals to help with your engine swaps, but this particular topic has generated so much interest that we offer this free download of the Speed Sensors chapter from our Chevrolet TPI & TBI Engine Swapping manual. Everything we know about it, yours for free. Enjoy!