Anyone experienced this issue?

When I turn the key to on the light is bright and working correctly. When I start the car the light goes out, but not completely out. The car is running great and I have no codes. I'm running an LS1 / T56 and LS1 harness.

I'm using an LED for my check engine light and it does have a resistor built into it already. It's acting like the pcm is not going completely open in normal state to turn the light off.

 

not very knowledgeable on this problem specically but in general when a light is dimly lite when it should be off it is a bad ground. It would not be a ground in that light but another light or circuit is searching for ground. just check everything. Good luck!

 

NO a dim lit LED is normal.
There is just enough residual current to dimly light the LED then when fully on it will be blindingly bright. I use LEDs for that purpose exclusively so I can ensure it is functional when a code IS set.
The residual current is not enough to illuminate a std blight bulb.

 

So it's normal? I see your point and that's not a bad idea at all. Maybe I will just leave it this way. Thanks for the reply.

 

LED CEL are dim when off, same issue here. If it bothers you, add a resistor in series. Or change to a regular bulb. Or just get used to it.

 

he said he already has a resistor in there.

OP - why dont you try turning the CEL on and see if it gets brighter?

 

He just needs another resistor to prevent the LED from lighting. The one is has isn't enough hence its still lit.

 

As an electronic tech I'd say this makes 100% sense. I like the idea of leaving the light dim to know it's functional too.

OP what color is the LED you're using? (is the casing tinted or the LED that specific color?) What's the resistor value? Do you know the resistance/ Watts of the stock light bulb? Maybe you want to make sure the ECM sees the same current as it would with the stock bulb?

 

Every time he turns the key on without the engine running, it gets brighter.
Not sure if a "resistor built into it" on a 12 V LED equals adding a resistor in series.

 

Correct, it will be dim even with a 10k ohm resistor. I use clear blue led that is quite dim and blindingly bright when actually on. Very dim with the key on and running. Sanity check for diagnostics for me. I have used red diffused and you can't even tell that it is dim unless it is at night. I mount them next to the obd2 port so it can only be seen by the driver upon cranking.

Now for a bit of electronics theory. Most newer high intensity led drop about 3-4 volts. Subtract that from 13 volts leaves you with 10 volts to drop across the resistor. Ohms law says, 10 volts dropped across 10,000 ohms is 0.01 amps or 10ma. 10 ma through the led will illuminate it brightly. Now with less than 12 volts applied (no code set). There will be considerably less current but just enough to forward bias the led and make it glow very dim.

Sorry but is the engineering stink coming out in me. ;-)

 

It's been a while since I was in class, but how about throwing a zener diode in there? So there's no current before the breakdown.
Just an idea.

 

Rather have it dimly lit to ensure functionality, IMO. It's not annoying even when dim and placed in drivers view.

 

Yeah it's really not annoying now that I know it's normal. I just wanted to make sure I didn't have a problem with my PCM. My CEL is built into my obd port under the dash so it's not bother at all.

 

I use a blue LED for my high-beam indicator. As-is, it was soooo bright it was blinding. I experimented with resistors and ended up with a 15k ohm in series to make it "just right" for night driving and it's completely off when not activated.

 

Had the same thing happening with my LED lights. Switched to an even more custom set up using stock "placards" for CHECK ENGINE and BRAKE and used regular 194 bulbs. Only issue is those things get so stinking hot. So Ill probably switch back to LED and deal with it.

J.

 

There is 12v or so at the LED at all times the key is on. The PCM provides a ground to light the LED. So its a leak of ground out of the PCM causing it not 12 volts or less. The LED gets enough ground to light but its not enough to light an reg. blub. Correct.

 

Correct