Current rating of the ignition switch

[John Mock]

I am doing a 5.0L conversion on a '73 Mustang coupe. I cannot find a reference that tells me what is the current capability of the '73 ignition switch. Can anyone help me or point me to reference material. I have Jim Olsons '73 Mustang guide and I have a set of '73 Ford shop manuals.

 

[midlife]

I've not seen a rating, but I can estimate it from various items. The main battery line has to support the two fuses (14 amps and 30 amps), as well as the two main ACC fuses (14 amps, 20 amps) and the extra accessory fuse (20 amps). The ignition also supports the main ignition coil line (say 5 amps), power to the brake master cylinder sensor (say 2 amps), and the starter relay (but when that is engaged, the ACC lines are down). So...a good estimate would be 105 amps maximum. I would guess that it is rated for 50 amps with 120 amps total for safety margin. The rating would be lower because it is extremely unlikely that everything is running at maximum current for all of the circuits. Typically, Ford designed the fuse for twice the nominal current load.

Another method is to look at the rating of the incoming battery line, approximately 8 gauge. That wire can carry about 50 amps in 29 feet of actual wire length.

Does that help?

 

[John Mock]

I've not seen a rating, but I can estimate it from various items. The main battery line has to support the two fuses (14 amps and 30 amps), as well as the two main ACC fuses (14 amps, 20 amps) and the extra accessory fuse (20 amps). The ignition also supports the main ignition coil line (say 5 amps), power to the brake master cylinder sensor (say 2 amps), and the starter relay (but when that is engaged, the ACC lines are down). So...a good estimate would be 105 amps maximum. I would guess that it is rated for 50 amps with 120 amps total for safety margin. The rating would be lower because it is extremely unlikely that everything is running at maximum current for all of the circuits. Typically, Ford designed the fuse for twice the nominal current load.

Another method is to look at the rating of the incoming battery line, approximately 8 gauge. That wire can carry about 50 amps in 29 feet of actual wire length.

Does that help?

It is a good running start...better than I have had so far. By your analysis I should be able to remove everything except the 5.0L engine management and computer from the ignition switch and run safely on the '73 wiring and switch. I'll run all the accessories off their own switch with a fuse on each...just have to remember to turn every thing off when I get out of the car.

 

[Don C]

Run all the accessories on a properly sized relay, requires minimal power through the ignition switch, all accessories will be disengaged when the ignition switch is turned off.

 

[John Mock]

Run all the accessories on a properly sized relay, requires minimal power through the ignition switch, all accessories will be disengaged when the ignition switch is turned off.

Run all the accessories on a properly sized relay, requires minimal power through the ignition switch, all accessories will be disengaged when the ignition switch is turned off.

 

[73 MACH 1 MAN]

Im adding a few accessories to my build and didnt want them on the Stock Fuse Box. So I went to the junk yard and started looking for an engine bay fuse/relay box. Ended up with one out of a 2004 Mustang. Heres' a schematic of my plans. The new fuse box will be powered from a 100 amp continuous solenoid that will be energized by the ignition on wire. Also Moved the battery to the trunk!

Wiring Mach 1.pptx

 

[John Mock]

Im adding a few accessories to my build and didnt want them on the Stock Fuse Box. So I went to the junk yard and started looking for an engine bay fuse/relay box. Ended up with one out of a 2004 Mustang. Heres' a schematic of my plans. The new fuse box will be powered from a 100 amp continuous solenoid that will be energized by the ignition on wire. Also Moved the battery to the trunk!

Im adding a few accessories to my build and didnt want them on the Stock Fuse Box. So I went to the junk yard and started looking for an engine bay fuse/relay box. Ended up with one out of a 2004 Mustang. Heres' a schematic of my plans. The new fuse box will be powered from a 100 amp continuous solenoid that will be energized by the ignition on wire. Also Moved the battery to the trunk!

Thanks!

Good information.

Especially the schematic.

One question...What is, and where did you get, the 100 amp continuous duty relay? I tried that on the bench in my garage with an ordinary starter relay (your drawing) and in about 15 minutes it was smoking the bench it was so hot. No contact load, just the relay coil on the battery. Same thing with the relay from a '93.

 

[73 MACH 1 MAN]

Thanks!

Good information.

Especially the schematic.

One question...What is, and where did you get, the 100 amp continuous duty relay? I tried that on the bench in my garage with an ordinary starter relay (your drawing) and in about 15 minutes it was smoking the bench it was so hot. No contact load, just the relay coil on the battery. Same thing with the relay from a '93.

Yeah a starter solenoid is basically an intermittent solenoid made for shourt burst only. The continuous is made to stay on when energized!

You can get them at several places just google 100 amp continuous solenoid. Here's one of them:

http://www.texasindustrialelectric.com/relays.asp

I got mine on Ebay! One like this http://www.ebay.com/itm/New-UNIVERSAL-12-Volt-Continuous-Duty-Solenoid-100-AMP-/230374480490?pt=Motors_Car_Truck_Parts_Accessories&hash=item35a3639a6a&vxp=mtr