Fuel pump voltage

[MikesStripedSVT]

Car cranks but won't start. Installed a new fuel pump and i'm still not hearing it turn on. I checked the voltage at the pump connector and i'm getting 7 volts. What should voltage be at the fuel pump and fuse? When I put 12volts straight to the pump I hear it turn on.

Thank you,
Mike

 

[MikesStripedSVT]

The fuse (14) and fuel shutoff both show 7 volts also (with ignition on). 2 of the relay (R8)connections on the box show 12 and 1 shows 7. I'm not sure which connections should show what. The ford service CD should be showing up in the mail soon. The wiring diagrams should help. I'm thinking its a bad connection somewhere. Anyone know where a jumper wire should be ran to the fuel pump at? It starts when running jumpers right from the battery.

 

[Big Jim]

The fuse (14) and fuel shutoff both show 7 volts also (with ignition on). 2 of the relay (R8)connections on the box show 12 and 1 shows 7. I'm not sure which connections should show what. The ford service CD should be showing up in the mail soon. The wiring diagrams should help. I'm thinking its a bad connection somewhere. Anyone know where a jumper wire should be ran to the fuel pump at? It starts when running jumpers right from the battery.

Mike,

Please tell us the needed specs on your car. What year, engine, transmission, etc.

This is critically important because depending on the year, there are two different fuel systems, return and returnless. The controls for each system are very different.

 

[gorman]

it could be the ground wire (inside the fuel tank on the sender unit) loose. If poss. test the unit at the plug to see if you have a good connection... otherwise you may have to take the sender unit back out to check it physically ..GL ...G.

 

[MikesStripedSVT]

Its a 99 SVT with return fuel system.

I just installed a new pump. I also checked the voltage at the connector going straight to the battery for ground and I get the same 7 volts. So ground doesnt seem to be the issue. I did clean the battery (relocated to trunk) ground and fuse box grounds.

The car has been sitting for about a year. I'm getting ready to swap in a 3L but want to solve this first.

Also when cranking it goes down to about 5 volts.

 

[gorman]

have you reset the inertia switch in the trunk a couple of times ? ...G.

 

[MikesStripedSVT]

have you reset the inertia switch in the trunk a couple of times ? ...G.

Is there one in the trunk? or do you mean the one behind the drivers side kick panel by the fuse box? The voltage here is also the same, 7 volts.

 

[gorman]

Yes your switch is behind the footwell panel.
The fact that you have 7v suggests you have a high resistance joint/connection somewhere. You could try swapping out the relay and making sure the fuse and relay are firmly seated.
When you apply 12v directly to the pump, are you getting fuel at the schrader valve ? ... Also; if the pump runs ...does it pressurize the system and stop or keep running ?.
If it keeps running, the ECU may be in limp mode .... G.

 

[MikesStripedSVT]

it should be in the trunk .. or at least it is on my mondeo.
The fact that you have 7v suggests you have a high resistance joint/connection somewhere. You could try swapping out the relay and making sure the fuse and relay are firmly seated.
When you apply 12v directly to the pump, are you getting fuel at the schrader valve ? .... G.

I swapped the relay with a different circuit, still the same. I also feel the relay click. Can a relay 1/2 way work? or does it work or not work.

I didnt open the valve but when I apply 12v directly I hear the pump turn on. I hear nothing with it getting 7v. The car will start with 12v going directly to the pump then shut off. With 7v I get nothing, just cranks.

I'm thinking i'll need to run a jumper wire from the relay if I can't find the high resistance connection. I just don't know what tab of the relay to tap into. Hopefully i'll be getting the ford service cd soon so I can take a look at a wireing diagram.

 

[gorman]

I swapped the relay with a different circuit, still the same. I also feel the relay click. Can a relay 1/2 way work? or does it work or not work..

as long as the relay you swapped in was the same .. that is ok. Normally a relay works or it doesn't.. a burnt contact inside can cause it to flutter or cause a volt drop to the output.

I didnt open the valve but when I apply 12v directly I hear the pump turn on. I hear nothing with it getting 7v. The car will start with 12v going directly to the pump then shut off. With 7v I get nothing, just cranks...

check the pump pressurizes the system and then stops.

I'm thinking i'll need to run a jumper wire from the relay if I can't find the high resistance connection. I just don't know what tab of the relay to tap into. Hopefully i'll be getting the ford service cd soon so I can take a look at a wiring diagram.

Make sure you operate the inertia switch a couple of times... also, disconnect the ECU and make sure you leave it long enough to reset.
One other thing .. double check to fuel pipe connections on the sender are the right way round ( no insult intended..just not taking anything for granted)...

One nagging question remains.. if you only have 7v at the relay .. this suggests that you have a main battery/ground or fusebox wiring problem.. only thing is you said you checked them .. might be worth a double check..report back ...G.

 

[MikesStripedSVT]

Ok checked everything again with ignition on....

With everything hooked up as normal there is no fuel pressure at fuel rail. When connecting the fuel pump directly to the battery I get fuel pressure at fuel rail. I'm just opening the schrader valve, I don't have a fuel pressure gage.

Pump connector: 7.4v

Pump connector positive going straight to battery for ground: 7.4V
This eliminates a bad ground.

Fuel shutoff switch: 7.4v

Fuse: 7.4v

Relay: See pic…. http://contour.org/ceg-vb/picture.php?albumid=288&pictureid=1684

Same conditions after reseting ECU.

How does power flow? Battery > relay > fuse > fuel shutoff switch > pump ?
If so there seems to be a issue between the relay and fuse but I don't know how the relay should work. Should the lower connector only read 7.4v? If not it seems to be a issue with the relay connector wiring.

 

[gorman]

short circuiting the fuel pump inertia switch is an inadvisable thing to do. Would it be possible find an ignition switched supply of 12v and connect it to the inertia switch ? ....7v at the fuse says bad connection for the fuse/relay box. Is the common rail in the fuse/relay box giving 7v ?

Just seen that F10 protects the power supply to the fuel pump relay. Try making sure this fuse is properly inserted ...G.

 

[gorman]

here's the control circuit.

Fuel pump power is fed from R8 thru F14

The R8 switching circuit is from R11 protected by F9.

The grounds for these circuits are
1. behind the LH headlight (fuse/relay box ground)
2. RH 'A' pillar (ignition supply control relay)
......G.

 

[2000ZxT]

short circuiting the fuel pump inertia switch is an inadvisable thing to do. Would it be possible find an ignition switched supply of 12v and connect it to the inertia switch>
.

Why not just hardwire the pump to the battery with a relay in between, the relay using the old fuel pump power and ground as 85 and 86 on the relay. It is how mine is and that way it retains the intertia switch. I would think the 7v would be good enough to trigger the relay.

 

[gorman]

Why not just hardwire the pump to the battery with a relay in between, the relay using the old fuel pump power and ground as 85 and 86 on the relay. It is how mine is and that way it retains the intertia switch. I would think the 7v would be good enough to trigger the relay.

From what i can gather, the 7v is on the pump power supply not the relay switch circuit. ... a permanant 12v supply must have a fuse before the relay or it would be an overcurrent/fire hazard.
. ... G.

 

[jd2-98s]

The terminal above the 7.4volt is your supply. When the relay switches on it should carry the voltage over.

But why 7.4 volt on the fuel pump terminal of that relay socket.... Well it does tie back into pin 40 of the computer. It also goes to that connector that is taped off near the main engine harness plug around the master cylinder. It is a purple wire with a female connector that is just floating there.

So reading what you have posted this is what I can come up with.

In all cases usally if you probe the terminal that runs to the fuel pump. You should be able to see ground, if the pump motor is not open and is also grounded. Which most likely it is not, the 7.4volts is coming from the computer.

What I would do is disconnect the plug at the fuel pump sender. 7.4 volt at violet/orange wire with the key on or off? Either way Have someone turn the key off for 10seconds then back on. Voltage should go to battery voltage 12-13.4volts for 2 seconds. If yes, fuel pump ground wire, or fuel pump connector on sender, or fuel pump itself open. If no, open fuse, bad relay, bad relay control circuit.

If it doesn't go to battery voltage at the fuel pump plug, then have someone jump the to larger terminals the ones you list as 13.4 and 7.4 volts and see if you get battery voltage to the plug. If yes then plug it back into the pump and listen for it to turn on. If it does you know you have an issue with the PCM suppling the turn on voltage to the pin you list as 0volts, which gets pulled to ground by the PCM to turn the relay on. If it doesn't turn on you know you have to check the ground wire of the harness side, or the pump motor is open.

Again the only way you would be reading 7.4 volts by using a VOM and messuring with the black lead to battery ground and the positive lead to that bottom terminal is an open in the fuel pump or ground for the pump and the computer is showing that voltage. If you can see that purple wire floating I bet if you probe that too you will get 7.4 volts, and if you have someone work the key and the relay is installed and working it too should go 13.4 volts (or battery voltage) for 2 seconds. This is the prime before cranking.

 

[gorman]

.... the ecu is the ground switch for the fuel pump relay control circuit. No power comes from the ecu to run the pump. As has been stated, he has 7v from the ignition switched supply that does power the pump (thru the relay)..... check ground connections for the fusebox (underneath the battery)...G.

 

[MikesStripedSVT]

Ok its been a busy 2 weeks, time to get back to work on the car! Thanks for the discussion!

What I would do is disconnect the plug at the fuel pump sender. 7.4 volt at violet/orange wire with the key on or off? Either way Have someone turn the key off for 10seconds then back on. Voltage should go to battery voltage 12-13.4volts for 2 seconds. If yes, fuel pump ground wire, or fuel pump connector on sender, or fuel pump itself open. If no, open fuse, bad relay, bad relay control circuit.

Key on. Your correct ( I didn’t notice this before) the connector reads 13v for 1-2 seconds.

If it doesn't go to battery voltage at the fuel pump plug, then have someone jump the to larger terminals the ones you list as 13.4 and 7.4 volts and see if you get battery voltage to the plug. If yes then plug it back into the pump and listen for it to turn on. If it does you know you have an issue with the PCM suppling the turn on voltage to the pin you list as 0volts, which gets pulled to ground by the PCM to turn the relay on. If it doesn't turn on you know you have to check the ground wire of the harness side, or the pump motor is open.

I jumped the terminals and yes I get 13v at the connector but when I connect it to the pump it does not turn on.

Again the only way you would be reading 7.4 volts by using a VOM and messuring with the black lead to battery ground and the positive lead to that bottom terminal is an open in the fuel pump or ground for the pump and the computer is showing that voltage. If you can see that purple wire floating I bet if you probe that too you will get 7.4 volts, and if you have someone work the key and the relay is installed and working it too should go 13.4 volts (or battery voltage) for 2 seconds. This is the prime before cranking.

Yes the purple wire is doing what you described. I removed the fuel pump and everything is connected. I don’t understand why this would be a issue if the bottom relay connector is reading 7v and it shouldnt be then why would this matter.

LOL I have been writing this the past 3 hours I have been diagnosing. Electronics has never been my strong point. But I started to understand what you meant by if the circuit is open the bottom relay connector will read 7v. So I started wiggling the fuel pump connector with the meter hooked up to the bottom connector. I noticed the voltage was changing and I got it to read 0v holding the connector towards the front of the car. Horary she started! I just don’t understand how this happened from just sitting. She has been sitting for about a year because of a engine knock and no inspiration to fix it. I guess I’ll try installing a new connector.

Thanks for all the help you guys! Time to start on the 3L sitting next to me

 

[jd2-98s]

.... the ecu is the ground switch for the fuel pump relay control circuit. No power comes from the ecu to run the pump. As has been stated, he has 7v from the ignition switched supply that does power the pump (thru the relay)..... check ground connections for the fusebox (underneath the battery)...G.

That is not what I stated.

There is 4 pins for a relay. 2 for the coil, 2 for the switch.

Coil- 1 pin will be positive, the other will switch to ground by the ECU.

Switch, one will be hot, live, B+ whatever (battery voltage) the source, the other will be the load or drain in this case the fuel pump.

When you probe a load terminal for the fuel pump, you should be able to to see 0volts or ground threw the motor windings. Seeing he was getting 7.4volts there was only one reason for this, that load terminal also goes back to pin 40 of the ecu, this is not the turn on output from the ecu to the coil. It is a pin that will get battery voltage, most likely to show the ECU the pump circuit is live. Why 7.4 volts, because there is a connection issue be it an open pump motor, open ground, or bad connection, the load was no there, and not meaning to drive the pump, it just the fact it wasn't grounded threw the pump motor.

I have a power probe with a display. What would happen when testing that terminal the display would of showed 7.4 volts, what should happen if everything is connected is 0.0volts and a green light showing ground. Why would you see ground. Because negitive electrons would be flowing from the ground wire, to the pump, threw the motor winding, thew the wire, threw the fuel cut off switch, and back to that terminal, waiting for positive to complete the loop. But in this case it was open at some point, and the fact that it ties to the ECU for whatever reason, with pin 40 completely unloaded, you get 7.4 voltage out of pin 40 of the ECU.

 

[jd2-98s]

Ok its been a busy 2 weeks, time to get back to work on the car! Thanks for the discussion!



Key on. Your correct ( I didn’t notice this before) the connector reads 13v for 1-2 seconds.



I jumped the terminals and yes I get 13v at the connector but when I connect it to the pump it does not turn on.



Yes the purple wire is doing what you described. I removed the fuel pump and everything is connected. I don’t understand why this would be a issue if the bottom relay connector is reading 7v and it shouldnt be then why would this matter.

LOL I have been writing this the past 3 hours I have been diagnosing. Electronics has never been my strong point. But I started to understand what you meant by if the circuit is open the bottom relay connector will read 7v. So I started wiggling the fuel pump connector with the meter hooked up to the bottom connector. I noticed the voltage was changing and I got it to read 0v holding the connector towards the front of the car. Horary she started! I just don’t understand how this happened from just sitting. She has been sitting for about a year because of a engine knock and no inspiration to fix it. I guess I’ll try installing a new connector.

Thanks for all the help you guys! Time to start on the 3L sitting next to me

Glad to hear it started, make sure the connectors inside the harness are not spread open, also look at the fuel pump side of the connection and make sure there is no burned pins, or signs of it melting inside that connection.

This was a good thread, figure would add this pic for future searches.