Auto Electronics: Tail Lights, Taillight malfunction, fuses

Question
1995 Geo Prizm.

My son's car.  This just started a few days ago. Brake lights work, tail lights will not come on when headlights are on.  When it first happened, he pulled all the fuses and put them back.  

It fixed the problem for about a week.

Checked bulbs, fuses - good.  Thought maybe relay.

Any suggestions?

Answer
Hi Theresa,

Sorry to hear your son is having car troubles.  Lets see if we cant get his car squared away.

My first thought here is to check and see if ALL the parking lights are out when the headlights are on or is it ONLY THE REARS.  If it is only the rears, then chances are we are dealing with a bulb or fuse/fuse connection problem, if all of them are out then chances are its a wiring issue (USUALLY A BAD GROUND).

If only the rear parking lights are out when you turn on the headlights, i would suggest removing the tail light assemblies, and removing EACH parking lamp bulb, usually on setups like the 95 prizm, the parking lights and taillights share a bulb, this is known as a multi-filament bulb (sometimes referred to as a step bulb).  To give you an idea how this works it is similar to how a headlight works when the high and low beams are controlled by one bulb.  Meaning that the filament for the brake light may be good but the filament for the taillights may be blown, this is easily noticed by inspecting the bulb itself, if the filament is blown you will see the coiled wire filament in two pieces or missing all together, remember on multi filament there are two filaments in the bulb so one can be intact and the other blown, this happens a lot with this style bulb as one filament gets more use then the other one does.

This is the easiest, fastest and most inexpensive problem to fix, so i recommend checking this first.

If the bulbs are good, then the next thing to check is going to be the fuses, sometimes a fuse can be good, meaning not blown, and still not supply power where it needs to, this happens when, over time, the fuse block gets corroded, thus corroding the fuse terminals (the silver color prongs on the fuse itself). Given what you said in your question about your son removing the fuses and it solved the problem temporarily, this may be where the issue is, so follow the below procedure to see if it corrects the issue.

Remove the fuses that power the taillights from the fuse block, check to see if the terminals are corroded, they should be silver in color, if not, use a wire brush or an old toothbrush to clean them thoroughly until they are silver again, or just replace the fuse with the same type, meaning if you remove a 20 amp fuse replace it with a new 20 amp fuse, do not replace it with a fuse that is rated for different amps, such as replacing a 20 amp fuse with a 15 or 30 amp fuse as this can damage the electrical system.  Also use a toothbrush or pipe cleaner to clean the terminals (or slots rather) where the fuse goes in the fuse block.  MAKE SURE TO DISCONNECT THE POSITIVE TERMINAL OF THE BATTERY BEFORE DOING THIS.  Do not use a q-tip as the cloth fibers can get stuck in or on the block itself and cause more problems.  Another thing to do is when you insert the fuses back into the block, coat them first with a light coat of di-electric grease, this is cheap usually 5 to 7 dollars a tube and found at most auto part stores, do not use alot just a light coating on the prongs of the fuse itself will do fine.

Now test the lights again and see if they work.

If the lights still will not work, then chances are you have one of three problems.

1. A bad ground wire in the taillight electrical system
2. A broken power wire or grounded (ground faulted) power wire in the taillight electrical system
3. A Bad switch (not likely if the front parking lamps work) however if the front lamps do not work this is a possibility.

either one of the above three issues is quite indepth, and i recommend having a certified mechanic look at it and make the necessary repairs, since it can get complex to trace a bad wire or grounded circuit, and you will want to have a professional doing this to ensure safety and proper working order.  however, if that is the only problem with the vehicle and it still runs good, it is well worth the investment to have it fixed in my opinion.

I hope this helps, try my steps above and if it doesn't work let me know and we will try some other things before you bring it to a mechanic.

Thank You
Please let me know how you make out.