Testing to determine whether headlights are operating at full power

One problem with the lights that’s particularly common with older RVs is low voltage to the headlights when under load.  Many motorhomes use an isolator between the alternator and battery, which costs you about 0.6-0.7V, and many also run power from the headlights through the headlight and dimmer switches.

It’s not uncommon to see only 10V at the headlights.  While that might seem like most of 12V, consider that power goes with the square of voltage–compared to a normal 14V system, your lights at 10V would only be half as bright!

As part of the process to fix your bad headlights, it’s worth checking to see if this is also part of your problem.  There are several ways to check, but most important is making sure you get a measurement while the wiring is under load.  I’m going to walk you through a series of measurements that can be taken with an ordinary voltmeter, and without piercing any wires.

To start, disconnect both headlights at the bulb connector.  Start the RV (or truck), and measure voltage at one of the connectors (1).  It should be very close to 14V.  Turn on a larger load (e.g. dash blower on high) and see if the voltage stays within 0.2-0.3V of the initial measurement (2).  If not, we need to have a closer look at the charging system.  You can continue, but even components in perfect condition can’t make up for a low source voltage.

Next, turn on the headlight switch, and again measure voltage at one of the headlight connectors (3).  It will likely drop some, but anything more than 0.1-0.2V starts to become a problem area.  Why would voltage drop with the parking lights turned on?  In some cases, a single power supply runs to the headlight switch, while in others the headlights and blower are supplied by different fuse boxes.  This test helps to narrow down where the source of the voltage drop might be.

Now, plug in the driver’s side headlight, and measure voltage on the passenger side (4).  Expect these numbers to be the lowest so far, but they’re still higher than what they would be with both headlights operating.  Now, plug in the passenger side bulb, unplug the driver side bulb, and measure at the driver side (5).  If the driver and passenger side measurements are nearly the same, the wiring between the dimmer switch and the lights is probably in good condition.  If they’re different (usually lower on the passenger side), the wire between the dimmer switch and the lights is problematic.

Finally, switch the high beams on and measure each bulb connector with the other side plugged in, just like with the low beams. Record these measurements as (6) and (7).

We still don’t have a direct measurement of voltage to the lights with both lights operating. But we’re going to get pretty close by doing just a little bit of math, and making a few assumptions.  With both lights plugged in, the current flowing is approximately double what it is when just one is plugged in–so take higher of the measurements from 4 and 5, and subtract from battery voltage with the RV/truck running (from 1).  This is the voltage drop with one light running.  Double that number, and subtract it from (1).  That’s approximately the voltage at the headlights when you’re driving down the road. Record this as (8).

How does that translate to power and brightness?  Take the number from 8 and square it.  Then divide by 196 (which is the square of the 14V that should be present).  As an example, if the measurement from 8 was 9.0V, 9 squared would be 81.  81/196=0.41, or 41% of the power your lights should be operating at.

Any measurement in (8) that’s lower than 12.8V should be considered a problem.  FMVSS108 requires headlights meet the performance standards when tested at that voltage.  If you’re lucky enough to be under warranty, address the issue with the manufacturer, notifying NHTSA of the defect as appropriate.  If you’re not under warranty, your recourse may be limited–the standard still applies, just a tougher road to a fix.

Either way, there are a number of approaches to fixing a low voltage problem, depending on where it exists.

  • If the voltage in (1) is lower than 14V, test the alternator output voltage and measure voltage at the battery.  If the alternator output voltage is low, it may be failing or overloaded.  If the alternator checks out fine but battery voltage is low, check the condition of the charge wiring, and see if you have an isolator installed.  If there’s an isolator, look for a guide soon on the choice between installing a separator or relocating the alternator’s sensing wire.
  • If the voltage is low anywhere else, you have two choices.  Either follow the wire, taking measurements to identify the problem wire or connector, or simply install a relay harness to take power directly from the battery to the headlights, and use the existing headlight wiring only to control the relays.

If any of the above doesn’t make sense, feel free to send an e-mail to [email protected].  Look for an interactive form soon to help in collecting and analyzing these measurements.  We’ll also try to keep up with publishing your measurements, to help in identifying problem motorhomes and/or chassis.


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