For some reason, I can't seem to get into the "diagnostic" mode of the car by shorting out pin A and pin B on the ALDL connector. I short the pins, turn ignition to ON, and nothing happens (no flashing SES light, fans stay off). Interestingly enough, my fan runs constantly ONLY when the engine is running. If the key is ON, but the engine is off, the fan doesn't turn on. The SES light is not lit when driving. Over the last few months I did some work to the car, including removing the A/C and "redoing" a few wire harnesses. It's possible that I messed up a connector, but I really don't think that I did that. What could cure these symptoms?

 

uM Have you ever actually SEEN your check engine light, come on? its possible you just have a burnt out bulb or the bulb is missing...

when you start the car it should come on for a few seconds. if it doesnt then the bulb is prolly bad or gone... or theres black electrical tape over it (happened to me)

 

The bulb works fine. When I start the car, it illuminates for a few seconds then goes away. When the pins are shorted and the ignition is switched to "ON," the light stays lit and does not blink (the same thing that happens when the ignition is ON and nothing is shorted). The fan is supposed to come on when in this mode, too. It doesn't.

 

Boy, do I feel stupid . Turns out that I shorted out the two LEFTmost pins in the top row, not the two rightmost pins. I guess I read the ALDL diagram backwards... or I'm just a forgetful moron who hasn't driven his car since September. It works now .

 

That's not so "stupid". If you were going to short two of the wrong pins, those are probably the "smartest" ones to short, since you can't cause any real harm in doing so. You could have really done some damage if you chose a couple of other pins.

 

So I hate to ask but what kind of damage could be caused by shorting the wrong pins? my car used to do diagnostic mode just fine half a year ago, but now it wont do it, and I fear I may have shorted the aforementioned wrong pins and hurt something. In any case, what are some common causes for no diagnostic mode?

 

Nobody knows?

 

bump

 

The ALDL is pin outs are configured like this (viewed from the front):
E = Serial Data
F = TCC Solenoid

I don't know if jumping these two would do any damage.

How do you know the ECM is not entering Diagnostic Mode? What does the SES light do with the key ON, engine OFF (ALDL not jumpered)? Steady light? No light? Flashing Code 12?

If you have a steady SES light and you jumper A-B what does the SES light do?