LS3 crate mil light on no code
Thread Starter
Teching In
Joined: Jul 2025
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LS3 crate mil light on no code Hello everyone,
New to the forum. I have installed several crate engines in street rods over the last few years. Latest is an ls3 in a 1937 Ford, everything is running good, only issue is a mil light that is always on with the engine running. No codes. When I test the mil function with HP tuner it will turn off when I turn it on and turn on when I turn it off. Will then come back on when ignition is cycled. Wondering if anyone has run into this. Stock ls3, GM harness/computer, t-56 transmission.
New to the forum. I have installed several crate engines in street rods over the last few years. Latest is an ls3 in a 1937 Ford, everything is running good, only issue is a mil light that is always on with the engine running. No codes. When I test the mil function with HP tuner it will turn off when I turn it on and turn on when I turn it off. Will then come back on when ignition is cycled. Wondering if anyone has run into this. Stock ls3, GM harness/computer, t-56 transmission.
On The Tree
Joined: Mar 2010
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Is the check engine light LED? I know many people disagree but I have dealt with an ECM that was looking for resistance from a conventional bulb.
I can't recall the vehicle but it did throw a fault. It might be something to try.
I can't recall the vehicle but it did throw a fault. It might be something to try.
I'm using an LED light and it glows a bit dimmer when there are no codes. Turn the key on and note the brightness of the bulb, then start the car and see if it doesn't dim a bit.
Thread Starter
Teching In
Joined: Jul 2025
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No light with just key on. It does not behave properly at start up, light up from key on then a couple seconds, then off. It is constant on when running. I can toggle it with an hp tuner, however it turns off when I use the tuner to turn it on, and on when turned off. My next move is to disconnect the temp sensor to see if the pcu will throw a code.
Just to make sure, the light should have an external power supply and the other wire goes to ground thru the ECU. Unlike a regular bulb LEDs have a + and - also. I'm sure you know this, just covering all bases.
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Thread Starter
Teching In
Joined: Jul 2025
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The gauges have a check engine light wire, when I used that and the "mil" wire from the GM harness it lit up. I double checked wiring, isolated it to a stand alone led, grounded the light and used the "mil" wire as the positive feed. If the GM harness/ecu grounds to complete the circuit, then this light should never come on. The crate engine wiring info from GM does not indicate that the mil wire provides ground. Haven't gotten to the car to see if it will throw a code and what happens when it should light the cel.
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Verdad Gallardo On The Tree
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I'll repeat the general consensus here. Typically, the PCM is throwing a ground to illuminate the check engine light.
Get a conventional bulb (non-LED), provide power to the positive side and your check engine wire to the negative.
Get a conventional bulb (non-LED), provide power to the positive side and your check engine wire to the negative.
Hello everyone,
New to the forum. I have installed several crate engines in street rods over the last few years. Latest is an ls3 in a 1937 Ford, everything is running good, only issue is a mil light that is always on with the engine running. No codes. When I test the mil function with HP tuner it will turn off when I turn it on and turn on when I turn it off. Will then come back on when ignition is cycled. Wondering if anyone has run into this. Stock ls3, GM harness/computer, t-56 transmission.
New to the forum. I have installed several crate engines in street rods over the last few years. Latest is an ls3 in a 1937 Ford, everything is running good, only issue is a mil light that is always on with the engine running. No codes. When I test the mil function with HP tuner it will turn off when I turn it on and turn on when I turn it off. Will then come back on when ignition is cycled. Wondering if anyone has run into this. Stock ls3, GM harness/computer, t-56 transmission.
So you just tap one end of the resistor into the MIL Control wire, other end of resistor to 12v+ key hot. (you can use same key hot that is going to the MIL light. )