anybody know how to check a bad ecu.

 

depends on the year of your LS...

please search.

 

one of the tale-tale signs that is nearly always a bad ECU is when the ECU will not communicate its error codes

so first try and scan the system for OBD codes

 

A method without pulling out the ECU:
To observe the ripple level using an oscilloscope is the most simple and quick way to judge. All you need is to hook up the prove to the coolant temp sensor terminals. As far as I have experienced, the worst one had more than 1000mV of ripples. The one with 250mV showed intermittent problems.

If an oscilloscope is not available, a headphone is another easy tool to check. Connect it to the coolant temperature sensor terminals in series with a film capacitor of 0.47uF or so and listen to the ripple as a sound by your ears. You will notice the ripple as a noise by this method. The impedance of a headphone is low and the sensor line has some higher impedance, but the 250mV of ripple can be mostly heard.

The reason why this is effective to judge.
The deteriorated capacitors cannot remove the ripples at the DC line in the ECU and some appears at the coolant temperature sensor line. If you can recognize any noise except the ignition noise, it means that the capacitors inside of the ECU are not working well enough and it's time for you to change capacitors.

The ignition pulses are strong and interfering some as an EMI (Electro Magnetic Interfere) to the coolant temperature sensor line and you can detect it some but just disregard them.

Some ECU problems are caused by failed semiconductors or resistors but as far as I know, 9 out 10 are caused by deteriorated capacitors.

 

Another sign is if you turn the key on and the Check engine light fails to come on AT ALL even with the engine off. Although i'm sure they can still be bad and the light will function normally as well.