Using a multimeter on headlights.

[HerNameIsRita]

I went out and bought a multimeter to test for voltage drop on my headlights, only to realize I don't even know what I'm doing.

Checking the battery was pretty easy. I set the meter to 20 and the battery read 12.34v But how do I go from the battery to the headlights? The longest lead extension I could find is 10ft which isn't long enough. Should I buy basic 12v wire and some alligator clamps to extend the length? And what should the multimeter read once I connect the red lead from the battery to the connector?

Thank you VW community for your help!!!

[skills@eurocarsplus]

read this

http://www.fme-cat.com/Docs/1519.pdf
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[Tcash]

If you are trying to determine the Voltage drop across the light. Measure it at the light.
Replace 'switch' with 'light'

Tcash

[rockerarm]

Hi. The voltage drop is the easiest test to perform and is the gold standard in automotive diagnostic's, in this area.
You will need a DVOM, which it seems you have and a long enough lead, with basic alligator clips, to go from the battery up past the front bumper a bit.
For your headlight circuit you will be performing 6 basic tests. 4 tests for the positive circuits and 2 tests for the negative circuits.
The 6 tests are performed with the headlights on, possibly with engine idling, if you wish.
Remove the headlight trim rings and remove the headlights to access the electrical plug in its rear.
You will connect your long test lead to the battery positive for the first 4 tests.
The DVOM, on its DC voltage selection, at a voltage scale above battery voltage, is good. attach the DVOM positive to your test lead.
Test 1- Attach the negative wire from DVOM to the back of the left headlight low beam terminal, yellow wire, and read/document the DVOM reading.
Test2- Attach the negative wire from the DVOM to the back of the right headlight low beam terminal, yellow wire, and read/document the DVOM reading.
Switch headlights to high beams.
Test 3- Attach the negative wire from the DVOM to the back of the right headlight high beam terminal, white wire, and read/document the DVOM reading.
Test 4- Attach the negative wire from the DVOM to the back of the left high beam terminal, white wire, and read/document the DVOM reading.

Now attach the wire from the battery positive terminal from the positive to the negative terminal.
Test 5- Attach the negative wire from the DVOM to the back of the left headlight ground terminal, brown wire, and read/document the reading.
Test6- Attach the negative wire from the DVOM to the back of the right headlight ground terminal, brown wire, and read/document the reading.

Turn off engine/lights, etc and turn off DVOM.
You should now have 6 voltage readings. Obviously, the lower the better. I have seen 2v plus on this test the owner was concerned his H4 headlights weren't bright.
Hope this helps, Bill.

[sestino]

Here is how I measured the volts at the headlamp socket with the motor off:

And with the motor on:

These voltages are when I am using a plug and play headlight relay loom that I purchased off of Ebay. The voltage increased by about 1 volt as compared to the voltage through the original sockets. The power from the new loom is from an auxiliary fuse box that is fed from the Starter post with 10 gauge wire.

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1977 Fuel-Injected Sage Green Westy

[telford dorr]

[HerNameIsRita]

[Tcash]

Just to clarify.
Voltage drop and No Load Voltage readings. Are two different measurements.
Voltage drop would be like the energy that the light is burning.
No load Voltage is performing no work so other than the resistance in the wire, you should see close to battery voltage.
Make sense?
Tcash

[rockerarm]

[telford dorr]

Expanded explanation: the input impedance of a meter (what the outside world sees when you connect a meter to something) is somewhere around 10 megohms (10,000,000 ohms). Thus, any typical resistance you might put in series with the meter (e.g. poor probe contact, upstream switch resistance, poor fuse contact, etc.) is generally insignificant.

For example, suppose you have 1,000 ohms in series with the meter probe. This will form a voltage divider with the meter input impedance:

Vmeter = Vsource x (10,000,000 / (1,000 + 10,000,000))

Vmeter = Vsouece x (10,000,000 / 10,001,000)

Vmeter = Vsource x 0.99990

Thus, your reading will be low by 0.01%

In short, the meter presents no load to the system being measured. In the example above, without the bulb in place, you should read the same voltage as at the switch. With the bulb in place, your measurement will include all switch, relay, and wiring losses.
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
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Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[SGKent]

rockerarm - resistance won't show up in any meaningful way with that meter at no load. Measure the voltage with the bulbs plugged in. That will tell you what the resistance is between the battery -> ignition switch -> headlight switch -> the headlights -> and back to ground then to the battery again.

It is well worth your while to ask Telford lots of questions if you would like to learn about using your meter. There are very few mechanics who know how to use one to locate problems. Every year we have many people here whose solution to electrical issues might be asking about replacing a wiring harness, when in fact their multi-meter might reveal that only one connection is corroded.

[rockerarm]

Steve, Hi. What meter? That's not my meter. I have spent time here explaining the voltage drop/loss test by using the easily available DVOM's. It was I that explained the 6 separate tests needed to determine if a voltage drop/loss exists in the OP's initial question,. I fully recognize and stated the tests needed to be performed with the lights on, or with any circuit the component being tested needed to be on. The DVOM is in parallel on the light circuit.

It's kinda like going to the doctor and stating "I'm outa breath, cant walk up stairs", etc. He does the basics, such as blood pressure and listening to ones chest, but makes a referral for a treadmill test with all the wires hooked up. The system will now test the patient under a load, or while operating, to diagnose the concern.

The voltage checks at the headlight plug is a failed method! The only value for that is if one headlight was not lit.

Maybe I'm not explaining myself correctly and/or in an understanding method. Please respond.
Hope this helps, Bill.