1996 Chev K pickup Cranks but wont start

Ok I have a 1996 Chev K pickup with a vortec 5.7L engine that cranks and wont start. I borrowed a GMC scan tool and found that I have no communication to the Engine control computer but communications to the other modules. I did get communications twice while trying to check for codes and the engine fired right up and the next time it wouldn't It intermittently will start but not very often and only when it gets communications to the ECM so my question is has anyone ever had this problem and where should I start ?


The battery is fully charged and all fuses has been tested and all relays.

Update

I have proper power to the Data link connector and grounds have been checked grounds from Data link connector to the ECM is good and resistance on the wires are withing specs. After testing connections today with this problem I did find that the GM scan tool ( TECH 2 ) dont have to be conected for it to start when it decides to work but when it does decide to not work there is no communications and have found that there is no power at the fuse box to ECM-I and EGN-I when it wont start but still power to ECM-B I am starting to think its in the ignition switch circuit that powers the ECM and has nothing to do with the ECM. The only thing I do to get it to work is cycle the Ignition switch sometimes once and the next time it takes like fifty tries

Post edited by: bubbagump, at: 2008/07/14 03:41


Update:

Fuel filter new The fuel pump is controlled by the fuse ENG 1 as stated above when no start condition happens there in no power to ENG 1 fuse or ECM 1 which leads me to believe ignition switch or wiring to the ignition switch as these both get there power from the ignition switch. and with no power to these it would let the enginge crank but no fuel pump operation or engine spark

Post edited by: bubbagump, at: 2008/07/21 05:25

scan tool put ecm default engine would start normal sounds like a bad connection or wire check first then suspect ecm is bad replace.remember cheap checks first then expensive sec.module.connectors are a good suspect start their first then broken wiring go.

scan tool put ecm default engine would start normal sounds like a bad connection or wire check first then suspect ecm is bad replace.remember cheap checks first then expensive sec.module.connectors are a good suspect start their first then broken wiring go.

suspect fuel filter change first to make sure you have a fuel supply is ok.

Fuel filter new The fuel pump is controlled by the fuse ENG 1 as stated above when no start condition happens there in no power to ENG 1 fuse or ECM 1 which leads me to believe ignition switch or wiring to the ignition switch as these both get there power from the ignition switch.

I don't have the schematic to this in front of me but don't all ECM-I, EGN-I and ECM-B get their power all from the same portion of the ignition circuit?

You probably have either a relay or solid state relay between the battery and ignition module since this demands greater power.

If all of those modules are switched by the same ignition switch position then you have to figure out where those modules which aren't showing power are supposed to get their power from.

You're on the exactly the right track, trace power first.

Ok It is Fixed by a new Ignition switch I traced the problem all the way there power to the switch but no switched power out to ECM-I and EGN-1 . ECM-B does not get the power from here. Thanks for all the help all

Bubbagump, good to hear you fixed it and thank you for updating and telling us what the problem was