'88 350 TBI Firewall 12v Junction to Bulkhead connection wires?

[chengny]

Hell yeah!!!! This is exactly what I needed!

Now I know why I was getting confused. the Bulkhead on the diagram is shown 90 degrees counter clockwise.

I really Appreciate your help with this! I'll post some photos of the work I've done..this weekend.

Yeah, I noticed that too. It seems to be only that way for the 88 dwgs. Normally you just visualize bringing one of the edges on both halves together (bottom as in the example) and then image flipping the opposite edges up to meet. That results in how the connector pins would actually lay out when assembled to the firewall. This shows a 1984 bulkhead connector as an example:

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The cab side of the connection block - for the 88 dwgs - is upside down and reversed.

 

[mrgto]

Ok, that confused me even further lol.

Here's a question. With the new ECM harness it comes with the Fuel Pump Relay. It has one wire that connects to the Tan/White wire from the old relay. I traced that wire through the old harness, it then splits off to three tan/white wires one of them going to the bulk head @ A3 shown in this pic. This wire needs to be fixed. I then noticed that from the Bulk head all the way on the right, we'll call it G3 , has the tan/white wire out going to the fuel pump. where the tan/white wire goes in at A3 = 120 - fuel pump circuit (ecm, relay, oil press switch) the ecm relay and oil pressure switch is not being run through the bulk head so the only thing the tan/white is being used for is for the Fuel pump. I'm able to bypass the bulk head all together for the fuel pump, and cut the damaged wire at A3, cut the tan/white at G3 and splice the two together, the only question I can't figure out is why it connects into the bulk head. is it for a fuse at the fuse box? If so, I can easily run an inline fuse.

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[mrgto]

this is confusing as well. On the Hot Fuel Wiring Diagram, it shows the bulkhead and to the left of the Bulkhead image is another what appears to be another type of bulkhead point. I marked it in the image. the tan/white wire that comes out of the bulkhead at "G3" it's not shown on the bulkhead image on the right, but it does show it on the image on the left.

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[chengny]

Ok, that confused me even further lol.

Here's a question. With the new ECM harness it comes with the Fuel Pump Relay. It has one wire that connects to the Tan/White wire from the old relay. I traced that wire through the old harness, it then splits off to three tan/white wires one of them going to the bulk head @ A3 shown in this pic. This wire needs to be fixed. I then noticed that from the Bulk head all the way on the right, we'll call it G3 , has the tan/white wire out going to the fuel pump. where the tan/white wire goes in at A3 = 120 - fuel pump circuit (ecm, relay, oil press switch) the ecm relay and oil pressure switch is not being run through the bulk head so the only thing the tan/white is being used for is for the Fuel pump. I'm able to bypass the bulk head all together for the fuel pump, and cut the damaged wire at A3, cut the tan/white at G3 and splice the two together, the only question I can't figure out is why it connects into the bulk head. is it for a fuse at the fuse box? If so, I can easily run an inline fuse.

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For future discussion BHC = bulkhead connector

I'm not really sure what it is you want to do. Let me see if I read this correctly;

As it is now; the 120 BRN/WHT enters the BHC at A3, makes a u-turn inside the cab, re-emerges at G3 and then heads back to the fuel pump.

What you propose to do is; cut the 120 BRN/WHT (that connects to A3) before it enters the BHC, cut the 120 BR/WHT after it has passed through the BHC at G3 and splice those two cut leads together.

First, the reasons they ran the 120 into and out of the cab:

1. The Hot Fuel Handling Module is located inside the cab in the area near the fuse block & BHC. It is activated by a branched off leg of 120 that enters the cab at A3. By-passing the BHC with the 120 would cause you to lose the hot fuel handling feature.

2. The lead to the fuel pump runs back in the same harness loom as the rear lighting group and that group transits the BHC in the small section on the far LH side (looking fwd). They probably just wanted to have the 120 to the fuel pump come out of the cab near the entrance to the loom.

It would still work if - rather than cutting and splicing the two 120's together, you spliced the fuel pump power into the 120 that connects to the A3 pin. But I can't see how that would help you.

You know that a lot of the wiring to/from the ECM and components located in the engine compartment don't transit the firewall at the BHC. There is a big group of ECM connected wiring that passes through the firewall over on the RH side - by the transmission dipstick.

Here is a brief explanation of the fuel pump relay circuit's operation:

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[chengny]

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this is confusing as well. On the Hot Fuel Wiring Diagram, it shows the bulkhead and to the left of the Bulkhead image is another what appears to be another type of bulkhead point. I marked it in the image. the tan/white wire that comes out of the bulkhead at "G3" it's not shown on the bulkhead image on the right, but it does show it on the image on the left.

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Are talking about that white space in the middle of the engine side of the BHC? If so that is the section that contains the wiring for the forward lighting group. Since that fwd light section has no wires involved with the fuel system, they left it out for clarity.

I see the 120 enter the BHC on the cab side and come out on the engine side at points G3 & A3. I marked the G3 branch below:

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[mrgto]

Ok, that makes a lot of sense. I was thinking about bypassing the BHC because of the shape of those wire terminals, but then after doing some online research after I made the post I saw I can pickup the GM 56 terminals at the NAPA right up the street. So I'll be removing all the wires to accessories that are not needed ie: lights etc. and then cleaning up the stripped wires and replacing the GM 56 terminals on the connections that are needed.

With the new harness in place, I'm slowly removing the old harness making sure not to eliminate something that's needed that wasn't included in the new harness. ie: the Hot wire to the coil, fuel pump wiring etc.

 

[mrgto]

The arrow was meant to be pointing at the entire diagram that looks like the BHC.

Maybe I'm looking at this wrong.

I see two "BRC" one labeled "engine" the other labeled "I/P" The one Labeled I/P looks like the BHC on the firewall... What does the the one on the left, labeled "engine" and has a wire on the bottom @ 39 going to Kick down Switch, represent?

 

[chengny]

The arrow was meant to be pointing at the entire diagram that looks like the BHC.

Maybe I'm looking at this wrong.

I see two "BRC" one labeled "engine" the other labeled "I/P" The one Labeled I/P looks like the BHC on the firewall... What does the the one on the left, labeled "engine" and has a wire on the bottom @ 39 going to Kick down Switch, represent?

Oh boy - I guess I assumed too much.

On these dwgs you'll notice a line labeled "cowl". That is how they refer to the firewall. Notice that it runs between the two halves of the main bulkhead connector.

The MBHC is essentially just a big, multi-pin socket/plug assembly, the two halves of which are secured together with a bolt. One half (the female/socket side) is mounted on the cab side of the firewall - behind the fuse block. The other half is mounted on the engine side. That side is where the bolt that holds the two halves together is accessed.

So what you have is a multi-pin socket on the cab side of the FW and a multi-pin plug on the engine side. The steel firewall separates the two and provides a surface on either side for sealing. The bolt squeezes the whole thing together.

In the cab the - where the wires enter to transit the MBHC - the back of the socket side looks basically like your engine side. Just hopefully not all burnt up. The big difference between the socket and plug sides of the connector is that while the cab side is just one piece, the engine side is broken up into 3 sections.

1. The section that carries the wiring for the engine control and power groups. It is on the far inboard side and is the biggest section.

2. In the middle is a medium sized section that is used to transit the fwd lighting group, horns, brake warning lead, etc.

3. The smallest section - on the far outboard side - is where the rear lighting, fuel pump and tank level indication wires transit.

See if these images help me explain:

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If you release the bolt you can easily unplug the engine (male/plug) side from the cab (female/socket) side.

So when you are looking at these dwgs, the right hand side is in the cab and the left hand side is in the engine compartment. Note that the engine side is divided into the 3 sections discussed above. Also because power might be flowing in either direction depending on what the particular wire is used for, I should not use the terms entering/exiting the MBHC. The wiring is connected to terminals on both sides and the electron flow varies from wire to wire.

 

[mrgto]

Wow, it's crazy, as soon as you said the line splits the two I immediately saw it. The left side is the female side of the "plug" looking at it from inside the cab. mirror image of the BHC on the firewall.

now that I see it, it all makes sense now.

 

[roundhouse]

This plug is also the source or a lot of wiring gremlins

If you have problems with the turn signals , brake lights etc
And have power under the dash and not under the hood on the same wire

This block is broken or something

We had that issue and it took forever to trace down
I also didn’t realize that it was a plug together deal

But there were several wires that just would not make a good connection no matter how many times we removed the pin and squeezed it tighter

So we just cut those problem wires , drilled a hole , installed a grommet and made a new section of wire to bypass the giant plug and re connected it back to the proper wire under the hood

 

[mrgto]

I wanted to thank everyone for their input, you guys really helped me out big time!

Chengny, you especially! Selflessly taking the time to help educate a fellow wrench turner.
Your last post gave me the Ah-ha moment. Those wiring diagrams were confusing the hell out of me.

I pretty much have the entire engine compartment cleaned up. The wiring that's needed on the MBHC got all new connection pins. All frayed wiring have been replaced, fuel pump now gets activated by the ignition switch not relying on oil pressure switch, the hot fuel module actually works now ;-) and the motor runs super smooth again.

I have a question regarding idle controls and the AC signal wire. I started another thread in the tech section that addresses it.

I'll upload photos of this project on another thread. I think youll get a kick out of this project..this was back in 2014 when I first began the build.

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[Charlie7985]

Does anybody know where I can get the larger male terminals in the bulkhead? I'm assuming they are Packard 59, but all I have been able to find are the female terminals.

 

[4WDKC]

Does anybody know where I can get the larger male terminals in the bulkhead? I'm assuming they are Packard 59, but all I have been able to find are the female terminals.

Check here, you may have to buy some other items also though.
http://terminalsupplyco.com/default.aspx

 

[Charlie7985]

Check here, you may have to buy some other items also though.
http://terminalsupplyco.com/default.aspx

Thanks I've been looking for them everywhere