how does the system work exactly to bypass the fuel pump resistor on 88-90s?

[Pete M]

looking for someone with better wire diagram reading skills than me to find something I can share with others that shows where to look should the Jeep start with the resistor in play, but not at all if the resistor is unplugged.  from my experience, unplugging the resistor should produce a fire-then-die condition, not a no-fire condition. 

[Ωhm]

Three conditions feed (B+) to the fuel pump motor. One (1) using the ballast resistor and two (2) bypassing the ballast resistor. Conditions are as follows:
1. During CRANK. B+ is supplied by the Starter Relay. Ballast resistor is bypassed.
2. During WOT. ECU knows when vehicle is in WOT and activates the 02 Heater Relay. Ballast resistor is bypassed.
3. At all other times, Fuel Pump Relay supplies B+ using the Ballast resistor in series with the fuel pump motor circuit.
 

[Pete M]

what would happen if an 87 computer was installed in an 88-90?  I'm assuming that the computer wouldn't know anything about the resistor, but where would it send power?  where could someone test for power to check that the bypass line is functional?

 

 

is the wiring anything like this overly simplified drawing apparently done by some random kindergartner?

 

 

[Ωhm]

3 hours ago, Pete M said:

is the wiring anything like this overly simplified drawing

No. Wire to the resistor, then the fuel pump motor is from the Fuel Pump relay. Wire to the pump is either from the 02 Heater Relay (WOT) or Starter Relay (CRANK).

 

3 hours ago, Pete M said:

what would happen if an 87 computer was installed in an 88-90?

Should work just fine. The ECU only controls the Fuel Pump Relay coil side and/or the 02 Heater Relay coil side when needed.

[Pete M]

I found another toddler to alter the art.  how about this one.  am I closer? 

 

 

[omega_rugal]

by how much is the voltage reduced with the ballast resistor? mine is MIA

[Eagle]

4 hours ago, omega_rugal said:

by how much is the voltage reduced with the ballast resistor? mine is MIA

 

IIRC the ballast resistor drops the voltage from 12 volts to 9 volts.

[cruiser54]

[Ωhm]

how does the system work exactly to bypass the fuel pump resistor on 88-90s?

---

 

Now your question has the word "exactly" in it. So keep in mind both FP relay and 02 Heater relay that the ECU controls the coil side of each relay. Meaning power (B+) to the fuel pump motor, with or without the ballast resistor, is switched from the fusible links. The same thing is applied to the Starter Relay except that's controlled by the IGN SW with the key in the CRANK position. Back to the drawing board or now are we good? Great question.

[knever3]

Soooo...  I am running a 4.7L stroker and it seems on moderate throttle input the A/F ratio stays around 14.7 like usual, it takes a LOT of throttle input to get down to 12.5/1 so I have always been concerned about not having enough fuel or maybe lacking some performance.  I do not have an adjustable MAP sensor, I run a '95 stock fuel pressure regulator at 38psi if I remember correctly?  I have 21lb/hr injectors.  So my question is if I bypass the resistor would it allow more fuel?  I know it is dependent on fuel pressure and duty cycle of the injectors, but worth a question.  If the fuel pressure isn't lacking it shouldn't have anything to do with it.  I just haven't had any tuning done except switching FPR's and injectors to accommodate the long and short fuel trims and go into closed loop.  I have a Snap On scanner and a wideband O2 monitor onboard.  Just a novice at tuning Renix.

[Ωhm]

Running engine without the ballast resistor just means more fuel through the fuel return line. Pressure will stay the same.

 

EDIT: I was thinking you had RENIX fuel pressure regulator.

[Eagle]

4 hours ago, knever3 said:

Soooo...  I am running a 4.7L stroker and it seems on moderate throttle input the A/F ratio stays around 14.7 like usual, it takes a LOT of throttle input to get down to 12.5/1 so I have always been concerned about not having enough fuel or maybe lacking some performance.  I do not have an adjustable MAP sensor, I run a '95 stock fuel pressure regulator at 38psi if I remember correctly?  I have 21lb/hr injectors.  So my question is if I bypass the resistor would it allow more fuel?  I know it is dependent on fuel pressure and duty cycle of the injectors, but worth a question.  If the fuel pressure isn't lacking it shouldn't have anything to do with it.  I just haven't had any tuning done except switching FPR's and injectors to accommodate the long and short fuel trims and go into closed loop.  I have a Snap On scanner and a wideband O2 monitor onboard.  Just a novice at tuning Renix.

 

You probably need 23-pound injectors.

[Ctwilson1]

What’s the heater relay got to do being tied in with the fuel pump resistor bypass?

[Ωhm]

Under normal engine operations, the fuel pump current, flows through the ballast resistor causing a voltage drop to the fuel pump motor. Now when engine is at WOT the O2 Heater relay is activated (O2 Heater OFF) and this will bypass the ballast resistor and apply full battery voltage (B+) to the fuel pump motor (max current). During a WOT condition there is really no need for a O2 Heater, but there is a need for max fuel pump.