Reverse Lights - Grrr

[CW4B]

70 C20 Longhorn. Replaced reverse light slider switch. Now 12V through switch but only 4V to bulbs. If I take one out, the other will come on - dimly. Not both. Help!-)
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[hjewell2]

ground issue

[HO455]

Check the grounds at the lights to start. If you have a good ground (ie to the frame and a known good ground from the battery to the frame) with your meter and only four volts. Then you have a bad connection some where in the circuitry.
I would visually inspect the entire run from the backup lights forward looking for any non-factory connectors that may have been installed.
A rusted fuse block can also cause a voltage drop in the system.
Good luck

[67GMCcampercruiser]

I just had an issue with backup lights. I had the light housing out and hooked up a booster cable to it and on the other end connected the cable to the frame where I first took a grinder to it to get to bare metal. Nothing with the test light until I put the ground cable on the light socket itself. Then I noticed a little play where the socket attatches to the rest of the light housing. I kept turning it back and forth letting the rust fall out. Now it works. I might tack a weld between the light socket and the housing. If you need a picture I'll take it apart again and get a picture to show what I'm writing about.

[tino0217]

There's a short in the wiring

[Sparky dave]

Yes a short drags the voltage down

[HO455]

Well technically a short is a hot conductor that is touching a ground or another hot conductor of a different phase. So it is true that a shorted conductor that is not properly fuse protected can cause the voltage to go low as the supply of voltage is unable to keep up with the load imposed. That generally leads to a major failure with smoke and sparks.
A better way of stating this is a "partially grounded" hot lead. In that the insulation has degraded and now there is a partial pathway for current to flow to the ground. This could draw the voltage low if the circuit was not properly fused. If you have a circuit that has a normal load of 3 amps with a 5 amp fuse everything will be fine. If the hot wire insulation is degraded so that current now flows to ground and now an additional 3 amps is added to the circuit the 5 amp fuse would blow. But if it was replaced with a 10 amp fuse the size of the wiring may limit the volts that are available at the bulb. In a short time usage circuit like backup lights you probably wouldn't melt the wiring and have a direct short. A circuit like the tail lights that may be on for extended periods would most likely overheat and melt the insulation. Resulting in a direct short which would blown the oversized fuse.
A more likely scenario in one our trucks is a bad connection is causing resistance in the circuit and causing the voltage to drop. It is possible to have more than one bad connection between the battery positive post and the negative post with each one adding to the problem. To troubleshoot this problem it is best to start at the source of the symptom. Like the OP did with finding only 4 volts at the bulb. Then divide the circuit in half. The OP found 12 volts at the switches power side. Next would be to check the voltage at the switched side of the switch when reverse is selected. If it is good are there any more factory connection points in the circuit? (Sorry I'm more versed in Suburban wiring harnesses) If so they are an easy way to confirm the positive side of the circuit is good. All of this leads us to the other half of the circuit commonly called the ground side.
The other half the circuit starts at the filament in the bulb which is connected internally to the metal base of the bulb. The bulbs base in turn makes connection in the bulb's socket. The socket is attached to the light housing by a crimped ring. The light housing is attached to the bed by screws. The bed has a ground strap to the frame. The frame has a cable to the negative post of the battery. As you can see there are a lot of connections with a lot of exposure to the elements so any or combination of these could be causing the low voltage.
Corrosion is the unseen demon here. Commonly referred to as a "Grounding issue" the lack of resistance free continuity from the filament to the battery negative is likely what is causing your problem. There is only enough current available to light one bulb dimly at a time. There are numerous ways to check for this problem. One of the easiest would be to run a test wire from the battery negative post to the bulb's base and place the gear selector in reverse. If the bulb lights at full brilliance then your problem is in the gound side connections. Move the test lead to the housing and check bulb operation, then to the bed and finally to the frame if needed.
Remember the test lead must be touching bare metal. Any paint or rust will prevent the bulb from lighting.

I'm sure I forgot something or stated something poorly so forgive me and ask for clarification.

[wilkin250r]

Ditto to what HO455 posted (because it's the internet, and people looking for solutions like to hear second opinions and confirmations).

Because a short circuit is a parallel connection, it generally doesn't drop voltage. Theoretically, it's possible, but in reality it just doesn't happen. Like, 2% chance.

98% of the time, low voltage is a connection problem. They can be mind-boggling if you don't understand them, because you can often see 12V with no load, and as soon as you add the bulb or otherwise load the circuit, the voltage drops. The reason it drops is because of the high resistance of the bad connection. Resistance has no voltage drop until current actually flows through it.

[CW4B]

Wow, thanks everyone. HO455, thanks for the details. I am confident I will be able to solve this riddle now!

[HO455]

Your welcome. And thanks to Wilkin250r for clarifying my rambling. Good luck and let us know what you find.

[wilkin250r]

However, I would disagree slightly with HO455's earlier assessment when he spoke about the positive and negative sides of the circuit, and he said "Commonly referred to as a 'Grounding issue' the lack of resistance free continuity from the filament to the battery negative is likely what is causing your problem. "

The tail lights/blinkers and reverse lights all share the same ground path (bed frame, grounding strap, ect ect) except for right at the sockets themselves. Since it's unlikely that you have identical grounding problems in both reverse light sockets, and also since you didn't mention anything about problems in the other tail lights, logic would dictate that your problem isn't in that portion of the circuit.

My money is on the positive side...

[HO455]

Originally Posted by wilkin250r

However, I would disagree slightly with HO455's earlier assessment when he spoke about the positive and negative sides of the circuit, and he said "Commonly referred to as a 'Grounding issue' the lack of resistance free continuity from the filament to the battery negative is likely what is causing your problem. "

The tail lights/blinkers and reverse lights all share the same ground path (bed frame, grounding strap, ect ect) except for right at the sockets themselves. Since it's unlikely that you have identical grounding problems in both reverse light sockets, and also since you didn't mention anything about problems in the other tail lights, logic would dictate that your problem isn't in that portion of the circuit.

My money is on the positive side...

Good sound logic.