Use "Choke" light as MIL/CEL in LS swap?

[BrandonHall10]

I recently LS swapped my '84 K10. I am looking to use my (now obsolete) "choke" light in the IPC as a MIL (or check engine light). I have done a little research into how the choke circuit works in its original stock configuration. Something about running through the oil pressure switch and switching ground to feed the choke. So on and so forth.

Anyhow, all that is moot now. I just need to know which wire (if possible) to feed 12v to to make the choke light illuminate. Can this light be used in a simple circuit?

Thanks! Picture of the truck in question:

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[Turbo4whl]

The choke light is another of GM's tricks. Key on engine not running, ignition feed to the light, to make the ground the current goes to the choke heating coil then ground. The small amount of amps the bulb needs does not heat up the choke. When the engine starts, the oil pressure switch closes. Current, also from the ignition joins the same wire to feed the choke. Since the light now sees to positive feeds, it goes out.

So this also makes the choke light a trouble light. It you turn the ignition on and the choke light does not light, then the choke coil is open (burnt out) or disconnected.

If you start the engine and the choke light stays on, then the oil pressure switch is bad or open wire.

This simple system could have relay to remove the higher amps required for the choke coil. This will remove the higher load from the oil pressure switch.

Now the answer to your question: The printed circuit on the back of the gauge set may/ will use that ignition on circuit for other items besides the choke light. The other wire looking for the ground going to the choke coil most likely is dark blue. Somewhere in the harness it joins the light blue wire that feeds the choke the switched current from the oil pressure switch.

My question, is the MIL light current from the LS engine PCM, looking for a ground? Or does it make the ground circuit complete? If it is looking for a ground the lamp socket must be isolated from it's hot feed and a ground added.

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Remember, there could be a relay added not shown on this simple diagram.

 

[Turbo4whl]

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[BrandonHall10]

My question, is the MIL light current from the LS engine PCM, looking for a ground? Or does it make the ground circuit complete? If it is looking for a ground the lamp socket must be isolated from it's hot feed and a ground added.

From what I understand, and my research shows, the PCM provides ground to complete the circuit. So the MIL or bulb in this example would need 12v keyed on power at the other lead.

 

[BrandonHall10]

FWIW, I'm not opposed to "nipping" the leads on the PCB that go to that bulb and just stringing in some wire. But I'd rather not pull the ICP if I don't have to. I tent to break things when I do dash work.

 

[Craig 85]

I had an '87 C30 years ago and recall it having a Service Engine Soon filter in the gauge cluster. You could maybe replace the Choke filter with the one shown below.

The other thing you could do is find a pigtail like in photo #2 to illuminate the light. It is used to illuminate 4WD. That way you don't have to modify the circuit board. Disregard the red box it was for a different thread.

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[Turbo4whl]

FWIW, I'm not opposed to "nipping" the leads on the PCB that go to that bulb and just stringing in some wire. But I'd rather not pull the ICP if I don't have to. I tent to break things when I do dash work.

So look for the blue wire in the taped pack coming from the gauge panel. Don't cut it, but just strip a little cover off to expose the copper wire. Use an inline fuse holder and make a test to ground. Key on, if you found the correct wire, the choke light will light up.

I made an alligator clip test lead with an inline fuse holder. Very handy test tool for many things. Very hard to make a costly mistake with the fuse inline.

 

[BrandonHall10]

I had an '87 C30 years ago and recall it having a Service Engine Soon filter in the gauge cluster. You could maybe replace the Choke filter with the one shown below.

I like this! I'm going to add this to my list of long(er) term fixes. I'll be on the hunt for that filter. Thanks!

The other thing you could do is find a pigtail like in photo #2 to illuminate the light. It is used to illuminate 4WD. That way you don't have to modify the circuit board.

I had a very similar thought laying in bed last night. "I wonder if I can just modify a bulb holder to wire it direct and bypass the PCB all together..." I didn't know that type of holder even existed. I will have to look into that also. Thanks again!

So look for the blue wire in the taped pack coming from the gauge panel. Don't cut it, but just strip a little cover off to expose the copper wire. Use an inline fuse holder and make a test to ground. Key on, if you found the correct wire, the choke light will light up.

Brilliant! I will give this a look next time I'm under there. Thanks!

 

[dsteelejr]

@BrandonHall10

Nice looking truck.

If all you want to do is have the CHOKE light illuminated by something else just splice the wire on the cluster pigtail to whatever wire you want to illuminate it.

Looking on the wiring diagrams for my '80, circuit 931 is for the electric choke light. Just splice that wire into whatever wire you want to have illuminate the CHOKE light.

The '80 wiring diagram says it's a dark blue wire, but it may be different for your year. On this spare pigtail I have from a later 80s model it looks like a brown & white wire.

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[BrandonHall10]

^^^ Awesome! Thanks!

I really appreciate all the help guys. I rigged up a temp light for now just so I have something. I promise I will do better when I have more time (and it's a little cooler outside). I have some options now. Thanks again for the information! This gives me something to work with.

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[TPISly-C10]

I had an '87 C30 years ago and recall it having a Service Engine Soon filter in the gauge cluster. You could maybe replace the Choke filter with the one shown below.

The other thing you could do is find a pigtail like in photo #2 to illuminate the light. It is used to illuminate 4WD. That way you don't have to modify the circuit board. Disregard the red box it was for a different thread.

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photo#2 thats what i did on mine!