Question about wiring coming through bulkhead firewall, including the resistance wire

Hi folks,

I’m in the process of replacing the wires that come through the bulkhead firewall into the engine bay, including removing the resistance wire (hiding it actually as per the help instructions).

I have a couple of questions but I will start with some info:

The following is the bulkhead firewall wiring based on Figure 12-108 in the Cadillac repair manual:

To starter:

18 G Yellow (goes from starter to coil directly)

12 G Dark Blue

10 G Red

To coil:

20 G White with orange and violet cross tracer

18 G Yellow (goes from starter to coil directly)

To thermostat:

18 G Dark Green

Here is what comes out of my bulkhead firewall:

12 G Dark Blue that goes to a connector. The connector also has the 18 G yellow coming in AND out of it. See photos below.

10 G Red

18 G Dark green

On my setup, the white resistance wire is connected to a 18 G Yellow wire via a clip that connects to the coil (see first photo). If I delete or disconnect the resistance wire, I assume I need to run a new 18 G yellow wire from the coil to the starter, correct?  

If yes, then I can delete the connector that splits the dark blue wire (first photo) and just run a new 12 G Dark Blue wire to the starter?

I also have a white wire in the bulkhead firewall harness that loops from one side of the loom to the other with a female connector on each end (see photo below). I don’t see this wire in the repair manual diagram, so I have no idea what it’s for. Any thoughts?

Thanks in advance for both your input and patience as I fumble my way though this.

Oh and here are a couple of progress photos for your entertainment!

Balanced crankshaft

My good friend Bruce checking tolerances.

Almost there...

New vacuum lines run, new brake hard lines, suspension work underway. Then everything else...

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If you scroll down the home page ( dashboard ) to --Dennis link -Jason has just given a complete explination of all the ignition wiring colors and what the wiring gets connected to
Dennis's issue was the car shutting down after start up so you find the correct post and info
Should help sort out you wiring issues a lot
I enjoyed the start up wiring info Jason explained a good deal myself as it was a bit of a refresser for me on the subject

Some very very extensive restoration work going on with your Antique Cadillac pride and Joy


Thanks for the suggestion Tony!

I think I found Jason's notes and it's good information but it doesn't address my two questions unless I didn't find the correct thread.

White wire is for trans kickdown switch on the 1964 cranking harness. Round black connector plugs into Turbo Hydramatic transmission.  Is this harness from a 1964 Cadillac with the THM?

Sorry, it's a '63.

I don't know that THM stands for.

Chris, I know your Cadillac is a 1963. I am asking if your cranking harness was sourced from a 1964 Cadillac with the Turbo Hydra-Matic Transmission (THM).
The 1964 cranking harness has a white wire with connectors on both ends. The large round black connector plugs into the 1964 THM transmission and the other end plugs into the electric kickdown switch.
You asked what the white wire is with 2 connectors and I am saying it looks like the kickdown switch white wire found on 1964 Cadillacs with the THM transmission. If you have a Hydra-Matic transmission (Standard for all 1963 body styles, and standard for 1964 Series 62, Commercial Chassis and 75 series), then it is not needed.

Ah ha! Thanks Jason.

Both connectors on the white wire look pretty dirty so I'm thinking that they were not being used. Plus the crimps used on the bulkhead wiring near the firewall are definitely not original so maybe it was replaced at some point.

I'm going to delete the white wire.

That was my thoughts Jason. THM kickdown wire, but I never looked what year the car was. The end looks like the transmission connection to me. 

That engine looks great by the way!

Thank you Clovis. I originally tried the Bill Hersch Cadillac blue but I didn't like the colour combo.

I just have to ask why you are using a 64 wiring harness designed for the turbo- hydramatic trans ---in a 63 that has a hydramatic trans which is a very different trans and does not require the wiring the THM trans does to operate

Is this something that has just been realized from the info being exchange in this tread ?


The wiring harness was in the car when I purchased it.

The crimps used on the bulkhead wiring near the firewall are definitely not original so maybe it was replaced at some point.

I very much like the photo of you hovering over the block with the dial indicator
The overalls in the photo have a nice look about them

One hell of a resoration project you have in the works there




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