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Headlight Tech

by John Sandhoff

Reprinted with the author's permission

A lister asked: > What has me stumped are the headlites. Mine broke tonite (one can only > expect so much out of a 33 year old relay) so I'm only guessing pull/twist > is hibeam?

Some -general- comments here. Print out the schematic (check Ross's site here! and find a buddy that knows electrons to really figure it out :-)

First, note that 1970 is DIFFERENT than prior years! On 1970, there are TWO fuses, one for each bulb, and +12 is fed to the 'common' (what you'd expect to be ground) terminal. To activate the lights, the hi or lo contact is brought to ground. So if you're using different bulbs, be careful as some of the HO stuff has the 'common' terminal grounded as part of its construction.

On the '68 especially, the fusebox tends to have a problem and cook the single headlight fuse. From what I can tell, there's slight resistance between the brass clip itself and the brass rivet. That point heats up, burns wires on the back and melts the fuse on the front. Several of us have ultimately installed a separate fuse that bypasses this one on the fuseblock.

The 'twist' portion of the headlight switch is for the FOG lights. All the later cars are pre-wired for fog lights (the connectors are with the turn signal wiring), just twist the knob to turn on the power. If you want to run high power fog lights, PLEASE use the existing wire to simply activate an auxiliary relay, and use the relay (direct from the battery, via an inline fuse) to run the lights. Let's not melt the original wiring harness, or cook the switch, OK?

The hi/lo setting: This is controlled by a relay that is activated by the stalk switch. Pushing/pulling the turn signal switch turns this relay on and off; the relay then provides power (or the gound path, for 1970) to the hi or lo connector as appropriate.

Many people note their headlights dim or go out at about 4000 RPM. This is because of a vibration that affects the headlight relay - it makes the points bounce and lose contact. In my personal experience, it affects only one setting (lo or hi, I forget - the setting in which the relay is NOT energized). Flipping the lights to the other setting at least gets them going again. The two cures: 1) Stop driving so fast. Especially at night, when you're using the lights! 2) Mechanically isolate the relay from the body with a block of rubber or somesuch. Be sure to maintain an electrical ground, however! (that is, if the case requires a ground. I don't recall, and I don't have my car parked here in my cubicle with me... Easy enough to check - unscrew the relay and see if things still work properly...)

If your headlights are weak, check and clean the fuses and fuse holder, check the connectors (especially underhood) for corrosion or almost- broken wires, and triple-check the gound points for the wires. Usually a partial ground on one bulb will make them both do weird things as the electrons try to find a 'back way' to sneak around... When I helped build a house 25 years ago, we'd blow lights when we turned on a space heater - bad grounds can do WEIRD stuff...

-- John