How to Troubleshoot LS Swap Electrical Issues

Most LS swap electrical problems come down to a few common causes. Bad grounds, wrong harness for the engine, missing tune, or cheap wiring that's failing. Before you start throwing parts at it, work through the basics in order. Nine times out of ten, the fix is simpler than you think.

Before You Chase Anything Else

Is the PCM tuned?

This catches more people than you'd expect. A junkyard PCM still has the factory calibration with VATS enabled, emissions settings, and parameters for a vehicle that doesn't exist anymore. If VATS isn't deleted, the engine won't start. Period. It'll crank, maybe even fire for a second, then die.

If you bought a harness and PCM package from PSI, it comes tuned and ready. If you sourced the PCM yourself, make sure it's been programmed for standalone use before you troubleshoot anything else.

Does the harness match the engine?

Gen III engines use a 24x reluctor. Gen IV uses 58x. The harness and PCM calibration have to match the engine. If you've got a Gen IV engine with a Gen III harness, the PCM can't read the crank signal correctly. It'll crank strong and never fire. Everything looks right but nothing works.

Check this before you go any further. A mismatch here wastes hours chasing problems that don't exist.

Crank No Start

This is the most common call. Engine cranks fine but won't fire. Here's how to work through it.

Check for RPM signal while cranking. Plug in a scan tool and watch the RPM reading while someone cranks the engine. If it shows zero or jumps around randomly, the PCM isn't seeing a valid crank signal. That means no fuel and no spark because the PCM doesn't know where the engine is in its rotation.

If there's no RPM signal, check the crank sensor connector, the wiring back to the PCM, and make sure the sensor gap is correct. A damaged reluctor wheel will also cause this.

Check for injector pulse. Get a noid light and plug it into an injector connector. Crank the engine and see if it flashes. If it does, the PCM is commanding fuel. If it doesn't, either the PCM isn't seeing a valid crank signal or there's a power or ground issue at the PCM.

Check for spark. Pull a coil connector and use a spark tester. If you've got pulse at the injectors but no spark, check power to the coils and the coil grounds. If you've got neither, go back to crank signal and PCM power and grounds.

Check fuel pressure, it should be 58 psi. Even if you've got injector pulse, you need fuel. Key on, you should hear the pump prime and see pressure on a gauge. If the pump doesn't run, check the fuel pump relay, the wiring from the harness to the pump, and make sure the relay is getting a trigger signal from the PCM.

Grounds

Bad grounds cause more weird problems than almost anything else. The symptoms mimic sensor failures, bad idle, misfires, random stalling. And because the codes point at sensors, people replace parts that were never broken.

Here's what you need for grounds on a high quality LS swap.

Large ground from the battery negative to the engine block. Ground from the engine block to the chassis. Ground from the engine block to the firewall. Head to head ground strap. If you're doing a truck, chassis to cab ground.

Every ground point needs to be clean bare metal. Not painted, not rusty, not just "pretty clean." Bare metal. Use a wire brush or sandpaper and make sure the terminal is tight and can't rotate.

If you're chasing a weird idle or intermittent misfire, check grounds first. Do a voltage drop test between the battery negative and the engine block while cranking. If you see more than 0.1 volts, your ground path has resistance and that's your problem.

Rough Idle or Unstable Idle

Check for vacuum leaks. A small leak at the intake gasket, throttle body gasket, or a cracked vacuum line will cause a rough or high idle. Spray carb cleaner around the intake while it's running. If the idle changes, you found your leak.

Check the IAC or throttle body. On drive by cable setups, the idle air control valve controls idle. If it's dirty or stuck, idle will be rough or erratic. On drive by wire setups, the throttle body handles idle and the PCM needs to learn the idle position. Some PCMs need an idle relearn procedure after install.

Check fuel trims on a scan tool. If long term fuel trims are way positive (adding fuel), you've got a vacuum leak or a lazy O2 sensor. If they're way negative (pulling fuel), you might have a fuel pressure issue or a leaking injector.

Stalling After Warmup

This one's frustrating because it runs fine cold, then dies once it's hot.

Check for heat soak issues. Connectors and relays can get flaky when they're hot. A relay that works fine at 70 degrees might drop out at 200. If the engine stalls after 10 or 15 minutes of running, check the main relay, fuel pump relay, and ignition relay while the engine bay is hot. Poor grounds can cause high resistance and increased heat.

Check the crank sensor. Crank sensors can fail when hot and work again when cold. If you're getting random stalls at operating temp and it restarts fine after cooling down, the crank sensor is a likely suspect.

Check grounds again. Heat increases resistance. A marginal ground that works when cold might fail when everything expands from heat.

Drive by Wire Problems

DBW setups add the throttle body, the accelerator pedal, and the TAC module into the equation. If any of these aren't communicating correctly, you'll get reduced power mode, erratic throttle response, or the engine won't rev past idle.

Check the pedal and throttle body connectors. Make sure they're fully seated. A partially connected plug can cause intermittent issues that are hard to catch.

Make sure you are using genuine delco, Delphi, or GM sensors. Knock off sensors can cause intermittent issues that lead to a dead pedal or drivability issues.

Check for codes. DBW problems usually set codes. A scan tool will tell you if the PCM sees a mismatch between commanded throttle and actual throttle, or if the pedal signal is out of range.

Make sure the harness matches. Drive by wire requires specific circuits that drive by cable harnesses don't have. If you've got a DBW engine and a DBC harness, it won't work.

Cheap Harness Failures

If you bought a $150 harness off Amazon, this section is for you.

Cheap harnesses use thin wire that can't handle engine bay heat. The insulation breaks down, wires short, and you get random problems that don't make sense. The connectors are knockoffs that don't seal properly. Moisture gets in, terminals corrode, and you get intermittent connections. The splices are hand soldered instead of crimped or ultrasonically welded. They crack under vibration.

If you've checked everything else and you're still chasing gremlins, the harness itself might be the problem. This is why buying a premium harness from the start matters. PSI builds their harnesses with TXL wire, real Delphi connectors, and tests every one before it ships. When you're troubleshooting, you can trust the harness and focus on other variables.

Using a Scan Tool

A scan tool is your best friend for LS swap troubleshooting. Here's what to look at.

RPM while cranking. Confirms crank signal.

Coolant temp and IAT. Should read reasonable values. If coolant temp shows -40 or 300 degrees, you've got a sensor or wiring issue.

Throttle position. Should read near zero at idle and climb smoothly as you press the pedal. Jumpy or erratic readings point to pedal or throttle body issues.

Fuel trims. Shows how much the PCM is adjusting fuel. Big corrections mean something's off with the air/fuel system.

Trouble codes. Read them, but don't just replace whatever part the code mentions. A code for an O2 sensor might actually be a ground problem or an exhaust leak. Use the code as a starting point, not a final answer.

Final Thoughts

LS swap electrical troubleshooting doesn't have to be a nightmare. Start with the basics. Is the PCM tuned? Does the harness match the engine? Are the grounds solid? Is there crank signal, injector pulse, and spark?

Work through it in order and don't skip steps. Most problems are simpler than they seem once you stop guessing and start testing.

And if you're still running a cheap harness, that might be your whole problem. Upgrade to something that's actually built right and half your issues disappear.