Headlight Relay Yes No

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The only reason to pass charging power thru the firewall twice is economy. A precision shunt or an inductive pick-up could have been used in the engine bay instead. Both cost more to do than how it was done, and of the Big Three Ma appears to have been the most, er.... frugal. There are those who desire an ammeter. More power to them, I'm not one of them and I'll take a voltmeter over one any day.

There is another, oft over-looked reason to employ relays: voltage drop. Headlights, even stock sealed beams of legal wattage, work better at higher voltages. We've all seen cars whose headlights dim and go more yellow when they drop to an idle. That's the effect of voltage drop even though that type of occurrence may not entirely be the fault of the wiring. The OE wiring was probably OK 50 years ago, but then 50 years ago were headlights even on people's radar as something that could be made to work better? I bet not.
Using relays allows the use of larger gauge wire in a shorter run than OE. If a Concors-type restoration isn't the goal I see no downside to relays for the headlights.
 
My own theology about car mods is that an Engineer or Designer designed it, but he (or she these days) had a good enough reason behind it to make it past the Accountants. Thus Accountants are the real designers of things, not the Engineers or the Designers.
Flip side is that we Engineers would still be perfecting the car while the world was all still driving '58 Edsels......
 
RedFish said:
My opinion... Issues with bulkhead connectors and ALT' gauges are blown waaaay out of proportion by the internet. l.
Click to expand...

Sorry Red, you are mistaken. I have personally found failures in these cars LOOOOONNNNGGGG before Al invented the internet, LOOOOONGGGG before "big headlights" and huge alternators got to be fashionable


I've preached this MANY times. "Ma" KNEW this was a problem. The proof is in "fleet wiring." Optional 60A alternator installations were supposed to be upgraded to "fleet wiring." This was an early form of "bulkhead bypass."

'n yeh.....meh.......You can find many cars that "still work." But if you were to REALLY dig into them, you'd find that the batteries might be boiling a bit much, why? Because of voltage drop to the regulator.

And, you might find that "even though" the lights still all come on, the combined drop in the connectors leaves them operating at substantially less voltage than they should

If you found a "bunch" of these cars used in northern climates, with shorter winter days, and which used the heater motors a lot more, used the lights a lot more, you will very very likely find more and more damaged and nearly failed bulkhead connectors.

One reason I believe you don't find EVEN MORE is that many of these cars that were in severe climates were in the RUST BELT of the U.S. and these cars simply have not survived period.

My own car is proof enough. This was a car that has never been substantially wrecked, was as rust free as you'll find, and has always had a mundane 35A or so alternator. No extra anything. Yet there was a ONE VOLT DROP in the ignition line to the regulator through the bulkhead. The charging voltage to the battery was OVER 15.5 volts!!!! This was not the fault of the regulator, it was caused by our old friend, poor connections in the bulkhead.
 
It is correct that if the car is at fast idle/cruise speeds, then the alternator is 'taking the load'. If the headlight relay source is the battery + terminal or the starter relay's big lug, then the current flows out of the alternator into and out of the bulkhead connector to either of those points. The battery only supplies the headlight current at the moment that the lights area turned on, and before the alternator can adjust to the new load. The battery voltage will slowly drop to a new, slightly lower level, and its contribution to the headlight current will drop to zero as that new, slightly lower voltage is reached.

Connecting the headlight relay's +12V feed directly to the alternator output will solve this. But it had better be fused or circuit breaker protected.
 
ALWAYS relay your headlights, horn etc. Anything that pulls a fair amount of power is best suited to run a relay on.
 
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