Headlight issue - 510

[James]

Hello - been slowly working on a problem on a '70 4door 510. Slowly - because it hasn't been of high importance, and because my back doesn't necessarily like being hunched in the engine bay (being 6'-6" has its issues).

I have searched and understand the basics of the negatively switched headlight system of the 510, but wanted some input on direction before tore everything out and went through it. The car - stock, low mileage (90K??) - its in the transition era in terms of its wiring: has the older style relay with the separated plug, 3 wire fuse box, and has the red/red-white/red-blue harness for the headlights (white needle gauges).

Symptoms - Low Beams work fine. Switch to high beams, and the drivers side works (both illuminated fully), and the passenger low beam goes dim, with the high beam not showing anything. I'm assuming a short or poor connection somewhere in that side.....

What I have done - took apart the relay - it was in amazingly good shape (the old design seems to be more weatherproof than the one in my '72). I cleaned the contacts. I didn't expect much here as the relay is obviously doing something. I do have a Bosch style relay that I could re-wire into the system if there is potential issue with the original relay - but would rather leave the wiring intact....

What suggestions can you give me as where to start.....nothing too stupid to throw out there.

Thanks a lot in advance.

[MikeY]

Remember when you have the relay apart to clean the grounding point. The screw that holds the relay to the base. Take it apart there and make sure the bottom of the relay and the base are clean. And when it is screwed to the car that is the ground. As well many headlight issues have been solved by soaking the fusebox in vinigar bath
I like a 50/50 white vinigar/water mix. 15 to 30 min and rinse in hot water and dry as quick as possible The wife's hair dryer comes to mind.
Good luck

[510rob]

James,

Power comes from the battery, through the fusible link, to the fuse box, to a pair of fuses (left hi/lo headlight pair, right hi/lo headlight pair), to the wires that supply power to each pair (left pair, right pair) of headlights, then out of both lights to the single relay (paired sides are now summed as high-beam circuit loop and low beam circuit loop), then from the relay to the headlight switch's 2nd position, which finally connects the circuit to ground. Lots of connections, lots of wiring, lots of potential points of failure, but it works well when everything in that path works properly...

The relay switches which headlight filaments are grounded, thereby completing the electrical circuit loop for either low beams (one low beam filament in each outer headlight) or for high beams (one high beam filament in each outer headlight as well as the filaments in the inner high-beam-only bulbs).

The relay's coil draws positive power from the running light circuit that gets its power from the 1st position of the headlight switch. This draws power through the running light wiring (another fuse in the fuse box).

The relay's coil is switched to ground through the column switch. No ground = low beam bulbs. Ground = high beam bulbs.


So, where is your problem? ...somewhere in there!

I tried to 'rebuild' my stock relay but after the 2nd attempt on that f%&king thing, I made an adapter harness to plug into the stock harness and allow me to use a typical ISO Mini relay (typical automotive relay).


How to troubleshoot the mess of wires.
(headlight switch (HLS) in position #1, running lights ---> HLS = #1)
(headlight switch (HLS) in position #2, headlights ---> HLS = #2)

• You should have +12V at [the fuse box's inner terminals for the headlights] at all times.
• You should have +12V at [the headlight connectors] at all times.
• You should have +12V at [the relay's positive coil terminal] when HLS = #1 and HLS = #2.
• You should have GROUND CONTINUITY at [the relay's common output terminal] only if HLS = #2
• You should have GROUND CONTINUITY at [the headlights' low-side terminals] if HLS = #2, depending on the column switch position.
• You should be able to hear the headlight relay clicking if HLS = #1 and you quietly move the column switch for hi/lo beams

Some ideas to start with anyway...

[James]

Thanks a lot. I will work out through these and report back what I find. I appreciate the methodology - its helpful.

[okayfine]

You've already got excellent advice above. I tried to run down a similar issue for a long time. Ended up being the headlight fuse. It was not blown in that the bar was solid inside the glass, but one of the ends was mostly disconnected from the cap, so putting amps through it raised the resistance to the point that only a tiny current could go through, hence the very dimness of the light.