Help needed! Tail lights not working but brake lights work

[TheHubbardBus]

Exactly. Like William said, I'd probably confirm your initial findings by applying 12V directly to the tail light circuit if it's convenient. But it sounds like you ruled the lights out IMO.

"oh well"... hell no. You should be stoked. You're smartly diagnosing the problem as you should without wasting $ on parts you don't need. You suspected (rightfully) it wasn't the bulbs anyway. A little time spent, but you're ahead, not behind, and now you can proceed knowing the lights aren't the issue.

FWIW the reason you're seeing different readings on the filaments is because they provide different resistance, which in turn is directly related to how bright they burn. When each circuit is activated, each is getting 12V, but with the lower resistance on the stop circuit, it's pulling more current (and burning brighter). A reading of infinity (infinite resistance) means no path for electricity to follow... a broken / burned filament.

[southhillkids]

I will test the bulbs by connecting to battery directly tomorrow anyways
but at this point it's not likely the bulbs as they have the right resistance, and I can see from the side of the lens that there are no broken filaments

I will check/test all related fuses and relays carefully tomorrow hopefully to find any defaults

The last solution I could think of is to wire a 12v line directly from battery positive to the tail lead and add a separate switch to it. What do you guys think?

I already finished my build inside the bus and can't open everything up just to fix this

Originally Posted by TheHubbardBus

Exactly. Like William said, I'd probably confirm your initial findings by applying 12V directly to the tail light circuit if it's convenient. But it sounds like you ruled the lights out IMO.

"oh well"... hell no. You should be stoked. You're smartly diagnosing the problem as you should without wasting $ on parts you don't need. You suspected (rightfully) it wasn't the bulbs anyway. A little time spent, but you're ahead, not behind, and now you can proceed knowing the lights aren't the issue.

FWIW the reason you're seeing different readings on the filaments is because they provide different resistance, which in turn is directly related to how bright they burn. When each circuit is activated, each is getting 12V, but with the lower resistance on the stop circuit, it's pulling more current (and burning brighter). A reading of infinity (infinite resistance) means no path for electricity to follow... a broken / burned filament.

[TheHubbardBus]

The problem could lie anywhere, not just in an inaccessible wire run (Like the switch, among other things, as mentioned by Kentucky). I'd use the meter to methodically determine where your problem lies. Start with the fuse. What's the voltage between the fuse controlling the tail lights & ground (with the lights on)? If you read battery voltage, then you know the problem is downstream between the fuse & socket. Otherwise, the switch would be the next likely culprit.

[mmoore6856]

Originally Posted by southhillkids

I haven't, but I will
It's not likely both bulbs are out at the same time?

i have replaced both tail lite bulbs and even headlamps that blew out at the same time. always check bulbs first. then if that dont work learn how to use a test lite as here you will get a lot of ........

[HamSkoolie]

Standard troubleshooting procedure (I'm a retired mechanic certified ASE on brakes and electrical and that was AFTER a career in Military auto and aviation maintenance).


Start at the symptom.
Work from the symptom towards the power source that it should have.
Don't probe wires with a pointy test lamp or MM "sticker" just go to
the accessible points "up stream". In your bus with build out done

you probably buried the wire harness. If not, there will be connecting

plugs in places along the route of the harness. Probe the pins for the

circuit you're tracing on both sides of each plug.
You can "jump ahead" if you know what you're doing. But is the fuse before or after the switch? If you don't know that you can't know what to look for.


Go to the same store where you got that screwdriver in the first pic and pic up a multimeter and a test lamp. Don't buy the cheapest MM, get one that has at least a 10amp current testing capability. You will end up needing that one day AND those meters are generally auto ranging which is nice....>READ THE MANUAL AND KEEP IT.

[burnerskooolie]

I just got a call back from Collins. He says the fuse is the trailer light fuse under the hood.