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

I have an '88 R30 that has been stripped down and being used in a custom off road application. I'm in the process of cleaning up the engine bay and eliminating a bunch of electrical components that aren't needed. All of the headlights, Taillights, blower motor, AC etc basically everything that's not vital to the engine running.

Needs some help with these.
On the Firewall there's a junction box with 12v wires going to the Bulkhead connectors. I looked at a bunch of wiring diagrams till my head was about to explode

What are these for? They all terminate on the left side of the Bulkhead. Thanks in advance.

• Left: The One I'm holding with a Red and Black wire
• 2nd from left: Solid Red
• 3rd from left: Red w/black and a fuseable link
• On the Right: I believe the larger Red wire hits the Battery.

mrgto said:

Needs some help with these.

On the Firewall there's a junction box with 12v wires going to the Bulkhead connectors.

What are these for? They all terminate on the left side of the Bulkhead. Thanks in advance.

• Left: The One I'm holding with a Red and Black wire
• 2nd from left: Solid Red
• 3rd from left: Red w/black and a fuseable link
• On the Right: I believe the larger Red wire hits the Battery.

Click to expand...

First, to be sure which wires are being called what - is the image below correctly labeled?



2nd question. When you say this:

They all terminate on the left side of the Bulkhead

Does that mean that they all transit the FW at the main bulkhead connector - specifically in the engine wiring section? Like this?




Is the truck equipped with A/C?

Did it come with an auxiliary cooling fan?

Right on! Thank you . Yes the wires are labeled correctly and terminate at the left side of the bulkhead like you pictured.

I don't believe the truck had AC or the Aux cooling fan.

I did install a new ECM wire harness and got rid of the old worn out harness. There were some janky-ass "repairs" done on this thing by someone, amazed at what some use to splice wires lol.

I did some radical custom modifications. Relocated the driver controls to an area "floating" above the hood. the ignition switch on/off on the column is the only thing I kept intact. Ignition Switch on and then use a temp on/off button to crank the motor. I kept the ignition switch operating at the time as I didn't need to open a can of worms locating and and making modification to the coil wiring.

Anyway, I guess your questions are;

1. Why are there 5 hot wires (wire L has 2 branch leads) coming off the firewall junction block going into the cab - via the bulkhead connector - when there should only be one?

2. What do each of these leads connect to?

Here is marked up image of the connector block. Provide the location of where each wire connects (like A-1 will be one of the big RED's) and we'll see if we can't figure out what is going on:



I know the rows are a little wavy but just follow them left to right.

You know, one of those red wires has to connect to the solenoid. That's how battery voltage is supplied to the JB.

Thanks again! Posting the image of that clean Bulkhead really helped.

Here's what I have:

• L wire - A1
• The Black Wire paired with L goes to what looks to be C4
• Disregard the 2nd Wire.
• 3rd Wire: 4B
• R wire, couldn't figure it out tonight

There are a few wires on the Bulkhead that have the insulation stripped off baring copper. That either can be removed or will be repaired.
A2
A3
A4

yeah this thing is a mess.

mrgto said:

Thanks again! Posting the image of that clean Bulkhead really helped.

Here's what I have:

• L wire - A1 = HL switch (main beams, tail/parking/marker lts, crtsy/dome lts) & BATT bank of fuse block - far left
• The Black Wire paired with L goes to what looks to be C4
• Disregard the 2nd Wire.
• 3rd Wire: 4B
• R wire, couldn't figure it out tonight

There are a few wires on the Bulkhead that have the insulation stripped off baring copper. That either can be removed or will be repaired.

A2 = 436 – AIR solenoid (ecm, solenoid coil)

A3 = 120 - fuel pump circuit (ecm, relay, oil press switch)

A4 = 39 – TCC control circuit (ecm, TTC relay)

yeah this thing is a mess.

You must be registered for see images attach

Click to expand...

C & D - 3 & 4 (yellow box below) are all associated with the windshield wipers



We'll do some more later.

chengny said:

C & D - 3 & 4 (yellow box below) are all associated with the windshield wipers

You must be registered for see images attach

We'll do some more later.

Click to expand...

Are you replacing the entire underhood harness? The wires on the left side of that bulk head almost burnt my car to the ground, i took the bulk head apart and they had melted the plastic in there from the heat of corrosion and wasnt making good contact. I suggest you split that bulk head and inspect it if not replace the entire under hood harness at a minimum.

Hey 4WDKC, Yes. I have already purchased and installed a new ECM harness. Back in 2013 I was looking for a 87-88 R30 flatbed Dually w/ the 350 tbi to be used as the base for a radically designed Art Project Vehicle. Which is is rig in my profile pic. I chose this model for the reliable and STRONG 350, plus for my application, the TBI was the best choice. The motor ran well, but as you can see by the condition of the Bulk head, the wiring was a mess. Two years ago I purchased and installed the new harness. The completed rig is strictly "off road" and used in Parades and festivals. Max speed is about 25 mph but she pretty much always travels at 5mph or less. When in operation the motor is always running turning two high output Alternators keeping the battery bank charged for all of the 12v electronics, LED Lighting etc.

When I installed the new harness I kept the old harness in place to use as reference just in case. I'm now sorting through and removing all of the old harness wires and original 12v accessories, lighting etc. basically if it's not essential to the running of the motor, it's gone.

Thanks again! Posting the image of that clean Bulkhead really helped.

Here's what I have:

• L wire - A1 = HL switch (main beams, tail/parking/marker lts, crtsy/dome lts) & BATT bank of fuse block - far left
• The Black Wire paired with L goes to what looks to be C4
• Disregard the 2nd Wire.
• 3rd Wire: 4B
• R wire, couldn't figure it out tonight

There are a few wires on the Bulkhead that have the insulation stripped off baring copper. That either can be removed or will be repaired.

A2 = 436 – AIR solenoid (ecm, solenoid coil)

A3 = 120 - fuel pump circuit (ecm, relay, oil press switch)

A4 = 39 – TCC control circuit (ecm, TTC relay)


Thanks Chengny! This is a huge help!

It looks like I can then remove these wires.

• A4 TCC Control Circuit... ecm and TTC relay... the new harness is not running off the fuse block.. so I can remove this wire... this is also the TCC relay. my Truck has the TH400. the new harness has a TCC wire. If I'm correct, the TCC wire is not used in with the TH400?

• The Black Wire paired with the L Wire looks like it goes to C4 which would be the 130 slot? is this the ignition wire to the coil?

Is there a link or a diagram chart that I can cross reference that would show the slot number on the Bulkhead and it's function instead of lines that you need to follow? Like you did with 436, 120 and 39 above.

mrgto said:

Thanks again! Posting the image of that clean Bulkhead really helped.

Here's what I have:

• L wire - A1 = HL switch (main beams, tail/parking/marker lts, crtsy/dome lts) & BATT bank of fuse block - far left
• The Black Wire paired with L goes to what looks to be C4
• Disregard the 2nd Wire.
• 3rd Wire: 4B
• R wire, couldn't figure it out tonight

There are a few wires on the Bulkhead that have the insulation stripped off baring copper. That either can be removed or will be repaired.

A2 = 436 – AIR solenoid (ecm, solenoid coil)

A3 = 120 - TAN/WHT fuel pump circuit (ecm, relay, oil press switch)

A4 = 39 PNK/BLK from 20 A GAUGE/IDLE fuse to Aux Cooling Fan and Kick Down Solenoid

Thanks Chengny! This is a huge help!

It looks like I can then remove these wires.

• A4 TCC Control Circuit... ecm and TTC relay... the new harness is not running off the fuse block.. so I can remove this wire... this is also the TCC relay. my Truck has the TH400. the new harness has a TCC wire. If I'm correct, the TCC wire is not used in with the TH400?

• The Black Wire paired with the L Wire looks like it goes to C4 which would be the 130 slot? is this the ignition wire to the coil? No, it to the alternator

Is there a link or a diagram chart that I can cross reference that would show the slot number on the Bulkhead and it's function instead of lines that you need to follow? Like you did with 436, 120 and 39 above.

Click to expand...

B1 = Empty
B2 = 3 PNK - to ignition coil primary
B3 = 6 PPL - to S terminal on starter solenoid
B4 = 2 RED - Battery power to ign switch BATT terminals

C4 = BRN 25 alternator terminal 1 to generator warning light on dash (if you have gauges this wire won't be there - I think)

D4 = 439 PNK/BLK common ignition switched power to a lot of components (see the attached diagrams)

I apologize but I had to do some stuff. What I did - instead of finishing - was pull all the wiring diagrams associated with your truck from the 1988 Electrical Manual. There are 16 that would have an impact on what you are trying to do. Some are important, others are not so important - but I understand that you want to clean up as much as you can. I can only upload 5 per post, so it'll take 3 at least.

First group - power distribution, start/charge, ignition and fuse block:

Fuel system:

ECM inputs & outputs:

Fwd lights and horns:




That should keep you busy for awhile. Good luck.

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.