Does anyone have any insight as to why the coil is typically not fused upstream of the ballast on earlier model Fords? I am not all that familiar with wiring schemes past '79, but would imagine the concept extended beyond that. Everything I'm familiar with has the ballast powered straight from the key. Why is this circuit not fused?

I've seen several cases where loads powered through a resistor wire are not fused, which generally isn't a big deal since the resistance is big enough. For example, the ICVR that powers the gas gauge is typically powered through a resistor wire of 8 to 9 ohms; and the back-up path for the ALT light in the charging system is 15 ohms. The ballast however is usually 1.4 ohms or so - if that shorts to ground under the hood, that's a lot of power to dissipate.

Why not fuse the coil? Ford did this for years, so I'm curious what the reasoning would be - does rapid pulsed current tend to blow fuses early? Has anyone tried fusing the coil with a points or Duraspark setup?

 

Plused current causes lower heat and thus takes a lot longer to burn out a fuse.

My understanding of an IVR is that it is just like a old style Bi-metal blinker can. It creates a 50% duty cycle square wave which gives a average voltage that is 6 volts, half of 12 volts. I have even seen the gauges pulsing (like a blinker) when testing by grounding the wire at the sender. I have used blinker cans to replace IVR on older trucks (IH) where parts were not available.

The whole idea of using 6 volt parts in 12 volt systems really seems kind of odd in the big picture, did they think 6 volt systems were going to make a come back. Did they have that many 6 volt parts to use up?

But on to your main question... I have never seen a coil that was fused in any car or truck. My guess is that essential systems like IGN and headlights were run HOT for safety reasons. The fuseable link is still in the system, but I cant see how that would protect the wiring any.
If you feel the need you could use a CB in the cab for a quick reset, but I wound not use a fuse.

I think the resistance in the ALT excitation circuit was used to slow the start up of the ALT, meaning the resistance limits current flow. The resistor in Parallel with the GEN light was so the ALT would still turn ON if the idiot light burned out.

Sorry, no real answers here........
Jim

 

I believe Jim mentioned something in his reply that I agree may be the answer. Fuses are known to blow out sometimes and be a nuisance. You do not want nuisance fuse blowing to cause the engine to stop during critical times of driving. It's the same with the headlights. I know now the newer trucks use fuses for the headlights, but all the older vehicles used a hot wire, some with a fusible link, some without, and a circuit breaker in the headlight switch.

The way some of the older vehicles are serviced and "restored", the wiring is sometimes terribly abused, and care is not taken in wrapping it, protecting it, and fastening it back in place like the factory had it originally. When changes are made, most people do not stop and think how they should re-route or deal with the wiring if it can't go back in it's original place or it needs extended or shortened. If the factory wiring is installed back properly, usually you have zero problems with the wires shorting out or melting fusible links. I am guessing the factory weighed the advantages versus the disadvantages of fusing the ignition, and decided against it.

 

Thanks for the responses guys - was hoping I would hear from you two. I think what you guys are saying about critical paths makes perfect sense.

The reason I ask is because I'm rebuilding the main harness out of my '73 Galaxie/LTD (slow restoration project). Believe it or not, the turn signals are not fused in this vehicle, but all other lighting circuits are. The flasher is powered directly from the ignition switch. This is confirmed with the factory wiring diagram. I pulled another harness out of a '77 LTD in the junkyard, and it does have the turn signals fused. Seems this was something that was "left out" and corrected later on. As I'm going through everything I'm looking at other areas to see if they should be fused, and the coil caught my attention. But based on this feedback, which I very much agree with, I'll leave it unfused, but routed just as was done at the factory.

 

http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3310.pdf


Old style Bimetal flasher cans will also act as a fuse as they are current sensitive and will limit current. Acting like a self reseting fuse. I would guess the stock Bimetal strip would melt before the wiring would be damaged.
A heavy duty flasher would allow higher current flow and risk damaging the wiring.
Jim

 

When I wire trailer brake controllers, I always use a circuit breaker off the battery to power the controller. If I have trouble on the road with a wiring problem on the trailer, my theory is I can pull over and fix the wire, the breaker will be reset, and I can go on my way without worrying about fuses.

And reading your post about restoring a wiring harness reminds me of a trick I read about. They recommended after a full restoration, instead of hooking the battery up and trying out all the electrical after the wiring was re-installed, hook a battery charger up to the wiring instead, something around a 10 amp charger. You won't be able to start the engine, but you will be able to test all the electrical and see if it works. If there is a problem somewhere, the charger has a circuit breaker built in and it will trip before there is any damage or melting done to your new harness.

 

Great advice; this is exactly what I was going to do