Thread: Vehicle Speed Sensors

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Does anyone know how the GM pulse sensor works?

if anyone has an circit layout/diagram that would be great.

Thanks
Zachariah

There's more than one, but the usual type is self generating (winding around a magnet) and plugs into the transmission tailshaft and looks at a multi-tooth gear on the tailshaft. Some have 20 or so teeth and some 40. There are only 2 wires and either can be connected to signal or ground.

I have one in an old TH400 I modified for electric speedo and the VSS in my '97 truck was just replaced recently.

Hope this helps some. Kitz

The following is a quote from the http://www.jtrpublishing.com/Pages/S...eedometer.html
web page that gives an excellent explanation of the VSS

(quote) Basically, the VSS tells the ECM how fast the vehicle is going. Most people think the VSS is only used for the lock-up torque converter. 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. This is all explained in the Chevrolet shop manuals.

It must be emphasized that the VSS is used to control the idle speed when the vehicle is moving. Without the VSS, a vehicle may have stalling problems under certain conditions. Needless to say, this is dangerous. Raising the minimum idle speed with the adjusting screw can eliminate stalling, but the engine will still not run optimally without a VSS.

A lot of people think that running "closed loop" is best for fuel mileage. Closed loop simply means that the oxygen sensor is being used to monitor the fuel/air ratio. Some of the Chevrolet fuel-injected engines are programmed to run lean under certain conditions (called "highway mode") to improve fuel mileage during steady cruise conditions. Without a VSS, the engine computer (ECM) will not get the correct signals to run the engine for best fuel mileage. There are a lot of programs in the ECM that depend on the VSS. For best operation, the VSS needs to be connected and functioning.

There are four types of VSS signals required by the engine computer (ECM):
1. The two-pulse (2000 pulses per mile) square wave (D.C. current or direct current) used on all TBI engines through 1992, all computer-controlled-carbureted engines, and on 1985–1989 TPI engines.
2. A four-pulse (4000 pulses per mile) sine-wave (A.C. current or alternating current) signal is required by the 1990–1993 TPI, 1992–1993 LT1 engines, and 1990–1993 Camaro 3.1/3.4 V6 engines.
3. A 40 pulse per driveshaft revolution speed sensor used on 1993 and newer trucks with automatic transmission, 1994 and newer rear drive cars (Camaro, Corvette, and Caprice) with the automatic transmission.
4. A 17 tooth per driveshaft revolution speed sensor used on 1993–1997 LT1 engines with the Borg-Warner 6-speed transmission. An 11 tooth reluctor ring is used on 1993 LT1 engines with the Borg-Warner wide ratio (3.35 First gear) 6-speed transmission. (end quote)

Note that you must match the pulse per revolutions to your ECM; otherwise it will not know how fast you are going.

all VSS that I know of are pulse generating, meaning they pass around a magnent and create a small ammount of electricity, maby up to a volt or so. the higher the voltage sent to the computer, the faster your going. also the VSS is primarialy there for ABS brakes, if a wheel stops and the VSS reads 0 then the antilock pulsation will activate.