No check engine light

[Mgardiner1]

I've adapted a 91 XJ wiring harness into my 87 YJ. Everything runs fine. I wanted to wire in a check engine light to the dashboard so i could extract codes via the MIL light (87 wranglers had no MIL/CE lights). I know the color of wire leaving the ECU and entering the XJ bulkhead connector is Black/Pink, however, i never have 12 volts on it with the key in any position, engine on or off. When i hook a lead of my DIM to 12v, and the other end to the Black/Pink, i get a reading that shows the MIL is grounded. If i cycle the keyswitch on and off 3 times, it doesn't act like it flashes any codes. Any idea's why this might be so? As far as i know, check engine light should illuminate when the engine is not running, and the ignition is switched on.

Maybe a required input for the ECU is missing?

[amcinstaller]

if its like the 92s (sorry, brain fart, cant remember the change in computer system) the check engine light will come on when you first turn on the key, but only for a few seconds, just to make sure it works. alternatively, it could trigger the light with ground, not power.

[Mgardiner1]

After digging through some electrical schematics, i agree with you, the ECU switches ground, not power. However, if i hook a DMM to 12v+ and the check engine lead, its always on, key off, key on, engine running. If i flip the keyswitch on, off, on, off, on (code check) it just remains 12v the entire time. Thinking it may need a resistive load, i tried the same procedure using a test light. I hooked a 12v test light to power, and the other end to the check engine lead. With a load on it, the test light NEVER illuminated. So i'm back to the drawing board....

[eliv1]

Ok, its like this.

1. The PCM ground the MIL, not supplying 12[v] to it.

2.locate the wire that goes out of PCM pin #32, and connect it to a Led/Lamp, other lamp side should be switched on power from ignition switch.

if this is what you have done, then your PCM MIL circuit is probably fried and you should try a diffrent PCM.

when you switch ignition on it should illuminate for about 3 sec's and then go off, 3 key switched will show trouble codes.

there shouldn't be a problem using a Led.

good luck!

[Mgardiner1]

Thanks Eliv1. I've noticed from schematics that it should work via switched power and activated ground. I'm going to get a 12v LED today and see if i can hook it up.

Again, what i have so far is, with a resistive load, it looked like the light would NEVER come on. When i used a DMM, it said it was ALWAYS on...