I plan to use a 1985 turbo ECU on my 280z. I am custom building a wire harness. Thing is the OE Z31 uses a switched ground for the pump. The S30 uses a switched positive. 

I want to make this a plug and play as possible, which mean using the under dash 4 pin plug that feeds the switched positive to the pump.

Below you can see how the ECU has constant power to the pump with a selectable ground. One ground is thru a relay (full pump power) switched by pin 20,  the other ground is thru the ECU (pin 108) which has a voltage drop across it.

My plan is to just use the pin 108 to control the relay coil ground and configure the relay as a normal switched battery positive. The logic table indicates pin 108 is active all the time so this should work. Just wondering if anyone as tried to mod the 85 ECU in this way.

 

 

this is the logic table for pin 108 

and here is the logic for pin 20 as well as the resulting voltages table. I am not interested in the low voltage operation so I figure the 108 is always working as a ground to operate the pump, my way is just use it as a relay signal (voltage drop across the relay coil should not be a factor) to trigger a switched  B+ to feed the pump.

Edited March 19, 20241 yr by Dave WM

I guess the only issue is if the pin 108 is a duty cycled ground vs just a resistive load to drop the current to the pump ground, with the duty cycle based on various engine operating conditions. If that were the case my relay may get "buzzy"... for that I could install a capacitor to smooth out the buzz.