Stumped By Headlight Problem - Help!
 

Something is wrong w/ my headlights that I am a loss to explain.

I have euro H4 conversions w/ Sucro relay kit. The driver's side headlight went out. I received a new 80/100w bulb from Pelican.

Removed the driver's side headlight assembly, disconnected (unplugged) the old bulb from the plastic connector, hmm bulb looks fine.

Plugged in the new bulb in the plastic connector, no light either low or high beams. Plugged the old bulb back in, no light.

Unplugged the bulb and used my VOM probes on the plastic connector plug, I get 11.7v from the center female spade terminal to either of the side female spade terminals. So I do have voltage as far as the female terminals in the plastic connector.

Try plugging in the bulb only half-way and touching my probes to the exposed part of the male spade terminals on the bulb - that are half-way inserted into the female terminals that I just got voltage on. No voltage on the male terminals!. Either the old or new bulbs.

WTF? I check. Terminals in the connector look clean. I get some extra male spade terminals, insert them in the female terminals in the connector, touch probes to the exposed ends of the male terminals, 11.7v. So the female terminals in the connector are clean.

I try plugging the bulb half-way in again. As best I can tell by feel, the male terminals are indeed inserted in the female terminals, but still no voltage on the exposed part of the male terminals.

Now I try fully plugging the bulb into the connecter, and touching probes to the exposed part of the female spade terminals that you can see on the backside of the connector. No voltage on the female teminals!

So (1) I have voltage across the connector's terminals when there is no bulb plugged in, but (2) I have no voltage across those terminals when a bulb is plugged in.

Repeat all of above, same result.

Remove the headlight assembly from the passenger side, unplug the existing bulb, plug in the old and new bulbs, they light fine.

What in the world is going on?

Is there something going on w/ the relays, like they can supply voltage with no current but once there is appreciable current the relay fails? I haven't heard of that failure mode before. I can go get a replacement relay and try that, hopefully it is a standard autopart store item.

One other thing - looks like I never hooked up the small "city light" bulb in the H4. When I (you guys) figure out the stump-the-chump problem, and I actually have two working headlights, seems I might as well get that city light hooked up. How do you do it - can I simply connect the male spade terminal on the city light bulb to the side marker light circuit?

Check the voltage and connections at your headlight relays. Try swaping relays.

Solved - thank you Pete. I unplugged and replugged the relays and the problem went away. I don't understand why - but oh well.

Corrosion on the relay terminals....

Jeez, going around the car I'm finding all kinds of light-related things.

Driver's side fog lamp bulb out - replaced.

Passenger's side taillight bulb out - going to get.

Both taillight assemblies very dirty on the inside - cleaned.

License plate light out, the light assemblies crumbled as I removed them, age or a rear impact? - ordered replacements.

The PO of this car appears to have accumulated some deferred maintenance. Wait, that's me.

Yup. The "city light" and the "side marker" circuit is the same, it terminates in the headlight bucket on the male spade of the city light bulb for the Euro cars (because Euro flashers only flash and aren't running lights as well, so don't need that circuit down to the bumper).

The US cars run the harness feed for each of the 2 front running lights per side to the same area and connect there instead.

If you want both, just make up two small Y-harnesses, one for each side, with two female spades (city/side lights) and one male spade (to connect to the feed from the chassis harness) and connect them all.

you were getting the voltage, but not the current.
if you had used a test light that would have showed a probelm(for those that dont believe in useing test lights). the bulb in a test light is a much lower wattage, it may have been on very dim letting you know there was a problem instead of the MM indicating there was not one.

when you measured across the 2 male plugs, with the bulb part way in, you should not read any (maybe a volt) voltage. its kinda like putting your test leads across a good fuse, there is no voltage drop across a short.

What you have or had...was a resistance connection.
The connection will show voltage when there is little or no current.
But, when current tries to flow through...the resistance of the joint will show as a very high value resistor.
This will drop the current flow to almost nothing.
I have said it many times....CLEANLINESS !!!
If there is any dirt or corrosion in a joint or connector....you will have trouble... either right away...or on that lonely back road 50 miles from nowhere...with no cell towers.
People....clean grounds....clean connections....maintainance!!!
Bob