How to wire an LS swap: what it needs to start and run right

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Minimum wiring to start it

These have to be right before it will crank, fire and idle. Items tagged for a generation, computer or throttle only apply to that setup; the picker above shows only yours.

• Constant battery power to the computer
  It keeps the computer alive and holds what it has learned. Without it the computer never wakes up: no fuel pump prime, no scan tool, no start. Fuse it, and don't put it on a switch you turn off every day: cutting it wipes the learned fuel trims and shift adapts.
  Check: Key off: battery voltage at the computer's battery feed. More: computer pinouts
• Key-on power that stays on while cranking
  This wire turns the computer on, and the same feed usually powers the coils and injectors through a relay. Take it from a wire that is hot in RUN and START. A feed that drops out while cranking (an accessory or run-only wire) is a common reason a fresh swap cranks and never fires. Find it by what it does on your car's wiring diagram, not by color: the purple wire at many ignition switches is the starter solenoid wire, not a key-on feed.
  Check: Meter it with the key on, then while someone cranks: it has to stay at battery voltage the whole time.
• Grounds: computer, engine, frame, body and battery
  The computer, coils, injectors and every sensor measure against ground, and the starter's current has to get back to the battery through heavy straps, not the small harness wires. Bolt the harness ground lugs to the block or the back of the heads on bare metal (never the valve covers: they're isolated on an LS), and run heavy straps battery-to-frame and engine-to-frame, 4 gauge or bigger, plus an engine-to-body strap. Don't count on the motor mounts.
  Check: Meter each ground lug to battery negative: close to 0 ohms. Tug-test every lug.
• Fused power to the injectors
  Every injector gets fused key-on 12 V, and the computer opens each one by switching its ground side. Many harnesses fuse each bank separately, so one blown fuse kills one side of the engine. The injector data in the tune has to match the injectors.
  Check: Key on: 12 V at one injector plug on each bank.
• Coils: key-on power, ground and the timing signal
  An LS coil is not wired like an injector. Each coil has its own power driver inside, fed by key-on 12 V and a ground, and the computer only sends it a low-current timing signal. Keep both coil sub-harnesses and their ground wires bolted to the heads.
  Check: Key on: 12 V at a coil plug on each bank; each coil sub-harness ground to engine ground under 3 ohms.
• Fuel pump relay: the computer feeds the coil +12 V
  On the 0411, P59, E38 and E67 the computer runs the pump relay by sending +12 V to the relay coil: computer wire to terminal 86, terminal 85 to ground. That's the opposite of the fan relays, where the computer grounds the coil. The pump's own current comes from a fused battery feed on terminal 30, out of 87 to the pump, and the pump's other wire goes to ground. Never run the pump straight off the computer wire. The computer runs the pump about 2 seconds at key-on to build pressure, then again once it sees the engine turning, so the pump stops if the engine dies. Cars built with a fuel pump control module (the supercharged LSA CTS-V is one) send a fuel-enable signal to that module instead: wire those the way the donor was.
  Check: Key on: you should hear the pump run about 2 seconds, and the computer's pump wire reads battery voltage for those 2 seconds (meter, not a test light). No prime: check the relay coil ground, the fused feed on 30 and the pump ground separately. Then put a gauge on the rail: a factory LQ9 truck system is specified at 55–62 psi; other rails and regulators have their own number. More: fuel pump and fan relay wiring
• Fuel pump power from the aftermarket ECU harness Aftermarket ECU
  Aftermarket harnesses usually give you a fused pump power lead instead of a relay-control wire. On a Holley Terminator X or X Max the green lead is +12 V pump power: it can run a pump drawing up to 15 A directly; a bigger pump gets its own relay and fuse, with that lead running the relay coil. Size the wire and fuse for your pump.
  Check: Key on: the pump primes. Rail pressure to the number your regulator and injectors were set up for. More: wire, fuse and relay sizes
• 24x crank sensor 24x
  The crank sensor gives the computer engine speed and crank position. No crank signal means no spark, no injector pulse and no pump after the prime. Your engine and computer are both 24x. A 24x sensor runs on 12 V, with its own low reference and signal wire. Use a good-quality sensor: a bargain one is a classic no-start.
  Check: Watch rpm on the scan tool while cranking: it should read above about 150 rpm (lower: charge the battery). Zero rpm points at the sensor, its wiring or its 12 V feed. More: crank and cam codes
• 58x crank sensor 58x
  The crank sensor gives the computer engine speed and crank position. No crank signal means no spark, no injector pulse and no pump after the prime. Your engine and computer are both 58x. A 58x sensor runs on 5 V. Use a good-quality sensor.
  Check: Watch rpm on the scan tool while cranking: above about 150 rpm. Zero rpm points at the sensor, its wiring or its 5 V feed. More: crank and cam codes
• 58x-to-24x converter box (or a 24x reluctor and 1x cam gear) 58x engine, 24x computer
  Your 0411/P59 only reads a 24x crank and 1x cam. A converter box turns the 58x crank and 4x cam signals into 24x/1x so the engine runs without being opened up. Some early Gen IV engines also need a cam sensor extension harness to reach it. (The other direction has no box: a 24x engine on a 58x computer needs the reluctor and cam gear changed.)
  Check: Scan tool rpm while cranking, as for any crank sensor.
• Cam sensor (front cover, 1x) Early LS2
  The 24x LS2 (2005 Corvette, 2005–06 GTO and SSR, 2006 TrailBlazer SS) moved the cam sensor into the timing cover but kept a 1x cam signal with its 24x crank. With the crank signal it tells the computer which stroke each cylinder is on. The front-cover sensor is the same part (GM 12585545) behind a 1x or a 4x cam gear: the gear decides the signal, so the gear, the computer and the harness have to match.
• Cam sensor (front cover, 4x) Gen IV
  With the crank signal it tells the computer which stroke each cylinder is on. A 58x Gen IV may not start at all with a bad cam signal. The front-cover sensor is the same part behind a 1x or 4x gear: the 4x gear is what makes it a 4x signal.
  Check: No start with good rpm: check the cam sensor plug, its feed and ground, and that the cam gear matches (4x).
• Throttle position sensor and idle air valve (cable throttle body) Cable throttle
  The idle air control valve is how the computer holds idle on a cable throttle: without it, and without a usable idle air path, the engine fires and dies unless you hold the pedal. The TPS tells it how far the throttle is open, for fuel, spark and shifting.
  Check: Key on: TPS reads 0% closed and sweeps to about 100% at full throttle.
• Drive by wire: pedal, throttle body and TAC module as a matched set Gen III drive by wire
  Gen III drive by wire uses a separate throttle control (TAC) module between the computer and the throttle body. The pedal, throttle body, TAC module and tune have to match: 1999–2002 truck parts with an 0411 (the Corvette TAC module looks the same but doesn't interchange), 2003-and-later truck parts with a P59. A Painless 60522 harness, for example, is built for TAC module 12578953 and pedal 12565643. The TAC module needs its own key-on power and a clean ground. Don't try to make up for a missing TAC or pedal circuit with a spare power feed. Mismatched or badly grounded parts start and idle but give no throttle.
  Check: Key on, engine off: press the pedal slowly and watch the blade open and return. Then check the pedal and throttle readings agree on the scan tool. More: drive-by-wire pinout
• Drive by wire: pedal and throttle body that match the ECM's calibration Gen IV drive by wire
  On Gen IV the ECM drives the throttle itself (no TAC module). The pedal and throttle body have to be ones its calibration expects: a pedal from the same family and years, and the throttle body it was set up for. A Corvette calibration with a truck 87 mm throttle body is a known no-start, and a mismatched pedal reads fine on the scan tool but never opens the throttle. Wrong parts give reduced engine power or no throttle.
  Check: Key on, engine off: both pedal sensor readings move together and the blade follows the pedal. More: drive-by-wire pinout
• Drive by wire: pedal and the 90 mm silver-blade throttle body E40
  The E40 drives the throttle itself and only works with the silver-blade throttle body; the gold-blade ones that look the same don't work with it. Keep the calibration year matched to the computer (a 2005 E40 with a 2005 calibration, a 2006 with a 2006) or you get throttle codes.
• Drive by wire on an aftermarket ECU Aftermarket ECU
  Use a pedal and throttle body on your ECU maker's list and run the throttle setup before first start: a Holley ECU injects no fuel until the TPS autoset is done. If the throttle loses a sensor it drops to its limp-home opening, which lets in more air than idle.
  Check: Key on, engine off: the blade opens fully and returns as you press and release the pedal. More: drive-by-wire pinout
• VATS (anti-theft) turned off in the tune GM computer
  A factory computer waits for the donor's body computer to say the key is right. In a swap that module is gone, so the engine cranks and won't start, or starts and dies after a second or two. How it's turned off depends on the computer: a setting on the 0411, P59 and E38, an operating system patch on the E67. PSI stand-alone harnesses, for one, are built for a computer with VATS removed. Turning off a trouble code is not the same thing as turning off VATS.
  Check: Starts and dies, or good cranking rpm but no injector pulse: recheck VATS first. More: where VATS lives on your computer
• A tune set up for this engine, injectors, throttle and transmission GM computer
  Know what is in the computer before you crank. A donor file still expects its own injectors, MAF, throttle parts and transmission, and firing an unconfigured swap risks damage. Match the calibration to the equipment actually on the engine, using the requirements for your exact computer and harness. More: first-start checklist
• A base map loaded for this engine Aftermarket ECU
  An aftermarket ECU needs its base calibration for your engine, injectors, crank/cam setup and throttle loaded before first start. More: ECU comparison
• Starter circuit, through the neutral-safety or clutch switch
  Wire the starter the way your setup is built. Some factory computers run the starter themselves (2003–07 P59 trucks ground a start relay; the 2007 E38 trucks have a starter-enable relay output); many swaps use the car's own key-to-solenoid wire instead. Either way, route it through the range switch (automatic) or clutch switch (manual) so it only cranks in Park or Neutral, or with the clutch down, and make sure the tune matches the way you wired it.
  Check: No crank: battery voltage at the solenoid trigger wire while the key is in START.

Connect before the first start

Not every one of these is strictly needed for the engine to fire, but you want them connected and reading right when it does, so the first start is controlled and the computer has what it needs. The brake input is wired one way or the other depending on the computer: use the item for yours.

• Cam sensor (back of the block, 1x) Gen III
  With the crank signal it tells the computer which stroke each cylinder is on. A Gen III will start on the crank signal alone, but plug the cam sensor in: the computer uses it for normal timing and to flag cam faults.
  Check: A cam sensor code (P0340-P0344) after first start means check its plug, feed and ground.
• Coolant temperature sensor (front of the driver-side head)
  Cold-start fuel, idle speed and fan control all key off it, and the computer reads it while cranking to decide how much fuel to give. Connect it and check it reads right before the first crank.
  Check: Engine cold, key on: the scan tool's coolant temperature should be close to the air temperature.
• MAP sensor
  It is the computer's main load input for fuel and spark, and the only one when there's no MAF. It can't be tuned out. A boosted engine needs a 2- or 3-bar sensor and a tune set up for it.
  Check: Key on, engine off: about 95–102 kPa at sea level (lower at altitude).
• Brake switch input: 12 V with the pedal released (0411 / P59) 0411 / P59
  An electronic automatic uses it to release the converter clutch when you brake, cruise uses it to cancel, and drive by wire uses it as a safety input. Wired backwards, the converter can stay locked as you stop and stall the engine, or a drive-by-wire throttle can stop working. The 0411 and P59 (pin C1-33, circuit 420) want 12 V with the pedal released and no voltage when you press it: a normally-closed switch contact, the opposite of a brake-light switch. Many brake switches carry both kinds of contact; use the right one, or the inverting relay your harness calls for. On Gen III drive by wire, wire the TAC module's brake input the way your harness sheet shows.
  Check: Meter the computer's brake wire: 12 V with the pedal up, 0 V pressed. Then watch the brake reading on the scan tool. More: brake pin on your computer
• Brake switch input: 12 V with the pedal pressed (E38) E38
  An electronic automatic uses it to release the converter clutch when you brake, cruise uses it to cancel, and drive by wire uses it as a safety input. Wired backwards, the converter can stay locked as you stop and stall the engine, or a drive-by-wire throttle can stop working. The E38 (pin X1-9, circuit 6311) wants 12 V when you press the pedal and none when it's released, like a brake-light switch. That's the opposite of the 0411/P59. Don't use pin X1-57: that's the vehicle speed output.
  Check: Meter the brake wire at the ECM: 0 V with the pedal up, 12 V pressed. Then watch the brake reading on the scan tool. More: E38 brake pin
• Brake switch input (E67: know your application) E67
  An electronic automatic uses it to release the converter clutch when you brake, cruise uses it to cancel, and drive by wire uses it as a safety input. Wired backwards, the converter can stay locked as you stop and stall the engine, or a drive-by-wire throttle can stop working. The PSI HAR-1099 harness (LSA/LS9, 2009–15, manual transmission) wants 12 V when you press the pedal, and a brake switch wired wrong on it can leave you with no throttle. For any other E67 setup, find out which computer, operating system and brake input you have, then wire it the way its harness instructions say.
  Check: On a HAR-1099: 0 V released and 12 V pressed at its brake lead. Any other setup: check the signal its instructions call for, then the brake reading on the scan tool.
• Brake switch input (E40) E40
  An electronic automatic uses it to release the converter clutch when you brake, cruise uses it to cancel, and drive by wire uses it as a safety input. Wired backwards, the converter can stay locked as you stop and stall the engine, or a drive-by-wire throttle can stop working. Find out which E40 application, operating system and brake input you have before you pick the switch contact, wire it the way the matching harness instructions say, and confirm the brake reading on the scan tool before you drive.
  Check: Watch the brake reading on the scan tool as you press and release the pedal.
• Brake switch input (aftermarket ECU) Aftermarket ECU
  An electronic automatic uses it to release the converter clutch when you brake, cruise uses it to cancel, and drive by wire uses it as a safety input. Wired backwards, the converter can stay locked as you stop and stall the engine, or a drive-by-wire throttle can stop working. Holley's drive-by-wire harnesses and its 6L80 transmission harness want 12 V from the brake switch when you press the pedal. Other ECUs: wire it the way their harness sheet shows.
  Check: Watch the brake input in the ECU software as you press and release the pedal.
• OBD-II port wired to the computer
  Not needed to fire, but without it you can't see rpm while cranking, read codes or confirm VATS is off. Gen III computers talk on one serial-data wire; Gen IV uses the two-wire high-speed GMLAN (CAN) pair. Wire the port's ground and battery pins too.

Before the first crank, go through the printable first-start checklist: oil pressure first, fuel pressure, grounds, and the tune.

What it needs to run right

It will fire without these, but it won't idle, drive, cool, charge or pass inspection the way it should.

• Front O2 sensors, both banks, heaters powered
  They're how the computer trims fuel as you drive (closed loop). Without them it runs open loop all the time: no fuel trims, rich or lean drift, worse mileage. Closed loop needs both fronts. The heaters need fused key-on power: a blown heater fuse sets heater codes on several sensors at once. O2 plugs differ by sensor and harness (a Gen III harness may have a flat or square 4-pin plug), so match the plug to your sensors.
  Check: Warm idle: both front sensors switch rich and lean on the scan tool.
• Rear O2 sensors: only if you keep the cats Emissions
  They only check the catalytic converters; they don't fuel the engine. No cats and no inspection? Turn their tests off in the tune. Keep them (and the cats) if your state inspects the OBD system.
• P59: you may have to keep the rear O2s plugged in Emissions P59
  Some P59 computers run a catalyst test that can't be turned off. With the rear sensors removed it swings the commanded air-fuel ratio during drive cycles. Leave them plugged in (they don't have to be in the exhaust) and turn their codes off.
• Knock sensors (two, in the valley under the intake) Gen III
  They hear detonation so the computer can pull timing before it hurts the pistons. Gen III uses two 1-wire sensors under the intake. Keep them wired so the computer can protect the engine.
• Knock sensors (two, on the sides of the block) Gen IV
  They hear detonation so the computer can pull timing before it hurts the pistons. Gen IV uses two 2-wire sensors on the sides of the block. Keep the matching sensors and their wiring. Relocation brackets solve a mounting clearance problem; they don't make other sensors interchange or make running without them safe.
• Gen III knock sensors on the Gen IV block Gen IV engine, Gen III computer
  Your Gen III computer expects the Gen III 1-wire sensors, and the Gen IV block mounts its own (2-wire) on the sides. Drill and tap the block for the Gen III sensors, or use an adapter harness, then check they work: tap the block lightly and watch for knock retard.
• MAF sensor and air temperature, or speed density in the tune
  The MAF measures airflow for the best part-throttle fueling with the least tuning. A stock P59 truck tune falls back to speed density off the MAP sensor when the MAF is unplugged, so going speed density on a naturally aspirated engine is a tune change, not a special operating system. Either way keep an air temperature sensor: on the 5-wire truck MAF and the LS3/LS7-style MAF (GM 15865791) it is built in, so removing the MAF removes it too (a breakout harness can move it to a separate 2-wire sensor); the 3-wire LS1 car MAF uses a separate one. Changing MAF type with an adapter harness means the tune's MAF table has to be redone.
• Vehicle speed sensor to the computer
  The computer uses road speed for idle and decel fuel, the converter clutch and shifting. Even with a manual or a non-electronic automatic, run one: without it you can get stalling under hard braking and, on drive by wire, a throttle that stops working. It isn't needed to start the engine. For the speedometer, use the computer's speed output wire and match it to the speedometer's calibration: adjust the output where the computer allows it, or use the gauge's own measured-mile calibration. Never tap the sensor's own wires.
  Check: Drive it: the scan tool's vehicle speed matches GPS. More: speed pins
• Electric fan relays from the computer (or a thermostatic controller)
  The computer grounds each fan relay coil (the opposite of the pump relay, which it feeds +12 V). Never run a fan straight off a computer pin. On a P59 the low-speed fan is pin C1-42 (circuit 335) and high speed C2-33 (circuit 473). The on-temperatures are tune settings: a 2002 Corvette comes from the factory at about 226 °F and 235 °F, and every calibration sets its own, so set them for your radiator. No computer fan outputs? A thermostatic fan controller works. Check the alternator can carry big fans.
  Check: Command the fans on with the scan tool, or let it idle up to temperature. More: fan relay wiring
• A/C request to the computer
  When the compressor kicks on, the computer bumps idle air, and it can drop the compressor at wide-open throttle and run the fans. Without the request wire the idle dips or stalls when the A/C engages. A stand-alone compressor circuit works, without those extras.
• P59 trucks: A/C request came over serial data P59
  2003-and-later trucks sent the A/C request from the climate control over serial data. In a swap you need a 12 V request wire and the tune set for a 12 V request.
• Alternator turn-on (Gen III 4-pin plug) Gen III
  The 4-pin Gen III alternator needs its key-on exciter wire on the L terminal or it never starts charging, and you drive on the battery until it dies. Feed it through a charge lamp or a resistor from switched run power: on the 4-pin alternator in Holley's LS accessory drive that's a lamp or a 560 ohm, 1/2 watt resistor. A 2-pin computer-controlled alternator wires differently.
  Check: Running: 13.8–14.5 V at the battery.
• Alternator control (Gen IV 2-pin plug) Gen IV
  Gen IV alternators are told what voltage to make by the computer. With the control plug unused they fall back to a fixed voltage of about 13.2–13.8 V and still charge. Full computer control needs the factory battery current sensor and setup, which most swaps don't keep.
  Check: Running: check the voltage at the battery with a meter before you call it done.
• Alternator Early LS2
  Wire it for its plug: a 4-pin alternator needs its key-on exciter wire (through a lamp or resistor), a 2-pin computer-controlled one falls back to a fixed voltage when its control wire is unused.
  Check: Running: check the voltage at the battery with a meter.
• Check-engine light
  The computer grounds the lamp; the other side gets key-on 12 V. It's how you'll know a code has set, and inspections want it to light with the key on and the engine off. An LED lamp only works one way round.
  Check: Key on, engine off: the light is on. Engine running with no codes: off.
• Tach output
  The computer makes a tach signal from the crank sensor, on its own engine-speed output wire (on a P59 that's C2 pin 10, circuit 121; check your computer's pinout). Match the computer's tach output setup to the gauge (where it's adjustable, set them to agree), and wire the tach the way its own instructions say (some need an adapter or a pull-up resistor). More: tach pin on your computer
• Gauges: their own senders
  The computer doesn't need the oil pressure sensor to start; it's there for the gauge. For aftermarket gauges, use the senders that come with them (a tee for oil pressure). Never splice into the computer's coolant, speed or other engine sensor wires. Oil pressure plugs vary: some E38 harnesses use a small 3-pin round plug with an adapter for the larger sender.
• EVAP equipment: check what your donor and state need Emissions
  The purge valve carries fuel-tank vapor into the intake to be burned. Keep the equipment your donor setup and your state's inspection need, and confirm the wiring and calibration for your exact computer and harness before you leave the purge or vent circuits off. A purge port left open is a vacuum leak.
• AFM (DOD) solenoids, or a delete AFM
  AFM engines shut off cylinders 1, 4, 6 and 7 at light load with oil-pressure solenoids under the valley cover. Keep them wired with an AFM tune, or turn AFM off in the tune (with or without the mechanical delete). Unplugged without the tune change, it sets codes. With the hardware deleted, tape off the valley cover plug.
• VVT phaser solenoid, or a delete VVT
  The ECM moves cam timing with a solenoid at the front of the engine. Unplugged without a tune change, it sets codes and the phaser parks in its locked position. A real VVT delete is mechanical (non-VVT cam or phaser lockout, timing set, front cover) plus turning VVT off in the tune.
• Manual transmission: speed sensor and reverse lockout Manual
  The tail-housing speed sensor feeds the speedometer and the computer. A T56 or TR6060 reverse lockout blocks reverse until its solenoid gets 12 V: wire it to switched 12 V and ground, or bolt on a delete plate (you lose the mis-shift protection). The 1-4 skip-shift solenoid is a factory shift feature. Some harnesses (the Painless 60522, for one) have a skip-shift circuit: connect it or leave it off according to the instructions and calibration for your exact transmission and computer.
• Non-electronic automatic TH350 / TH400 / 700R4
  The computer doesn't shift it, but it still needs a road-speed signal (see the speed sensor), and the tune is set for a manual or non-electronic transmission so it doesn't look for an automatic.
• Electronic 4-speed: the computer shifts it 0411 / P59
  The 0411 and P59 run the engine and the transmission. Wire the transmission connector (the 4L60E plug changed from 13 to 15 to 17 pins over the years, and pins moved, so match the harness to your transmission's year), the range switch, the speed sensor, the brake switch and the transmission's fused 12 V. The tune has to be set for your exact transmission.
  Check: Starts in second and won't shift: check the connector is fully seated and its pins straight, the transmission fuse, and that the tune matches the transmission.
• 4L80E: two speed sensors and the tune 4L80E
  A 4L80E has two speed sensors: the front one is input speed, the rear one is vehicle speed. Putting a 4L80E behind a computer that ran a 4L60E takes more than picking 4L80E in the tune: its transmission settings have to come from a 4L80E file, and a wire or two may move.
• Electronic 4-speed: a separate T42 transmission controller E38 / E67 / E40
  Gen IV engine computers don't shift a 4-speed automatic. It needs a T42 controller wired to the transmission, with its own power and ground and the CAN link to the ECM. The ECM and T42 operating systems have to be a matched pair.
• 6L80E / 6L90E: the controller is inside the transmission 6L80
  Its controller (T43) lives in the transmission and talks to the ECM over CAN, so there's no solenoid wiring, but it needs constant battery power, a good ground, key-on power, the brake input, the park/neutral starter circuit and the CAN pair. The ECM sends it engine torque, which the transmission relies on to shift smoothly and look after its clutches. Tire size and gear ratio are set in the transmission controller.
• Automatic on an aftermarket ECU Aftermarket ECU
  The ECU has to shift it, or you add a transmission controller. Holley: the Terminator X Max and Dominator shift a 4L60E/4L80E, and the Terminator X Max runs a 6L80E/6L90E with Holley's 6L80 harness; the base Terminator X doesn't control an automatic. Haltech's plug-and-play LS kits are for manual transmissions. More: ECU comparison

What changes between Gen III and Gen IV, and what interchanges

Which LS2 is which

The LS2 is a Gen IV engine either way, but its crank and cam pattern depend on the car and year, not the year alone. This is how they left the factory; a rebuilt or crate engine can have a different reluctor, so check the crank sensor.

Power, grounds and fuses

• Main feed. A heavy, fused feed from the battery (or the starter's battery stud) to the harness fuse and relay block. Everything else is fused from there.
• Constant 12 V to the computer. Fused, straight from the battery side. Don't run it through a switch you use every day: the computer forgets its learned trims.
• Key-on 12 V. From a wire that is hot in RUN and START, to the computer and the coil of the main (ignition) relay. The relay then feeds the coils, injectors and sensors.
• Relays for the heavy loads. Fuel pump, fans and the coil/injector feed run through relays; the computer only switches relay coils. On a GM computer it feeds +12 V to the pump relay coil and grounds the fan relay coils, so wire each relay for its job. Fuse each circuit for its load (coils and injectors often one fuse per bank; O2 heaters on their own key-on fuse). Size wire and fuses for the actual load with our wire, fuse and relay chart.
• Grounds. Harness ground lugs to the block or the back of the heads on bare metal (a GM crate-engine harness uses three eyelets on the block), never the valve covers. 4-gauge-or-bigger straps battery-to-frame and engine-to-frame, plus engine-to-body; don't count on the motor mounts. Bolt lugs down separately, not stacked.
• Testing. Never probe computer pins with a test light: it can damage the driver. Use a digital multimeter and back-probe.

Harness options

Stand-alone harnesses we checked

What each one is built for, what comes in the box and what you still have to buy. The picker narrows them by engine, computer, throttle and transmission; confirm the exact donor, computer, plugs and options before you order. Prices are the makers' advertised base prices as of October 10, 2026, before options, shipping and tax, and they change: check before you buy. These prices cover the wiring products listed; budget separately for computers and programming unless the package you pick says it includes them.

Comparing harness-plus-GM-computer with an aftermarket ECU? See the LS swap ECU comparison. Wiring a specific harness: Speartech · Painless 60522 · Holley EFI.

Troubleshooting by symptom

Codes? What each LS swap trouble code means and what to check.

Questions

Next: every pin on the 0411, P59 and E38 · fuel pump and fan relay wiring · drive-by-wire pinout · first-start checklist · wire, fuse and relay chart · trouble codes