Headlight Circuit Evolution | MOTOR

In sealed-beam headlamps, which have been part of the automotive landscape for decades, the filament(s), reflector, housing and lens are incorporated into a single assembly. Early sealed-beams were round and incorporated both the high-beam and low-beam filaments, so vehicles needed only two such headlamps. Older techs will recall that the high/low-beam switch (dimmer switch) was on the upper left corner of the floorboard, near where the driver’s left foot rested. The high-beam and low-beam headlamp current passed through this switch and, given its location, was prone to trouble. I recall replacing quite a few floorboard-mounted switches, especially during the snow/road-salt season. Today we have steering-column-mounted dimmer switches. I’ve replaced a number of these as well. The more things change…

Sealed-beams evolved into quad headlamps on some vehicles as U.S. vehicle lighting regulations changed. In this arrangement, typically, the outer lamps are dual-filament (high/low-beam) while the inner lamps are high-beam only. When the high-beams are switched on, the low-beam filaments go out. I have two vehicles with this headlamp arrangement, and I must say that the four high-beams provide darn good illumination on rural roads, especially if the inner, high-beam-only lamps are aimed a bit high. A great advantage of four-headlamp systems is that they typically provide independent aiming of the four lamps. Quad headlamps were also sometimes arranged vertically and canted.


Sealed-beams evolved into rectangular lamps, both dual and quad, again with changing U.S. lighting regulations. While current and less recent vehicles have replaceable-bulb (composite), HID or LED headlamps, you may still see vehicles with old-style sealed-beams. As we shall see, the circuitry for sealed-beam and composite headlamps is similar, although modern composite-headlamp circuits can be considerably more complicated, especially if daytime running lamps are also on the vehicle.

Headlamp control has also evolved over the years. Fig. 1 above shows a quad sealed-beam headlamp circuit for a U.S.-spec passenger car from about 60 years ago. Note that there are no fuses, relays or control modules in the circuit, and the high/low-beam switch is located on the floorboard. Also, the headlamps work regardless of ignition switch position. The dimmer switch selects either high or low beams, but not both. Overcurrent protection is provided by a circuit breaker, integral to the headlamp switch, which supplies current to all four headlamps. So, in an overcurrent situation, when the circuit breaker opens, all four headlamps go out and stay out until the circuit breaker cools off and resets. This can take several seconds! Having experienced this, I can tell you that there had to be a better way to provide headlamp overcurrent protection!

Headlight Circuit Evolution | MOTOR

Looking at a 1977 copy of Motor Auto Engines and Electrical Systems, I see a headlamp circuit similar to that in Fig. 1, except the headlamps and all the parking lamps/taillamps are on a single fuse. So in the case of an overcurrent situation where the fuse blows, almost every exterior lamp goes out and stays out! This is not a better way to control headlamps.

Headlight Circuit Evolution | MOTOR

Fig. 2 on page 36 is a simplified representation of a more modern headlamp-control circuit from a late-2000s Asian vehicle with composite headlamps (separate low-beam and high-beam bulbs) and daytime running lamps. This circuit uses four relays, and the left and right headlamps (both the low-beam and high-beam bulbs) are individually fused. In the event of an overcurrent situation on either headlamp, both the high-beam and low-beam bulbs will go out if the fuse blows, but the other headlamp will not be affected. This is certainly better than the circuit in Fig. 1! Referring to Fig. 2, note the following (for the sake of discussion, I omitted the flash-to-pass portion of the circuit):

  • The heavy lines denote the bulb current paths. Note that no bulb current passes through the headlamp and dimmer switches. All bulb current is relay-switched.
  • The daytime running lamp function is provided by operating the low-beam bulbs at a reduced voltage courtesy of the daytime running lamp relay and a resistor.
  • When the headlamps are switched on low-beam, the daytime running lamp relay coil is energized and the relay cuts the voltage-dropping resistor out of the circuit, thereby operating the low-beam bulbs at full voltage.
  • If the voltage-dropping resistor burns out (or someone unplugs it), the daytime running lamp function will be lost but the low-beams will still work. Some vehicles use a separate resistor for each low-beam.
  • If fuse F4 supplying control (coil) voltage to the high-beam and the daytime running lamp relay blows, the headlamps will remain in daytime running lamp mode but the low-beam and high-beam functions will not work.
  • The headlamp relays, when energized, supply power to both the high-beam and low-beam bulbs in each headlamp. The ground for the headlamp-relay coils is provided by the body control module (BCM), so to have any headlamp operation, the BCM must provide this ground. So even though we have two headlamp relays and separately fused headlamps, the BCM can still cause loss of all headlamp function.
  • The BCM also has inputs from the parking brake switch and fuse F4 (ignition in RUN). With the ignition in RUN and the parking brake applied, the BCM removes the ground on the headlamp relay coils and the daytime running lamps go out. Similarly, if the headlamps are on when the ignition is switched to OFF, the BCM again removes the ground from the headlamp relays and the headlamps go out. Some BCMs will, upon sensing that the ignition has been turned off while the headlamps are on, will delay the headlamps turning off for a predetermined time.
  • Note that the high-beam bulbs are both grounded at G3, while the low-beam bulbs are grounded at G2. So if either ground is bad, the high-beam or low-beam will be lost or the lamps will be dim. Remember this when we look at the problem-child Jeep discussed next, which has a common ground for the high- and low-beams on each headlamp—a different grounding arrangement.
  • No serial-bus communication is needed for the headlamps to work. This is not the case for all vehicles; on some, the BCM input for the parking brake comes from the instrument cluster via bus communication.
  • When the low-beams are on and the headlamp switch is moved to the high-beam position, the low-beams stay on.

Fig. 3 on page 38 is a representation of the headlamp circuit in our problem vehicle, a Jeep Cherokee with dual halogen sealed-beam headlamps. Note the following:

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  • The four headlamp filaments are individually fused and wired. In the event of an overcurrent situation in one of the four filament circuits, the appropriate fuse will blow. In this arrangement, it’s unlikely that all headlamp function will be lost simultaneously. A blown fuse will result in the loss of either the high-beam or low-beam on one headlamp only.
  • The two headlamps are grounded separately, at G1 and G2, but both filaments of each headlamp share a ground. Keep this in mind when we get to the diagnosis.
  • The bulb current passes through the column-mounted headlamp switch.
  • The headlamp switch will provide either high-beam or low-beam operation, but not both at the same time. This is typical of dual-filament headlamps.
  • There are no relays or modules in the circuit, and the headlamps work regardless of the position of the ignition switch.

The owner’s complaint was that “I can’t see well at night, and I just replaced the left-hand headlamp, but the high-beam still doesn’t work.” I asked the owner what he meant by “left-hand headlamp,” and the answer was “the driver side.” This is a very important question to ask when interviewing a vehicle owner, as some will think the left side is the passenger side, as it would be if you were facing the front of the vehicle. I avoid using “right” and “left,” instead using “driver-side” and “passenger-side” when talking with vehicle owners.

In the shop, I switched on the headlamps and moved the switch through the low-beam and high-beam positions. The following is what I observed as part of the complaint verification:

Left Side Out Bright
Right Side Bright Dim

The left headlamp appeared to be new. Given the schematic in Fig. 3, what might we conclude prior to breaking out the DVOM and before disturbing anything?

  • Since both high-beams are powered from the same pole of the headlamp switch and the left high-beam is out while the right is bright, the problem is probably not the headlamp switch.
  • Since both low-beams are powered by the same pole of the headlamp switch and the left low-beam is bright while the right is dim, the problem is probably not in the headlamp switch.
  • Since the headlamps are individually grounded and each headlamp has one bright filament, the problems are not likely to be on the ground side of the circuit.
  • A likely cause of the new left-side high-beam not working is fuse F3, which supplies only the left-side high-beam filament.
  • The dim right-side low-beam could be caused by excessive voltage drop between points A and B.

Okay, let’s break out the DVOM and do some testing, prior to disturbing anything in the problem circuit. The first thing I did was connect my meter negative lead to the negative battery terminal and probe the positive terminal with the DVOM’s positive lead. The meter showed 12.6V, suggesting a fully charged battery. Perhaps more importantly, it verified that my meter was working and that I had continuity in both leads. I began performing this very basic check after recently struggling to diagnose an electric fan circuit where my meter was reading 0V on what I thought was a known-good point. It turned out I had a bad meter lead!

Headlight Circuit Evolution | MOTOR

Looking first at the left-side high-beam and leaving the light switch in the high-beam position, I probed point D in Fig. 4 (page 39), leaving the DVOM negative lead clamped on the battery negative terminal. It’s hard to see in the diagram, but I was able to probe directly at the high-beam terminal on the headlamp. On some composite headlamps, it’s very difficult or impossible to probe this point without disconnecting things, so you may have to compromise and backprobe the bulb connector at point E. Of course, probing point E will not catch a high resistance or open circuit between points D and E.

I measured full battery voltage at point D. Had I measured no or low voltage, I would have moved my probe to points E and G, and so forth until I found the problem. The owner’s new headlamp did not work on high-beam, yet it worked fine on low-beam, suggesting that ground G1 was good (both filaments share G1). To double-check, I moved my meter probe to point F and switched back to low-beam, ensuring that the low-beam was back on. Measuring negligible voltage at point F with the low-beam shining brightly, I confirmed G1 to be good and concluded that the owner’s new sealed-beam headlamp was defective. I installed a new headlamp on the left side, and both the high-beam and low-beam worked. I had already established that the left-headlamp ground was good, so I moved on to the dim right-side low-beam problem.

Probing point B showed full battery voltage with the low-beams on so, again, I moved the probe to point H to verify ground G2. I measured negligible voltage at H, so I concluded there was some kind of internal problem with the low-beam filament within the right-side headlamp. Strange that the high-beam was seemingly unaffected. A new right-side headlamp restored proper low-beam brilliance, but I was not quite ready to ship the vehicle.

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It bothered me that the owner’s new left-side headlamp had not been working on high-beam. I was unable to learn from the owner if the new headlamp had worked on high-beam when it was first installed or if it was bad from the start. So I did two final checks.

With the engine running and having left the DVOM negative lead clamped on the battery negative terminal, I reprobed the battery positive terminal, measuring about 14.1V with the low-beams on. Although there could have been an intermittent high-system-voltage problem, system voltage was okay when I checked it. However, an intermittent high system voltage would more likely have affected the low-beams, which are on more than the high-beams. There had been a problem with the right-hand low-beam, after all.

I also did a quick check of the left-headlamp housing mounting screws to make sure the housing was securely affixed to the vehicle and not bouncing around on rough roads. Again, a problem here would more likely affect the low-beams, but I checked anyway.

Headlight Circuit Evolution | MOTOR

The Jeep has been back on the road for some months now, and the owner reports that the headlamps are working properly. So I conclude that the problem was two bad sealed-beam units.

I hope you’ll take some time to further compare the headlamp circuits shown in Figs. 2 and 3. How would you have approached the Cherokee’s headlamp problems if the symptoms had been the same but the vehicle had a headlamp circuit like Fig. 2’s? Imagine trying to troubleshoot headlamp problems on a vehicle like Fig. 2’s (definitely not your father’s headlamp circuit!) without a schematic! This is yet another example of where the schematic is your best friend. Even with the Jeep’s relatively simple circuit, with one glance at the schematic I knew about the four headlamp fuses and that there were no relays in the circuit. [external_footer]