Fuel Pump Relay Troubleshooting (2 Viewers)

[CharlieS]

Hi All,

Could I get a sanity check on normal behavior of the fuel pump relay (FPR).

I'm trying to determine if I should be observing cycling of the FPR back and forth during engine warmup. By observing, I mean hear it clicking and feel the relay cycle periodically by putting my hand on it.

Based on reading here and factory docs posted on 'mud, it seems like the FPR has two settings: one delivering full power to the fuel pump delivering full pump speed and the other going through the fuel pump resistor delivering a lower fuel pump speed. If I read it right, it should engage full fuel pump pressure at startup, and then drops to a lower pump pressure once the vehicle is running.

What I'm wondering is if a normal FPR should be actively "clicking" switching from low to high during the engine warm up cycle? Probably normal, but wanted to ask others that have more experience.

_________
Other stuff:

I get that one can bypass the FPR and Fuel Pressure Resistor by jumping B+ and FP in the diagnostic connector or jumping the same terminals in the FPR plug. I tested this while the vehicle is warmed up and running there is no change in the engine speed. The Fuel Pressure Regulator bleeds off whatever additional fuel that is more than the set pressure back to the tank, so it doesn't matter as long as the pump is delivering enough pressure.

"The fuel pressure regulator adjusts the pressure of the fuel from the fuel line (high pressure side)to a pressure 284 kPa (2.9 kgf/cm2, 41 psi) higher than the pressure inside the intake manifold, and excess fuel is returned to the fuel tank through the return hose and pipe."​

What I haven't tried yet, and will once the engine cools all the way, is jumper +B and FP at startup and see if it changes the way it warms up.

FSM section EG-302 goes through troubleshooting the relay. Mine failed several resistance tests, but I have a bad track record of not testing electrical bits correctly. I have some evidence of arcing on the contacts of the FPR, which I kind of expect at 300K miles. I haven't tried burnishing them per the Landtank method, but will once I get a new one and this one goes into backup service. I have a replacement on the way (it is kind of unreal how pricey this relay is!).

Nice to know I can bypass this whole system in the field if I need to!

_____
Related question:

Is there any trick to unplugging the connector on the fuel pump resistor? It isn't working like other connectors on the vehicle. I haven't forced it because I don't want to damage the connector. I want to test the resistance per the FSM - Approx. 0.73 0 at 20 C (68 F)

 

[BILT4ME]

Hi All,

Could I get a sanity check on normal behavior of the fuel pump relay (FPR).

I'm trying to determine if I should be observing cycling of the FPR back and forth during engine warmup. By observing, I mean hear it clicking and feel the relay cycle periodically by putting my hand on it.

Based on reading here and factory docs posted on 'mud, it seems like the FPR has two settings: one delivering full power to the fuel pump delivering full pump speed and the other going through the fuel pump resistor delivering a lower fuel pump speed. If I read it right, it should engage full fuel pump pressure at startup, and then drops to a lower pump pressure once the vehicle is running.

What I'm wondering is if a normal FPR should be actively "clicking" switching from low to high during the engine warm up cycle? Probably normal, but wanted to ask others that have more experience.

_________
Other stuff:

I get that one can bypass the FPR and Fuel Pressure Resistor by jumping B+ and FP in the diagnostic connector or jumping the same terminals in the FPR plug. I tested this while the vehicle is warmed up and running there is no change in the engine speed. The Fuel Pressure Regulator bleeds off whatever additional fuel that is more than the set pressure back to the tank, so it doesn't matter as long as the pump is delivering enough pressure.

"The fuel pressure regulator adjusts the pressure of the fuel from the fuel line (high pressure side)to a pressure 284 kPa (2.9 kgf/cm2, 41 psi) higher than the pressure inside the intake manifold, and excess fuel is returned to the fuel tank through the return hose and pipe."​

What I haven't tried yet, and will once the engine cools all the way, is jumper B+ and FP at startup and see if it changes the way it warms up.

FSM section EG-302 goes through troubleshooting the relay. Mine failed several resistance tests, but I have a bad track record of not testing electrical bits correctly. I have some evidence of arcing on the contacts of the FPR, which I kind of expect at 300K miles. I haven't tried burnishing them per the Landtank method, but will once I get a new one and this one goes into backup service. I have a replacement on the way (it is kind of unreal how pricey this relay is!).

Nice to know I can bypass this whole system in the field if I need to!

_____
Related question:

Is there any trick to unplugging the connector on the fuel pump resistor? It isn't working like other connectors on the vehicle. I haven't forced it because I don't want to damage the connector. I want to test the resistance per the FSM - Approx. 0.73 0 at 20 C (68 F)

Be aware that the fuel pump changes speed based on the Temp sensor and what it tells the ECU.

Once warmed up is when it slows because it goes into closed loop and pays attention to the O2 sensors.

 

[devo]

I like this subject and would like to read more on it. Good post @ *CharlieS !
In 298,000 miles my FPR has failed 2 times and would have left me stranded if I didn’t have a backup on board. Always wondered why mine konked out where others have not?

 

[BILT4ME]

I like this subject and would like to read more on it. Good post @ *CharlieS !
In 298,000 miles my FPR has failed 2 times and would have left me stranded if I didn’t have a backup on board. Always wondered why mine konked out where others have not?

What year and model is your truck?

 

[CharlieS]

As a field fix, at least on my '94, you can easily jump pins +B and FP (bottom row left and top row left) in the diagnostic connector (DLC) on the passenger side firewall, and that bypasses the FPR and Fuel Pressure Resistor and pumps battery voltage direct to the fuel pump. I haven't looked at other years.

 

[devo]

@BILT4ME …… I drive 1997 80 series 40th

 

[alia176]

saw this in the EWD


FUEL PRESSURE CONTROL SYSTEM
THE FUEL PRESSURE UP SYSTEM CAUSES THE VSV (FUEL PRESSURE CONTROL) TO COME ON FOR HIGH TEMP. STARTS AND
IMMEDIATELY AFTER STARTING IN ORDER TO INCREASE THE FUEL PRESSURE, IMPROVE STARTABILITY AT HIGH
TEMPERATURES AND PROVIDE STABLE IDLING. THE ENGINE CONTROL MODULE EVALUATES THE SIGNALS FROM EACH
SENSOR (INPUT SIGNALS FROM (1), (2), (5), (11)), OUTPUTS CURRENT TO TERMINAL FPU AND CONTROLS THE VSV.

* FUEL PUMP CONTROL SYSTEM
THE ENGINE CONTROL MODULE OPERATION OUTPUTS TO TERMINAL FPR AND CONTROLS THE
FUEL PUMP RELAY AND THUS CONTROLS THE FUEL PUMP DRIVE SPEED IN RESPONSE TO CONDITIONS.

 

[CharlieS]

saw this in the EWD


FUEL PRESSURE CONTROL SYSTEM
THE FUEL PRESSURE UP SYSTEM CAUSES THE VSV (FUEL PRESSURE CONTROL) TO COME ON FOR HIGH TEMP. STARTS AND
IMMEDIATELY AFTER STARTING IN ORDER TO INCREASE THE FUEL PRESSURE, IMPROVE STARTABILITY AT HIGH
TEMPERATURES AND PROVIDE STABLE IDLING. THE ENGINE CONTROL MODULE EVALUATES THE SIGNALS FROM EACH
SENSOR (INPUT SIGNALS FROM (1), (2), (5), (11)), OUTPUTS CURRENT TO TERMINAL FPU AND CONTROLS THE VSV.

* FUEL PUMP CONTROL SYSTEM
THE ENGINE CONTROL MODULE OPERATION OUTPUTS TO TERMINAL FPR AND CONTROLS THE
FUEL PUMP RELAY AND THUS CONTROLS THE FUEL PUMP DRIVE SPEED IN RESPONSE TO CONDITIONS.

(1) ENGINE COOLANT TEMP. (WATER TEMP.) SIGNAL CIRCUIT
(2) INTAKE AIR TEMP. SIGNAL CIRCUIT
(5) THROTTLE POSITION SIGNAL CIRCUIT
(11) STOP LIGHT SW SIGNAL CIRCUIT

 

[alia176]

Looks like the Circuit Opening Relay closing/opening would cause the FPR to switch. When the COR opens/closes, it is switching +B 12vdc power to the FPR, just follow the yellow wire to the previous page. The Yellow becomes Y/R wire that goes all the back to the EFI main relay.

 

[CharlieS]

I don't think I mentioned it, but I put a new EFI main relay the other day and saved the old one as a spare. The COR seems to be working correctly, so I didn't touch it.

 

[alia176]

I'd be curious to see if you can find out the reason the FPR is clicking on/off during idle.

 

[CharlieS]

I'd be curious to see if you can find out the reason the FPR is clicking on/off during idle.

Yes that was my primary reason for this thread. I'm still searching. Lots of good information though.

Best guess is that one of the input data sources is telling it parameters that don't make sense. My ECT (coolant temp) is new, VAF (for IAT) is used but replaced (original bench tested fine too). TPS (throttle position) checks out good in bench testing.

 

[TomH]

Be aware that this system changed in the later models. The diagram @alia176 posted is for a later model where the fuel pump is not wired directly to the data link connector. In the later models I don't think you can jump +B and FP at the data link connector. Not sure when the changes were made.

 

[CharlieS]

Be aware that this system changed in the later models. The diagram @alia176 posted is for a later model where the fuel pump is not wired directly to the data link connector. In the later models I don't think you can jump +B and FP at the data link connector. Not sure when the changes were made.

That is a great callout, I had no idea. Makes a big difference.

 

[jonheld]

Looks like the Circuit Opening Relay closing/opening would cause the FPR to switch. When the COR opens/closes, it is switching +B 12vdc power to the FPR, just follow the yellow wire to the previous page. The Yellow becomes Y/R wire that goes all the back to the EFI main relay.

View attachment 3428390

View attachment 3428393

Negative. It is the logic on ECU pin E7-13 that causes the FPR to switch. FPR=fuel pressure relay.
The COR just feeds +12 to the FPR and is also controlled by the ECU E4-14 FC=fuel control.
All +12 for this and most other ECU logic comes from the EFI MAIN relay. All it does is supply voltage. The ECU runs the show.

 

[jonheld]

Be aware that this system changed in the later models. The diagram @alia176 posted is for a later model where the fuel pump is not wired directly to the data link connector. In the later models I don't think you can jump +B and FP at the data link connector. Not sure when the changes were made.

One of the major differences between OBD1 ad OBD2.

 

[CharlieS]

Is there any trick to unplugging the connector on the fuel pump resistor? Lift up on the tab.

Test resistance per the FSM - Approx. 0.73 0 at 20 C (68 F)...

Mine is 1.4 ohms at 72 F.

Resistor replacement time, I think.

 

[frankenyota]

That resistance for the resistor seems too high, hopefully a new one sorts your issue. Lift up the tab and wriggle the terminal usually works for me.
You can also bridge the resistor plug wiring to test your fuel pump operation, its puts 12 volts to the fuel pump all the time thou.
I'm chasing a cutting out problem which l thought was related to the Fuel Pump Relay, so bought a new one but its still happening.
It only happened a few times but seems to be getting more regular. It played up yesterday and l swapped in another fuel pump resistor and it fired up but as soon as l got home it did it again (5 mins drive)
I then swapped the Circuit Opening Relay, still the same. Then swapped the EFI relay which was very hot to touch and it fired up and seems good.
It seems the earlier 80 series had an issue with thin wire to the EFI relay, but not sure if the later models have this problem?
Mines a 96 model. I do run bigger fuel pump and thinking this might draw more current so wondering if l need to upgrade all the wiring?

 

[jonheld]

Test resistance per the FSM - Approx. 0.73 0 at 20 C (68 F)...

Mine is 1.4 ohms at 72 F.

Resistor replacement time, I think.

For reference, I have a brand new FP resistor, new in the box. It measured .75 ohms @ 73F.
Please remember to zero out your test leads when measuring low resistances to get an accurate reading.

 

[CharlieS]

I didn't know about the zeroing the meter thing. Thank you for mentioning that. I googled how to zero my meter and retested the old and new (used) resistors. They both settled out at 0.7 ohms. Thank you for sharing this information.