confused voltage readings

[terrywinchester]

Hi guys,

Trying to troubleshoot a brake light problem on a 2000 Avalon.

None of the brake lights are working, including the rear window light (it went out first). Tail lights work, as do flashers and turn signals.

Replaced all Bulbs, checked fuses; OK, checked brake switch; OK.

Checking for voltage at the 3rd rear window light with a DMM shows +/-11v, while checking it with a light tester shows 0v. Any idea why the discrepancy? I assume there's no voltage there as the bulb doesn't light.(??) Both testers have been checked to verify that they work. Am I getting a type of AC Phantom Voltage reading with the DMM? Is it possible that there could be voltage there but the bulb won't light due to a loose ground? I'm confused - Help!

Thanks,

Terry

[TheSilentChamber]

that car may use a relay on the tail lights, if it does you might want to check it.

[curtis73]

Often times when that happens there is a bad connection somewhere. What happens is the volt meter tests for 12v. In your case it found it. But that doesn't mean that there is any amperage available. Check grounds, corroded connectors, etc.

A multi meter is a great tool, but sometimes (like in this case where you really need a load) a good old fashioned test light will help diagnose better.

Did you ever have a loose connection at the battery? You turn the switch on and get radio, lights, and everything, but when you try to hit the starter it acts like a dead battery? What's happening is a bad connection is supplying enough juice to run remedial things, but when you need that 200 amps to run the starter it supplies 0 volts.

This sounds like the same thing. It supplies 12v under no load, but 0v under load.

[terrywinchester]

Originally Posted by curtis73

Often times when that happens there is a bad connection somewhere. What happens is the volt meter tests for 12v. In your case it found it. But that doesn't mean that there is any amperage available. Check grounds, corroded connectors, etc.

A multi meter is a great tool, but sometimes (like in this case where you really need a load) a good old fashioned test light will help diagnose better...

Curtis,

This sounds completely reasonable to me. Thank you.

One more thing, from an electrical theoretical standpoint, I hope you or someone can answer is, whith the bulb in the socket, I read 0v across the terminals; with the bulb out, it went back to near full voltage. Is this a theoretical effect of voltage drop when measuring across a resistance, i.e; I measured 0v across the filament, therefore 0vd?

Thanks again and I welcome more responses.

[curtis73]

Yep, that's the exact thing I was talking about. Under no load it shows 12v, but with a load it shows 0v. I had a similar thing happen with a 110v outlet. The wire wasn't clamped in the lug of the breaker, it was just resting in there. It showed 110v at the outlets, but if I plugged in a lamp and turned it on, it showed 0v.

[terrywinchester]

Originally Posted by TheSilentChamber

that car may use a relay on the tail lights, if it does you might want to check it.

Where would this relay be located? I don't have an Owners Manual or a Repair Manual.

Can anyone give me some guidance as to tracing down and locating a broken ground or hot wire?. The third light's wiring seems to go through the center console from behind the rear seat.

Thanks for any help.

Terry

[TheSilentChamber]

Probably under the dash somewhere, if it has one. Most dont use them but I'v been seeing more and more newer cars using them. If its in the wiring (and probably is) it would more than likely be at the harness plug up close to to the headlight switch.

[UncleBob]

your car has what is called a "bulb failure sensor", this is what operates the little light on your dash that tells you when you have a bulb out. All of the brake light wires go to this device, and it acts like a relay also. Most likely this component is faulty. Its located in the trunk, behind the carpet on the right side.