Low beam voltage too low

no, doesn’t make sense. Analog or digital voltmeter, there is no battery in the circuit. The voltmeter is an entirely passive instrument, it’s only effect is that it loads the circuit slightly with it’s impedance (resistance), which is >20kΩ for the analog meter and usually 10MΩ for the digital.

Someone is getting confused with an ohmmeter which does put a battery into the circuit.


no, doesn’t make sense. Analog or digital voltmeter, there is no battery in the circuit. The voltmeter is an entirely passive instrument, it’s only effect is that it loads the circuit slightly with it’s impedance (resistance), which is >20kΩ for the analog meter and usually 10MΩ for the digital.

I agree here completely. My comments regarding 9 volts across a sensor are in regard to testing resistance only, using the meter as an ohm meter. Current is limited, as can be readily seen by touching the two meter leads together. No sparks result, but there is obviously still a current / voltage present.

I believe you are incorrect with your voltage. It does not send out 9V on any ohm setting. It’s more in the neighborhood of .29 volts (that’s point 29V). I just tested it with two multimeters.

Modern meters use a fraction of a volt for the ohmmeter; and ~1.5V for the diode check function.
The old SImpson 260 analog meter, very popular when I was repairing TV’s in the '70s, used 1.5V for the lower ohmmeter ranges and a 30V for the highest ohm range.
This could cause problems when older techs started working on transistorized equipment.

Ok, so I replaced the ground wire. Still no light.

I went back to original ground and tapped the positive line of the working passenger light and the headlights work fine now.

BTW, digital multimeter & lots of irrelevant discussion on the thread…


What did you expect from a bunch of old car nerds that don’t have a life, but spend it on this forum.:grin:


I think your daughter will like the PT Cruiser. Something a little different is always a good look for a college student.

If the bulb is getting 11 volts it should still light up, might just be a little dim is all. You say it wont’ light up at all? It isn’t an LED bulb is it?

Suggest to measure the voltage at the bulb connector with it plugged in. You may need to back-probe the connector or pierce the wire insulation with a sharp pin. Try this with the meter ground onn the battery - post, and again with the meter ground on a chassis ground near the bulb. And if possible with the meter ground on the actual bulb ground in the connector. The reason to go to this trouble, you need to rule out a bad connection at the bulb connector, or that the other side of the bulb filament isn’t properly grounded to the chassis for some reason.

My guess, the bulb connector is heat-fried or corroded.

Note: When cleaning electrical connectors, cutting or modifying wiring, or in general working with the electrical system or electrical components at all, always a good idea to disconnect the battery negative connector first. It can save you a lot of grief later, should you accidentally short something out.

Trailer lights always seem to be a problem for me, In preparation for dragging the boat out today, tried emergency flashers, for first check. Left side fine right side nothing, Pull the bulb out, solder thing broken and dangling, new bulb, right side good left side bad, found a wire with maybe 3 strands left, patched in a new piece, now both work. I Have had more trouble with trailer lights than anything else that should be so simple. Keep looking!

1 Like

The lack of power to the bulb suggests that there is a failure in the Totally Integrated Power Module as mentioned early on. Having two headlights on one power module driver will likely shorten the life of the module.

This might not be proper for this site but here goes.
I have a Chevy 5500. Headlights are dim. I checked ground and not the issue. I actually ran a jumper wire to new ground to test. I checked voltage and get 11.7 volts. I ran a wire from battery which read 14 volts while truck running. Jumper wire to both low and high beam brightened the lights. I replaced headlights with new ultra bright Still dim. What is the issue? Headlights are so dim I installed HID drive lights but on dim with HID off it is like really dark. We run two of these trucks and both seem to have same issue.

You will get more responses starting a new thread of your own instead of piggybacking on a 2 year old thread.

Since the voltage is 14 volts at the battery but only 11.7 volts at the headlight, there’s obviously some resistance in between the battery and the headlight. The current to power a headlight on low beam I’d guess is about 10 amps. Since you got a 14- 11.7 volt = 2.3 volt drop, that means there’s 2.3/10 = 0.23 ohms resistance somewhere between the battery and the headlight. Electrical connections like that would rarely have more the 1/10 ohm, and generally more like 1/20 ohm. So you’ve got a corroded connection at some point between the battery and the headlight. Usually the problem is the headlight connector itself, b/c of its location at the front of the vehicle, where there’s water spray from the car ahead hitting that area. That connector can pr,bably be replaced, you buy something called a “pigtail”.

Thanks for reply. I am going to start checking connections back. Starting with pigtail. I did think 11.7 volts was standard voltage to headlights and they should be bright. Obviously losing voltage somewhere. I met this truck at night in line of vehicles. The headlights were very dim compared to others which I already suspected but that confirmed. 2008 Chevrolet 5500. Considering we have another truck with same problem I thought this might be a common issue that somebody knew about.

If I don’t figure this out, I may start new post to see if I get more response. Thanks!

Let’s clarify something here, please . . .

Are you referring to an older Chevy C5500 Topkick/Kodiak. . . in other words a class 5 truck?


We have lots of those in our fleet, and they all have sealed beams, which provide adequate lighting, but no more than that, in my opinion

Yep! C5500 truck w/single sealed beams. I would say lights are half bright as should be. 1 is fire truck and 1 ambulance. If I didn’t have aux drive lights on them, I don’t think we could drive them safely.

Are you familiar with the terminology “voltage drop test” . . . ?

If so, you need to perform a voltage drop test to determine where you’re losing available voltage, as you already stated

for all I know, you might have significant voltage drop through a connector . . . in other words, 14V on one side of a connector, but significantly less on the other side

here’s a silly question . . . you are actually turning the headlight switch to the on position, not just relying on the drl to do its thing. I specifically mention drl because, as far as I’m aware, this truck never had auto headlights, just drl. I’ve seen situations where a guy would start the truck with the headlight switch in the off position, the headlights would come on dimly . . . until he actually turned the headlight switch to the on position. On some vehicles, drl alone doesn’t provide full voltage to the headlights

I’ll try to help as much as I can . . .

go to gmupfitter.com

now go to body builder manual

on the right side of the screen, choose medium/heavy duty vehicles, specifically, “click here for model year selection”

Once you’ve done that, you have more choices

Choose “electrical medium duty C-series” . . . it’s a pdf

It’s not perfect, but it has component locations, pinouts, schematics, and so forth

maybe it’ll give you an idea where you need to look for that “missing voltage”

another thing . . . exactly how are you measuring available voltage to the sealed beam?

Directly at the sealed beam 3-pin connector, or somewhere further back, teed in, for example?

Are you sure the sealed beam ground is okay?

Have you hooked up your meter positive to the positive terminal for the sealed beam, and hooked up the meter negative to another ground, for example battery negative post?

If you now see 14V at idle, then your sealed beam ground has a problem

Please forgive me if you’ve already done all this or if it sounds as if I’m talking down to you. I’m just throwing some ideas out there

No need to worry. Any info I get is a plus because this just doesn’t make a lot of sense to me.
Checked that website. Thank you. I will dig into it.
I am using multi-meter. Checking right at headlight prongs.
I ran a separate ground to frame to test the quality of ground using a jumper wire. No difference.
Lights do come on “dri” when light switch is in off position. But I am turning them fully on to test. Although I didn’t test voltage in the switch “off’ position. You have me curious now because, yes they seem dimmer on “dri” by my memory.
Again, thanks! I appreciate all input. It’s like going to fire training. You might have done it a million times but you nearly always take away something new or hadn’t thought about.
Since we are volunteer fire/rescue I don’t spend all my time working on the trucks. Just a lot. I will get back to it probably tomorrow.

1 Like

The brightness of the headlight is proportional to the electrical power delivered to it. The difference in electrical power comparing 14 volts to 11.7, since power goes as the square of voltage, is 11.7^2/ 14^2 = 70%. So you are losing 30% of the available power as resistance-heating somewhere between the battery and the light bulb. That makes the headlight appear 30% dimmer than it should.