Get an old sensor/engine loom and cut the grey plugs off both of them. then wire them together with a long bit of wire between them making and extension.
Now if you plug this into your car's sensor and into the 2nd car's engine loom and start both of them, The 2nd car will display your Rpm - if it works your sensor is fine and the problem is the petrol dash loom
Now if you plug this into your car's sensor and into the 2nd car's engine loom and start both of them, The 2nd car will display your Rpm - if it works your sensor is fine and the problem is the petrol dash loom