Check Engine Light doesn't illuminate at all

[sarwaria23]

Hi,

I went to pass smog in CA and everything went well except for the visual. I was told that the check engine light is supposed to illuminate when the key is in the on position, mine does not. I pulled the cluster out and shecked the bulb and bulb holder by plugging them into the ABS light plugs since they're the same and they work, so I know the problem isn't with those but rather the wiring or something electrical. What should I check and how? I tried to follow the two wires (green and white; brown and black) as far as I could and they seem ok. I checked bentley for a diagram but couldn't find anything. Car is 88 325iC and it starts and seems normal. Thanks!

[bmwman91]

If you have a multimeter, you can try pulling the cluster and checking continuity between the pin that controls the CEL and the 2 terminals that the bulb holder contacts. One of those should have continuity, and if not then a PCB trace broke somewhere. You can also check to make sure that the other contact gets 12V supplied if you plug the cluster in while it is still partially removed and accessible. Lastly, you can check the terminal on the wire harness to make sure that it has ~0V on it when the key in in the ON position since the CEL is activated by the ECU connecting that line to ground.

[sarwaria23]

If you have a multimeter, you can try pulling the cluster and checking continuity between the pin that controls the CEL and the 2 terminals that the bulb holder contacts. One of those should have continuity, and if not then a PCB trace broke somewhere. You can also check to make sure that the other contact gets 12V supplied if you plug the cluster in while it is still partially removed and accessible. Lastly, you can check the terminal on the wire harness to make sure that it has ~0V on it when the key in in the ON position since the CEL is activated by the ECU connecting that line to ground.

Hey, thanks for your input. I have a few questions, forgive my ignorance.

1. I have the cluster out, by two terminals do you mean the two wires that connect to the bulb holder and where and what is the pin that controls the CEL?

2. What is continuity? Sorry worked on cars but never electrical stuff.

3. Other parts do get power as the ABS light powers on when the key is in on position.

4. Can you please elaborate on the checking the terminal and ~0V and ground please?

Thanks, I really appreciate.

[bmwman91]

1. There are 2 contact patches on the PCB where the bulb holder makes electrical contact when it is installed. It iccurs to me that this is probably on the inside-face of the PCB, so you would either need to toally disassemble the cluster, or you could leave the bulb in and test its exposed contacts since those should be contacting the PCB on the inside.

2. Continuity just means that there is an unbroken electrical circuit (you can also think of this as "zero Ohms").

3. I am not sure how the 12V supply is distributed internally, but it is likely that it is used across various lights and stuff, so sure it is probably fine.

4. Ignore my first suggestion about this. Once you find the bulb terminal that reads 12V (per #1), check the other one to see that it goes to 0V when the key is in the ON mode. If it just stays at 12V, then there is either an issue with the wiring to the ECU, or in the ECU itself.

[sarwaria23]

1. There are 2 contact patches on the PCB where the bulb holder makes electrical contact when it is installed. It iccurs to me that this is probably on the inside-face of the PCB, so you would either need to toally disassemble the cluster, or you could leave the bulb in and test its exposed contacts since those should be contacting the PCB on the inside.

2. Continuity just means that there is an unbroken electrical circuit (you can also think of this as "zero Ohms").

3. I am not sure how the 12V supply is distributed internally, but it is likely that it is used across various lights and stuff, so sure it is probably fine.

4. Ignore my first suggestion about this. Once you find the bulb terminal that reads 12V (per #1), check the other one to see that it goes to 0V when the key is in the ON mode. If it just stays at 12V, then there is either an issue with the wiring to the ECU, or in the ECU itself.

I dont think the check engine light is related to the PCB the bulb and bulb holder pop out of the cluster and are connected to two wires from the harness.

[AndrewBird]

I dont think the check engine light is related to the PCB the bulb and bulb holder pop out of the cluster and are connected to two wires from the harness.

This doesn't sound like stock wiring. Do you have a picture to clarify?

The light might not work because someone did something fucky.

[earthwormjim]

Check engine light wiring doesn't go through the cluster PCB, it has its own direct wires, one of which leads to C101 and then to the Motronic ECU.



There was a service bulletin which added a diode inside C101, if your car had that done, perhaps the diode is damaged?

Other place to look is at your ignition relay. ECU needs to be getting power when your key is in position 2, in order for pin 15 of the ECU connector to go low, to pull current through the CEL on startup.

Try probing the CEL bulb holder, see if BR/BK wire is going low (continuity to ground) when your key is in position 2, and verify the GN/WT wire has ~12V on it.

[bmwman91]

Aaah right, the CEL is a plug-in one, not a screw-in one. Duh. Earthwormjim has the right info to be using here.

[sarwaria23]

Check engine light wiring doesn't go through the cluster PCB, it has its own direct wires, one of which leads to C101 and then to the Motronic ECU.



There was a service bulletin which added a diode inside C101, if your car had that done, perhaps the diode is damaged?

Other place to look is at your ignition relay. ECU needs to be getting power when your key is in position 2, in order for pin 15 of the ECU connector to go low, to pull current through the CEL on startup.

Try probing the CEL bulb holder, see if BR/BK wire is going low (continuity to ground) when your key is in position 2, and verify the GN/WT wire has ~12V on it.

Thanks a lot for the input. Where exactly is the ignition relay and could the main relay have anything to do with this? Also all the other bulbs light up in on position. And what tool should I use to check the wires, sorry never done anything electrical?