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Help me calculate voltage drop to headlamps


Robert92867
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I read 26 ohms from my lo beam (blue wire) headlamp bulb connector (removed connector from lamp), through headlamp switch and hi/low switch to the load side of the 25a headlamp fuse (fuse removed). What is the voltage drop?  Hi beam reads about the same resistance.

Tried to find an answer on the internet, but I'm still confused.

Resistance to ground is good at .4 ohms

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Current (Ampere) = Voltage/ Resistance
I (Ampere) = V / R

https://circuitdigest.com/tutorial/dc-circuit-theory

12v / 26 ohms = 2.166666666666667 current in circuit

to calculate voltage drop you will need the size and length of wire.

https://www.jcalc.net/voltage-drop-calculator-nec

To measure Voltage drop with meter hold the probes across the connection where you’re trying to measure the voltage drop, and energize the circuit. ie. put the red and black probe leads at two different points on the same side of the circuit, measure between the starting point, (switch) and ending point, (lamp).

find the spots with the largest voltage drop, clean / repair and re-test.

https://www.hagerty.com/media/maintenance-and-tech/measuring-voltage-drop/

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Robert 26 ohms is a lot of resistance. An ohm reading taken with a typical meter with probe resistance zeroed out over a short circuit length as yours should be zero, i.e. less than test setup resolution with a typical hand held meter. Accurately measuring resistance of less than one ohm is a multistep process best left to the bench techs. Your voltage at the source is going to be actual battery voltage. Not the nominal battery rating of 12v. The 25 A is the rating of the fuse and not the actual load that the circuit is going to carry. Recommended design practice as to voltage drop across an electromechanical relay or switch is no more than .300 MV at full rated load. I would suggest that you measure the voltage at source and load with the headlamp energized and leave the ohm's law calcs to the math nerds.🙂

Edited by Gary Cole
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If there is heating inside the resistive connections you could have more voltage drop than just Ohm's Law. 

Best thing is to probe multiple points along the wire path.  Use the same point for ground reference, even if it takes a longer DVM "ground" wire.  Stick the probe inside each connector, where the wire goes into the connector, on both sides of the connector.  And just because you get voltage on the connector that doesn't mean the voltage is getting into the wire ie bad crimp on the wire.  It's a long road. 

And if you do find a bad crimp don't be afraid to solder the wire to the connector verrrry carefully.  I even had a crimper break the wire under the insulation.  That one was fun. 

Good luck,

- bob

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There's no starting the diesel for 2 weeks, until parts come in so actual engine running voltage is out.  Troubleshooting the rats nest of wire under the dash is not going to happen either.  My headlights have been weak since day 1, when I bought the coach new 20 years ago. I avoid driving at night, or drive with high beams on (nobody has ever flashed me).

Sorry if I misled you guys into thinking this was a degenerative wiring problem. In retrospect, I think what I really want to know is will my headlights get brighter if I install relays, and as Gary mentioned... 24 ohms is a lot of resistance.

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You could do that . . . . or try LED headlamp "bulbs".  They draw less current so more voltage will be at the bulb.  OTOH, the beam pattern might not be the same as a halogen bulb.  LED's are plug n play and you'll know pretty quick if they're brighter.  Just don't confuse whiter with brighter.

- bob

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Weak headlights is a very common complaint.

Make or find a good ground for your meter. Run an extension cord and plug your meter negative lead into the ground hole if needed. But get a good ground for the meter. 

It does not matter if you have shoreline or generator power, just battery power is fine.

What is important is to get measurements at the input to the bulbs and then on the ground side of the bulbs. IF you have any voltage at all on the ground side of the bulbs you have a ground problem. However, the most common issue is the voltage to the hot side of the bulbs. Take a reading on your battery terminal and then on the hot side of the bulbs. Of course do this with them turned on. You will see a difference between battery voltage and the voltage at the bulb. How much is determined by the resistance of the long long run of wires, the size of the wires, the condition of any connections along the way such as a headlight switch etc.

Lets assume your ground side voltage is zero. Ok, ground function is fine. But your voltage is a couple of volts or more lower at the bulbs hot side. You may find below the driver in the outside compartment some heavy battery cables coming in to post there. Use your voltmeter to find one that is close to battery voltage. Do this with headlights on still. Once you establish a good source of power there compared to battery readings you can run a temporary jumper to the hot side of the headlights. If you feel better go ahead and cut the wire to the bulb or just buy a replacement bulb for the test but feeding power in parallel this way should do no harm. I do like to put a fuse inline with a setup like this. Make sure you don't use a wimpy alligator lead for this test. It has to carry 5-15 watts of power. Don't get confused here with the watts. Just my point that it takes some current carrying capacity. 

IF you notice a significant improvement in the brightness we can go from there. If just a bit better then you need to measure the voltages again with that jumper in place and look at the ground side voltage as well. If that has come up to some readable amount you need to address the grounds as well which is very common. 

Getting those voltage readings is paramount but as I mentioned, this is a very common complaint and yes from folks that bought theirs new. There are just too many long skinny water hoses in the path of the sprinkler so you have low pressure there. 

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