1950 mercury is low voltage at headlights

[BobO]

6.1 Volts at the battery 2.3 V at headlights. What would cause this

[Matt Harwood]

Bad grounds, worn out switch, frayed wires, undersized wires, stray ground, dirty connections--a whole bunch of stuff. Fresh wires of the appropriate size, clean grounds, and a good switch should definitely help.

[Bloo]

Stray resistance on the way to the headlights or back through the ground. It is a big loop, and the missing voltage will be found somewhere along the way. Ohms law and Kirchoffs laws are laws of nature that make that always true. Without going in too deep, 6.1-2.3 is 3.8 volts, so a total of 3.8 volts is being dropped across unwanted resistance in the wires, connections, switches, crimp joints, etc. Literally everything the current flows through on the way to the bulb and back to the battery has resistance. Some resistance is inevitable, but that is way too much.

 

Start by splitting it into halves to see where the worst loss is.

 

With the lights on, measure from the hot battery post (with one meter lead) to the hot side of the light (other meter lead). Now measure from the grounded battery post to the grounded side of the bulb. If you get any negative numbers, just consider them as positive. Add both up. It should be about 3.8 volts.

 

Now go looking at the side which is worst, the side that had the highest measured voltage drop. You can check individual components or connections the same way, to find the worst ones. For instance with the lights on you could put one lead on the hot terminal of a dimmer switch, and the other lead on the terminal that feeds the lights that are lit (high or low). The voltage you measure is the voltage being dropped in the dimmer switch.

 

The same trick could be used on a light switch, or really any part of the circuit containing one or more possible failures. It isn't only things like switches that can be bad. Any spot where something connects to something else on the way from the battery to the bulb and back could be bad. It is hard to measure the spot where a wire bolts to the body all by itself, for instance. By testing sections and process of elimination, you can usually figure out where the worst problem is.

 

Once you fix the worst problem, If you still don't have all the voltage that you think should be there, the process starts over. Another thing guaranteed by Ohms law and Kirchoffs laws, which I am not going to delve into deeply, is that once you fix something in a circuit everything else will appear to get worse. You can't assume that if you measured 0.5 volts at the second worst spot before you fixed anything that it will still be a 0.5 volt drop still after you fix the first thing. It will not. It will be worse. So, fix the second worst thing (which is now the worst thing) and recheck. Continue until you have enough voltage at the lights that you can live with it. There is always some voltage drop just because of the nature of wires, switches, and connectors. The trick is to get it as low as possible.

 

 

Edited October 7, 2023 by Bloo (see edit history)

[Fordy]

I am going to put money on a bad ground connection. Pull the bulb and check voltage of "hot" wire to the opposite battery terminal (- if negative earth). if the voltage is up then it's a grounding issue. If it's still low - I eat humble pie and bow to Bloo's superior knowledge.

Steve

[JACK M]

99% of all electrical issues are groundless.

[37_Roadmaster_C]

12 hours ago, Fordy said:

I am going to put money on a bad ground connection. Pull the bulb and check voltage of "hot" wire to the opposite battery terminal (- if negative earth). if the voltage is up then it's a grounding issue. If it's still low - I eat humble pie and bow to Bloo's superior knowledge.

Steve

@Fordy,  This will NOT work like you think. With the bulb pulled the voltage WILL read proper voltage. The only way this will tell you anything is if one wire is completely open. In the case of low voltage there is increased resistance in the circuit as @Bloosaid above. Because of the resistance of the meter being EXTREMELY high, the resistance of the circuit will appear as virtually 0 ohms and therefore the voltage reading will be proper. What you are wanting to do is what @Bloosaid... Make the measurement with the bulb INSTALLED. This will provide the load which will cause the voltage drop you are looking for.

[Grimy]

My go-to diagnostic tool, besides a VOM, for a 6V system is a length (6-10 ft) of 10-gauge wire with an alligator clip on each end.  Try grounding-to-frame each bulb socket with the lights turned on and note any improvement.  Often I find that I need to improve the socket's ground, sometimes by soldering a wire to the socket and running a short ground wire.

[Oldtech]

Remember the ground fault could be anywhere. Body to fender, fender to light bucket, body to frame.  Troubleshooting, like Bloo said, is the way. 

 

[Grimy]

13 minutes ago, Oldtech said:

Remember the ground fault could be anywhere. Body to fender, fender to light bucket, body to frame.  Troubleshooting, like Bloo said, is the way. 

 

Agreed!  In this headlights case, I think I'd check output from the switch first, then jump ahead to the sockets *if* non-sealed beam headlights.  My experience with non-sealed beam headlights and their tendency to have defective grounds.  If sealed beam, I'd work from the switch forward, perhaps with early attention to the dimmer switch.

[Angelfish]

Way back my Dad drove a 50 Mercury, it had a similar problem.  I ended up tracing it to the rear license plate light.  Peculiar things can happen in wiring.