I hardly ever drive my car in the dark, mainly because when I first got our car back together on an evening cruise, I lost headlights while high beams were on, figured it overheated the switch which was new at the time. then last year the car began to get lazy eye syndrome,
where one light would need a hand coming up. I never diagnosed the problem, just drove it during the day. after 3 years I wanted to just drive the car. Anyway fast forward to last night, I wanted t take the car out for a cruise in this overbearing heat we have here. it was late in the afternoon / early evening, I was cruising with the running lights on until I had to put the lights on. I pulled the **** to lift the headlights, figuring I would have to assists one of the lights. As I got out I noticed for the first time in forever they both came up together. So we started our drive home. while on our final drive, the lights went off! then came back on, then off again. While driving blind I pushed in the **** and the lights didn't even go down. I pulled it back out and then in again. lights still off and lights still up. I was going to pull the other under dash valve when the lights came back on. I drove home as quickly as I felt safe doing so, the lights flickered two more times but remained on until I got home. I pulled the car into the garage, shut it off and shut the lights off, this time the lights that won't come up also wouldn't go back down without assistance.

So I could use some help here, I'm leaning to a new headlight switch but could use some advice.

 

Does it only happen when the high beams are on? Did you change your bulbs to halogens? Have you replaced your headlight switch lately? This is the classic over-current condition. The switch has a thermal circuit breaker and when too much current flows through it this CB will open then close when it cools off. Aftermarket switches from China are terrible.
First check your grounds as that will contribute to a high current situation. Then I'd look into putting relays on the lights as the original design, good for it's day, is just a fire hazard with all the current going through the switch.

 

I have not upgraded to halogen lights. I changed the switch about 4 or 5 years ago when I got the car. It was an aftermarket switch. I have a switch out of an 81 car I’m going to see if it will work in the 79. It only happened when I used high beams but last night it happened on low beams. And what I found odd was the weird vacuum issues as well.

 

The headlamp switch has a built-in self-resetting circuit breaker. That's why the lamps went on and off.

Check the front lamp harness grounds before replacing the switch! A poor ground connection will cause amperage to rise. The grounds are not weatherized in any way and get rusty. A good cleaning of the heavy hex headed sheet metal bolts and their holes with a wire wheel will solve the problem unless there's serious corrosion. One ground is in the top center of the nose between the vacuum headlamp door relays--that one's easy. Another is at the lower-left (driver) corner of the radiator support member--that one's a royal pain because it's blocked by the big vacuum tank that doubles as a structural member. Sadly that one (and it's the largest) is likely one of the two most likely to be in SEVERE need of a cleaning. There is a third grounding point for the front lamp harness on the alternator. The other grounding point likely to be in sever need of a cleaning is for the rear lamp harness and it's located near the radio antenna.

Also GM certainly did not over-spec the headlamp wiring! If you've replaced the dual-filament low beams with a higher wattage halogen (or other) lamp (ALL '79s came with "regular" low-beam lamps with later production having halogen high beams) that too can cause amperage to rise due to wiring inadequate to the load. Combine that with a dirty ground and amperage can jump high enough to trip breaker.

 

Just saw your reply that they're the original style lamps. Regardless I still suggest you clean those grounds before buying another headlamp switch. "Bypass" relays will stop the problem with with the breaker in the headlamp switch cutting in and out but they won't solve a root problem of dirty grounds. Sadly I suspect many just install those breakers with a direct source to power without using proper (or perhaps any) circuit protection which means a problem can take out the entire electrical system.

 

Just to add what Mr Mike said above regarding the grounds, take a look at all the headlight connectors. My 78 had corrosion from 40 years of use. To the point that half of the plastic connector was melted. That could also be adding to your current overload, which as he said will trip the breaker in the switch.

 

The melted plastic connectors are a result of a too high current draw. I suggest you put the headlights on relays.

 

High current draw and melted connectors go hand-in-hand with both corroded grounds and dirty connectors! Again those relays will do NOTHING to correct dirty/corroded connectors and grounds! The low-resistance lamps try to draw their full wattage--a poor ground and/or dirty connectors causes voltage to drop as amperage increases. The increased amperage causes excessive heat--especially at the poor connections both ground and power--that can melt the connectors and wire insulation which eventually results in the sort of "ground leakage current" that makes a fire

If the grounds and connections are clean connector bodies are extremely unlikely to melt unless you've greatly increased the load on the circuit to the point that any proper circuit protection for the design load and wire gauge trips.

 

When I went through Marine Corps electronics school in 1962 the formula for current was I=V/R. An increase in resistance, such as corroded and dirty grounds, will lower the current flow. Example: 12 divided by 20 = 0.6 while 12 divided by 120 = 0.1. Chrysler had this type of problem up into the 70's with the wiring of their ammeter since they didn't wire the ammeter through a shunt circuit like GM did with the Corvette.

 

Relays already done.

 

Although I agree 100% about good, clean grounds usually when installing relays the grounds are messed with and cleaned when they are installed. Relays also take the high current out of the switch which is a good thing.
Here is my wiring diagram for my relays I installed. Everything from the lamps back to the relays is upgraded wiring. You can get by with just two relays but I wanted the added safety that 3 create. Also I don't use fuses where it says 20 amps I use reset-able circuit breakers (not self resetting). With this setup and the 12 gauge wiring I installed from the relays to the lamps (and the lamps grounds) it works well and I have no voltage drop to the lamps.I get full alternator output voltage to my lamps and it makes a huge difference.


Circuit breaker:



Here is a good explanation:
Daniel Stern Lighting

 

Lots of info to read, I pulled the car out and like under the rear by the antenna I noticed a ground strap not connected to anything. The wiring harness is connected to the frame, grounding strap from the antenna is connected at the same point and another strap connected at the same frame point but doesn’t go anywhere.

 

Oooh-Rah to my fellow Marine.

Yes, Ohm's law IS a law! In a simple and directly connected circuit it's very easy to compute. Not so easy when there are multiple points of resistance with the load--NEVER forget the load as it's just that-essentially defining the current in the system with a controlled voltage. A poor connection in a complex circuit has it's own I=V/R calculation independent of the load. The result of a poor connection in such a system is heat! Note that Ohm's law does not include heat. Why? Because heat is an artifact of an imperfect load that does not completely turn electrical energy into mechanical energy.

 

Heat is a byproduct of work and that is why a larger gauge wire in a circuit will produce less heat because of the lower resistance of the wire. Corroded grounds increases the resistance or total impedance of the circuit resulting in less current draw.
Were you in the Corps and if so what was your MOS? Mine was 2741 ground radar repair until it was changed to 5931 and now it has been discontinued.

 

Ohm's, Relay's you guys are speaking German to me. Am I going to have to buy an electricity for dummies book ? If I remember correctly I do believe the headlight switch connector might have had a slight melted plastic but I can't remember exactly.
I will start by cleaning the back connector, I would love to know where that second strap goes, I'm guessing the exhaust clamp? does that even make sense?

 

The exhaust clamp, no, any other metal yes.
You might want to look into something like this https://www.corvettemods.com/C3-Corv...s_p_16146.html

 

Thanks for the reply, that strap in the picture is connected at the frame ground in the rear, there are two straps back there, One strap goes from the frame to the antenna this one goes no where, but it appears that it was connected somewhere.the only other connection back there is the wiring harness for the lights connected at the same point on the frame.

 

If you have an alarm horn mount it there.

 

This is in the rear by the antenna, I do not have an alarm horn.

 

It could be an extra put on at the factory, but I would try to find an attachment point on something metal. You can't have too many grounds especially on a Corvette