On my drive in to work this morning I noticed that the check engine light was on on my 91. I recently (2-3months ago) upgraded to MS3 from my MS2extra. I have not seen a check engine light in my 91 since I installed the original MS1 back in 2010.


I was under the impression that the check engine light was just driven by the factory ECU. Wiring drawings show it on terminal 1E which is no longer connected to anything and is not driven by the MS3.


The light was on for a while. Came to a stop and got on the gas a little as I was leaving the light and the check engine light went out. A mile or two down the road the check engine light started flickering a little, more dimly than before. I hit the brakes and it went out again. Seems like the light will come in at cruise but go out if I accelerate or hit the brakes.


Kinda made me wonder if there was a fluid sloshing or moving that would cause the issue. Only thing I could think about on my 91 was the brake reservoir level switch. Checked reservoir and it is full.


What could cause the check engine light to come on like this?


There are so many useless freaking posts on the site these days that my searched were fruitless.

 

Your CEL will only be triggered by MS3. Therefore, it can't be the brake fluid. The best thing you can do is datalog, and either post here or figure it out. Find the section in Tunerstudio about setting up the CEL (I think it's in the furthest right hand tab), and figure out what could've triggered it. It could be fail to sync (early signs of a failing sensor), coolant temperature too high, afr too high, etc.

In your datalog, CEL will most likely have a column (I don't have MS3 so idk) and if it's 1, the CEL is on. Find where it's on, then check to see which sensor is triggering it based on how it's configured in TS.

 

Who built your MS3? I know if you have an MSLab's one that Reverant turns on the CEL for different warnings. Would have to go in TS and lookup what it is set to to go off. I am not sure if the Braineack, or PRO's from DIY have the CEL setup by default.

 

Nope, I never hook it up when I build them unless asked.

 

Maybe the pin is wiggling around and grounding on something?

 

I confirmed that nothing CEL related was turning on. My brother built my MS3 with no additions for the CEL.

On the drive home it only came on when I was going over bumpy roads.

I moved the connector around and didn't see any change on the CEL.

 

start looking for loose wires and other wiring related junk

 

That fact that it's flickering/changing intensities is a strong indicator to me that it's something shorting somewhere.

 

More info.


I get in the car this morning and it wont start. I get one click and then nothing. No dash lights and I don't hear the fuel pump priming pulse. I figured I left the radar detector or something else on, but found everything off. Just before I hook up the charger I take a voltage reading at the battery and find 12.3V, more than enough at the battery. This made me think a I blew the main fuse or something. Checked the fuses and nothing was blown.


At this point I was running late so I just jumped in the GLI, will have to troubleshoot later. I am thinking there is a short or something causing this bad behavior.

 

Unplug the gauge cluster and try again.

 

Yippee, another opportunity to crack the 24 year old gauge hood!

 

Just got home and did a little trouble shooting. I still get 12.3V at the battery, no drop throughout the day so I am not having a constant drain or short. What is strange is that I only get 9V at the alternator and starter. I pulled the main relay and only get 9V at the relay. Haven't figured out which pin is which on the main relay yet. MS doesn't power up. I need to check the MS voltage.

What would cause that big of a voltage drop with no load on the system?

 

I am an idiot! I had the positive cable off of the battery to replace the alternator 4 weeks ago. I must have put it back on without tightening the bolt. Tightened it up and it started just fine. I bet the loose connection was causing all kind of issues like the flashing CEL.

 

Drove it in to work this morning with no issues. No CEL, even when I run over the real rough section of pavement on the back roads that made the CEL flash before.


Oh well. You live and you learn!

 

Glad you figured it out!