Gen 4 ignition wiring questions

[DM72C20]

Getting close to finishing up the wiring on my swap. It’s a 09’ 4.8 from a Silverado going into my 72’ C20. I’ve reworked the original harness, retained most of the 09’ fuses, am using the 09’ alternator, modified the tank for the 09’ fuel pump module and am basically after a reliable daily driver.

What I am completely lost on is how to get the ignition switch inside the cab to communicate with the ECM to start the engine. It would be running through one of the 09’ relays - I tried a pink wire from the ignition switch to pin 30 of the run/crank relay. It clicks and the MAF clicks as well when key ON, but no crank. Coils have power and the fuel pump primes. I thought the run/crank relay would be the one to get the rest of the relays going in order to fire. But maybe it should be wired to the starter relay? Maybe I need the purple solenoid wire instead? I thought I read that the ECM needs constant power which is why I started with the pink one/key hot.

Going to get into HP tuners tomorrow to get rid of Vats in case that is interfering. I thought it would still crank with it enabled. Any suggestions would be great!! So excited to be at the stage of even trying to start this thing. I know lots of these swaps have been done, but my goodness what an effort!

[jjzepplin]

I would also like to know this. Some guys use the original purple wire. I would think you just trace the relay trigger wire and connect it to either the purple wire or the ignition where the purple wire goes.

[ls1nova71]

Originally Posted by DM72C20

Getting close to finishing up the wiring on my swap. It’s a 09’ 4.8 from a Silverado going into my 72’ C20. I’ve reworked the original harness, retained most of the 09’ fuses, am using the 09’ alternator, modified the tank for the 09’ fuel pump module and am basically after a reliable daily driver.

What I am completely lost on is how to get the ignition switch inside the cab to communicate with the ECM to start the engine. It would be running through one of the 09’ relays - I tried a pink wire from the ignition switch to pin 30 of the run/crank relay. It clicks and the MAF clicks as well when key ON, but no crank. Coils have power and the fuel pump primes. I thought the run/crank relay would be the one to get the rest of the relays going in order to fire. But maybe it should be wired to the starter relay? Maybe I need the purple solenoid wire instead? I thought I read that the ECM needs constant power which is why I started with the pink one/key hot.

Going to get into HP tuners tomorrow to get rid of Vats in case that is interfering. I thought it would still crank with it enabled. Any suggestions would be great!! So excited to be at the stage of even trying to start this thing. I know lots of these swaps have been done, but my goodness what an effort!

You say you are using most of the '09 fuses, but you didn't say whether or not you're using the '09 fuse box as well, or if you made your own.

If you are going to use a starter relay, then you would need to use the purple wire coming from the ignition switch to turn on the relay. It would need to be hooked to either 85 or 86 on the relay. 30 would be from the battery and 87 would go to the starter relay.

You are most likely going to need to not use the ECM to control it though, because without the BCM, which actually tells the ECM when to crank it, it probably isn't going to work.

It's possible that hooking the yellow/black X1-52 wire to the relay on pin 86 and grounding orange/black X1-1 through the NSS will work, but I have never personally tried it, but might be worth a shot just to see if it will work or not.

[DM72C20]

It is the 09 fuse box as well. Hoping for OEM reliability and parts interchange if possible.

Will try the purple wire to the run/crank relay. It didn't make sense to me that the key would turn on the relay but then there was no signal for crank. Would the pink supply key on power to the ECM through some other input?

Today's work yielded a bit of good but more bad. I got HP Tuners connected and disabled VATS and wrote that. No problem there. The bad was when I tried to connect the pink wire to 86 on the starter relay and promptly melted that area of the fuseboard

In other good news, I have the speedo swap in place and it looks great! Thanks for the kit!

Originally Posted by ls1nova71

You say you are using most of the '09 fuses, but you didn't say whether or not you're using the '09 fuse box as well, or if you made your own.

If you are going to use a starter relay, then you would need to use the purple wire coming from the ignition switch to turn on the relay. It would need to be hooked to either 85 or 86 on the relay. 30 would be from the battery and 87 would go to the starter relay.

You are most likely going to need to not use the ECM to control it though, because without the BCM, which actually tells the ECM when to crank it, it probably isn't going to work.

It's possible that hooking the yellow/black X1-52 wire to the relay on pin 86 and grounding orange/black X1-1 through the NSS will work, but I have never personally tried it, but might be worth a shot just to see if it will work or not.

[ls1nova71]

Originally Posted by DM72C20

It is the 09 fuse box as well. Hoping for OEM reliability and parts interchange if possible.

Will try the purple wire to the run/crank relay. It didn't make sense to me that the key would turn on the relay but then there was no signal for crank. Would the pink supply key on power to the ECM through some other input?

Today's work yielded a bit of good but more bad. I got HP Tuners connected and disabled VATS and wrote that. No problem there. The bad was when I tried to connect the pink wire to 86 on the starter relay and promptly melted that area of the fuseboard

In other good news, I have the speedo swap in place and it looks great! Thanks for the kit!

After looking at the schematic again, there is no way that I can see you being able to use the ECM to control the starter. The starter is controlled by the BCM, then that info is sent via the CANbuss to the ECM. Without the BCM, I just don't see it working.

There are several circuits on a '72 ignition switch. The red wires are constant hot, the pinks are ignition controlled power and the purple is the actual "start" wire that goes to the starter solenoid. Hooking up an ignition controlled wire to the starter relay will just start the starter cranking any time the key is turned on, so that won't work. You need to use the purple wire for that. The pink ignition controlled wire should be used to power up the ignition relay that essentially powers up the engine harness, which it sounds like is working.

Not sure why hooking power to the 86 on the starter relay would have melted anything though? It should have just kicked the relay on which would have started the starter turning over. Are you sure you weren't on 85? That wire is the ground, so I could see it doing that there. Or it is possible the schematic I have is wrong, but Mitchell is usually pretty accurate.

[DM72C20]

There were some anomalies when reading voltage through the fuseboard where the starter relay was. The 09' manual showed it opposite of where battery hot was and I reversed the relay to align with the manual orientation. I don't have it all in front of me to reference. Essentially battery hot was supplied in the opposite corner.

In terms of fuses and relays, the unneeded ones were not put into place. ABS, AC, etc. Don't think that the fuses would need to be there but maybe?

Curious and will try the X1-52 wire and X1-1 through the NSS.

So if I attempt to bypass the ECM and relays, how do I get this thing to run?

I'm a couple hours away from the truck, so won't be able to try things immediately.

[DM72C20]

The truck runs!

What we ended up doing was using the pink key on wire and purple solenoid wire from the ignition to get it started. By carefully checking the battery hot through the fuse board, we determined which way to orient the run/crank, starter, powertrain and fuel pump relays.

It was weird since the battery hot positions did not align with the 09’ manual and didn’t align with the new donor fuseboard setup or with my dad’s stock 09’ Silverado. By positioning all the relays with 30 at battery hot, and then attaching the pink wire to 86 for run/crank and purple to 86 on the starter relay, it fired up! This was after programming out VATS and disabling the starter check relay in HP Tuners.

It was stalling out but there was a loose coil connector and the brake master cylinder vacuum line was off, which meant a big vacuum leak.

We then had air trapped in the coolant system which was an issue until it was filled through the upper heater hose. After that it starts up every time and we took it for it’s first drive in almost two years! I am so excited to be at this point!
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[DM72C20]

Oh, the next little puzzle will be getting the alternator to output more than 13.8V. The battery monitor is around the big black negative cable, but since there’s no BCM, I’m not sure where to run it’s wires. I have a PWM signal generator if I need to go that route but ideally the stock system can be setup somehow.
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