Background. 94 civic. B18c all motor. Race only chassis. No dash, cluster, underdash fuse panels, etc. I built a race only harness for the car and need some help getting the MIL to work properly.

I have a wire from the MIL (A13-OBD1) going to one side of a LED. The other side I have going to a switch 12V power source. I have the SCS (D4-OBD1) going to the SCS connector. The other side I have for grounding to check the codes. When I test this harness out, the MIL stays on even when the SCS is not grounded.

Now if I test this setup in a car that still has the cluster, fuse panels, etc......run a wire from the MIL (A13-OBD1) to one side of a LED and the other side to a 12V switched source, the LED functions properly. Goes off when no codes and on when has a code. I measured the voltage across the LED, I get 12V on both side when no codes and when the LED is on, I have 12V on the power side and approx 3 volts on the MIL side.

BUT if I try this harness independant, supply 12V to one side and measure the voltage on the MIL side and it is down to around 3 volts and light is SOLID!!

I have to be missing something here. Help please.

 

Try using a bulb instead of an LED. I bet terminal A13 is some kinda output transistor that isn't completely totally open when it's supposed to be 'off'. So there's a very small current flowing thru the bulb even when it's 'off'.

See if EE_Chris chimes in here... I bet there's a small resistor you can put in parallel with your LED to take that current so the LED doesn't light up.

 

EE_Chris....thanks for PMing me. It will not let me send one back so here it is:

I made a race only harness for my race car. It has no cluster, etc.

I made a P28 ecu with a LED mounted on it to check the codes. I have one side of a LED to a 12V power. The other side of the LED I have going to the A13(MIL). When I send power to the harness, I get a solid LED. When I jump the SCS, it remains solid. I have tested the ecu and it works fine. I have also tried another ecu.

Now if I install this into a stock EG,.....stock everything..normal daily driver...if I install this ecu into that car, the LED works properly inline with the stock gauge cluser's check engine light. They both work perfectly! I can jumper the SCS and get the codes, etc.

BUT, if I remove the MIL wire from the A13 side of the harness.....I still have my LED mounted in the ecu to A13 on the ecu itself. If I remove the MIL wire from the chasssis side, the LED remains solid.

I have also tried using a gauge cluster bulb and get the same results. I have used a 1kohm resistor inline with the 12V power wire to the LED as well. This turned the LED off, but would not let me jumper any codes. I have not tried using a resistor inline with the MIL side of the LED.

Thanks for all your help.

 

bump. Help.

 

It really doesn't make any sense - why can't a normal stock bulb setup be replicated?

If you remove the LED and place a normal stock bulb in its place and that configuration doesn't work, then there is something up with your wire job, it would seem anyway.

If you remove A13 from the ECU but still have A13 connected to one side of the bulb (not LED) and then the other side connected to 12v and this doesn't work, then I dunno....are you really on A13?

 

If it doesn't work with a bulb & your harness, what about other connections missing? Mine has like 4 ground terminals (A23, A24, A26, B2) & I don't know which is used to complete the ground circuit for the MIL. You got any of those still open in your harness?