Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Brake light problems when headlights are on


[INTRO]I recently purchased a 2000 Chevy Express 1500 cargo van. I have converted the van into a mini-RV for my honeymoon (which I’m now on). In the process of this conversion, I cut all of the wiring to the rear, re-ran it through the frame and then resoldered the wires together. I labeled the wires to avoid any mistakes but one could have still occurred (it was a real pain in the butt and I am unlikely to try doing that again). After doing this I checked the turn signals and brake lights and everything seemed to work okay untill…I got pulled over the other night by a cop.

[PROBLEM] When the cop pulled me over, I had my headlamps on so my back lights were all on. However, the cop noticed that when I stepped on the brakes the center rear brake light went on but the rear left and right brake lights went dark, rather than becoming brighter. When the headlights are off, the brake lights work fine. As a result of this problem I can’t drive at night since it would be unsafe.

[ADDITIONAL INFORMATION] I have read a few other forums here with similar problems and the answers given all seem to suggest there’s a faulty ground somewhere. While I’m a scientist by training, I have a very limited understanding of electrical systems and have a hard time understanding how this could be the case since the rear middle and left and right brake lights are all on the same circuit (I know this because my Chilton’s manual indicates this and when resoldering the wire I discovered that all three (+) lines were already spliced together downstream (?) from where I clipped the wires).

[A CRY FOR HELP] Could someone explain to me what’s going on? If it is a faulty ground, what is it about the structure of the circuit that could lead to this kind of behavior. Could you point me to the right resource or the key terms I should search for for figuring out how to locate the faulty ground? Alternatively, could it be I got the wires crossed somewhere or have a bad connection when I did the resoldering? Any help would be greatly appreciated.



You certainly don’t need problems like this on your honeymoon so lets get this fixed ASAP so you can focus on other things.

From what you describe of the trouble it does sound like there is a grounding problem. The headlights and the brake lights may be sharing the same ground connection.

When a bad ground connection happens there is extra resistance added in series with the normal load to the connection to ground and a voltage drop occurs across that added resistance when current passes through it. In order to understand what is happening it is important to realize that in a series circuit the current is the same anywhere in the circuit but the voltage drops across the loads in the circuit can be different. Also, the sum of all the individual voltage drops in the series circuit will add up to the total supply voltage. In this case, 12 volts. In a parallel circuit the voltage is the same and the current can be different in each leg depending upon the resistance of each load.

The added resistance in the ground connection to the lights may not be enough to cause a problem when just the brake lights are on. Lets say the bad ground connection only has 1 volt across it when the brakes are on but the headlights are off. That will leave 11 volts for the brakes lights, enough for them to work. Now when the headlights are on a lot more current is needed to supply the load they require to work. That will mean the bad ground connection will also have a lot more voltage across it. The voltage across the bad connection may now be 6 volts instead of 1 volt. With only 6 volts left to run the brake lights it won’t be enough to make them work as they should. Now you know how the problem occurs but now you need to find the trouble.

I would start by cleaning the battery connections. This is a good first step though it most likely isn’t going to fix the trouble. Clean the battery to chassis ground connection, the problem may be there. Try to find the ground connection for the headlights and clean it. Hopefully cleaning those areas will solve the trouble. If it doesn’t then you may need to find the ground connection in the rear and go over it.

You can also make a long temporary ground jumper to help you find the problem. Place one end of the jumper lead to the negative battery connection. Then tie the other end of the jumper to areas you suspect may be causing the trouble. If the lights now work ok try cleaning that ground connection.

The reason the why the middle light works could be due to a couple of reasons I think. Either the power to light them is a lot lower than the other brake lights so it can tolerate more voltage drop and still work or the middle light has a seperate ground connection that is making a good connection. So there is no voltage drop on that leg of the circuit.

Typical light wiring in a car has most lights having only positive wires going to them. The light socket is grounded. Supply power to one wire and the light comes on as the socket ground completes the circuit.

If the ground is faulty, and the bulb has 2 connections, (brake light and running light) then the other wire can become a ground. (DC flows from high to low potential. This is supposed to be ‘ground’ but a wire without any power to it can become that lower potential.) Usually the light is weaker than normal, sometimes taking a second to appear. Since your brake lights come on until you turn on the headlights, I suspect that’s what is happening. Turn on the headlights and that turns on the running lights, putting power to the formerly ‘dark’ wire.

Try running a jumper wire from one of the light bulb sockets to a good ground on the chassis somewhere, then step on the brake with and without the headlights on. If it works with both then the missing ground is your problem. You’ll have to run a ground wire to the taillight area.

Hi Cougar and richw46,

Thanks for the responses, they’ve helped me gain a better understanding of what’s potentially going wrong. I’m a bit confused as to where the most likely places the fault could be. If this was happening to just one of the brake lights then it would seem like it would be at that point. However, if each light is being grounded independently, then it doesn’t make sense to me. Nevertheless, I’m going to take a shot at this this afternoon.

Many thanks.


One easy thing you may want to try is running a temporary ground to the rear lights from the battery and see if that makes them work.

One possible issue often overlooked.  Many brake lights are combo lights where a single bulb works as both a turn/brake light and a tail light.  Try replacing the bulbs.