The onboard serial connection almost certainly uses standard RS-232 signalling where the voltages are plus and minus 12V. The voltages might be less than that e.g. +/- 10V but they are not compatible with direct connection to the Arduino and you will be very lucky if you have not already blown the Arduino's two serial port pins or even the whole chip. RS232 signals are also the logical inverse of what the Arduino uses, which is TTL levels of zero and five volts.
You must use some sort of conversion between the voltage levels. This can be as simple as a pair of transistors (with resistors) or a MAX-232 (or similar) chip.