Voltage required to start fuel pump?
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Voltage required to start fuel pump?
Short story is that my 2000 f150 4.6L is starting intermittently (now its rarely) for the past week and turns out the fuel pump isn't always kicking on during startup. When I hear it kick on, the truck will start immediately. Odd thing is that the fuel pump and truck will always start with a battery jump starter. Checking voltage with a (slow) DVM at the fuel pump harness connector gives me:
Startup with battery:
12.05 for two seconds, then down to 6.83
Startup with jump starter on battery:
12.48 for two seconds, then down to 7.06
So, anyone know if this half volt difference might be the cause (bad ground? I dunno), or is the fuel pump not supposed to be that sensitive and it's going bad?
Things checked but no change:
New battery
Battery terminals thoroughly cleaned
All fuses checked, any fuses associated with fuel pump and PCM was replaced
Tried new relays for fuel pump and PCM, returned them
Anti-theft light doesn't flash
New fuel filter two months ago
Crankshaft connecter cleaned & dielectric greased
Fuel pump connector cleaned & dielectric greased
BTW, great forum, learned lots from you guys.
Startup with battery:
12.05 for two seconds, then down to 6.83
Startup with jump starter on battery:
12.48 for two seconds, then down to 7.06
So, anyone know if this half volt difference might be the cause (bad ground? I dunno), or is the fuel pump not supposed to be that sensitive and it's going bad?
Things checked but no change:
New battery
Battery terminals thoroughly cleaned
All fuses checked, any fuses associated with fuel pump and PCM was replaced
Tried new relays for fuel pump and PCM, returned them
Anti-theft light doesn't flash
New fuel filter two months ago
Crankshaft connecter cleaned & dielectric greased
Fuel pump connector cleaned & dielectric greased
BTW, great forum, learned lots from you guys.
Lead Driver
Joined: Sep 2006
Posts: 7,907
Likes: 143
Could be a ground.
Have you eliminated the Crash sensor behind the passenger kick panel?
Half voltage is too low indicating there is to much resistance in the pump circuit feed or ground return.
Most pumps draw an average of about 7 amps.
At 12 volts cranking voltage the total circuit resistance would be close to 1/2 ohm.
How this is is 12v / 7a =.58 ohms.
If the circuit resistance rises for some reason, using the voltage you see as 6.83, the resistance would be .98 ohm or nearly double.
Taking the sum of the voltage drops for 12v - 6.83v would mean the circuit fault is dropping 5.17 volts. 6.83+5.17 = 12v.
This amount of drop is not tolerable in that circuit same at it would not be for lighting at any point on the truck.
The pump has to generate pressure at near 90 to 100 psi and won't do that on low voltage.
Keep looking. It's there some where.
Another item to keep in mind is if the return line is blocked, the pump is trying to pump against it and may draw excess current that will drop the voltage if there is any substanical resistance in the circuit.
Good luck.
Have you eliminated the Crash sensor behind the passenger kick panel?
Half voltage is too low indicating there is to much resistance in the pump circuit feed or ground return.
Most pumps draw an average of about 7 amps.
At 12 volts cranking voltage the total circuit resistance would be close to 1/2 ohm.
How this is is 12v / 7a =.58 ohms.
If the circuit resistance rises for some reason, using the voltage you see as 6.83, the resistance would be .98 ohm or nearly double.
Taking the sum of the voltage drops for 12v - 6.83v would mean the circuit fault is dropping 5.17 volts. 6.83+5.17 = 12v.
This amount of drop is not tolerable in that circuit same at it would not be for lighting at any point on the truck.
The pump has to generate pressure at near 90 to 100 psi and won't do that on low voltage.
Keep looking. It's there some where.
Another item to keep in mind is if the return line is blocked, the pump is trying to pump against it and may draw excess current that will drop the voltage if there is any substanical resistance in the circuit.
Good luck.
Lead Driver
Joined: Sep 2006
Posts: 7,907
Likes: 143
Second part.
At key on to run, the PCM is powered up and closes a solid state switch to offer a ground to a relay for about 1 second.
This relay passes the fused power source to the pump circuit for a short time.
Under cranking load, the battery voltage 'drops' to about 12 volts.
Even at this point the pump puts out a high pressure to fully pressureize the fuel rails so the regulator can reduce it to the rated pressure range with some control by engine vacuum, after starting.
At key on to run, the PCM is powered up and closes a solid state switch to offer a ground to a relay for about 1 second.
This relay passes the fused power source to the pump circuit for a short time.
Under cranking load, the battery voltage 'drops' to about 12 volts.
Even at this point the pump puts out a high pressure to fully pressureize the fuel rails so the regulator can reduce it to the rated pressure range with some control by engine vacuum, after starting.
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Half voltage is too low indicating there is to much resistance in the pump circuit feed or ground return.
Most pumps draw an average of about 7 amps.
At 12 volts cranking voltage the total circuit resistance would be close to 1/2 ohm.
How this is is 12v / 7a =.58 ohms.
If the circuit resistance rises for some reason, using the voltage you see as 6.83, the resistance would be .98 ohm or nearly double.
Taking the sum of the voltage drops for 12v - 6.83v would mean the circuit fault is dropping 5.17 volts. 6.83+5.17 = 12v.
This amount of drop is not tolerable in that circuit same at it would not be for lighting at any point on the truck.
Most pumps draw an average of about 7 amps.
At 12 volts cranking voltage the total circuit resistance would be close to 1/2 ohm.
How this is is 12v / 7a =.58 ohms.
If the circuit resistance rises for some reason, using the voltage you see as 6.83, the resistance would be .98 ohm or nearly double.
Taking the sum of the voltage drops for 12v - 6.83v would mean the circuit fault is dropping 5.17 volts. 6.83+5.17 = 12v.
This amount of drop is not tolerable in that circuit same at it would not be for lighting at any point on the truck.
Once the truck is started everything is normal at any speed, even quick starts and stops so I'm assuming there is no blockage.
Lead Driver
Joined: Sep 2006
Posts: 7,907
Likes: 143
To answer for once the truck is started.
Once the motor is started, the pump runs full time so the pressure gets up high enough.
Then at shutdown, the check valve in the pump holds the pressure for some amount of time before bleeding off, so a hot restart may not show to be a problem.
The proper way, though few ever do it, is to measure the pump flow rate or volume to determine if the pump has an issue.
Once you think a bit about it, pressure is not the only factor, but flow rate under that pressure has to be up with it.
The reason is the pump line pressure will rise if you squeeze off the return {but} the volume flow rate goes down. It's an inverse function.
Same as putting your finger over a water hose to increase the exit pressure. The amount of water coming out per unit of time is reduced.
For the fuel pump, the pump's capacity and line size determined by design is the sole control of pressure and volune at a specified design operating voltage.
Good luck.
Once the motor is started, the pump runs full time so the pressure gets up high enough.
Then at shutdown, the check valve in the pump holds the pressure for some amount of time before bleeding off, so a hot restart may not show to be a problem.
The proper way, though few ever do it, is to measure the pump flow rate or volume to determine if the pump has an issue.
Once you think a bit about it, pressure is not the only factor, but flow rate under that pressure has to be up with it.
The reason is the pump line pressure will rise if you squeeze off the return {but} the volume flow rate goes down. It's an inverse function.
Same as putting your finger over a water hose to increase the exit pressure. The amount of water coming out per unit of time is reduced.
For the fuel pump, the pump's capacity and line size determined by design is the sole control of pressure and volune at a specified design operating voltage.
Good luck.
Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
Bad battery, replace it. The charger curing the problem told you that. Why does every one try to make this so hard.
During starting, if the battery drops below 10.5 volts, the battery is bad.
During starting, if the battery drops below 10.5 volts, the battery is bad.
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The voltage test I noted wasn't at the battery, the test was at the harness connecter going to the fuel pump (black and pink wires).
I just tested voltage at the battery at startup about 20 times and this is what I got...
First 4 tries-
before turning key: 12.38v
turn key on (not start) and fuel pump kicks on for about two seconds: 12.21v
fuel pump stops priming: 12.25v
From the 5th - 14th(ish) try-
before turning key: 12.36v
turn key on and fuel pump doesnt kick on: 12.11v
after about two seconds then voltage is: 12.22v
About the 15th(ish) - 20th try:
before turning key: 12.30
turn key on and fuel pump kicks on for about two seconds: 12.16v
fuel pump stops priming: 12.20v
Looks like the fuel pump is trying to draw more power when it doesn't kick on. Perhaps fuel line blockage or bad pump? I may be able to try a fuel volume test today to help narrow things down.
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Post Fiend
Joined: Mar 2004
Posts: 6,056
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From: Union, Washington
A fully charged battery should read 12.8 volts before you do anything with it. 12.38 volts is way to low.
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Post Fiend
Joined: Mar 2004
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From: Union, Washington
The ignition switch does not control the pump voltage, the computer does. It turns on the pump to prime (pressure up) the system, the the pump shuts off. Once the engine starts the pump runs again. The voltage difference is negligible.
Senior User
Joined: Mar 2005
Posts: 115
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From: Powhatan, Virginia
I am having the same issues with my 2001 F150 Lariat 4.6l. I am trying to fugure this one out as well. If you figure it out let me know and I'll do the same. I am getting ready to replace the fuel pump. All relays have been replaced
Fleet Owner
Joined: Mar 2003
Posts: 24,724
Likes: 74
From: Blue Hill Township
Check the 12v feed from the fuel pump relay to the fuel pump. Have you tried a jumper wire from the battery directly to the fuel pump to see if it will solve the problem? You probably have a corroded connection or a bad wire.
Cargo Master
Joined: Apr 2003
Posts: 3,476
Likes: 224
From: Frequently frozen MN
To clarify some stuff.
Your first note talked about a voltage drop after 2 seconds -- that's about the time that the fuel pump is shut off on startup. ( It runs for 2 seconds then the computer shuts it off until it senses that the engine has started).
So, you might have just been seeing your voltmeter drift downward.
Your battery voltage does seem a touch low - although not horribly so.
Full battery charge under controlled conditions is 12.65 v. However, temperature is a factor. Also, some DVMs aren't all that accurate.
You might try checking your charge voltage. Leave the meter on after the truck starts and see what it goes to. 13.8 v is pretty common, but it does vary. Could be that your alternator is not quite up to it.
Could also be worthwhile to make some jumper wires so that you can monitor battery voltage from the cab. Use some ring tongue terminals and try to sneak the wires into the cab with some kind of a connector in the cab so that you can get a reliable connection to your meter. Then you can see what's going on when it fails.
Good Luck,
hj
Your first note talked about a voltage drop after 2 seconds -- that's about the time that the fuel pump is shut off on startup. ( It runs for 2 seconds then the computer shuts it off until it senses that the engine has started).
So, you might have just been seeing your voltmeter drift downward.
Your battery voltage does seem a touch low - although not horribly so.
Full battery charge under controlled conditions is 12.65 v. However, temperature is a factor. Also, some DVMs aren't all that accurate.
You might try checking your charge voltage. Leave the meter on after the truck starts and see what it goes to. 13.8 v is pretty common, but it does vary. Could be that your alternator is not quite up to it.
Could also be worthwhile to make some jumper wires so that you can monitor battery voltage from the cab. Use some ring tongue terminals and try to sneak the wires into the cab with some kind of a connector in the cab so that you can get a reliable connection to your meter. Then you can see what's going on when it fails.
Good Luck,
hj
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Man, good suggestions, I'll do some of these in the morning with what else I got planned, which is...
Remove the truck bed and rebuild the fuel pump assembly.
Don't scream at the computer, I know, I know...
Can't remember how many years ago it was rebuilt so might as well do the preventive maintenance even if it doesn't sound like the culprit. But, if it's seizing, that would explain the extra battery voltage drop at all the times the pump doesn't start, which is the main thing stuck in my head.
Also, it could be a blocked return as Bluegrass suggested. With the bed off I can more easily do a volume and pressure test on the entire return. Of course check all the electrical as well.
If all this doesn't cut it then perhaps Monday or Tuesday will be battery swap day. Hopefully an Expletive Touretts Day will not happen afterwards.
Remove the truck bed and rebuild the fuel pump assembly.
Don't scream at the computer, I know, I know...
Can't remember how many years ago it was rebuilt so might as well do the preventive maintenance even if it doesn't sound like the culprit. But, if it's seizing, that would explain the extra battery voltage drop at all the times the pump doesn't start, which is the main thing stuck in my head.
Also, it could be a blocked return as Bluegrass suggested. With the bed off I can more easily do a volume and pressure test on the entire return. Of course check all the electrical as well.
If all this doesn't cut it then perhaps Monday or Tuesday will be battery swap day. Hopefully an Expletive Touretts Day will not happen afterwards.