I started today on what thought would be simple project for a Sunday afternoon.The battery on my El Camino has been dying a slow death so decided to get new one and some new cables.After attaching the cables to the starter and the other to ground I put the battery in and attached the positive cable to the battery I stated to connect the negative cable to the battery post and engine began to crank.I drove the car into the garage on the old battery and simply changed the battery and cables so don't know what's wrong.I have so idea's but before I start changing parts can someone tell me what when wrong.Thanks

Is the large cable on the starter solenoid touching the S terminal on the solenoid? This is the terminal closer to the block.

That was it.Thank you very much for your help.