Author Topic: LY6/6L80 into 79 Stepside or "Someone Kill Me Now"  (Read 3654 times)

Offline Ikarus

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LY6/6L80 into 79 Stepside or "Someone Kill Me Now"
« on: April 26, 2015, 08:12:17 PM »
Okay, here goes.  I'm building a 79 Stepside with an LY6 from an 08 Sierra and a 6L80 from a Pontiac G8.  I'm finally trying to start her up, and all it does is crank all day long.  I've checked every ground, every power wire, tried using every single wire in the ignition switch harness as my "Hot in ON and START" wire (I'm using a PSI Conversion harness), but no matter what, I get no spark.  I've checked voltage at the coils, and all is well.  I have fuel, and my PCM was just reformatted with a new tune to rule out any tuning issues.  I don't see any recessed pins in the PCM connectors, and I don't see any obvious broken wires anywhere.  I've checked voltage and resistance pretty much everywhere.  I replaced the Crank Position Sensor because I wasn't getting RPM, and now I am.  I haven't replaced the Cam Position Sensor, but it should start up regardless of that. 

My wiring, as of right now (using this diagram):

Grounds: 

Engine to chassis
Battery to chassis
Engine to body
Two PSI harness grounds to the back of the driver's side head

Positive:

Battery positive to starter battery positive post
Starter positive post has connections to Pin 2J on the firewall
Starter positive to junction block
Starter positive to two battery positive terminals from the PSI harness
Junction Block goes to alternator and Pin 2E on the firewall
Starter S post goes to Pin 6 on the firewall

The "hot during ON/START" has been tried on the cab side of Pins 3 and 25 (as well as almost every other one in the ignition harness)

I can't figure out what I'm missing.  I had to re-run brand new wire and Type 56 terminals to the firewall because the original harness in this truck was completely trashed.

The weirdest part is, earlier this week, the truck started up for a couple of seconds, 5 or so times in a row.  I had it connected to a laptop and an EFILive autocal so I could log whether or not it was even commanding spark, but it fired up instead.  I've tried the same thing multiple times since, and I can't get anything from it.

Any help or direction you guys can give me would be appreciated.  This is the first time I've ever built a vehicle, and until now it was going great.
« Last Edit: April 27, 2015, 06:48:38 PM by bd »

Offline travisr1988

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Re: LY6/6L80 into 79 Stepside or "Someone Kill Me Now"
« Reply #1 on: April 26, 2015, 09:14:08 PM »
If it cranks all day long but won't start I would check for fuel issues first, then check for spark and timing.
1979 Chevrolet Silverado C20 rclb cammed 355/th400/4.10 - sold
1977 Chevrolet Custom Deluxe c10 rcsb stepside 355/th350/3.42
Vortec heads lunati cam edelbrock intake headers

Offline Ikarus

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Re: LY6/6L80 into 79 Stepside or "Someone Kill Me Now"
« Reply #2 on: April 26, 2015, 09:17:23 PM »
It's got good fuel.  Pump turns on and is putting out good pressure.

Offline travisr1988

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Re: LY6/6L80 into 79 Stepside or "Someone Kill Me Now"
« Reply #3 on: April 27, 2015, 05:13:18 PM »
Do you get spark at the plugs? No blockages in the air intake? Throttle blade is actually opening? If all that is good double check timing. If timing is good check voltage and resistance at crank, cam, throttle position, and IAC sensors.
1979 Chevrolet Silverado C20 rclb cammed 355/th400/4.10 - sold
1977 Chevrolet Custom Deluxe c10 rcsb stepside 355/th350/3.42
Vortec heads lunati cam edelbrock intake headers

Offline VileZambonie

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Re: LY6/6L80 into 79 Stepside or "Someone Kill Me Now"
« Reply #4 on: April 27, 2015, 07:39:20 PM »
You have scanned it and checked for DTC's? You have verified cranking rpm signal with a scan tool?
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Offline Ikarus

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Re: LY6/6L80 into 79 Stepside or "Someone Kill Me Now"
« Reply #5 on: April 27, 2015, 07:54:05 PM »
You have scanned it and checked for DTC's? You have verified cranking rpm signal with a scan tool?

Yes, I have.  No DTC's and it gives me RPM signal during crank.  Up to about 150-160RPM. 

And no spark at the plugs.  Throttle body moves with pedal actuation.  I've checked voltage at the coils and they have the 12V constant, but the 5V exciter voltage isn't there, and goes negative during cranking.

Offline Captkaos

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Re: LY6/6L80 into 79 Stepside or "Someone Kill Me Now"
« Reply #6 on: August 12, 2015, 11:52:51 AM »
Did you disable the Theft Lock in the ECM?  It is going to be looking for that key and if it isn't there it won't crank.

Offline roundhouse

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Re: LY6/6L80 into 79 Stepside or "Someone Kill Me Now"
« Reply #7 on: August 13, 2015, 08:21:53 AM »

Also gotta disable all the emission stuff
EGR.  Rear O2 sensors , etc or when it does crank it won't run very well


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