Quick question 4 & 5 pin ECU’s

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The 5 pin used a dual ballast resistor while the 4 pin used a single ballast. Later 5 pins just had 1 dummy pin so they could be used where a 4 pin was as long as u have a 5 pin harness. Kim
 
Here's why.
The pin at the top and the one at the bottom left are both for powering the unit.
When first put into general use, the ECUs received power through a 5 ohm resistor. That's why there is a second ballast resistor on those cars. The 5 ohm resistor reduced the operating voltage for the transistors in the unit.
But during start up, voltage is already low. So I believe the pin at the top powers the ECU from the main half ohm ballast resistor when the key is in start position.

Later, Chrysler provided power regulation internally (or maybe just transistors that could tolerate 14 - 15 Volts but I think not). So only one power feed is needed, and it can be at system voltage.
 
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................And you cannot tell by looking. Obviously a module with only 4 pins is a "4 pin" module, but one with 5 physical pins might still be a 4 pin module
 
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