Posted as separate thread rather than reviving this old one:

https://www.corvetteforum.com/forums/c3-tech-performance/3727033-fuses-for-a-1973-a.html

My headlights open and work fine when starting cold, but after 10 minutes or so on the road, they start flickering / blinking on and off.

Garage missed this since it looks fine if you just start the car for a quick look.

Any ideas what could cause this?

Any input very much appreciated!

 

You probably have newer halogen sealed beams. These draw more than the old style T3 bulbs ever did so the they overload the system and momentarily trip the breaker in the headlight switch.
Good news, it's an easy fix. Just add a pair of headlight relays to your underhood wiring, reasonably close to the alternator, like all newer cars have. This will take the load off your vintage wiring and your headlamps will be much brighter.
I do this simple and inexpensive upgrade to all my vintage cars.
​​cars. Only takes about 20-30 minutes. For the procedure, just do a search here for "headlight relays".
Cheers, Greg

 

The halogen headlights are really not the issue- it was GM's beancounters that tried to cut corners with the least expensive way.

Reproduction headlight switching on/off circuit breaker is a known issue.

The fix is as Greg said- headlight relays- replacing headlights will not solve the problem.

Here's a test- the "new" Halogen bulbs actually pull less current-

 

You probably have a crusty headlamp switch. Even when new there is a large voltage drop across the switch, so best to convert to relays. Your lights will shine brighter. This diagram will support up to a 100 amp ECE headlamp conversion.

 

check your grounds and plugs. Make sure they are all clean and plugged it tightly. If you jack up the nose you may be able to get to them from underneath, if its too tight you may have to pull the grills out. once you can get to it try wiggling everything to see of you can reproduce it. If it was flickering all the lights then I would suspect the headlight switch and its plug

 

If you wanna do your own relay set up-

Bosch 332019150 RockAuto has them for less $2- made in Portugal- not Chinah

https://www.rockauto.com/en/parts/bo...ing+relay,3380

It is a 5pin relay- but it has TWO output terminals-

Makes wiring soooo much easier.

I use 12GA for power- under the hood with the extra heat- going up a size is a rule of thumb

For circuit protection- you might wanna stay away from fuses- unless they are easy to replace at night-
The LED fuses- light up when they blow would be a great choice.

I also like the combo relay/ fuse block-

https://www.autoelectricsupplies.co....01/category/37

And if you have fiberoptics- Delcity sells a female plug that has the fiber optic cavity

https://www.delcity.net/store/Headli...98105.h_818652

Here's a clean way to use three relays- each headlight has its own terminal- simplifies wiring

 

What Richard454 said. You also eliminate the high current fire hazard going through the headlight switch. Even way back when I can't believe GM would design this circuit like this. It's just a disaster waiting to happen.
Here's my diagram and I use the Bussman self resetting CB.

 

Instead of all the guess work how about some info from the owner?
Are the headlights stock? If you dont know how about a picture of the headlights? Sounds like you just bought this car from a garage? If so there could be anything going on from the previous owner as well as just a dirty wiring connection or faulty headlight switch.

As mentioned the stock wiring on these cars was not the ideal setup but if if its blinking and cutting out you may want to find out why first instead of just installing the relays which may or may not resolve the issue but for how long? If the switch is starting to fail its probably best to replace it... but first I would start with checking and reseating the connections to it. as well as your ground to the headlights.

 

Here is a picture of the inside of the headlight switch showing the bi-metalic "switch". This is the only current control device in the circuit and IMO is a poor attempt at it. What happens when the circuit draws current and if that draw becomes excessive this "switch" heats up and since they are two different metals one heats faster and distorts causing the points style contact to open. This open stops current flow (of course) and the strips of metal start cooling. Once they get cool enough the "switch" is made and the current starts flowing. This is what causes the lights turning on and off or "flickering" if you will.
Now, if that points style contact gets welded closed you've just lost all your current control in this circuit and since it doesn't have a fuse or anything the next thing to control the current it the wires or the headlight switch starts to heat up and burn. A loose ground could cause the light to flicker too but to me that is highly unlikely.


This is the reason for relays. The relay mod does a number of things but the most important IMO is getting that high current draw out of the headlight switch and out of the passenger compartment. The second best thing is now all the voltage coming from the alternator (in my case 14+ volts) goes directly to the headlamps which increases the brightness.
One word of caution, don't just install relays using the existing forward headlight harness. The wire gauge is too small to handle the increased voltage which will cause the wire to get hot and potentially catch fire as in the original scenario.

 

Thanks for all the info - very useful!

I just bought the car from a private seller, who didn't make the best impression (handed me a bottle of water "just in case", 20 minutes later I was stuck on the motorway with a dead 'vette...), so I'll be going over everything as I get to it. Most important is the flickering lights, since driving lights are mandatory here, and I don't think the fuzz will be happy with my "blinking" kind.

What you describe here with the two metals heating and cooling, would that happen quick enough for the lights to blink several times per second? Heating probably could happen very fast, but not sure metal would cool down that quick...

I'll get some pictures of the headlights (I don't know whether they are original, but previous owners changed all kinds of things, including paint (yellow->black) and engine, so I wouldn't be surprised if the headlights are aftermarket too.

I was hoping to make a video of what I mean by blinking/flickering, since I realize that is very vague...

 

The turn signal/hazard flasher is the exact same design- so yes.

 

This kind of sounds like what you might find on an HID headlight system if the ballast of bulb is faulty... is it both headlights that flicker at the same time or just one? HIDs also slowly come on and up to full brightness so if your headlights behave that way that might be your issue

 

I see why he is questioning it as analog turn signal flashers metals are lighter (thinner) than the thermal switch in the headlight switch as it has to control more current than a flasher unit. With that said, yes, I'm pretty sure this is your problem as there is nothing else in the circuit that would cause this except for a loose ground. Yes, the metal switch does cool off fast enough to make the lights flicker off and on quite rapidly. I went through this with two Chinese aftermarket switches before I installed relays.
Does your light start flickering just sitting in the driveway without moving? If so I'd rule out a loose ground as one would think it would only happen while driving around hitting bumps etc.
Good luck with your project. There are many members here that have a lot of knowledge about these cars so don't hesitate to ask anything.
My father once said "The only dumb question is the question you do not ask". Wise Man - R.I.P

 

What the OP is describing is classic headlight switch overload.
Install the relays.
It will solve your problem.

 

For those that have installed the relays; Where are you mounting them and inserting/wiring them into the original harness?

 

You can get a pre-made male 3 pin plug - (yellow one in pic) or you can make your own.

Plug it into the low beam plug- you'll get a hi/low and a ground- which will work great to control the relays.

You can tuck the relays up under the front of the nose- and get your power from the horn relay or alternator.

 

I thought it was not wise to power such things directly from the alternator?

 

It's not the best place - but headlights came on the car- and when designed- were calculated as part of the "system's" current draw.

Adding additional devices- like fans (which easily pull more than triple the power of what the low beams pull) to the stock system's capacities- I can't recommend connecting those to the alternator.

Richard

 

I got some pictures and videos together to show more clearly what I'm talking about, but it looks like relays and maybe a new switch would be a good bet for a solution.



Corvette 1973 Headlights

Corvette 1973 Headlights

Corvette 1973 Headlights

Not sure if you can tell from these pictures whether they are new?

I put some videos up here:
http://molk.ch/fun/corvette/1973/vid...hts-inside.mp4
http://molk.ch/fun/corvette/1973/vid...ts-outside.mp4

But reading this thread (and also the owner's manual...), it seems clear that relays will probably fix it.

 

Again, thank you so much to everybody who took the time to post these very detailed answers - if we were on the same continent, I would buy everyone beer for sure!

The diagrams are very nice, but how do I find out exactly which wires to mess with on the car? I've checked my fusebox (which is irrelevant if I understood correctly), and of course I know where I put my foot to put the lights on, but how do I get to where the relays need to be installed?

I have the haynes manual, and some other books I can check, but maybe this is not as hard as it seems?

I have an experienced mechanic I can get to help with this, but I would like to understand what we'll be doing before diving in.

Thanks!