# 89 Subaru justy vehicle speed sensor testing

**URL:** <https://community.cartalk.com/t/89-subaru-justy-vehicle-speed-sensor-testing/92083>\
**Category:** Maintenance/Repairs\
**Created:** [December 29, 2015, 4:44am UTC](https://community.cartalk.com/t/89-subaru-justy-vehicle-speed-sensor-testing/92083 "2015-12-29T04:44:56Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![deegore](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/deegore/32/2986_2.png) [@deegore](https://community.cartalk.com/u/deegore)\
**Post date:** [December 29, 2015, 4:44am UTC](https://community.cartalk.com/t/89-subaru-justy-vehicle-speed-sensor-testing/92083/1 "2015-12-29T04:44:56Z")

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My Chilton manual says “1. switch the ignition on and check the voltage on VSS terminal 29 of the ECU. 2. The voltage should vary above and below 2 volts with the vehicle moving slowly.”  
I think I’ve found the correct wire, but I’m not sure about the testing procedure. Would I unplug the wire harness and test the wire? If so, would I push the car to check the voltage, as it will not start with the harness unplugged?  
Thanks for any advice.

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**Author:** ![Tester](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/tester/32/15976_2.png) [@Tester](https://community.cartalk.com/u/Tester)\
**Post date:** [December 29, 2015, 5:01am UTC](https://community.cartalk.com/t/89-subaru-justy-vehicle-speed-sensor-testing/92083/2 "2015-12-29T05:01:06Z")

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It’s called back-probing.

It’s done without disconnecting the connector.

Tester

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**Author:** ![Cougar](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/cougar/32/21348_2.png) [@Cougar](https://community.cartalk.com/u/Cougar)\
**Post date:** [December 29, 2015, 5:02am UTC](https://community.cartalk.com/t/89-subaru-justy-vehicle-speed-sensor-testing/92083/3 "2015-12-29T05:02:13Z")

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Leave the plug connected and back probe the connector to make the connection. If you don’t have some needle point probes you can use a pin and stick it into the back side of the plug where the wire comes out. Then use a jumper wire to connect the meter probe to it or tape the pin to the probe tip.