How to Wire a Secondary Fuel Pump: Relay, Trigger, and Controller Setup for Dual-Pump BMW Builds

The second Walbro 450 in our dual pump LPFP kits pulls 18.9 amps at 80 psi on TI Automotive's own flow sheet, and the activation harness in the box is what carries it. Most of the emails we get after a Stage 3 or 3.5 kit arrives ask the same two things: which wire goes where, and how does it hook up to a Motiv ReFlex+.

This is the support guide we send those buyers, and anyone who buys the harness on its own for a dual pump build: every page of our installation diagram, the four ways to wire a secondary fuel pump trigger, a first start test and a troubleshooting table. Keep our full dual fuel pump activation harness installation diagram (3 page PDF) open on your phone while you work.

We build these kits and drew that diagram ourselves. Pump current comes from TI Automotive's Walbro flow sheets, controller settings from Motiv's ReFlex+ manual and FAQ, REV2 details from bootmod3's documentation, and kit details from our listings.

Shop the Activation Harness

The Short Answer

Wire a secondary fuel pump on a BMW with our harness by running its fused 10 AWG lead to battery positive, grounding it to bare metal and connecting the pump harness to the top hat. Then pick one trigger: the included 13 psi pressure switch, a ReFlex+ low side output on the green wire, a bootmod3 REV2 board, or switched 12V. Run the relay at 0 or 100 percent only.

Which Setup Do You Have?

Your trigger depends on what is already on the car: no controller means the pressure switch in the box, a ReFlex+ means one of its outputs, and a full load only car can use switched 12V. Find your row, then jump to that option.

JB4 owners: Burger Motorsports describes its JB4 port injection controller as having an extra output to run a second fuel pump in place of a Hobbs switch. It is only a trigger, so it still needs our relay. We found no BMS document stating that output's polarity, so confirm it switches to ground before using it like Option B.

MHD owners: MHD's user guide lists antilag, map switching and flex fuel as its main MHD+ features, and we found no secondary pump output in its documentation. Use our switch or a ReFlex+ output. Any other controller output that switches to ground at 0 or 100 percent, Pro EFI included, wires like Option B.

What Is in Our Activation Harness

Our activation harness splits the second pump's circuit into plug together sections, and every dual pump kit we sell includes all of them plus the 13 psi pressure switch. We also sell the same harness on its own for $83.99, with every section below, the switch and the test plug, for any 12V secondary pump. Here is what each piece does, straight from our installation diagram.

• Main harness. A normally open on/off relay and a 30 amp fuse on 10 AWG wire. The fused wire goes to battery positive, the ground to the battery or good chassis metal.
• Fuel pump harness. A 30 amp quick connect ending in two M6 ring terminals, on fuel compatible wire that can pass through a bulkhead into the tank.
• Negative trigger harness. Carries the 13 psi switch on a two wire connector, and becomes a controller input once it is cut back.
• Positive trigger harness. For a switched 12V source.
• Test wire. Forces the pump on for setting pressure or troubleshooting; not a long term control method.

The relay uses standard terminal numbers: 30 is battery power in, 87 is power out to the pump, and 85 and 86 are the coil. A negative trigger closes it by grounding the coil, a positive trigger by feeding it 12V, and the trigger never carries pump current.

TI Automotive rates the Walbro 450 (F90000274) at 17.2 amps at 60 psi, 18.9 at 80 psi and 20.2 at 100 psi (13.5 V), so the 30 amp fuse and connector leave about 11 amps of headroom at 80 psi. If you lengthen a run, stay on 10 AWG and keep the fuse at the battery end.

Our Installation Diagram, Page by Page

Our installation diagram is three pages: the whole circuit, the ReFlex+ hookup and the bootmod3 REV2 re-pin. Each is below with what it tells you to do; the full PDF zooms cleanly if you need the small print.

Page 1: the whole circuit and the four harness sections

Page 1 is the map. The header warns that routing and locations are for illustration only, since every car routes differently. Read it in this order:

1. Follow the red dashed main harness from the battery through the fuse, then the relay, to the junction beside the fuel pump. The fuse sits at the battery end, and the fused wire lands directly on battery positive.
2. Follow the black fuel pump harness from that junction to the pump.
3. Pick one negative trigger route. Option 1 (green) and Option 2 (blue) are two ways to run the same harness forward to your pressure switch or controller.
4. Or use the pink positive trigger to a switched 12V source, and the pump runs whenever the car is ON.
5. Note the test wire: for setting fuel pressure or troubleshooting only.
6. Read the red note twice: the included relay cannot be run PWM and must be used as on or off, 0 or 100 percent duty cycle.

Page 2: connecting a Motiv ReFlex+ or other controller

Page 2 converts the negative trigger harness from pressure switch to controller use:

1. Cut the harness about 2 inches back from the two wire electrical connector.
2. Set the pressure switch aside. Keep it in the glovebox if you might ever go back to it.
3. Remove the loom and heat shrink from the wire. A razor helps; do not nick the conductors.
4. Cut the brown wire flush with the rubber seal, or tape it up if you plan to use it again.
5. Connect the green wire, the negative trigger for the pump activation harness, to your controller's negative trigger output.

The bottom of the page repeats the PWM rule: the relay will fail if controlled PWM, so use only 0 and 100 percent duty, at a recommended 20 to 40 Hz. Option B covers the ReFlex+ side.

Page 3: re-pinning for a bootmod3 REV2 board

Page 3 re-pins the gray four pin connector on the negative trigger harness for a REV2 board. It applies only where bootmod3 supports REV2: S58 and B58 cars, never the N54.

1. Untuck the looped orange wire from the negative trigger harness, next to the "Trigger" label.
2. Pull the orange terminal lock out of the inside of the connector with needle nose pliers.
3. With a de-pin tool or other small tool, de-pin both ends of the orange wire and remove the brown wire. Discard the orange wire.
4. Install the rubber seal, then insert the brown wire into the orange wire's hole on the mating connector side, diagonal to the green wire.
5. Cut the round connector off, then connect the brown wire to Positive on the REV2 and the green wire to Pulsed Ground.

The footnote matters when matching colors: red is positive and yellow is pulsed ground on the REV2 harness, which bootmod3's own wiring page confirms.

Safety Before You Start

This job puts a 30 amp circuit next to an open fuel tank, so disconnect the battery and work where fumes can clear.

• Battery first. Disconnect the battery negative terminal before any wiring, and reconnect it last.
• Fuel vapors. Work outside or with the doors open, keep sparks and flames away from the open pump access, and keep an extinguisher close.
• Fuse at the battery end. It protects the whole run only next to the battery.
• Protect the wire. Grommet it through metal, keep it off exhaust and sharp brackets, and zip tie it so it cannot chafe.
• No splices in the tank. Only the fuel compatible pump harness goes into the tank, through a proper bulkhead.

Install Steps Every Setup Shares

Every trigger option starts with the same power, ground and pump connections, done with the pump assembly already in the tank.

1. Disconnect the battery negative terminal.
2. Mount the relay somewhere dry, solid and away from exhaust heat.
3. Run the fused main wire to battery positive and tighten the ring terminal fully.
4. Land the ground on battery negative or bare chassis metal. Scrape paint off a chassis point first.
5. Connect the pump harness ring terminals to the secondary pump terminals on the top hat, matching the marked polarity, and tighten the M6 nuts snugly.
6. Plug the pump harness into the main harness quick connect until it locks.
7. Route the trigger harness toward your trigger source, then follow Option A, B, C or D.

Option A: The 13 psi Pressure Switch

The 13 psi pressure switch, often called a Hobbs switch, is the simplest trigger and the right one for most street cars: it closes to ground once boost passes 13 psi, with no software involved.

1. Find a line that sees intake manifold pressure, so the switch reads boost, and tee the switch in with clamps on every joint.
2. Mount the switch where its hose cannot kink and away from turbo heat.
3. Plug the switch into the two wire connector on the negative trigger harness, and plug that harness into the main harness.
4. Reconnect the battery. With the engine off, the second pump stays silent until you unplug the switch and bridge its two harness pins; then the relay clicks and it runs.
5. Log a pull and confirm low side fuel pressure stays flat as boost crosses 13 psi.

Below 13 psi the primary carries idle, cruise and light throttle alone, keeping tank heat and pump hours down. Our switch has no part throttle cutout issue.

Option B: Motiv ReFlex+ Output

A ReFlex+ replaces the pressure switch by grounding the green trigger wire from one of its auxiliary outputs, so the second pump comes on at whatever boost, rpm or ethanol point you set. Motiv's FAQ says the ReFlex+ can replace a Hobbs switch.

Wiring the green wire to a ReFlex+ output

Motiv's ReFlex+ wiring diagram (v2.2) lists four low side auxiliary outputs, each of which switches to ground. Motiv's two FAQ pages disagree about AUX 1 and AUX 2 for a standard relay, but both list AUX 3 and AUX 4, so that is where we put the second pump.

1. Cut and prep the negative trigger harness exactly as page 2 of our diagram shows: brown trimmed or taped, green bare.
2. Join the green wire to your chosen AUX wire with a crimped, heat shrunk splice. Solder and heat shrink also works.
3. Run nothing else to the ReFlex+. The coil's 12V comes from our main harness. Motiv says loads over 2 amps, auxiliary fuel pumps included, need a relay.

Setting the output in TunerPro RT

The ReFlex+ is configured in TunerPro RT on a laptop (Motiv prefers Windows) over a USB data cable, or wirelessly if you have Motiv's ReFlex Link. Load the starter map for your engine first, as Motiv recommends for every BMW.

1. Set the Auxiliary Frequency Selector for your output to 20 Hz, option 0. Motiv lists that as the setting for a fuel pump on a standard relay, and it sits inside the 20 to 40 Hz our diagram allows.
2. Set the Auxiliary Mode Selector to 0 for a boost vs rpm table, or to 1 for a boost vs ethanol table if an ethanol sensor is installed.
3. Fill that table with 0 and 100 only. We start owners where our switch works: 0 below 13 psi, 100 at 13 psi and above.
4. Save the map and upload it to the ReFlex+.
5. Log the auxiliary output channel on a pull. It should read 0 or 100 and nothing in between; any middle value means the relay is being pulsed.

Motiv's bench test, on firmware V4.00 or newer, can fire an auxiliary output for a set time: a quick way to hear the relay click before the car moves.

Option C: bootmod3 REV2 Board

A bootmod3 REV2 board can switch the second pump through its secondary fuel pump relay output, but only where bootmod3 supports REV2 and CustomROM V2: S58 and B58, with N55 and S55 listed as to follow, and never the N54.

1. Re-pin the harness as page 3 of our diagram shows.
2. Connect brown to the REV2 red wire (12V) and green to the yellow wire (pulsed ground) on the output bootmod3 labels LPFP.
3. Never wire the pump straight to the REV2. bootmod3 warns it can blow the fuses in the REV2 harness's bulk connector.
4. In the bootmod3 Map Editor, set the Aux Fuel Pump Activation table to 100 percent wherever the pump should run, with little or no interpolated scaling, so it acts as an on and off toggle.
5. Listen at idle and while driving. bootmod3's own tell: a buzzing relay means the calibration is pulsing it.

Option D: Switched 12V Positive Trigger

A switched 12V positive trigger runs the second pump any time the ignition is on, which suits a car that lives at full load and costs a street car extra tank heat and pump hours.

1. Find a fused 12V source that is live only with the ignition ON. A fuse tap on an ignition switched fuse is the tidy way.
2. Connect the positive trigger harness to that source and plug it into the main harness.
3. Turn the ignition on: the relay clicks and the second pump runs. Turn it off: both stop.

Why the Relay Must Run On or Off, Never PWM

The standard mechanical relay in our harness is either fully open or fully closed, so any controller driving it must sit at 0 or 100 percent duty and nothing in between. Our diagram puts it bluntly: the included relay will fail if controlled PWM.

PWM, pulse width modulation, switches power on and off many times a second, and a mechanical relay's contacts arc on every pulse. bootmod3 says pulsing a standard relay shortens its life. A variable speed second pump needs a brushless pump with its own controller.

Testing the Second Pump After Install

A ten minute first start test proves the second pump runs, stops and comes in on time before a full pull.

1. Before reconnecting the battery, recheck every ring terminal and connector.
2. Reconnect the battery and fit the test wire. With the ignition on, listen for the relay click and the second pump humming under the rear seat. Set or check fuel pressure now.
3. Check the top hat and fuel lines for leaks while both pumps run.
4. Remove the test wire and connect your real trigger. At idle, a switch or controller setup leaves the second pump off.
5. Log a third gear pull with low side fuel pressure, its target, boost and, on a ReFlex+, the auxiliary output channel.

Low side pressure should hold near its target, about 72 psi (5 bar), as boost passes 13 psi; BMW's N54 bulletin SI B12 55 06 treats 4.75 bar (about 69 psi) as the minimum. The N54 has a factory low side sensor, and a ReFlex+ with Motiv's 0 to 10 bar sensor logs it as auxiliary pressure.

Troubleshooting: When the Second Pump Stays Quiet

A quiet second pump is usually a trigger, a connector pin, the fuse or the ground rather than the pump, and a multimeter finds it in minutes.

Multimeter checks, in order

1. Jumper the switch. Unplug it and bridge the two harness pins. If the pump runs, suspect the switch or its boost line.
2. Check the fuse. Probe both sides with the circuit live; voltage on one side only means it has blown.
3. Check terminal 30. Black lead on battery negative, red lead back probed at terminal 30: battery voltage at all times.
4. Check the coil. With the trigger active, terminals 85 and 86 should read about battery voltage across them, and the relay should click.
5. Check terminal 87. Battery voltage only while triggered. Voltage here but none at the pump rings points at the pump harness.
6. Check the ground. On the ohms scale, the ground ring to battery negative should read close to zero. A higher reading usually means paint or corrosion under the ring.
7. Check the pins. Tug each wire at every weatherproof connector. A pin that slides back has pushed out of its lock; push it home until it clicks.

If pressure still sags with both pumps confirmed running, the fault is upstream of the second pump. Our guide to BMW 29F2 and 29DC low fuel pressure codes separates a weak primary pump or EKP from a wiring fault.

What Owners Actually Say

SpoolStreet owner threads show a clear pattern: a misbehaving second pump is almost always a connector, a switch contact or the trigger wiring.

The pump that never came on. An N54 owner on SpoolStreet moved from a single pump setup to a dual pump assembly and suspected the second pump was not switching on at 13 psi. The cause was a weatherproof connector pin that had pushed out of the pressure switch harness. Once it was reseated, shorting the switch pins fired the pump right away, and the low side pressure drop disappeared.

Another owner in that thread offered a garage test: jumper the switch, start the car and unplug the primary pump. If the engine keeps running, the second pump works.

The mixed result: a SpoolStreet member adding a Walbro 525 as a second pump found it ran with the engine off. Replies blamed the relay wiring or a three terminal switch wired normally closed; no fix was posted.

The negative one: an E90 335i owner running dual Walbro 450s on a single turbo N54 lost pump prime after weeks of short garage starts. Three EKP modules later, a meter still read 1.75 V at the pump; members explained the EKP output is pulsed, so that reading proved nothing, and suggested checking the top hat for a burnt connector. No fix was posted.

Mistakes We See

Most second pump wiring faults come from treating a signal device as a power device or a pulsed wire as a steady one.

• Running pump current through the switch or controller. The switch and a ReFlex+ output are built to ground a relay coil, not carry a 19 amp pump.
• Triggering off the factory pump wire. On an E chassis BMW that wire is pulsed by the EKP, so a relay hung on it chatters.
• Leaving a controller output at PWM. A 20 Hz output with a table full of in between values pulses the relay. Use 0 and 100 only.
• Grounding to paint. It can test fine at idle and starve the pump at full flow.
• Driving on the test wire. It is for priming and setting pressure only.

The Kits This Harness Ships With

Every dual pump kit we build ships with this harness, the 13 psi switch and both trigger harnesses. Ratings are wheel horsepower from our listings, pump gas and full E85; prices as listed.

The pump on our harness is always a 450; in both Stage 3.5 kits the 535 is the primary and runs full time on the factory circuit. Our N54 listings warn that an aging EKP2 can overheat with upgraded pumps, and also cover the 2011 to 2013 E chassis N55 135i and 335i.

Buying the harness on its own

Already running a dual pump hanger, or building one? Our secondary fuel pump activation harness is $83.99 by itself, with the main harness and its 30 amp fuse and relay, the 13 psi pressure switch, both trigger harnesses, the pump power lead and the test plug. It runs one 12V brushed pump such as a Walbro 450 on or off, on any make, and it is not for brushless pumps that need their own speed controller. The pump, the boost line tee and the hose are not included. It carries our 1 year house brand warranty, and tuning does not void it.

For where each stage fits by power target, see our N54 LPFP upgrade guide and the N55 LPFP lineup. We are an authorized dealer for every brand we carry, and in stock hardware ships fast. Our dual pump kits ship free in the lower 48, and we ship worldwide with shipping calculated at checkout.

Who This Is For (and Who Should Skip It)

This guide is for anyone installing our Stage 3 or 3.5 dual pump assembly on an N54, N55 or N20, or adding a ReFlex+ to a car that has one. It also covers wiring a second 12V pump on your own build with our standalone harness, including S55, B58 and S58 cars with a bootmod3 REV2 board.

Skip the second pump if your target fits a single pump. Our single pump kits are rated 575 whp on pump gas with a Walbro 450 and 650 whp with a 535, with no extra circuit at all. B58 owners on our B58 Stage 3 kit have nothing to wire either: it uses one 500 LPH brushless pump on the factory pump control.

If It Were Our Car

On an N54 street car, we would run our N54 Stage 3 dual Walbro 450 kit on the 13 psi switch as it ships, plus an EKP3. With a ReFlex+ already on the car, we would move the trigger to AUX 3 or AUX 4 and keep the switch in the glovebox.

On an F series N55 chasing the most fuel, we would run the F series N55/N20 Stage 3.5 kit on the pressure switch, and not plan around a REV2 until bootmod3 documents it for the N55.

On a B58, we would skip the second circuit and fit the B58 Stage 3 brushless LPFP, rated 850 whp and up on E85 from one pump. On a full throttle only drag car, we would use switched 12V.

Shop BMW Fuel Pumps and LPFP Kits

The Bottom Line

The second Walbro 450 in our kits adds about 19 amps at 80 psi, so our harness feeds it from the battery through its own fuse and relay. The trigger only tells the relay when to close. The 13 psi switch suits most street cars, and a ReFlex+ owner moves the green wire to AUX 3 or AUX 4 at 20 Hz with a 0 and 100 table. When the second pump goes quiet, check pins, fuse and ground before blaming the pump. Still stuck? Email us your car, trigger type and a log.

Frequently Asked Questions

Which wire on the harness goes to my Motiv ReFlex+?

The green wire on the negative trigger harness goes to a ReFlex+ low side AUX output. Cut the harness about 2 inches back from the two wire connector and trim or tape the brown wire, as page 2 of our diagram shows.

How do I set up the ReFlex+ output for the second pump?

In TunerPro RT, set that output to 20 Hz (option 0), choose the boost vs rpm or boost vs ethanol table, and fill it with 0 and 100 only. We start at 100 from 13 psi, matching our switch.

At what boost pressure does the second pump turn on?

Our included pressure switch closes at 13 psi, so idle, cruise and light throttle run on the primary pump alone. It has no part throttle cutout issue, and the second pump runs whenever boost is above 13 psi.

Can bootmod3 trigger the second pump on an N54?

No. bootmod3 does not support the N54 at all, and its REV2 board is documented for S58 and B58 cars only. N54 owners use our 13 psi switch or a ReFlex+ output.

How do I test whether the second pump is working?

Fit the test wire, or unplug the pressure switch and bridge its two harness pins; the relay should click and the second pump should run. If not, check the fuse, relay terminals 30 and 87, the ground and the connector pins.

Why is my relay buzzing?

The output driving it is pulsing instead of switching on and off. Our diagram says the included relay will fail if controlled PWM, so set any controller table to 0 or 100 percent only.

What wire size and fuse does the harness use?

The main harness uses 10 AWG wire, a 30 amp fuse and a standard normally open on/off relay, and the pump connector is rated for 30 amps. A Walbro 450 draws about 19 amps at 80 psi.

Can I buy the activation harness without a fuel pump kit?

Yes. We sell the secondary fuel pump activation harness on its own for $83.99, with the 13 psi pressure switch, both trigger harnesses, the pump power lead and the test plug. It runs one 12V brushed pump on or off on any car; the pump, boost line tee and hose are not included.