Dim headlights, new bulbs, good ground

I’ve got a 1991 F150 and the low-beam headlights are so dim driving in the city at night is a bit dangerous. Out in the country they’ll do, but in the city I can’t even tell that my lights are on.

I’ve replaced the bulbs with new Sylvania Xtra Vision (65W), and no difference. I’ve sanded/cleaned the headlight glass. And I’ve checked/cleaned the grounds, they all seem good.

So I unplugged the lights and took some volt/ohm measurements, and maybe someone here can help interpret them for me. I’ve got a good, reliable DMM, so these measurements should be good. There’s three wires to the connector and three holes in the receptacle. I’m taking measurements at the receptacle, but labeling according to what appears to be the corresponding wire.

(1) Lights Off, Truck Off
Wire Volts Ohms
Black 0.008 7.4
Red 0.008 8.7
Green 0.008 8.6

(2) Lights On, Truck Off
Wire Volts Ohms
Black 0.082 80.9
Red 0.144 142.5
Green 11.51 OL
Battery Voltage 12.17 (I left the hood open overnight, with its accompanying light, which is probably why it’s a little low.)

(3) Lights on, Truck On
Wire Volts Ohms
Black 0.052 ~45
Red 0.125 ~120
Green 14.05 OL
Battery Voltage: 14.77

I hope you’re not reading resistance with the lamp sockets powered.
At any rate none of it matters except the voltage readings with the lamps on and engine running.
Leave the lamps installed and back probe the sockets.
The voltage readings without load can be misleading. A bad connection will read good voltage without any current being drawn.
Be careful not to touch the lamps while doing the measurements.
Report back with the updated readings.

Truck on, lights on, backprobed with lamp in, voltage reads 13.27.

The bulb is pretty bright, enough that I’m seeing spots right now. And like I said, I can drive and see alright when I’m the only thing on the road. But compared to other cars on the road it’s like I barely have any light at all. So I’m probably looking for one of those annoying hard-to-find small problems.

So this is only a 1.5 volt drop from the battery to the bulb, and that’s small, but maybe enough? What do you think?

I had a similar problem with a 2003 Taurus. I cleaned and polished the headlight assemblies and upgraded the bulbs but driving at night was still difficult. I ordered new headlight assemblies and with the same bulbs night vision was much improved. My best guess is the reflective coating in the OEM assemblies had degraded over time.

Ed B.

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New headlight assemblies made an enormous improvement in the lighting of my 1999 Honda and 2007 Town and Country. And they make the car look much better, too. Their effect on a 1991 vehicle would probably be quite remarkable.

In my estimation, that is pretty normal voltage at the connector. I do not believe you have an issue with power to the lamp. Two things- as mentioned, the headlamp housing has degraded some and also probably your vision some as well. It happens slowly so may not be noticeable. Of course, brand new vehicles have pristine headlamp assys AND they are far brighter than what came as OEM equipment on your '91. The best you could do is to replace the lamp assys and use the highest power lamps available. I believe that will still be a lot less light than a newer car is emitting…unfortunately.

"Two things- as mentioned, the headlamp housing has degraded some and also probably your vision some as well. It happens slowly so may not be noticeable. "

hey now! I may sound like an old geezer but I’m not! ;p

I did consider these older cars might just be not as bright, but I couldn’t think why. Same voltage, amperage, and bulbs between this and a new car. Of course, the headlight assembly might not be as clear or reflective. And I’ll buy that.

It sounds like the issue is probably the housing, and that’s a pretty easy fix. Just gotta get in the spirit to spend the money.

Many new cars have HID or LED lights. Big difference from halogen lamps.

If you are getting 13.5 volts across the bulb (between the bulb filament input and bulb ground input), with the bulb installed and everything connected, that’s within spec. That’s probably as bright as the bulb is going to light. You can always remove the bulb and connect it up directly to the battery using test leads of course as a test. Careful, as it might get very hot very fast. So you are left with something is blocking the light from the bulb, or the reflector behind the bulb isn’t doing its job. Or the new bulb is faulty. Or the bulb is mis-aimed.

Has the reflector surface inside the lamp housing degraded and lost its shine?

The Lincoln Marks from the 90s were horrible about this. One can sit under a single orange street light and not be able to tell if the lamps are even on while turning them on and off.
Sad thing about the Marks is that no one manufactures an aftermarket housing and the used ones from eBay or the boneyard are just as bad. Not one of Sylvania’s better efforts…

If this is the issue with your truck I’m sure replacements are available.

I have a 1990 F-150 and my headlights would quit then start again. Then I lost power to other things and what I found was the headlight switch at some point overheated. The plastic where numerous wires went into the switch housing was melted and blackened in places. melted
There is also an inline fuse on the main voltage feed (a black with purple stripe wire) and those wires in a harness there is no schematic for in the motor manual.
If you are getting voltage is one thing and clean and tight grounds are as well. Check to be sure the ground braids from the engine to the firewall. are there too. Are your taillights dim too or go dim when headlights are activated from parking light postion, also instrument cluster lights, are they dim . Does the ammeter show good voltage at all times. Test alternator with a volt meter too. Battery terminals clean and tight, battery load test too.
But a look at the switch is needed to rule that out since it was a common problem, Sparky has a guide with pictures on how to remove it here
Electrical problems are confounding but really the answers are always simple. Dim lights are a poor ground or a drain from loose connections (such as the melting did to mine). Headlights draw amps and that is why DC wire connections need to be clean and tight. Headlight taillights dashlights should be the same circuit so they all should exhibit problems.
The worst thing is someone added a radio or something and tapped into a wire for juice.