Goofy Tail Lights


#1

My '85 Chevy Van has got me scratch’n my head. When I turn on my lights at night, the rear blinkers don’t work and the brake lights stay on. When the lights are off, everythings functions properly. I’ve changed the bulbs and got a new flasher fuse already. Anyone have a y ideas ?


#2

Is this vehicle new to you? If it is, start by checking the bulbs to be sure they’re the correct ones, that they’re properly seated, and that the sockets aren’t corroded.

If it isn’t new to you and the problem is new, you may have a short to ground in the lighting wires. Typically these will occur where the harness goes through a hole on the body. Sometimes on an old vehicle there’ll be rust involved. To find the short, start at the light and work toward the fusebox. Another option is to run new lines. You’ll need the schematic, but sometimes that’s easier than finding the short.


#3

I concur with TSM, this used to be very common on rust belt cars and trucks. Trace the ground (black) wire to where they attach to the body. Clean the rust off the screw and terminals. Follow the other wires to where they pass through the body to see if the insulation if damaged. Remove the bulbs and if they are the right ones for the sockets, clean or replace the bulb and the socket.

Good Luck, this job is easy but tedious.


#4

Years ago on this forum. someone had a car on which the the ground for the tail light assemblies ran all the way up to the headlight relay. When it lost that ground, the tail lights (and probably headlights) grounded by flowing current backward through the brake light bulbs (to a ground it found somewhere in the brake light circuit when it was inactive), causing the brake lights to illuminate dimly. Could it be something like that?


#5

Thanks Guys. Looks like I have some work ahead of me, as this thing is a Rust Bucket !


#6

If this is a cargo van it should only take a few minutes to inspect and test the tail lamp grounds, there is usually little or no trim to remove.


#7

From what you stated about the problem it seems to that you have a ground connection problem causing the trouble. It also seems that the ground connection has a high resistance which is basically the opposite of a short. A short to ground should cause a fuse to blow out. Since the rear lights work okay when the headlights are off that means the rear wiring should be okay. What seems to be happening is when the higher current that is needed by the headlights is flowing it is causing a voltage drop to occur across the bad connection to chassis ground. This makes less voltage available to the rear lights and to function correctly. One easy way to prove this is to run a temporary ground between the negative battery post and a good clean connection to the car chassis. If the lights now work okay then you have proved that is the problem. Clean all the ground connections near the battery.


#8

Here’s another possibility. I believe this vehicle has bayonet bulbs. If they’re installed backwards (180 degrees out of rotation) you’ll get exactly the symptoms you’re describing. The pins are offset to prevent this from happening but you would be surprised how many people manage to do this by forcing them in.


#9

Well, I went through all the ground connections I could find and they’re all good. Could the problem be in the headlight switch itself ?


#10

You might want to try replacing the the turn signal/hazard flasher unit. There’s only one flasher unit in the lighting circuit.

The head light switch is a single throw/double pole switch. This means that when the switched is turned on, not only are there contacts for the head lights, but a secondary set of contacts are also closed. These secondary contacts leave the head light switch to two different circuits. One circuit goes to the front marker/turn signal lights. The other circuit goes to all the rear lighting.

But I would try first replacing the flasher unit as this would be cheaper than replacing the head light switch.

Tester


#11

Per my post have you checked the orientation of the bulbs? If they’re in backwards the voltage for the tail lights would be applied to the brake/stop light filament and the voltage for the brake/stoplight would be applied to the tail light filament. Do your tail lights seem extra bright? Do your brake/stop lights seem extra dim? There are two contact buttons on the bottom of the bulbs and if they are in backwards this is exactly what will happen.


#12

Check the bulbs like TSM and 2 Cents recommend. I had a strange problem like this many years ago. Turns out a dual filament bulb had one filament break and weld itself to the other filament. When the other filament got power very strange things happened, power was going backwards through various circuits, brake lights came on with turn signals, turn signals and tail lights on at same time were dim, baffled me until I found the bulb.

Try this, pull all the bulbs and put one in at a time. Check the function each time. Once the suspect bulb/socket is found you can start to trouble shoot from there.


#13

I have replaced the flasher unit and the bulbs seem to be in there correct(dark and raining out now). Heck, buld sockets maybe worn out after 30 yrs ! I’ll check again in the morning light…


#14

If you’ve replaced the flasher unit, then replace the head light switch.

That’s the only other component in the circuit that can effect the brake lights/turn signals.

Tester


#15
Per my post have you checked the orientation of the bulbs?

Bayonet bulbs are made to go in either way. Try it.

Edit: Got my bulb bases corn-fused…


#16

@insightful Yes and no, On a dual filament bayonet bulb such as a 1034 or 1157 one pin is set higher than the other to prevent them from being installed backwards. On a single filament bayonet bulb such as a 1056 it can be installed either way.

Now the newer flat based dual filament bulbs, such as a 3157, yes, they can be installed either way.


#17

@“MY 2 CENTS” , quite right!


#18

Here’s an illustration. Note the pin offsets.


#19

What happens if you just turn on the parking lights but not the headlights? If the same problem happens, I would suggest that the problem is one of the dual filament bulbs in one of the parking/turn signal lamps in the front of the car. As another poster said, the filaments in one of the bulbs could be touching. In most vehicles, the parking lights come on with the headlights. If the problem happens with just the parking lights on, you have isolated the problem.


#20

I understand what @“MY 2 CENTS” is talking about and that may be what the trouble is. Check the bulbs and the sockets for the brake lights. If you have a double filament bulbs make sure they are installed correctly. Also check the socket base and see if there are two contacts at the bottom of the socket. If that is so and the bulbs are only a single contact at the base then I could really see how this problem is happening. If this is the case, then you may see your dash lights turn on when you press the brake pedal even though the lights are turned off.