I have an erratic idle and I was maybe suspecting the BAC was totally bad or my ECU was going haywire. How can I tell if my S4 T2 ECU is working properly or if the problem is something entirely different.

Do I just have to shell out $100 for another one and maybe just have a spare or is there an actual test I can do?

 

You get your digital meter out and the Fuel section of the fsm. Go to the Control Unit section. It has the values of each pin on the ECU with the engine running or the engine just with the key to ON. Read and compare.

 

I just use the light method for S4s.. It gives a 2 second diag light to tell if the ECU is running correctly.

Of course HAILERS method is more methodical and will point out small faults in an otherwise running ECU. I have had to resort to this before, not fun..

 

The 'ole standby...

https://www.rx7club.com/showthread.p...leshooting+ECU

 

Actually that just tells you if any of ten input sensors are causing a problem. It doesn't tell you anything about the ECU itself. Even checking the ECU voltages like Hailers said is generally just checking what's on the other end if the wire. That's okay though because most of the time that's where the problem is. ECU's just use a bunch of input devices to control a bunch of output devices. It's usually those devices that cause the problems, not the ECU itself.

Erratic idle is usually caused by a vac leak or an out-of-adjustment TPS.

 

the ecu is probably fine as they dont go out very often at all ready..most of the time erratic idle is caused by eithe the bac, a vac leak, or somthing such, if u go ta stock tubo II check and see if the turbo inlet duct is leaking, as they ussually crack and that creats a pretty crazy idle!

there are ecu tests u can do i belive measuring certain resistances and voltage outputs , not sure where the write up is exactly but i did come across one for testing the ecu before....

 

It's all in the FSM. Wayne just posted a link above to his thread on the topic.

 

http://www.teamfc3s.org/info/articles/idle.html

 

Well, let me clarify..

Whenever I get a car that wont start/etc, I always just plug the lights in real quick. If you have a fried ECU, there is a good chance that the 2 lights at the key startup wont illuminate, and now you have a good starting point. I just use the 2 lights at the beginning of the start sequence as the ECU saying "Hi I am here!!"...

Just another diagnostic tool.. Whats the first premise in electronics? KISS....