ACE347

Tried searching and came up empty. My first question is, do I need a pcm for the ecm to be happy?

What I got is a 71 Chevy c20 with a non-electronic truck muncie. It currently has a 6L Lq4 from a 07 2500hd ready to go but it just will not fire up. I bought and installed a painless PRF-60526 stand alone wiring harness and had them flash just the ecm since I dont have a tcm (didn't think I needed it). I've been trying to get it running for a while now and no go.

All the grounds are good. Made sure I had the right harness. Still no injector pulse and no spark. I know a head chevy mechanic and asked him to help. He came over, hooked up his tool and began diagnosing. The ecm saw good cranking rpm and there where no codes at all but still no payoff. He tested the ignition circuits and the injector circuits to find they where in perfect working order. Even he said the motor should have been running.

Painless turned out to be a useless source of info since they have no idea how to tune without a tcm. I don't want to buy another ecm especially if it wont fix the problem. Can anyone help me get my truck running?

JoeNova

They can disable the trans all together in the PCM. The LQ4 didn't have a separate TCM.
Are you using DBW or DBC?

Is the neutral safety circuit disabled in the PCM?

ACE347

I didn't think there was a tcm in the truck that I got the motor from. Thanks for confirming this.

It's a dbw

Not sure about neutral safety circuit. I don't have one hooked up to the ecm but I did make my own that's wired inline with starter wire.

JoeNova

There is a neutral safety circuit enabled in the PCM on the gen 3 trucks. If its not disabled, you'll have to wire it up somewhere.

ACE347

Ok I found the pin for the neutral but how would I complete the circuit

JoeNova

I'm not sure off of the top of my head lol. I always turn it off and wire one from the shifter or column that goes to a relay inline to the starter.

You only have 2 options, power or ground. Try ground first.

2xLS1

iTrader: (25)

He said it is getting a cranking rpm signal so if it cranks a neutral safety is not an issue. Plus I've never had to change anything in any transplant P01 or P59 PCM having to do with neutral safety. Just disable VATS. On E38s I disable the starter checks.

JoeNova

The neutral safety in the PCM won't disable the starter in a swap. If your ignition switch is the only thing between the battery and the starter, its going to crank over even if there is no wiring/PCM or heads on the engine.

It WILL keep it from starting.

The neutral safety isn't enabled by default in every PCM. Some vehicles have the neutral safety wired in the body harness which will disable the engine from cranking completely if the shifter is not in park. I believe the F-bodies didn't have the neutral safety enabled in their PCM, nor did some of the vans.

JakeFusion

iTrader: (7)

I don't know about the neutral safety. But you don't need a transmission for the vehicle to fire. I've fired my car up and run it before connecting the trans (didn't want to have to go through removing it if the car didn't fire).

2xLS1

iTrader: (25)

All I can say is I've done probably over 100 PCMs for swaps and have never had to change anything as far as a neutral safety setting. Just VATS. Are you talking about the PRNDL Equipped setting?

ACE347

Thanks for all your input. Before a read all your replies, I tested this theory. There was no wire at all in the new harness so I added my own. Made no difference unfortunately.

Iv'e seen Ls's run on engine stands no problem yet mine won't even hiccup.

My Chevy guru says the ecm has bad drivers, and painless says it's the absence of tcm. Though, it seems like the tcm isn't necessary and I'm sure that that donor truck didn't have one because it didn't have the transmission loom in the original harness.

I attempted to cut down the original harness myself only to find that junk yard butchered too much of it. When I got the new harness, I cut out the PRNDL loom thinking that if the original didn't have then I didn't need the extra wires.

2xLS1

iTrader: (25)

If Painless thinks you need a TCM then they think you have a Gen 4 6.0L, not a Gen 3. Gen 3s did not use separate TCMs. The transmission control is part of the PCM. In 07 they made both Classic and New Body Style trucks. Gen 4 NNBS trucks used a separate ECM and separate TCM. Can you verify what your 6.0L came out of? Does your PCM have 2 long 80 pin connectors? Does the harness for those connectors have one with blue and one with green plastic pin retainers?

ACE347

The first pic is the vin plate for donor truck (knew it would come in handy one day)

The second is a pic of the motor in my truck. From it you can see the computer sitting on the vavlcover.

2xLS1

iTrader: (25)

You have a NNBS 6.0L Gen 4 with a E38 ECM. Although it is a 6.0L it is not really a LQ4. Those were last used in the 07 Classic trucks. And that truck would have has a separate T42 TCM. At this point without seeing the calibration in the PCM it's unclear if the VATS was really disabled. As well as the starter checks. And even if whoever did the initial tune disabled the VATS, if they did only a Write Calibration and not a Write Entire it would not be disabled. In those ECMs the VATS is part of the OS and a Write Entire has to be performed for it to be disabled. I think your problem is VATS.

JoeNova

At least once a day someone comes here looking for help, and it's because they aren't actually sure what engine they have lol.

ACE347

I did scope the cylinders and noticed that the pistons were flat tops. Always thought that was strange for an lq4.

So the tcm is needed or at least tuned out then, right?

Wouldn't painless performance know the difference and tune accordingly though?

2xLS1

iTrader: (25)

It should start and run without a TCM. I've found that some of the companies that make harnesses aren't always up to speed on the programming part of swaps. I've seen companies swear they disabled VATS only to find it still enabled. You may need to terminate the + and - CAN bus lines but I'm not 100% sure if that would keep it from running. And I would think that would only be if there was a TCM connector there with the wiring and no TCM leaving the CAN bus unterminated. . I know vehicles can lose communication with a TCM and set a code for that and still start and run. That is essentially the ECM not seeing the TCM there. Again I'm betting your VATS is still enabled.

ACE347

There wasn't any coeds though. None for the tcm and no code for vats. That's what has me puzzled. No indication why in doesn't run. It just doesn't send the spark or injector pulse.

2xLS1

iTrader: (25)

At this point you need to have someone with tuning software/cable read the ECM and see what's been done. Without that I can't help any more. You could send it to me and I could look at it. Just pay shipping both ways.

ACE347

I'll see if a can find a local tuner. I may have some leads to look into.