Fuel Pump Question 87 TII
 

I have been having a whole lot of problems with my 87 TII For a while. I have checked almost everything and was checking the fuel pump wires yesterday and the red one going into the top of the fuel pump got constant 12v which I assume is good, but there was a blue wire too that would go between 0 and 2 volts. Does anyone know what this blue wire does and if this is normal? Help would be much appreciated.

I'd take a wild guess and say it is the Fuel level sender.
put a meter on it and move the float assembly..if the voltage varies then that is what it is for.

I was thinking that it was the sender but the voltage varies without moving anything. Just sitting still it moves up and down between 1 and 2 volts. Id also like to add that I get good fuel pressure so do you think that it is even possible for this to be what is wrong with the car?

At the fuel pump connector:
Blue: Pump 12v+
Black: Pump AND Sender Ground
White/Red: Sender output to Warning Light
White/Green: Sender output to Gauge

Thank you! I am guessing i need to replace a relay or something somewhere? my blue wire obviously isnt getting the right voltage.

Especially with age, our cars tend to have a bit of voltage drop at the fuel pump connector. Before my 20B Conversion, my car showed about 10.5-11.0v on average. Simple solution is to use the blue wire to trigger a relay that feeds the pump directly from the battery. Fused properly, of course.

What I did with my car as part of its 20B swap was to add in a pass-through patch harness (sourced from the stock GXL stereo :D) with the haltech's fuel pump relay output spliced in parallel to the stock blue power feed wire. The pass-through plugs right into the stock connectors (Yazaki 0.250" ones, can't remember exact name) for the fuel pump. Since I've been using a FD pump since 2008, I used 16awg wire for it and it works like a charm.

keep in mind that the fuel pump is only on in three conditions.

1. the engine is running, ecu sees RPM signal.
2. the key is in "start" position, this directly triggers the "circuit opening" relay, which turns on the pump.
3. the fuel pump test connector is jumped with the key on. this is a yellow 2 pin connector, its on the engine harness on the passenger side, by the airbox.

the fuel pump circuit looks like this.

battery -> circuit opening relay -> fuel pump speed relay -> fuel pump

the turbo S4 and all S5's use a resistor, switched by the ecu with a relay to vary the voltage to the fuel pump. with the engine warmed up, at idle voltage should be 8-9v, and then under high loads it should switch to 12v.