Electrical diagnostic tool

1. A diagnostic device comprising:

two rigid control leads;

two rigid load leads;

an internal relay mechanism configured to selectively conduct electricity between the two load leads based on whether an electrical current is applied across the two control leads;

a sensor configured to provide measurements of voltage or current sensed at the load leads,

a data memory,

a processor configured to input the measurements from the sensor and store the measurements in the data memory,

a computer interface configured to send the stored measurements to a computer, and

a housing that houses the internal relay, the sensor, the memory, the processor and the computer interface;

wherein the control leads and the load leads have the same pin configuration as an existing relay that lacks a sensor, lacks a data memory and lacks a computer interface, such that the diagnostic device is configured to be plugged into a relay receptacle in place of the existing relay.

2. The diagnostic device of claim 1 , wherein the computer interface comprises a USB port.

3. The diagnostic device of claim 1 , wherein the computer interface comprises a wireless transmitter.

4. The diagnostic device of claim 1 , wherein the measured parameter is current.

5. The diagnostic device of claim 1 , wherein the measured parameter is voltage.

6. The diagnostic device of claim 1 , wherein the internal relay includes (i) an electromagnet connected across the control leads of the diagnostic device and (ii) a magnetically-activated switch connected across the load leads of the diagnostic device.

7. The diagnostic device of claim 1 , wherein the internal relay is a solid state relay.

8. The diagnostic device of claim 1 , wherein the control leads and the load leads are blade connectors with beveled ends.

9. An apparatus comprising:

the diagnostic device of claim 1 ;

a motor configured to be powered by motor-driving current; and

a relay receptacle into which the diagnostic device is configured to be plugged, wherein the relay receptacle is configured to conduct a control signal across the two control leads and to conduct the motor-driving current across the two load leads for the motor-driving current to be controlled by the control signal.

10. The apparatus of claim 9 , wherein the relay receptacle is part of a motor vehicle.

11. The apparatus of claim 9 , wherein the motor is part of a fuel pump.

12. A method performed with the apparatus of claim 9 , the method comprising:

unplugging the existing relay from the receptacle; and

plugging the diagnostic device into the receptacle in place of the existing relay.

13. The method of claim 12 , further comprising:

transferring the data from the memory to a remote computer via the computer interface.