Check engine light went on while my brother was on the way home from an errand (we share the car). I honestly have no damn idea what's wrong. Car is a '90 GXL. The only thing I can possibly think is that the battery posts measured a few hundredths shy of 15V when I got the hood open.

List of things done in the last 48hrs:
-Topped off the trans with gear oil.
-There were some loose wires next to the trans, used a bit of electrical tape to bundle them together.
-Re-affixed a vacuum line that had been loose the past several months (one of the lines that runs from the bracket behind the intake manifold.)

 

Battery should be about 12.5 maximium. Try taking a reading off of the alternator while engine is running. Red meter lead to the B+ terminal (single wire bolted to alternator) of the alternator and black meter lead to the negative battery post. Should read about 14.7 volts maximum.

 

Yeah, 15.16 volts from +alt to -bat.

 

Check engine light means the ECU is seeing something wrong and is setting a trouble code.
Find out what code is being thrown.

Pop the hood and somewhere behind the driver's headlight, near the leading coil, you'll find a green connector with ONE pin and ONE wire in it. Connect that to ground.

Check Engine light will flash, some long, some short...

Get us the codes so that we can figure out what's going on.

http://www.banzai-racing.com/FD&S5_error_codes.htm

 

So the alternator is putting out a little more than 15 volts and the battery itself is almost 15 volts?

 

Well, according to the FAQ thread, 15V+ is not good for the electronics.

@Pele, relatives just arrived, may have to wait for morning.

 

Update; just pulled the code/s (dinner finished)

Either code 10 and code 1, or code 11.

And a 15.16V output from the alternator.

...?