Why does Starter turn engine when I try to connect the negative cable to battery.

[elevator]

1990 Buick Century with a 3.3 V6
I installed a new (+) positive cable with a pigtail on it. I attached the secondary wire to the pigtail and connected it to the battery.
When I try to connect the (-) negative cable to the battery, I get a spark at the (-) negative battery terminal and the engine turns over.
I have the key in my pocket, why would the car want to start from the battery?

[94century]

Check the starter solenoid for wires touching each other. If not that it could be the starter solenoid is grounding out (bad).

[elevator]

Originally Posted by 94century

Check the starter solenoid for wires touching each other. If not that it could be the starter solenoid is grounding out (bad).

You got it right. The cable end was touching the solenoid terminal.
I adjusted the cable end to avoid touching anything. I was able to attach the (-) cable to the battery. Thank you very much for your quick response.