Any 66 guys using the additional 12R wire with your FWD Light Harness - Chevelle Tech
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post #1 of 25 (permalink) Old Sep 17th, 19, 8:02 PM Thread Starter
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Any 66 guys using the additional 12R wire with your FWD Light Harness

American Auto Wire tells me that the additional 12R wire used only in 1966 was an add on item to the FWD Light harness. And was not wrapped in the harness and just sat along side of the harness. I was wondering if it had no purpose and may have been used at some assembly plants; but not all. I can't find any reference to it in the AIM; but see it is some diagrams.

The one photo is of a harness I removed from a 66 that did not have the additional 12R wire. On my car, it also does not have that addition wire. I wonder if this was a 1st attempt at a fusible link that started in 67.
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post #2 of 25 (permalink) Old Sep 19th, 19, 3:52 PM Thread Starter
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Re: Any 66 guys using the additional 12R wire with your FWD Light Harn

41 views and no comments as to whether any 66 owners have or do not have the wire?


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post #3 of 25 (permalink) Old Sep 20th, 19, 6:41 PM
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Re: Any 66 guys using the additional 12R wire with your FWD Light Harn

My 66 does not have an extra red wire. Just a thought, where is it connected to, Could it be for the ammeter if the car had gauge package ??
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post #4 of 25 (permalink) Old Sep 21st, 19, 10:00 AM Thread Starter
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Re: Any 66 guys using the additional 12R wire with your FWD Light Harn

No ammeter wire is part of the FWD light harness and wrapped with the other wires in side the tape. This would be a stand alone 12R wire from horn relay to firewall junction box and run alongside the FWD light harness. Clearly indicated in the wiring diagram in the AIM and you see indications of it two places in diagrams in the AIM; but no reference to it.


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post #5 of 25 (permalink) Old Sep 21st, 19, 10:43 AM
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Re: Any 66 guys using the additional 12R wire with your FWD Light Harn

The only stand alone 12R I can trace on the AIM schematic routes from the bus bar on the horn relay to the small Junction block on the core support adjacent to the battery. (page 220)

My guess is that the one that parallels the light harness (which I haven't found on the schematic) might be for police or taxi accessories such as lights and sirens.

What pages are you referencing?
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post #6 of 25 (permalink) Old Sep 21st, 19, 3:10 PM Thread Starter
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Re: Any 66 guys using the additional 12R wire with your FWD Light Harn

Hi Cory. If you look in the AIM on page 12D It shows a 10R from the junction box to the split in the harness. It also shows a 12 R from the horn relay to the junction box. So both a 10R and a 12R on the buss of the horn relay. American Auto Wire tells me the 10R is not in the FWD Light harness; but a stand alone wire outside the harness from the horn relay to the junction box. I remember at one time finding a part number to the stand alone wire ( but not in the AIM) In addition to the part number for the FWDlight harnesss which IS in the AIM.

If you look on page 12 A6 you can see what looks like two wires on the the horn relay.

If you look on page U14 A1, you can see what looks like 3 wires because of the additional wire for the ammeter.


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post #7 of 25 (permalink) Old Sep 21st, 19, 3:21 PM
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Re: Any 66 guys using the additional 12R wire with your FWD Light Harn

Just dawned on me i do have a stand alone wire that runs from the horn relay along the driver side wire harness to the circuit breaker block on the firewall for my convertible top. Could that be what you are seeing??
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post #8 of 25 (permalink) Old Sep 21st, 19, 3:27 PM Thread Starter
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Re: Any 66 guys using the additional 12R wire with your FWD Light Harn

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Just dawned on me i do have a stand alone wire that runs from the horn relay along the driver side wire harness to the circuit breaker block on the firewall for my convertible top. Could that be what you are seeing??

No this wire would go from the horn relay across the front of the radiator support to the junction box where the lead from the battery is connected.


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post #9 of 25 (permalink) Old Sep 21st, 19, 3:32 PM
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Re: Any 66 guys using the additional 12R wire with your FWD Light Harn

I see all of the reference pages from your post but can't identify any 12R wires other than what you've pointed out.


On this color schematic at the top of the pic there is a red wire that appears to bypass the firewall bulkhead connector that's routed from the horn relay bus bar to the convertible top switch.
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post #10 of 25 (permalink) Old Sep 21st, 19, 3:40 PM Thread Starter
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Re: Any 66 guys using the additional 12R wire with your FWD Light Harn

Cory, of the two wires 10R and 12R that go from the firewall juntion box to the split and horn relay respectively. American auto wire tells me there is only ONE 10R wire in the FWD light harness. And that the other 12R wire is outside the harness and you have to add it to make the wiring as in the AIM.

My question is does anyone have an original 12R wire in there 66 as the AIM would seem to indicate it should have.


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post #11 of 25 (permalink) Old Sep 21st, 19, 3:48 PM
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Re: Any 66 guys using the additional 12R wire with your FWD Light Harn

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Originally Posted by Jim_66SS View Post
Cory, of the two wires 10R and 12R that go from the firewall juntion box to the split and horn relay respectively. American auto wire tells me there is only ONE 10R wire in the FWD light harness. And that the other 12R wire is outside the harness and you have to add it to make the wiring as in the AIM.

My question is does anyone have an original 12R wire in there 66 as the AIM would seem to indicate it should have.

I see that and can't figure out why the AIM schematic has both a 10R and 12R routed to the Junction Block since they both originate from the Horn Relay bus bar.


Once routed through the firewall, the 12R splits to the Ign Sw., Cigar Lighter and wiper sw.
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post #12 of 25 (permalink) Old Sep 21st, 19, 4:02 PM
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Re: Any 66 guys using the additional 12R wire with your FWD Light Harn

Quote:
Originally Posted by dream66 View Post
I see that and can't figure out why the AIM schematic has both a 10R and 12R routed to the Junction Block since they both originate from the Horn Relay bus bar.


Once routed through the firewall, the 12R splits to the Ign Sw., Cigar Lighter and wiper sw.
The 10r wire is the shunt wire from which the amp meter gets its input to function. A wire runs from each end of this wire to the amp gauge. The difference of voltages at each end of this wire is shown by the movement of the amp gauge needle.

I can't explain why some wires were in the loom and one was outside.

Look up shunt wire in this forum and you'll get more explanation.
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post #13 of 25 (permalink) Old Sep 21st, 19, 4:06 PM Thread Starter
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Re: Any 66 guys using the additional 12R wire with your FWD Light Harn

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Originally Posted by DUTCH MAX HEADWORK View Post
The 10r wire is the shunt wire from which the amp meter gets its input to function. A wire runs from each end of this wire to the amp gauge. The difference of voltages at each end of this wire is shown by the movement of the amp gauge needle.

I can't explain why some wires were in the loom and one was outside.

Look up shunt wire in this forum and you'll get more explanation.

Thought the shunt was in the 14B wire from the junction box to the amp gauge


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post #14 of 25 (permalink) Old Sep 21st, 19, 4:12 PM
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Re: Any 66 guys using the additional 12R wire with your FWD Light Harn

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Thought the shunt was in the 14B wire from the junction box to the amp gauge
Those 14 wires run to the gauge. They measure the difference in voltage between both ends of the 10r wire. The 10r wire is used because of it's known resistance at a specified length which corresponds to what the gauge needs to function semi accurately. I say semi accurately because the resistance of that wire varies with its temperature. So the gauge is only semi-accurate but good enough to show general charge/discharge levels.

The shunt is used to keep high currents out from under the dash. In earlier amp gauges, the full amperage of the system would need to run to the in-dash gauge through much larger gauge wires. The shunt keeps these amperages out from under the dash allows the use of smaller gauge wires to run to the gauge without he hazard of running full system amperage to the dash.

https://www.chevelles.com/forums/27-...-location.html post 4
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post #15 of 25 (permalink) Old Sep 21st, 19, 4:31 PM Thread Starter
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Re: Any 66 guys using the additional 12R wire with your FWD Light Harn

More confused, so if don't have the 12R wire; but do have the ammeter guage, then I must somehow add this wire and the shunt it contains.


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