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Questions about LS Harness Connection Harnesses? Check Here for the most common FAQ!



LS Harness Connection harnesses are built around simplifying the daunting task of electrical control of the LS engine and transmission in any vehicle of your choosing. The use of our harnesses does make connection and operation extremely easy and each harness comes with a manual specific to it's installation, but there can still be some questions that come up.


Part of your benefit when using a LS Harness Connection harness is the technical support that comes with it - if you have a question about your harness, please feel free to contact me and I will get back with you as soon as it is possible. As the years pass, I have accumulated a small list of common questions and concerns, and I have either modified the installation guide to clarify these questions or I will post them here in hopes of helping everyone who has questions.




Q: My car won't start.... What do I do?


A: This has to be my most common question, and it can be scary and frustrating to deal with, especially after having wrenched all of those late nights in your garage. The list of answers to this question is as long as you would expect, so I will include some of my most common fixes below.


VATS. This is GM's acronym for Vehicle Anti Theft System. This requires a specific signal to unlock the computer to allow the engine to operate in an attempt to prevent theft of vehicles. When performing a LS swap, the OEM key, and VATS/Passlock circuitry will no longer be present which will cause this Anti Theft System to trigger an enabled state. To prevent this, you must have your computer tuned to remove this system in order for your engine to successfully start. LS Harness Connection can provide you with VATS Removal


and Startup tuning for your vehicle using either a computer provided by you, or one provided by us. This is the most common situation which results in a non-start condition after swapping a vehicle.


The first most important thing to do is check that your installation follows the installation guide and that you have power and grounds in the correct spot. One common mistake is forgetting the ground that is on the back of the driver side cylinder head. This controls a lot of the sensor grounds and is required for operation, so just forgetting this will leave your car unable to start. Another common issue is ignition switch wire termination. This is the thin red wire that comes off of your fuse block, and though it is small, it has the critical task of sending power to all of your engine sensors and powering up your harness, so just like the ground on the back of the cylinder head, hooking this up incorrectly can prevent you from starting. Some older vehicles have power that drops out between the ignition cylinder switch points which can require finding another point to find a +12v signal when rotating the key to the Ignition position.


All of the LS/Vortec series engines are OBD2 compatible, and all LS Harness Connection Harnesses include an OBD2 port for diagnostics and tuning. If your power and grounds are absolutely correct, you can use this port to receive any codes that are stored in the engine. This can help you diagnose your issue and pinpoint what is wrong with your setup.


All engines require 3 things to operate - Fuel, Air, and Spark. Many swaps are the "junkyard dropout" style, and there is nothing wrong with that... except you might have snagged an engine that was unfortunately sitting for awhile. Due to the ethanol content in todays fuel, there has been an issue with


injectors getting gummed up and sticking shut, preventing fueling. You can use a cleaner and soak the ends to break up the ethanol, or remove/replace or send off for cleaning. Another common cause of no-start is injector coils. These can become faulty, or the sub harness can become damaged or improperly installed if they are switched from side to side, which causes the incorrect cylinder to receive spark during the combustion cycle. This can lead to either a rough, missing idle, or inoperable situation.


Q: On your harness, there are a bunch of loose wires- what are they for?


A: Depending on the build you require, there will be a series of loose wires for connecting the harness to the vehicle which vary in accordance with the build requirements. If you have chosen to have your harness labeled, each of these wires will come labeled with their function, otherwise, you may consult your manual for details by color. There are single multiple colored wires, and 3 white wires that are included (at the time of publication of this post) in my Gen 3 wire harnesses which are detailed in the installation manual. If you did not choose to have labeling added, you will need a multimeter to ring out/test the resistance of each wire to the corresponding computer terminal to determine what the function is. The termination point of each wire is detailed in the installation manual that is emailed out with each harness, and is also available by scanning the QR code located on the PCM connector.



Q: Where do I connect all of the "under dash" wires?




A: The under dash wires are used for relaying engine and transmission functions to your swap vehicle. The wires included are dependent upon the harness, but can include:

- Check Engine Light

- Speedometer

- Tachometer

- Park / Neutral Signal

- Brake Signal (Brakes Applied)

- Brake Signal (Brakes NOT Applied)

- Engine Coolant Temperature

- Oil Pressure

- A/C Request

- Cruise Control RESUME/Engage

- Cruise Control COAST/Inhibit

- Fan 1/2 Trigger Wires

- Fuel Pump 12v Output

- Ignition Switch Trigger


The place where each of these connects differs per vehicle for almost all, and you will need to consult your vehicle wiring diagrams for details on connection. Please be advised that vehicles with mechanical gauge systems will need a signal converter to drive the factory gauge.


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