308 headlights will not come up on lo-beam

i drive with my high beams all the time. your car is telling you something.

 

There is no "flaw" in the stock system that could cause the symptom that you report (i.e., selecting the low or high beams lamps should have no effect on the headlight bucket action). My guess would be that someone was trying to make the modification where, when rotating the stalk knob with the lever in the full up "headlights off" position, the external parking/running lights come "on", but the headlight buckets do not come up. My suggestion would be to have a look at the connections to:

relay E = relay 87 on the wiring diagram
relay M = relay 105 on the wiring diagram
relay N = relay 75 on the wiring diagram
relay O = relay 76 on the wiring diagram

and see if any obvious "non-stock" wiring modifications have been made that don't match what the wiring diagram shows.

 

Could be a simple problem such as having two wires reversed. First check that the relays are correct. Note that E and M are the headlight and master lift relays, and both should 'click' when the headlight stalk is rotated regardless of position (park, lo, hi). I would be inclined to remove E and M to verify that both do 'click' with rotation. N and O are lift relays but apparently are ok since both pods lift together.

If as Steve theorized, a previous owner has played games with the wiring...this should be a lesson to all about the ramifications of not leaving notes about such changes. Incidently, most such changes involve adding diodes...that is immediate conformation the headlight circuit was altered.

As far as circuitry...the headlight switch has 3 sections, one for flashing which we can hopefully ignore, one which sends a ground to the two relays, turning them on, and a third section which decides which beam gets voltage, hi, low, or neither. But when the stalk is rotated and the relays click, the headlight relay gives voltage to the parking light circuit.

Drawing below from the 1978NA wiring diagram.
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Paul -- Relay M (aka relay 105) should be shown (colored) as a ...101 relay.

 

Thanks, Steve
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I agree.