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Originally Posted by cdvs
TTL levels are +5 to -5 and fall within the RS232 spec.
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The voltages for TTL are either +5 or 0.
Technically, small ranges near those two points are accepted, e.g. +4.5 to +5V. Each device's specs will list the minimum voltage seen as a logic "1". Likewise for the maximum accepted as a logic "0".
If a device
were to swing from +5 to -5 that would indeed be "legal" for RS-232. But as Phil said, the data output signal emitted by the ECU is standard TTL, 0 or +5 volts. Since it does not swing negative, that won't work for a "true" RS-232 device. Hence using the MAX232 chip (or similar) to convert to -12 and +12V RS-232 levels.
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Originally Posted by b3lha
Would it not exceed the maximum rating by a factor of over 100% if your PC serial port supplied a 12 volt signal to these pins?
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YES! Do
NOT connect a computer's RS-232 output directly to the ECU, TCU, or anything else in the car!