Hi George,
Perform the test according to the GM procedure sent, but at point No. 2 the DTC P351 that should have appeared did not appear. You have a INTERMITENT CONDITIONS under SELF-DIAGNSTIC SYSTEM?

 

This means that you do not have a P0135 hard failure, the communication between the CPK, PCM and the ICM or the PCM/ICM/CPK operation is intermittent. This can be caused by:

Poor quality power or grounds at the PCM or ICM. Its not good enough just to prove that pwr/gnd is there
Poor quality connectors at the CPK/PCM/ICM
Poor quality wiring between those items
Failing components

Thats why I asked you to perform your LED test lamp trick without the test lamp because I think you have an intermittent problem and every time you disconnect and reconnect everything the truck starts having nothing to do with the led test light.

I would start by examining all connectors and wires very carefully then we need to test all of these things fully loaded and connected. Ill explain what I mean when your ready. You will need a digital multimeter so we can measure resistance and voltages.

George

 

We will start here:

"I place the led bulb negative to the cable that goes from the computer to the ignition module (ICM) and the led bulb positive to the battery positive, when connecting it the bulb does not turn on or it makes it very weak, After 10 seconds the light from the led bulb becomes intense,"

George

 

I am writing some of the explanation now while I have a few minutes in front of my computer before I have to get to work today.

A basic check for power, ground and wire continuity is OK for simple problems but when vehicles get decades old and start having intermittent problems then testing must prove that power, grounds and continuity of wires and connectors are adequate under the loads of normal operating conditions. Lets say that a wire is partially corroded and that corrosion puts 5 ohms in the wire. You check for power or continuity with an led test light which draws almost no current through the wire and the corrosion and you get 12v or ground. But now lets say that this wire must carry 1 amp of current while everything is connected. 1 amp through 5 ohms means that you lose 5v of voltage drop at the site of corrosion and now the wire only supplies 7V, under operating load

. An LED test light is very good around computer modules like the PCM so that you dont fry a non protected driver circuit but its not the ideal test tool for everything. You also need an old school incandescent test light, a digital multimeter and if you really want to get good at this, an oscilloscope (there are some small inexpensive portable ones).

This might be useful:

https://blazerforum.com/forum/2nd-ge...59/#post732668

George

 

I had that in mind, I removed the protective covers from the wiring and looked for signs of cut or burned wires, found nothing. Clean the connectors. I think now I must do it in a more exhaustive way.
If you have some voltage or resistance values that should be normal I can start checking them. Likewise, if you have any suggestions where to start or if I can do some testing with direct or temporary wiring, it would be of great help.
Thank you very much for your attention and help. Sorry to bother you on the weekend

 

Lets start by measuring the actual net voltage to the ICM while it is connected. Ignition in run, back probe or pierce the two power wires (A pink and C blk/wht) at the ICM connector and read the voltage. Move the wires and connectors around and watch the net voltage and see if it changes.

After that we will move on to the wht/blk wire connecting the PCM to the ICM.

George

 

can you hear the fuel pump com one when the key is turned to the on position? check the fuel pressure, and the fuel pump relay.

 

This was the original post in this thread from 8 years ago and that OP has not posted since so he probably does not even own the truck by now. I went along with yordanoper essentially hijacking this thread because he is a new member and the OP post was so old. We have now reverse hijacked the thread and mixing problems which can lead to confusion. The new poster is having ignition problems, lets stick to helping him.


George

 

ok thanks .

 

No problem, if you have any issues, start a new thread and we will be glad to help you.

George