91 Skylark SES light on, but can't get codes

I have a 1991 Skylark with the 2.3 Quad 4 engine in it. The car only has 33,000 miles on it but the Service Engine Soon light came on 2 days ago. The car now idles extremely rough but runs ok at once it's off idle. The speedometer also quit working at the exact time the light came on and the cooling fan runs constantly as well.
I tried to jumper wire the A and B terminals on the diagnostic port with the ignition on but not running but the SES light stays lit constantly and won't flash any codes, not even the 12 code.
I'm wondering if I have a bad ecm at this point. I've never found a car that wouldn't give up any code at all and I'm lost now. I hate just randomly replacing stuff hoping for a cure... :banghead:

Any ideas?

Just to update everyone, it was the ecm that had went bad.