1967 – 1972 Chevy / GMC Truck Installation Manual

Decide where you will get your speedometer speed signal. If you have a transmission port where the mechanical speedometer cable is installed, you will need a pulse signal generator. GM style signal generator is shown below in Figure 1. Pulse signal produces 16 pulses per revolution and has three cable connections. Usually, cable transmission speedometer gears produce 1000 revolutions per mile. Therefore; when using a pulse signal generator, you will need to calibrate the speedometer in accordance with 16,000 pulses per mile (ppm) chart on page 14 of this manual. If you have a transmission that was built on the vehicle speed sensor (VSS) or computer (ECM), you will need SN74 signal interface box, which is shown below in Figure 2. Speedometer cable Step 1: Connect the 12 VDC switched from a dedicated source (we recommend the accessories trigger) for the color pink / white wire to the 6-pin connector [position B] of the instrument wire harness. If the accessory of the ignition is not available, connected to a switched and dedicated fuse in the fuse panel. This will help prevent disruption to the speedometer of the power source. Step 2: Connect the red wire from OR pulse signal generator power cord from SN74 to purple / white wire to the 6-pin connector [position B] of the instrument wire harness. Step 3: Connect a dedicated chassis ground (ie do not stack with other land lines) to the black / white wire to the 6-pin connector [position A] of the instrument wire harness. This will help prevent disruption to the speedometer from the ground. Step 4: Connect the black wire from the pulse signal generator OR ground wire from SN74 to the yellow wire on 2 pin connector [position B] of the instrument wire harness. Step 5: Connect the white wire from the pulse generator signal output from the OR to SN74 purple wire on 2 pin connector [position A] of the instrument wire harness. Step 6: If you use the SN74 speedometer signal interface: Connect a transmission cable to gnd SN74 VSS sensors and other cables to the transmission SN74 VSS inputs. OR Connect the signal cable to the speed of SN74 ECM input. Step 7: If you use the speedometer signal SN74 interface: Connect the two wires from the pushbutton to the SN74 calibration “button press” position …

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