I want to measure the voltage of a headlight while it is connected to a vehicle and running. I wanted to know if there was a device or technique I could use to do this. I'm looking to take the measurements with the headlight connected and intact and want to measure the voltage at the bulb.

 

Yes it's called a Voltmeter, or Dvm...

 

Yes it's called a Voltmeter, or Dvm...


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Yes but my problem is I need to measure but don't have bare wire to measure from. I wanted to know if there was a non invasive way to measure from the housing or something.

 

You would have to jump through some tight hoops to measure the voltage wirelessly. I know it is not the same but why not measure battery voltage? Most vehicles have an auxiliary outlet for a simple wired connection.

 

I wanted to know if there was a non invasive way to measure from the housing or something.

Why? It will be essentially the same as the voltage at the battery.

 

Yes but my problem is I need to measure but don't have bare wire to measure from. I wanted to know if there was a non invasive way to measure from the housing or something.

The headlight voltage will be the same as the battery voltage, unless:

1. You are using undersized wires supplying the headlight.
2. There is severe corrosion between the battery terminal and battery cables
3. There is significant corrosion or loose connection between ground terminal and frame

In case 2, measure at the terminal and cable. Lead oxide build-up can be cleaned if needed.

 

If the car is a modern one with LED headlights they may be PWM-controlled for dim/dip functions. If so, your bog-standard DMM will give misleading readings.

 

It would help if we knew why you want to directly measure the bulb voltage.
What are you concerned about?

 

It would help if we knew why you want to directly measure the bulb voltage.
What are you concerned about?

Why? It will be essentially the same as the voltage at the battery.

You would have to jump through some tight hoops to measure the voltage wirelessly. I know it is not the same but why not measure battery voltage? Most vehicles have an auxiliary outlet for a simple wired connection.

It has to be at the headlight terminal because it is a question about the function of the headlight in particular. We need to verify the exact voltage being delivered to the headlight and as accurately as possible.

 

Then why don't you study the characteristics of the headlight on a test bench where you will have direct access to the current, voltage, temperature, luminance, etc.?

 

I would connect a twin cable directly to the lampholder (How you connect it would depend on the lampholder.) I would run the wire back to the inside of the car to your measuring equipment. An alternative would be to have an A to D converter close to the headlight and transmit the data back to the inside of the car using Bluetooth. You could then display the readings using a tablet inside the car. If you used an INA219 for the A to D conversion that could also give you current readings.

Les.

 

Then why don't you study the characteristics of the headlight on a test bench where you will have direct access to the current, voltage, temperature, luminance, etc.?

We have we just also need the readings with it directly connected to the car.

 

We need to verify the exact voltage being delivered to the headlight and as accurately as possible.

Why? The 12V in a car is unlikely to ever be exactly 12.000V. It's going to depend on engine revs, battery condition, and the total load.

 

Why? The 12V in a car is unlikely to ever be exactly 12.000V. It's going to depend on engine revs, battery condition, and the total load.

That is exactly why we want to measure it while functioning and connected

 

That is exactly why we want to measure it while functioning and connected

When the car engine is running the voltage on the battery will be determined by the alternator, voltage regulator, and the load on the system. Most automotive charging systems will be charging the battery with a voltage somewhere between 13.8V and 14.5V.

You can replicate all of this on a test bench. What are you hoping to discover by testing the headlight on a running vehicle?

 

That is exactly why we want to measure it while functioning and connected

H4 bulbs have like spade lugs, and usually a matching connector block - I've never had much difficulty wedging DMM probes into the back of the connector housing.

As others have hinted - if its LED, you probably shouldn't mess with that bit.

I'd add that HID uses high voltage, at somewhere around 35 - 40W - I'd assume its probably a lethal dose.

Make sure you know which type you're dealing with before sticking your fingers in there.

 

Depends on the headlight. My truck uses a 9005 Lamp with a molded socket and gasket. The only way I can measure the headlight voltage at the actual headlamp would be to tap into the wires right at the headlamp socket. The next option would be to sacrifice a new lamp or have an old failed lamp and break the envelop so I have access to the terminals. Then I go to the store and buy one of these which is the headlight socket for my 9005 lamp. Then I make a short extender cable using my headlight plug and socket. My truck happens to use the same lamp for both High and Low beam. So you make a fixture which serves as a short extender on the factory headlight cable. I have no clue what headlight you have so this is the best I can tell you.

That or I can go and buy a mating male (headlight) and female (connector) and build the same breakout I just mentioned. Again, totally depends on your headlight system. I can only speak for my and my wife's trucks.

Ron

 

That is exactly why we want to measure it while functioning and connected

And what will you do with the measurement? It seems a pointless exercise.

 

And what will you do with the measurement? It seems a pointless exercise.

Then it would become a classic exercise in futility. While we are at it we could also measure headlight current, temperature and intensity. Then we could save all the data somewhere so we could have it to view sometime.

Ron

 

Use the Voltmeter probe and poke a hole in the wire. Repair the hole with high temp RTV sealer.