The Ford Barn

PGBuick · 12-09-2023, 10:58 AM

I’m building the wiring harness for my 36 coupe, which is 12 V negative ground. Stock flathead powered, power-master alternator and starter, and electric fan via a fan relay. Headlights TBD. Also an under-dash Arvin heater with 12v fan. I’m guessing with 12v power my current draw across the stock on/off column drop switch won’t be a problem, but figured I would ask those who have been down this path before. I know I can add relays and or an additional master on/off switch for devices powered by “ignition” (aka not constant 12v) power, but it’s simpler to just put them all through the stock switch if it’s robust enough. And simpler is always better.
Thanks!
Pat

dmsfrr · 12-09-2023, 12:23 PM

It might be better to ask on the Early V8 forum, but in general... original 6 volt parts (in good condition) have double the current carrying capacity of similar '12 volt' designed parts.

KULTULZ · 12-09-2023, 01:48 PM

IF it were me (IMO) -

I would put all on relays as there will be a better draw (lower resistance) and high amp is kept out of the cabin, especially with circuits like EFI, HOT IGN, ELECTRIC FUEL PUMP(S), ELECTRIC FAN(S), HDLPS, SOUND, etc,

rotorwrench · 12-16-2023, 04:30 PM

The old pre 1948 V8 Fords were set up for the ignition switch to only power the ignition with the instruments added after 1938. Lights and horn were connected directly to the power buss. Lights had a circuit breaker. The horn just had a relay by the end of the 1930s.

The ignition on the pre-1948 V8 Fords had a ballast of around 0.8 Ohms. This helped control the current through the old Mallory coils that were designed to attach to the helmet and early crab distributors. The post war rabbit ear distributors still used the ballast even though they have a remote mounted coil next to the generator. The whole system would draw near 4 amps and those switches are rated for near that figure with just a little fudge factor.

fordor41 · 12-16-2023, 11:43 PM

On our '41, we had stereo & amp, ign, heater blower, elect fuel pump, charging ports for phone , elect. radiator fan & fog lamps all running thru the stock switch for 24 yrs and never had any problems.

rotorwrench · 12-17-2023, 11:55 AM

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The switch was changed in 1938 to include the instruments. The OP was referring to the 1936 version that only has an two terminals and the single contactor bridge. It may work but it will likely run a higher temperature as more and more amperage is pushed through it. The old bakelite ones could take more heat but I don't know about the modern reproductions. Few reproductions are the quality of the old original parts. The studs are rather short as well to they may be limited by how many ring terminals they can accommodate. Overloading electrical components is something I would never recommend. It can lead to a fire that could destroy the car. At what point does the luck run out?

PGBuick · 12-17-2023, 02:23 PM

Quote:

Originally Posted by rotorwrench

The old pre 1948 V8 Fords were set up for the ignition switch to only power the ignition with the instruments added after 1938. Lights and horn were connected directly to the power buss. Lights had a circuit breaker. The horn just had a relay by the end of the 1930s.

The ignition on the pre-1948 V8 Fords had a ballast of around 0.8 Ohms. This helped control the current through the old Mallory coils that were designed to attach to the helmet and early crab distributors. The post war rabbit ear distributors still used the ballast even though they have a remote mounted coil next to the generator. The whole system would draw near 4 amps and those switches are rated for near that figure with just a little fudge factor.

Thank you! This is incredibly helpful information. I’ve already put everything except ignition through a 100a. relay, which was extremely easy and inexpensive. Since I’ve already built an entire new fuse panel and harness I only had to move a couple of wires.

rotorwrench · 12-17-2023, 07:56 PM

Aircraft use a continuously operating relay to power their electrical buss. It is controlled by a Master Switch. It sounds like you have something similar, so this is a reasonable way to power the buss and then apply fuses or circuit breakers to protect all the loads in the system. I still do all my wiring in accordance with the FAA Advisory Circular AC43.13-1B chapter 11. There is more info in that AC about standard electrical system practices than a person can easily find on regular google searches.

12-09-2023, 10:58 AM	#1
PGBuick Member Join Date: Nov 2021 Location: Central Mass. Posts: 96	Max. current draw across stock ‘36 ignition switch? I’m building the wiring harness for my 36 coupe, which is 12 V negative ground. Stock flathead powered, power-master alternator and starter, and electric fan via a fan relay. Headlights TBD. Also an under-dash Arvin heater with 12v fan. I’m guessing with 12v power my current draw across the stock on/off column drop switch won’t be a problem, but figured I would ask those who have been down this path before. I know I can add relays and or an additional master on/off switch for devices powered by “ignition” (aka not constant 12v) power, but it’s simpler to just put them all through the stock switch if it’s robust enough. And simpler is always better. Thanks! Pat

12-09-2023, 12:23 PM	#2
dmsfrr Senior Member Join Date: Aug 2012 Location: Abq, NM Posts: 3,607	Re: Max. current draw across stock ‘36 ignition switch? It might be better to ask on the Early V8 forum, but in general... original 6 volt parts (in good condition) have double the current carrying capacity of similar '12 volt' designed parts.

12-09-2023, 01:48 PM	#3
KULTULZ Senior Member Join Date: Aug 2012 Location: IN A 'GALAXIE' FAR FAR AWAY Posts: 6,484	Re: Max. current draw across stock ‘36 ignition switch? IF it were me (IMO) - I would put all on relays as there will be a better draw (lower resistance) and high amp is kept out of the cabin, especially with circuits like EFI, HOT IGN, ELECTRIC FUEL PUMP(S), ELECTRIC FAN(S), HDLPS, SOUND, etc, __________________ *** - MULTI-VEHICLE SYNTHETIC TRANSMISSION FLUID - Multi-vehicle transmission fluids are becoming more and more popular in the marketplace. Oil marketers design these fluids for a wide range of automatic transmission types. While they are not licensed by any specific auto manufacturer ... ??? ... enough said ... DIAGNOSED CDO** - (OCD In Correct Alphabetical Order)

12-16-2023, 04:30 PM	#4
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 16,440	Re: Max. current draw across stock ‘36 ignition switch? The old pre 1948 V8 Fords were set up for the ignition switch to only power the ignition with the instruments added after 1938. Lights and horn were connected directly to the power buss. Lights had a circuit breaker. The horn just had a relay by the end of the 1930s. The ignition on the pre-1948 V8 Fords had a ballast of around 0.8 Ohms. This helped control the current through the old Mallory coils that were designed to attach to the helmet and early crab distributors. The post war rabbit ear distributors still used the ballast even though they have a remote mounted coil next to the generator. The whole system would draw near 4 amps and those switches are rated for near that figure with just a little fudge factor. Last edited by rotorwrench; 12-17-2023 at 11:57 AM.

12-16-2023, 11:43 PM	#5
fordor41 Senior Member Join Date: Oct 2011 Location: elmira,ny Posts: 1,518	Re: Max. current draw across stock ‘36 ignition switch? On our '41, we had stereo & amp, ign, heater blower, elect fuel pump, charging ports for phone , elect. radiator fan & fog lamps all running thru the stock switch for 24 yrs and never had any problems.

12-17-2023, 11:55 AM	#6
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 16,440	Re: Max. current draw across stock ‘36 ignition switch? Sponsored Links (Register now to hide all advertisements) The switch was changed in 1938 to include the instruments. The OP was referring to the 1936 version that only has an two terminals and the single contactor bridge. It may work but it will likely run a higher temperature as more and more amperage is pushed through it. The old bakelite ones could take more heat but I don't know about the modern reproductions. Few reproductions are the quality of the old original parts. The studs are rather short as well to they may be limited by how many ring terminals they can accommodate. Overloading electrical components is something I would never recommend. It can lead to a fire that could destroy the car. At what point does the luck run out?

12-17-2023, 07:56 PM	#8
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 16,440	Re: Max. current draw across stock ‘36 ignition switch? Aircraft use a continuously operating relay to power their electrical buss. It is controlled by a Master Switch. It sounds like you have something similar, so this is a reasonable way to power the buss and then apply fuses or circuit breakers to protect all the loads in the system. I still do all my wiring in accordance with the FAA Advisory Circular AC43.13-1B chapter 11. There is more info in that AC about standard electrical system practices than a person can easily find on regular google searches.