why could cause a computer not to read the or connect to the P.C.M.in my car on 1997 Toyota Avalon

I have checked the crank and cam position sensors all thr fuses and relays the igniter the coils and so on tried to hook it up to 2 different computers and they say that it cannot establish a connection to the powertrain control module any help would be appreciated and yes all connections are good with key on afterwards.a profestional tried yesterday.do you think the pcm could be bad

by in Corinth, MS on December 11, 2013
2 answers 3 comments
ANSWER by on December 11, 2013
It could be, have it checked at the PCM connector for all related pwr and grounds be fore replacing. Make sure DATA lines to the DLC are good as well.
COMMENT by on December 11, 2013
thanks, how do you check for all power going to it.i have checked all the grounds
ANSWER by on December 11, 2013
Diagnosing communication issues is something that is new for all of us. The right tool to use is an oscilloscope so that you can see the voltage on the data circuit over time. To communicate between the cars computers and the scan tool the signal has to rise above a specific voltage and drop below a specific voltage. If one of the computers has lost power, it alone would be missing from the network, but if one has lost its ground, it will back feed power to the data bus pushing the voltage high and disrupt the entire network. The factory scan tools also have additional functions built into them that assist technicians with diagnosing communication problems. You're going to need a specialist to help you with this problem.
COMMENT by on December 12, 2013
thanks for the info.i did solve the problem.i changed the PCM and that was the issue found one at the salvage yard for $40.00 works fine. thanks for the response and the info swampman johnny
COMMENT by on December 12, 2013
Great; Happy Motoring