2001 Aurora 3.5
I have a strange problem where the start 1 relay will not actuate even though it'* getting 12 volts from the ignition switch and grounding by the PCM
I placed leads in the fuse box and had someone turn the ignition switch. I can read 12 volts in the correct pins for the relay, but It will not actuate.
It will actuate if I use another ground and not the YEL 76 ground that comes from the PCM.
I tested resistance on underhood Fusebox slot 85 to PCM YEL 76 connector, and the resistance seems normal at around 4 ohms.
What could be causing this issue where voltage is there, but the relay won't actuate on the PCM ground?

Run a redundant ground and have a beer.
Merry Christmas.

I have confirmed that there is a voltage drop on the the circuit that powers the start 1 relay when the ignition switch is at start/run. The voltage drops to zero. That'* why I can read 12V at the relay socket, but the relay won't actuate. The problem is that theYEL 75 ground wire comes from the PCM. How can I make a redundant ground for YEL 75 since it is grounded by the PCM only at start/run. Maybe the wire that grounds the PCM needs to be made redundant?

Moving the ignition switch to the CRANK position sends a 12 V signal to the PCM Ignition Crank Sense. The PCM verifies that the transmission is in the PARK or NEUTRAL position. The PCM then grounds the control circuit of the START relay. When the START relay is energized it allows voltage to the starter solenoid * terminal.
Make sure the PCM see'* the ignition switch input.
PCM connector C2 terminal 23 should have 12v with the ignition switch in the start position.

It works with a redundant ground so he don't need to check any of that.

I guess I misunderstood. When he said, It will actuate if I use another ground and not the YEL 76 ground that comes from the PCM, it meant by grounding that circuit that the pcm provides, the start relay will turn on. So to verify that the PCM has the proper inputs to "turn on the ground" when in the start position would be worth checking.

So, I was going to start looking at the PCM harness diagram to find how many grounds go into the PCM and grounding diagrams to find different ground points on the vehicle to check and clean and decided to check YEL 23 (crank voltage pin on the PCM) again. I'm getting 0.5V. I've checked this before during this problem and got full 12V when the ignition is in start/run. So now the problem has changed into something different. I checked the fuse for the ignition switch under back seat and it is good and getting 12V across its pins. Will probably pull out the ignition switch and check voltage there, which was done a while ago to troubleshoot the problem and it was good. Something I didn't check was the voltage on the neutral safety switch, but I haven't found the location for it yet.

Based on the schematic provided. PCM connector 2 terminal 23 should have 12v in the START position only, and 0 in the RUN position. I believe with the 3.5 L, you have a 4T65-E trans and it uses a Internal Mode Switch not a neutral safety switch for trans range input to the pcm. Just a Question, Did you check at the pcm?

I may have inferred something. @atentaten You did or did not check that the PCM was grounding the relay? If not then we need to make sure it see'* everything needed to ground the relay. I assumed the PCM was grounding but it was getting lost between the pcm and relay. My statement about running a redundant ground was that you would do new from the PCM and feed it the easiest way to the relay and tie it in. The problem I've seen with some is the wiring will get bad at a junction or at the bulkhead connector if it goes through there. Rather than taking it all apart and trying to find the culprit is to leave it as is a run the new redundant wire.
Clear as mud?

Yes, checked 23 at the PCM. I did check the start position only as I did before. This time it is half a volt instead of 12V. Regarding not having a neutral safety switch but an Internal Mode Switch-- I think the internal mode switch is in the transmission case, which is a terrible location for repair work.