Why do my headlights and instrument lights get brighter when I rev the engine harder!?!?

[mrtask]

I have noticed when driving after dark that if I rev my V8 a little higher and hold it in a lower gear a little longer, the headlamps and the instrumentation light up considerably brighter. When I back off the throttle they return to their usual dim and paltry glow.
What might be the cause, and how might I go about improving things so that my driving lights and dashboard are always nice'n'bright at night, regardless of engine speed!?!?
Is my alternator perhaps old and tired?

 

[PeterZRH]

Voltage drop along the wiring. The voltage along a wire is I*I*R (current square times the resistance). So as the current load increase, the voltage drop in the wire increases.

When you rev, the alternator does 2 things. Firstly it ramps the voltage up to over 14V rather than the 12 from the battery and secondly the output to the lights is supplied from the alternator rather than the battery, so you don't get the drop over the battery cable.

A 65 watt alternator will help but a compete cure would also need heavier wiring to the lights - which many people do anyway with a relay to protect the switches.

 

[mrtask]

Thanks for your prompt reply, @PeterZRH.
I have had separate fuses for the headlights installed after the first fusebox melted at the headlamp fuse.
So the next step is to replace all the headlamp wiring entirely? Okay. What grade of wire would you recommend?
I appreciate your illuminating me!

 

[PeterZRH]

Thanks for your prompt reply, @PeterZRH.
I have had separate fuses for the headlights installed after the first fusebox melted at the headlamp fuse.
So the next step is to replace all the headlamp wiring entirely? Okay. What grade of wire would you recommend?
I appreciate your illuminating me!

Going a gauge higher would be a good start.

 

[jp928]

When I fitted relays to my headlights I soldered a take off lead into the heavy wire from the shunt to the headlight switch, partly in order to keep the ammeter reading true drain, ran that out to a hot point close to the lights, via a 40A circuit breaker. If you didnt mind the ammeter not seeing the headlight drain you could pick up a 12v feed from the starter connection , or the through bolt on the drivers floor, in 8SWG wire, to a point near the lights, as a source for the relays. With relays the std light fuses will only carry relay energising current, which is trivial.
It would be useful to check your battery connections are clean and corrosion free- both pos and neg, and where the neg lead bolts to the chassis - undo, wire brush, apply Deoxit, refit. Then check the alternator connections as well. A good test would be to put a voltmeter across the battery (or cigar lighter), reproduce the lights going dull, see what the volts are, and what happens when you lift the revs until they are bright.
This my offside relay cluster.