Novice seeking help (wiring/engine rebuild)

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Amsterdamned96

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Fellow GM truck enthusiasts,
Some time ago I made a thread dubbed “502 Accel 3500” with a different username. I was handed down a truck that was turned down by various garages/mechanics where I live. The harness was damaged severely due to a fire/excessive heat. I thought the air-intake was a twin root supercharger and the ‘502’ valve head covers had me fooled thinking I had a crate-engine. Since then I’ve learned a lot about automotive engineering and have decided that I want to study mechanical engineering. In an attempt to get this beauty back on the road, I’ve taken out the engine so that I can fully revamp the harness. Where necessary. I’ll be making a build thread as well but from what I’ve understood build threads do not allow for replies?

I was hoping to get some help with my engine rebuild and wiring questions.
The oil pressure/ temp sensor on the driver’s side of the block (7.4L / 454 Gen V) has three wires coming out of it, the center one is still in the connector the other two are missing. I’m about to put two new wires on the connector but I need to know if they go to the ECU or to the gauges. I’ll attach photos below.
Additionally I’d like to ask about these mysterious holes in the engine heads, I think they’ve been made to allow coolant to pass because the intended holes were blocked the gasket.

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badco

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Get a pin out wiring schematic and do some reading before jumping in to it. Also im still looking for picture of exhaust headers. Ive never seen coolant inbthe exhaust headers unless you have a gasket/cracked something issuse or its a jet boat. The intake side of the head has a exhaust crossover and depending on where you live is the determining factor if its needed at all. I personally plug all mine off from under the valve to the intake but i dont use egr or any emissions on mine
 

Amsterdamned96

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Get a pin out wiring schematic and do some reading before jumping in to it. Also im still looking for picture of exhaust headers. Ive never seen coolant inbthe exhaust headers unless you have a gasket/cracked something issuse or its a jet boat. The intake side of the head has a exhaust crossover and depending on where you live is the determining factor if its needed at all. I personally plug all mine off from under the valve to the intake but i dont use egr or any emissions on mine

Regarding the coolant in the heads, I made a very dumb mistake. I meant to say oil. I may have let a small amount of coolant run into the cylinders by turning the engine on the stand.
I was shocked to see such crude holes in the engine heads.

I’m glad that you mention the EGR & emissions, come to think of it there are 2 holes at rear of the engine on the heads that are plugged by the intake manifold

Edited: My mistake, I didn’t realise they are called engine heads. Not headers like the exhaust manifold. I’ve corrected this in the previous posts.
 

Amsterdamned96

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Looks like you have Accel part # 74106, which has been out of production for some time.

Right, this has been brought to my attention. I’ve been told the ECU dates back to ‘87. I’ve purchased
MAP CAL software that I received on a floppy disk. The ECU has a sticker on it which reads ‘Digital Fuel Injection’. I will add an image of it at a later date.
 
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Schurkey

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The oil pressure/ temp sensor on the driver’s side of the block (7.4L / 454 Gen V) has three wires coming out of it, the center one is still in the connector the other two are missing. I’m about to put two new wires on the connector but I need to know if they go to the ECU or to the gauges.
1. Is it an oil sensor, or a coolant sensor? They're NOT combined.
2. Is it in the head or the block?

I'm guessing you mean the OIL pressure sensor, in the block near (above) the oil filter--not the coolant temperature sensor just visible in your first photo of the cylinder head. (There is no oil temperature sensor.)
One wire will be for the dash oil pressure gauge.
Two wires are the pass-through to power the fuel pump if the fuel pump relay or it's wire harness fails. The sensor also contains a normally-open switch that closes at some minimal pressure--5 psi, perhaps--and then power is routed in one wire, through the switch, and out the other wire to the fuel pump.
YOU NEED A SERVICE MANUAL WITH WIRING DIAGRAMS. I've gotten them on eBay for reasonable prices, or you can get copies of the manual set on DVD-ROM for about $50. The CD-ROM or DVD-ROM copies are hard to navigate, but they've generally got the same info as the paper manuals they replace.

On some engines, this three-wire oil sensor will be on the top of the block, behind the intake manifold, next to the distributor. Nearly impossible to see.

Additionally I’d like to ask about these mysterious holes in the engine heads, I think they’ve been made to allow coolant to pass because the intended holes were blocked the gasket.
efa8401d-7d1f-4d79-96cb-82dae017b147-jpeg.229117
The long, thin, irregualr "holes" above and below the exhaust crossover port? They're blind casting voids, intended to shield the rest of the head from exhaust heat. they're essentially an air-gap insulator. There is no coolant or oil in those "holes". And some gaskets don't even cover the top one, it's open to atmosphere.
 

Amsterdamned96

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1. Is it an oil sensor, or a coolant sensor? They're NOT combined.
2. Is it in the head or the block?

I'm guessing you mean the OIL pressure sensor, in the block near (above) the oil filter--not the coolant temperature sensor just visible in your first photo of the cylinder head. (There is no oil temperature sensor.)
One wire will be for the dash oil pressure gauge.
Two wires are the pass-through to power the fuel pump if the fuel pump relay or it's wire harness fails. The sensor also contains a normally-open switch that closes at some minimal pressure--5 psi, perhaps--and then power is routed in one wire, through the switch, and out the other wire to the fuel pump.
YOU NEED A SERVICE MANUAL WITH WIRING DIAGRAMS. I've gotten them on eBay for reasonable prices, or you can get copies of the manual set on DVD-ROM for about $50. The CD-ROM or DVD-ROM copies are hard to navigate, but they've generally got the same info as the paper manuals they replace.

On some engines, this three-wire oil sensor will be on the top of the block, behind the intake manifold, next to the distributor. Nearly impossible to see.


The long, thin, irregualr "holes" above and below the exhaust crossover port? They're blind casting voids, intended to shield the rest of the head from exhaust heat. they're essentially an air-gap insulator. There is no coolant or oil in those "holes". And some gaskets don't even cover the top one, it's open to atmosphere.

It’s the oil-pressure sensor then & it’s located lower down on the block besides the crank/harmonic balancer. The car has aftermarket injection (MPFI) and the harness has been spliced into left, right & center. I’ve had a Carbed or TBI 7.4 Haynes manual and as far as I can recall it looked nothing like what I have in front of me.
I’ll look into getting a better idea of the stock-wiring diagram, so as to not have to ask about every other cable.

I’ll look into the pass-through wire powering fuel pump situation, as this is most likely what I’m missing.
If anyone has got a digital copy/link to an eBook version. I’m willing to purchase one. Although I’m doubtful it’d be of any help with an aftermarket ECU, MSD ignition and a spliced & crimped harness.

It may not be clear in the photos but the holes in the engine head I was referring to aren’t the neat ones around the rectangular port they harder to see in the picture so I’ve added another below to clarify.

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Amsterdamned96

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I had hoped that with the engine out I’d be able to simply trace wires and figure out where they out to go that way. Here are some images to show you what I’m dealing with.

Spliced harness

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Components damaged by excess heat/fire

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Amsterdamned96

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