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Thread: Fuel Pump Resistance Wire Bypass

  1. #1
    Red Captain MikeFleming's Avatar
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    Fuel Pump Resistance Wire Bypass

    The resistance wire is in the ground side of the 4-wire harness in the hatch area inside the car. The black wire turns into a white resistance wire a few inches from the lower floor grommet. Tag into the far side of the white-black junction (towards the fuel pump) with a 12 gauge wire and run that back to a good chassis ground.

    Additionally run a 12 gauge (or larger) wire to the fuel pump relay from the harness take-off that goes to the chassis-mounted fuel pump back to the power wire to the in-tank pump.

    The stock, low-pressure pump will pull about 4 Amps at peak power and the 255 LPH pump will pull about 17 Amps at full pressure. So the stock power AND ground wires are not large enough.

    Some pics of the ground resistor bypass.
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  2. #2
    Red Captain MikeFleming's Avatar
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    A few more details here. The fuel pump power wiring on the dual-pump systems comes from the fuel pump relay (under the drivers seat, with the harness end coming through a cut in the carpet) and travels toward the left side of the inside chassis rail. From there it goes rearwards to the lower frame area of the rear seat, and rearwards over the left rear inner fender. Very easy to trace with the carpet and inner plastic panels removed (as shown in the pics below).

    There is some padding installed over the wiring that travels from the left to right side - to provide power to the chassis-mounted fuel pump. This wire splices into a lower gauge of wire and heads rearwards over the inner fender hump to the fuel tank harness (and where it contains the resistance wire mentioned above).
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    Be sure to plug that hole with a rubber chassis plug (1-1/4").

    Chassis pump wiring removed and brought to the left side for extension and re-routing. It's the dirty harness section in the top of the pic.
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    Wiring going rearwards.
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    Unwrapping the harness shields to expose the wire to be extended. The chassis pump harnesses is in the lower part of the pic now.
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  3. #3
    Red Captain MikeFleming's Avatar
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    Fuel pump relay harness. The pink-red stripe wire is the power out to the fuel pump. I splice a heavy-gauge wire into that wire and run it rearwards to the fuel pump harness near the resistance ground wire we bypassed previously.
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    Expose the associated harness section in the hatch area.
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    Route the new wire along the same harnesses that already run to the rear. Here I'm adding a 10 gauge white/red hash wire. Shrink wrap the parallel connection.
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    Shrink wrap the rest of the wire as it goes rearwards for some added physical protection.
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    That wire feeds all the way to the rear hatch area and splices into the appropriate wire in the 4-wire connector to the fuel tank harness. This gives a large gauge wire to the rear pump from the fuel pump relay and we previously installed a large gauge ground wire from that same harness connector to a good ground in the rear hatch.
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  4. #4
    Red Captain MikeFleming's Avatar
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    Lastly we need to increase the wiring size for the power in and power out (ground) wire in the tank harness. Here's some pics of the modified, assembled harness, ready to install.
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    The later harnesses - that were originally installed into a single-pump system - may already have larger gauge wiring in them. I do not know - but it's worth checking it out. The fuel pump and level gauge connectors should be the same through 1993.
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  5. #5
    Noob
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    I didn't realize that the SVO had essentially the same setup as the XR4Ti, with the resistor-ground wire. What are the implications of swapping out the resistance ground with a "normal" ground, but still using the stock two-pump setup with the low-pressure in-tank pump? In the interest of time, I'm running that way at the moment and it's been fine. So far only put about 500 miles on the setup though.

  6. #6
    Red Captain MikeFleming's Avatar
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    It draws a wee bit more power and the in-tank pump gets a bit noisier. That's about it.

    The in-tank pump [of the dual-pump system] only reaches 6 PSIG, so it's not a big power draw.
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  7. #7
    Some Boost
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    Hi Mike,

    A couple of questions about this:

    - why are you bringing the chassis pump's wires back to the hatch area? I don't see any mention of using it/tapping it

    - is the wire from the relay going directly to the in-tank pump or somehow to the chassis wire ("Additionally run a 12 gauge (or larger) wire to the fuel pump relay from the harness take-off that goes to the chassis-mounted fuel pump back to the power wire to the in-tank pump. " is what I'm confused about.)

    - is there a reason to not cut the wire from the relay and run it directly to the in-tank pump vs. doing the splicing? Likewise cut the ground wire and run a new wire vs. tapping it?

    - any pictures of the tapping at the in-tank pump? The ground picture makes it clear which one to to tap into for the ground, but I'm not sure of the power side.

    Thanks,

    Chris
    1984 Mercury Capri RS
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  8. #8
    Red Captain MikeFleming's Avatar
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    In the dual-pump system, there is a 12ga wire fed to the chassis pump and 16 or 18 gauge to the in-tank. Obviously the 16/18ga tank wire is inadequate for a 255 pump (pulls ~17 Amps at full chat) and since I'm no longer using the stock chassis pump, I just extended that wire to the tank harness.

    There's four wires in the tank harness: two for the fuel pump and two for the level sensor.
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  9. #9
    Building Boost
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    This is a very well written and accurate. I do have a questions though. I am in the process of converting my 85 SVO to a single in-tank 255 lph pump. Here is what I planned to do... cut the connector off the stock chassis pump. take that same pigtail that I have created by cutting off the connector from the pump. I have then added two 12 gauge wires (pos and neg) off the from this pigtail and run them back to the hatch. I will then remove the connector from the pump. On the original harness to the low pressure pump, I will remove the resistor wire, substituting it with appropriate gauge wires in its place. At this point, I will cut the positive and negative wires going to the in-tank pump. I will combine both the positive wires and the negative wires that were going to the in-tank pump and the chassis pump and run them to the new 255 lph in-tank. This should allow for plenty of amps for the new pump.

    Can anyone see a down side to this? Will it cause any problems?

  10. #10
    Red Captain MikeFleming's Avatar
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    - why are you bringing the chassis pump's wires back to the hatch area? I don't see any mention of using it/tapping it
    I've slept dince I wrote that (and even more since I did it) did that but, iirc, my original plan was to take the ~14 gauge wire from the chassis pump and extend it to the in-tank pump, saving on the amount of wire I needed to use. Then I later decided to replace all of the power wiring from the relay to the in-tank pump harness with 10 gauge wire. A 255lph in-tank pump will pull about 17Amps peak, iirc, at full chat (pressure/flow) and the stock wire was not rated for that at any reasonable duty cycle (read as track time). Iirc the in-tank pump on the dual system uses relatively small wires to/from the in-tank pump, ~16-18ga, which is way too small for 16+ Amps.

    - is the wire from the relay going directly to the in-tank pump or somehow to the chassis wire ("Additionally run a 12 gauge (or larger) wire to the fuel pump relay from the harness take-off that goes to the chassis-mounted fuel pump back to the power wire to the in-tank pump. " is what I'm confused about.)
    I need to clear up that post. In my copious free time. Perhaps my comment above helped?

    - is there a reason to not cut the wire from the relay and run it directly to the in-tank pump vs. doing the splicing? Likewise cut the ground wire and run a new wire vs. tapping it?
    No. The only issue are gaining access to the original crimped connectors in the assorted harness & relay plugs. Some of them are old and brittle and may not handle being "expanded, old wire extracted, new wire inserted, re-crimped". Fortunately the ones I was working with, and having spares available, were successful. Ideally that FP relay and socket should be changed for a 30A Bosch relay and socket.

    - any pictures of the tapping at the in-tank pump? The ground picture makes it clear which one to to tap into for the ground, but I'm not sure of the power side.
    Not that are handy but it's easy enough to probe for (there's only four wires and we already know which one is the ground):

    - Power to in-tank fuel pump (switched by the FP relay)
    - Ground from in-tank fuel pump (the resistance wire) that attaches to the rear inner hatch area
    - Signal to the fuel level sensor
    - Signal from the fuel level sensor

    Wires/cables that need to be made larger (12 ga min; 10 ga more better):
    - (Ideally) run a separate power wire from the starter solenoid to the FP relay (with a 20A fuse) to replace the yellow power wire
    - Output power wire from the FP relay all the way to the in-tank pump (two pieces - the chassis harness and the tank harness)
    - The ground wire from the in-tank pump back to the chassis in 12 ga. (two pieces - the chassis harness and the tank harness)

    NOTE:
    I have always disliked the concept of using hte steel chassis as the ground return path in these era vehicles. I much prefer to run separate power AND ground wires to/from each powered device (like how we do it in aircraft and GFRP applications), but, alas, this is what we have to work with. I would recommend using a bolt/nut and star washers on the ground at the hatch rear versus the sheet metal screw with scratched paint.

    Then again, I like to solder stuff too.

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  11. #11
    Building Boost
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    Mike,

    Thanks for the clarity on that. I don't have my wiring diagram in front of me, but what relay feeds the chassis pump? Do they get power from the same relay as the in-tank pump?

  12. #12
    Red Captain MikeFleming's Avatar
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    ... , but what relay feeds the chassis pump? Do they get power from the same relay as the in-tank pump?
    Yes, there is only one Fuel Pump relay, located under the drivers seat behind the front rail on the SVO. Feeds both pumps for dual-pumps and the single pump in single-pump installations.

    The yellow wire is full-time battery positive through a fuse link. That same yellow wire feeds Keep-Alive power to the EEC and power to the EEC relay.

    The ground side of the FP relay coil gets completed by the EEC after it passes through the inertia switch in the rear hatch (takes the long. over-water route).

    The power side of the coil in the FP relay comes to the FP relay from the energized side of the EEC relay.
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    Red Captain MikeFleming's Avatar
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    Can anyone see a down side to this? Will it cause any problems?
    I'm not clear if you're planning on keeping the tank harness separate from the chassis harness. It's nice to have a 4-wire disconnect back there so the tank can be removed and installed.
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  14. #14
    Building Boost
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    For some Clarification...

    Does anyone know why Ford used the resistance wire to start with? I know it will lower the voltage to the factory in-tank pump, but why?

    For my applications, I opted to completely remove the resistance wire and replace it with a section of 12 gauge wire and ran it as a separate ground rather than leave it with the bundle of grounds it was originally with. I made sure all contacts were clean, and soldered all connections with proper shrink-wrap over all as well.
    Last edited by phils85svo; 04-10-2014 at 08:37 AM. Reason: correction

  15. #15
    Red Captain MikeFleming's Avatar
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    Quote Originally Posted by phils85svo View Post
    For some Clarification...

    Does anyone know why Ford used the resistance wire to start with? I know it will lower the voltage to the factory in-tank pump, but why?
    NVH - the pump runs slower so it's more quiet.
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