problem with headlights
Safety Car
Joined: May 2001
Posts: 4,757
Likes: 1,647
From: Fairfax Virginia
Can you remove the dimmer switch and try to make a connection manually with the harness? I believe it should only be two, maybe three wires connected to the Dimmer. Try and make the connection down there simulating the high beam dimmer switch.
Have you checked the engine ground? Is the headlight switch grounded when it is installed? Have you checked the individual headlights for a bad ground connection? Part of my dashboard and the headlight switch were not grounded when I first bought my C3. I made a new cable and supply a good ground signal to the dashboard, now some of the parts are working better including the headlights.
Try Measuring the battery voltage using the engine block for a ground (0 Vdc) and the alternator output as the positive (12 Vdc). Then measure the actual battery voltage (at the battery) and compare. I would also test the grounds on the dashboard and windshield wiper switch, as well. They should be the same or very close, if not then you have a faulty engine ground. The headlight system needs a good solid ground connection to allow it to work so again I would go through the grounds and be sure they are well connected and there is no corrosion on them.
There is a tool I use regularly on electrical issues with my Corvettes. It is called a Power Probe and it lets you supply at the tip either 0 Vdc or 12 Vdc by simply using a rocker switch. This allows me to test the circuits in the car which is very helpful. When testing components like electric window motors it is really useful as I can run the motor first to see IF it works, and then find out why it doesn't have power.
Some of the 1968 wiring information may not be up to snuff. DocRebuild sells really nicely laminated wiring diagrams that are easy to read and follow. even for color-blind people like me. The Factory Service Manual doesn't have that great of schematics in it. The later C4 FSM's were awesome as they came with a mechanical and the electrical manuals seperate. You would have a really hard time without one on a C4 and their EFI systems.
RockAuto might have some guides for you as they sell the FSM's on a DVD for a fraction of the price of printed copies. They sell much more than just the FSM's.
Have you checked the engine ground? Is the headlight switch grounded when it is installed? Have you checked the individual headlights for a bad ground connection? Part of my dashboard and the headlight switch were not grounded when I first bought my C3. I made a new cable and supply a good ground signal to the dashboard, now some of the parts are working better including the headlights.
Try Measuring the battery voltage using the engine block for a ground (0 Vdc) and the alternator output as the positive (12 Vdc). Then measure the actual battery voltage (at the battery) and compare. I would also test the grounds on the dashboard and windshield wiper switch, as well. They should be the same or very close, if not then you have a faulty engine ground. The headlight system needs a good solid ground connection to allow it to work so again I would go through the grounds and be sure they are well connected and there is no corrosion on them.
There is a tool I use regularly on electrical issues with my Corvettes. It is called a Power Probe and it lets you supply at the tip either 0 Vdc or 12 Vdc by simply using a rocker switch. This allows me to test the circuits in the car which is very helpful. When testing components like electric window motors it is really useful as I can run the motor first to see IF it works, and then find out why it doesn't have power.
Some of the 1968 wiring information may not be up to snuff. DocRebuild sells really nicely laminated wiring diagrams that are easy to read and follow. even for color-blind people like me. The Factory Service Manual doesn't have that great of schematics in it. The later C4 FSM's were awesome as they came with a mechanical and the electrical manuals seperate. You would have a really hard time without one on a C4 and their EFI systems.
RockAuto might have some guides for you as they sell the FSM's on a DVD for a fraction of the price of printed copies. They sell much more than just the FSM's.
Last edited by ctmccloskey; Jan 8, 2025 at 09:48 AM.
Melting Slicks
Joined: Feb 2018
Posts: 2,326
Likes: 721
From: Marietta GA
You didn’t supply the model year of your Corvette! Not every year Corvette has the same wiring scheme. If you want to fix your problem then purchase a year specific Chevrolet Service Manual ( look on eBay) it will have a wiring schematic. Good Luck.
Thread Starter
Racer
Joined: Jul 2016
Posts: 287
Likes: 15
I have wiring diagram from DR.Rebuild also said all four come on but not dim..if all four comes on it seems to me the grounds are good ...I have a suspicion the problem is on firewall where the headlight plugs into fuse box..just hard to work with...I could unplug the headlight harness and see if I have power out of fuse box[pretty sure I do since they all four come on]then I would supply power to wires for dim side..Dimmer continuity is good..I hate getting under car to do this but don't have any choice..like the idea of power probe...Thanks......Malcolm
Thread Starter
Racer
Joined: Jul 2016
Posts: 287
Likes: 15
68 bbc,air 4spd.,conv,,,,2024colorado blue,,header side pipes///..restored myself to have something to do{eight years now..should be painted by next month}new chrome 20,s for back//18,s for front//no tires yet
Safety Car
Joined: Nov 2002
Posts: 4,181
Likes: 1,177
From: out of nowhere
@ctmccloskey Excellent information, and very thorough (as usual). I actually screen shot and file away a lot of the info you share! I’m sure some of it, if not all, will be very useful at some point. Thanks!
