Trying to get information about detail operation of chevy tbi system

Hello guys.

I was hired to give some engine maintenance to a fleet of chevy tbi cars (cadillac devile, buick grandmaster, chevy caprice) i think almost all of them have the same ecu (the one with two black connectors).

I am looking for detail information about ecu operation, i mean:

*which is the coolant temp to take this thing into closed loop?
*which are the conditions to cpu sets idle speed at 1000 rpms instead of 650?

Any advice/link/book/tbs/ buletins will be very appreciated.

Regards.

By the way. I have checked three of them and they are idling at 1000 rpm no matter ECT temp, shift position (A/C off).

Those systems are probably one of the simplest you'll ever find, but you still need scan data for some of your questions

Closed loop requires more than just engine temp...

It would also be helpful to know the IAC counts

Do you have a scan tool?

Thanks flatrater,

Yes i have. In fact i have bought one two days ago, just to give service to that fleet (please watch attached picture in my first post). OTC 3211 scanner.

I have studied pin by pin configuration (some of inputs are power side switch and some ground side switch).

Some of those cars can get into closed loop and other can't.

I want to start with good information about that system. I also have simulated pin by pin with test light and scanner, the response of every PID.

At this point i feel it would be great to have that key detail information which tells me for instance, which signals and conditions PCM needs to set target idle. Until now, i have never seen that clues in a service manual.

Best regards.

By the way.

If you take a look to the first PID on that capture, you can see O/D was active, but actually transmission was in parking. Then I'm suspecting target idle (@1000 rpm) is set because PCM is not "seeing" the whole picture.

I forgot to give response about IAC steps.

Due to target rpm is 1000 rpm. IAC is sorrounding 108 step, back and forth one or two steps. If I partially block IAC passage with my finger (trying not to stall engine), pintle rectracts to something about 191 step. I thinks IAC is responding as it should (actual rpm is also good 975 - 1025).

guafa wrote: If you take a look to the first PID on that capture, you can see O/D was active, but actually transmission was in parking. Then I'm suspecting target idle (@1000 rpm) is set because PCM is not "seeing" the whole picture.

That may just mean O/D is enabled. Try turning it off and see if the PID changes.

Thanks Matt T,

I'll do it once i back there.

Guess what? I have found a very interesting book.

"Computarized engine control" by Steve V. Hatch

The conditions to get this PCM into closed loop are:
*A programed time
*Coolant temp above certain limit
*A voltage variation above 450mV in O2 sensor (which also indicates is hot enough)

Regards.