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ttodhunter

My headlight fix

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Hey all,

 

I just thought I would share my experience troubleshooting a problem with my headlights. Basically, all of my lights were functional, except my headlights were not working. My first step was obviously to check the fuse. No such luck that it would be that easy. Next, I checked voltage at the fusible link. Voltage checked out.

 

I busted out my test light and started probing at the plugs in front of the radiator on the lower support. Make sure that the headlight switch is on before testing. No voltage coming through.

 

I did a bunch of testing in the junction block, but what I tested next is probably a better place to start. Pop the steering wheel cover off and expose the headlight switch. There is a brass colored box on top of the base of the headlight/wiper switch. It should have a couple of smaller green wires, a larger white with red stripe and a larger solid red.

post-12144-056158000 1324435537_thumb.png

 

Touch the white/red with a test light and make sure that it has voltage. This wire is direct from the fusible link, and if the link has power, it should as well. If not, the problem is in the wiring from the fusible link.

 

Next, with the headlight switch turned on, touch the solid red wire with the test light. If it doesn't light up, your switch is dead as it was in my case. You can also do a continuity test between white/red and red. No continuity with switch off, continuity with switch on is normal. No continuity with switch on is bad.

 

I was able to find a '77 scrap car and pulled the combination switch assembly for $5 or so, not bad. I brought it home and did a continuity test between the white/red and solid red with the switch turned "on". Got continuity, switch is good. The connectors didn't match up with mine ('76), so I cut the box off both the donor assembly(good) and my assembly(bad). post-12144-038263500 1324435510_thumb.png

 

Then, I spliced the wires together. I did electrical tape the connections (not in picture). post-12144-085113100 1324435528_thumb.png

 

Once all back together, I gave it a shot and sure enough, the headlights worked! post-12144-080024500 1324435519_thumb.jpg

 

To summarize, the wiring goes like this: Battery -> Fusible Links -> Headlight Switch -> Fuse block -> Headlights - And that is the order I would perform checks in.

Edited by ttodhunter

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nice post, are you sure you are correct on the routing of the power though?

 

Battery, Fusibles, Fuseblock, Fuses, Headlight, Switch, Master Ground on the Steering Column is I think correct.

 

This may have changed on later models, to prevent the fuses from burning out like in the early cars...but both my 76 and 77 have had the 'dead light' syndrome from the corroded master ground connection under the combo switch which leads me to think Nissan still was switching the ground side of the filaments and sending fused power to the socket directly from the box.

 

Don't have my diagram to consult, but I'm pretty sure that's it. If someone could post the wiring/schematic and solve the riddle posed, please?

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Nice job on the troubleshooting!

 

nice post, are you sure you are correct on the routing of the power though?

 

Battery, Fusibles, Fuseblock, Fuses, Headlight, Switch, Master Ground on the Steering Column is I think correct.

 

This may have changed on later models, to prevent the fuses from burning out like in the early cars...but both my 76 and 77 have had the 'dead light' syndrome from the corroded master ground connection under the combo switch which leads me to think Nissan still was switching the ground side of the filaments and sending fused power to the socket directly from the box.

 

Don't have my diagram to consult, but I'm pretty sure that's it. If someone could post the wiring/schematic and solve the riddle posed, please?

 

Tony, both you and the OP are partially correct. I referenced page BE-9 of the '78 FSM:

 

Battery -> Fusible Links -> Light Switch -> Fuses (one for each headlight) -> Headlights -> Dimmer (high-beam) Switch -> Ground

 

The high-beam switch switches grounds between low- and high-beams.

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I was more getting at the 240 - 260/280 differences.

Indeed something changed as later boxes didn't routinely melt like early boxes did. So obviously something changed.

 

But it does look like ground switching is still being accomplished. The probing at headlight socket can prove confusing and misleading to actual circuit function/configuration as normally power is switched.

 

Proper probing of the socket. For instance requires clipping the ground lead into the ground socket to get actual light voltage available. The "dim light" is almost always found with this

Method, whereas probing the socket to local chassis ground gives "good power everywhere"...

 

And that ground point may change depending on what position the dimmer is...

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I'm still pretty green at electrics... From my rookie observations, it seems the dimmer switches ground between low/high, but the headlight switch supplies power to the system? On the actual switch itself, there are the power supplied from the fusible, the power out to the fuse box, and the two small green wires. The big black ground is not connected to the headlight switch, but to the dimmer assembly. This is (I think) what Leon was saying, right?

 

Diagram for reference...post-12144-046750800 1324570908_thumb.jpg

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Yes, the power comes in the 'ground' terminal of the headlight connection, and filament 'hot' is grounded through the dimmer switch to the master ground on the column.

 

This is EXACTLY OPPOSITE of how most everybody else does it! The single pin is a common ground (where we have hot) and then they power the respective filament for either high or low beam.

 

Most headlight relay setups swap to that configuration when you install them (which is really what you should do to keep all that amperage from being switched by the contacts directly in that little column switch!)

 

Relays run about 0.5-1.0A, as opposed to 10A and up for direct switching of the headlights. The repair you just did will last a LOT longer if you can swap to relay control of the headlights now, while everything is cleaned, fresh, and working correctly!

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You can also pop the circuit board out of that switch by bending the four metal tabs back and clean up the contacts, without removing the switch from the steering column. Worth a shot before replacing.

 

I took the halfway approach on the relays, and spliced one in to the red wire that feeds the fuse box. This takes the load off the headlight power contacts, which is where I found the pinhead size pit and corresponding metal deposit from years of sparking, but leaves full power through the fuse box and dimmer switch. Not as satisfying as protecting all of the old hardware but it takes one weak link out of the system. The power switch was where I was losing voltage anyway and is the part that was getting warm. Plus the wire is easy to get to and close to the battery positive terminal.

Edited by NewZed

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Another thing to add is you can usually swap switches from year to year within the early-late model groups simply by resoldering the wires for the correct plug onto the switch. This also works when you use LHD Switches in RHD Assemblies (which works wonderfully!)

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Don't overlook the fusible links and their mounting block.

I got to look at an early '76 after the owner had torn his hair out for weeks because one headlight was dull.

They'd tried everything, new lights, fuses, even the column switch.

The clue he missed was to sit a good voltmeter on the output of the column switch and then use the beam change switch.

I could see the voltage sag on switch to high beam and then slowly rise to about 10V.

Select low beam and voltage would immediately come back to about 11.8V even though the battery was showing 12.4V.

Pulled the fusible links, dismounted the block and gave the block a brief wash in mild muriatic acid, a water rinse and a mild soap solution rinse.

Nice bright terminals in there after that so clean the links the same way, stuffed silicon grease into the contacts and reassembled.

All fixed, lights working real good. The owner reckons they're brighter than they have been for years!!

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