LED check engine light w a GM ECM/harness?

[Ron in SoCal]

GM LSX swap experts step in please:

I'm going to check it out tomorrow, but I was told not to run an LED check engine light with an LED. Seems the LED has no resistance and therefore the computer thinks it's out and goes nuts (limp mode).

Ever heard of this? The fix is to wire up a light bulb.

[samckitt]

Originally Posted by Ron in SoCal

GM LSX swap experts step in please:

I'm going to check it out tomorrow, but I was told not to run an LED check engine light with an LED. Seems the LED has no resistance and therefore the computer thinks it's out and goes nuts (limp mode).

Ever heard of this? The fix is to wire up a light bulb.

Well I can tell you that the check engine light is burnt out in my truck & when I put it on a scanner, there is a codes is for the burnt out bulb. Couldnt you measure / find out the resistance of the bulb & replace with an LED & resister to match?

[Ron in SoCal]

Hey Scot - I wasn't getting a code for a burnt out light, but did get one for crank trigger learning (not critical). I was also thinking of putting a resistor in line but man, this is diriving me nuts. I'll read the OBDII first, do the bulb test and go from there.

[WSSix]

Hmmmm, I haven't heard that before but I know the SES light for my LT1 messing up the alternator if there isn't a resistor in line. LEDs in general don't have much or any resistance so if it is required to keep the PCM out of limp mode, then you'll have to get a load resistor in there some where.

I'd be interested to hear a reason behind this if it is indeed true.

[OBeer-WAN-Kenobi]

If you just have a LED by it's lonesome (as it would come to solder to a circuit board for example) you would need a resistor. If you have an LED "bulb" usually the resistor is built in. This is in general, wherever you are using it. I don't know how it would relate to a CEL in particular but it's possible the ECM wants a certain resistance there I suppose.

[TomM]

Never heard of that. The only issue I have heard of is LED not working because polarity is wrong, as some don't come marked. I have used nothing but LED's and never set a code for them.....

Detroit Speed makes some really nice ones, very small and very bright. You almost cannot see them in your dash.

T,

[Ron in SoCal]

Thanks Tom. I am using an E38 ECM and DSE LEDs

I'm off to the shop and will report back.

[samckitt]

Originally Posted by Ron in SoCal

Hey Scot - I wasn't getting a code for a burnt out light, but did get one for crank trigger learning (not critical). I was also thinking of putting a resistor in line but man, this is diriving me nuts. I'll read the OBDII first, do the bulb test and go from there.

So you are not getting a code for the burnt bulb, but rather causes it to run erratically? Hmmm, wonder if that is what is causes my missfire.....?

[HwyStarJoe]

I build test fixtures for a living, with multiple LED's.
LEDs have one lead longer than the other. This is the anode. This longer lead must be connected to the (+), and the shorter lead (cathode) must be connected to return, or NEG.
I don't know how an LED would effect a PCM, but there's always a resistor inline with the cathode... say 1K Ohm.

I know when I did my swap I read a lot about the ODBII connection to the ECM needing a resistor. I used the factory harness and ECM so I didn't worry about it. It works fine that way. But, my memory could be failing and I may have added one. I'd have to look at my notes.