Low DC Voltage

I tried to replace my recently departed step light with a shiny new LED fixture the same size.  The PHYSICAL removal and install went flawlessly.  I turn the light on and get a flash.  It is supposed to be "non polar", but I reverse wires anyway.  Same result. HMMMMMM
I check voltage at the wires and get 8.5 DCV.  WHAT???
I check voltage at the battery and get 13 + DCV.  I test the light at the battery - works great.
I had initially cut the wires off the old light and connected the new light to them.  Now, I go back another 4 inches to the original light connection / wire nuts and try connecting there. One original wire nut was loose!  MAYBE...  The light flashes.  Test voltage - 8.6.

I assume the circuit goes from the battery to the power center, thru a fuse to the switch located 4 ft above the power center with several other controls, then directly to the light, about 8 ft away.

I have verified all screws in the power center, both 120VAC and 12VDC, are tight.

Where do I go next?  Check voltage at the Power center?  Could it be a bad switch?  Loose switch wiring?

Should I expect the same issue when I replace the two high porch / scare lights?

the step light is operated by the step (electric) controller on my RV.  that's a computer. may have a bad pass transistor.

On mine however LED's work fine

Most of the LED's I have will work on six volts however, they are a bit dim, but they work.

Thanks, John.  I got it figured out, but now have a new problem.  I checked voltage at the power panel.  Batt in was 13+V, all circuits - EXCEPT ONE - were 13+V.  That one was 8?!  ???  :-[  A partially blown fuse????  Isn't that like a little bit pregnant???  It appeared blown, but had a burnt section of the fuse wire still in place.  After removal, the ohm meter said blown.

The light is installed, and works fine.

PARTIALLY BLOWN FUSE ??????

Did you try the step light with the fuse out?  It may be that there is a ground problem or some cross leakage applying enough to measure 8.5 volts with no load and allowing the light to flash before loading the circuit down.  You may have had a blown fuse all along instead of a "dearly departed" light.

The light was definitely shot, and its' plastic scattered around the tree which departed it!  The fuse in question was clearly bad.  The other light on the circuit also did not work - 4 incandescent bulbs. (did not realize it was a common circuit)  With the new fuse, the light functions as designed, and the other light works again.

My question is how do I get 8.5 VDC through a blown fuse??  Measured both in the fuse box and at the light.

grashley said:

My question is how do I get 8.5 VDC through a blown fuse?? 

Click to expand...

It wasn?t 100% blown, only 32% 

A possible answer to that question......when the fuse blew, it wasn't a huge overcurrent event.  In other words, just enough current to make the fuse blow.  After it cooled off (we've all seen light bulbs blow out then work after they cool off), the fuse link made contact through it again.....but this time, the fuse had a high resistance connection where it went back together.  And oddly enough, you could put an ohmeter on the fuse and check it, and the ohm reading might be normal because there is no load on the circuit.  If the fuse was put back in the fuse holder and you tried to use it, one of two things might happen....
1.  It blows again because of the amount of current it's trying to pass and the fuse link is not new...it was previously blown and no longer has is rated ampacity.
2.  It doesn't blow, but the high resistance in the fuse link drops some of the applied voltage (13VDC) and you end up with, let's say.....8.5 Volts at the load side of the fuse and at your light.  In this situation, remember this; the high resistance connection acts as a load on the circuit, and the light ACTs as a load on the circuit....but they are now a series circuit.  Ohms law states that in a series circuit, each load drops some portion of the applied voltage (depending on the load,'s resistance) and the other load will drop the remaining voltage that was applied to it.  So the defective fuse dropped 4.5 volts and left you with 8.5 at the light......which wasn't enough voltage to power it.

See, how simple was that!    ;D

grashley said:

My question is how do I get 8.5 VDC through a blown fuse??  Measured both in the fuse box and at the light.

Click to expand...

The series circuit explanation has it's merits, but consider your measurement.. 8.5 Vdc to what?  the frame or the battery post.  I don't have much experience (yet) on MH wiring but have seen some really funky thinks on TT hitch wiring, all due to the intermittent ground or high resistance ground. Perhaps there is a faulty ground path somewhere causing the "frame" to be a higher potential than the battery post.

signed, Sparky 

I like the "32%" answer
Fuses not only blow but they fail. and sometimes the failure is a poor (not bad) connection.  I have seen that a few times. And even 3AG glass fuses (the clear glass tube) fuse looks pristine but it's "open circuit" on a meter. They simply get old and die is the only explanation I can give

_Rusty_ said:

The series circuit explanation has it's merits, but consider your measurement.. 8.5 Vdc to what?  the frame or the battery post.  I don't have much experience (yet) on MH wiring but have seen some really funky thinks on TT hitch wiring, all due to the intermittent ground or high resistance ground. Perhaps there is a faulty ground path somewhere causing the "frame" to be a higher potential than the battery post.

signed, Sparky 

Click to expand...

Your explanation might make sense except.......when the original fuse was replaced with a new fuse, the circuit worked correctly.  If there was a grounding issue, there would still be a low voltage issue.

xrated said:

Your explanation might make sense except.......when the original fuse was replaced with a new fuse, the circuit worked correctly.  If there was a grounding issue, there would still be a low voltage issue.

Click to expand...

As I mentioned before, there is likely a secondary 'leakage' path which causes the lower voltage measurement.  Your meter places almost no load on the circuit so you read a voltage from a high resistance source.  A common way for this to happen is poor grounding.  Then as soon as you increase the load (turn on the lights), the voltage drops... a lot.  One way to test this is to remove the fuse and remeasure the voltage.  Assuming it repeats, it should measure roughly the 8.5 volts you reported before.  Then, with the fuse still out, turn on the lights and measure the voltage.  I think you will find it drops to a very low reading.  This would be because what current flows will be through a high resistance, causing a drop in voltage.


Now, just where this leakage is will be the fun part.  But, I would look for another circuit (could be lights, could be CO2 detector, could be most anything that is drawing current) that might be seeking ground through the defective light circuit.  Possible the easiest way would be to disconnect each 12 volt string and see which (if any) causes the leakage to go away.  But, before you do that, if your trailer is connected to your truck, unplug it and measure the voltage (again, with the fuse out).  Vehicle wiring is notorious for grounding issues.

When a fuse blows, it's metal filament melts, scattering debris all around the inside of the fuse.  On a glass fuse, some of the vaporized metal can deposit on the glass, creating a high resistance bridge from one end to the other.  This can allow some leakage current to pass through.

With an intact load like a light bulb on the other side, you'll never see the leakage because the bulb is shorting it to ground.  But a voltmeter is designed to have a very high resistance so it won't load the circuit you're measuring.

Even so, if you're measuring 8.5 volts, it's saying the leakage resistance through the fuse (or via the dirty surfaces around the fuseholder contacts) is slightly lower than the voltmeter's internal resistance.

I like Scott's answer the best, but I now believe Xrated nailed it.

With more time to think it over, it is NOT a ground issue.  The 8.5V was measured from wire to light to chassis ground, wire to ground wire to light, fuse out to BIG wire to battery.....

When I pulled the fuse, instead of a loop of shiny wire, half of it was a "brown residue".  I suspect by the time I got off the floor and to the counter, enough shaking had occurred to break any connection, so the meter read open on the fuse.  As X said, this residue likely maintained a high resistance circuit, eating enough voltage to keep the LED or ceiling light from illuminating.

Thank you all!  I learn something new here every day!

grashley said:

Thanks, John.  I got it figured out, but now have a new problem.  I checked voltage at the power panel.  Batt in was 13+V, all circuits - EXCEPT ONE - were 13+V.  That one was 8?!  ???  :-[  A partially blown fuse????  Isn't that like a little bit pregnant???  It appeared blown, but had a burnt section of the fuse wire still in place.  After removal, the ohm meter said blown.

The light is installed, and works fine.

PARTIALLY BLOWN FUSE ??????

Click to expand...

Yep, exact same situation and this is exactly what happened to me. I must have accidentally shorted it somehow installing the LED bulb, and then I couldn't figure out the voltage drop until I found the partially blow fuse. It was really weird.

) )  I know this does not prove I'm not nuts, but it sure helps!  Thanks!