One dim headlight

[Brohate]

'70 GT.... Right side headlight is dim. Checking voltage, and not excactly sure which it is, but when the high beam switch is on or off I lose voltage to either the double white connector or the single yellow wire connector at the relay. Left side headlight is okay at all times (high or low). Checking voltage at the right headlamp connector depending on high or low beam, I get voltage to either the white or yellow wire, but the headlight remains dim. Pulled the relay apart and cleaned the contacts but something seems askew with high/low beam switch. One last point, theres a black wire with a connector coming from the steering column thats not connected to anything and just hanging. I'm no electrician and trying like heck to figure it out, but most of the posts I've read (and re-read many times) here confuse the heck outta me. HELP!

[GoinManta]

Check the headlight asembly for rust.. sounds like a poor ground. The headlights usually ground through them or a wire running from the plug to a screw near the headlight..

Charles

[guyopel]

Also check the headlight connector at the bulb .. the Opel connectors use a spring to make contact and rust up and loose tension at the terminal . I replaced both of mine with the newer design connectors ( Most Auto. Parts Stores have these in stock app. $3.00 ) HTH
John

[Brohate]

Could be either or both. Will try a new connector. The ground wire runs from the connector into the harness, where does it exit? I found the bunch of ground wires on the front left ahead of the radiator, but the main harness (which is very rigid) blocks to see where they go to. Any clues?

[namba209]

According to my DESTEC schematic, there's only one Black wire in the steering column harness, and it is the RUN wire that powers all switched circuits. Look at the White connector and see if you have a Black wire in the #1 position, which would be top left. All the grounds for the front circuits ground outboard of the MC in front of the radiator support. Check all those brown wires for cleanliness and corrosion and make sure the connections are tight. HTH.

[Brohate]

The black wire from the steering column is rather short and has a "O" connector at it's end. It looks like it should be connected to a bolt which holds the column in place(?). The inside diameter of the connectore sppears to be the right size to do so. The last time I connected a black wire to where it wasn't suppose to be resulted in a small "meltdown." I was wondering if could be related to my headlight/dimmer switch problem I mentioned earlier? Thanks for the post!

[namba209]

That could be the main ground wire for the steering column, that is also the ground for the horn button, originally it is a brown wire though.

[wrench459]

Originally Posted by Brohate Checking voltage at the right headlamp connector depending on high or low beam, I get voltage to either the white or yellow wire, but the headlight remains dim.

One small strand on the ground side will allow the proper voltage,But will not handle the current load.

[Brohate]

Horn works, so what else could that danged black wire on the column be?

The other ground wires (brown) are all bolted(?) under the wire harness in the front right? The harness is tightly secured by clamps and sealant making it very difficult to move in order to inspect and/or repair the ground connections. I'll do what I have to do, but before so want to make sure I'm in the right place. This drivin me nuts! It's the only light on the entire car that doesn't work properly, except some of the dash lights which is a whole different project!

[namba209]

Just for grins and giggles, and maybe a bit of troubleshooting. How about swapping the headlights just to see if the lamps are O.K., or if in fact it is a wiring problem.

[wrench459]

Originally Posted by namba209 Just for grins and giggles, and maybe a bit of troubleshooting. How about swapping the headlights just to see if the lamps are O.K., or if in fact it is a wiring problem.

Good point Ron!
What kind of test instrument are you using?
The headlamps ground in a different location than the horns.

[namba209]

Originally Posted by wrench459 The headlamps ground in a different location than the horns.

He was asking about a short Black wire at the steering column connectors with a eye terminal on the end. I suggested it may be the main ground for the steering column which also grounds the horns, when you push on the horn button. IIRC the grounds for the front harness are all outboard of the MC and in front of the radiator support, including the headlight grounds. My DESTEC schematic only shows the headlight grounds as being short and attached to the body, but I'm not sure that's where the grounds are. I do know the headlight grounds are 12 gauge and Brown, so if there are one or two Brown wires at the location I specified, those would the ones to check.

[Brohate]

Bulb switch it is, just for S & G's. Originally I thought it was the bulb and bought new, but one never knows. As for the grounds in front of the MC, there's LOTS of 'em up there! Checkin those are on the list. I'm using a volt/ohm meter. So as a new day begins, as does the snow here in the N'east, it's back outside for another day leisure time activity!!! Thanks for your help on this!

[wrench459]

As I recall both the left and right headlamps ground at the same point. Left of the master cylinder. Could you back probe the headlamp connector(lights on)on all three wires and give the readings?
Very important . The neg. probe from the meter needs to be placed on the battery center terminal.

[Dennis Texas]

Originally Posted by wrench459 As I recall both the left and right headlamps ground at the same point. Left of the master cylinder. .

I just got into that harness as I'm upgrading my headlights in my little 72 project they both use the same ground point it is just to the left of where the master cylinder is.
There are a lot of obvious issues here to look at but I'm sure you have looked or checked by now . If it is dim all the time it may also be wired incorrectly ..make sure the ground lug on headlight plug is the same on both headlights if it's not the light will be dim because you have both filaments trying to turn on, other than that if one headlight works they both should headlights are wired together in the harness right in front of the master cylinder originally it your wiring has been changed then that lets other possibilities enter.

[Brohate]

Wrench, maybe I'm missing something but "the battery center terminal?" I assume you mean the on the battery, negative terminal. If my assumtion is correct, may I ask why on the battery and not on the connector at the ground connection? Is this to insure the ground connection of the headlight isn't bad? I do have to say that after cleaning the grounding wire connections near the master cylinder, the voltage at the headlight connector increased but the headlight remains dim. Leads me to believe the connector isn't getting a good connection to the headlight. Though it's a new headlight and I get get good voltage readings at the connector. Could the voltage meter be getting a better connection than the headlight? Still need to switch the headlights around to check the bulb itself, despite it being brand new and should work correctly. I will leave no stone unturned until the challenge is met!!!! As for the last post, I am only the second owner after buying the car in '76. The wiring hasn't been tinkered with other than me, but the car sat for the past 20 + years without running. All worked fine then. The wife has been left, but not the GT (aka: Bessy Lou)!!!

[Jeepsbeme]

Wrench459 is absolutely dead on with his diagnostic recommendation. Checking the voltage drop on the LOADED circuit is the best possible electrical diagnostic test. Please note the headlamp must be plugged in and the headlamp circuit must be powered for this test!!!

1) Put the red lead of your DVOM at the center of the positive battery post (this is just a good practice, but in your case with only one dim lamp you could probably go to the battery terminal itself) and put the black lead on the powered terminal of the headlamp connector. The voltage reading on your DVOM should be no more than 1/2 volt.

2) Next put the red lead of your DVOM on the ground terminal of the headlamp connector, and the black lead on the center of the negative battery post. Once again - the voltmeter should read no more than 1/2 volt.

Instead of measuring the available voltage, you're now performing a "voltage drop test" which shows how much voltage you're losing over the loaded circuit. To put things in perspective, image the heavy battery cable going to the starter motor and cut every strand of the cable but one. Using a DVOM in the conventional manner, this single strand of wire will still show that you have 12 volts available to the starter although it obviously could never carry enough current for the starter to function.

[Brohate]

Fighting the onset of darkness again, I did as suggested. Head lights powered, dim as usual, and got NO reading either way. We are talking a voltage reading as opposed to a ohm reading, right? Am I stupid and not getting these suggestions right? Tried recleaning (just to be sure) the ground wires. No improvement. aargh4: Is there a trick to cleaning the ground wires? Wire brushed and sanded 'em all. This is drivn me nuts!

[Jeepsbeme]

Brohate, you definitely want to measure the voltage. You never want to measure ohms in a live circuit.

Check out this link: Starter Voltage Drop Test

Now just envision your headlamp bulb in place of the starter motor that's shown. It's exactly the same concept.

Of course if your getting frustrated by the entire problem, walk away from it for a few days. Or have a friend help out for a "fresh eyes" approach. It's all to easy to miss the obvious when you're struggling. We've all been there ourselves!!

[Brohate]

Thanks for the info/link and encouragement! I'll step back and go it again with a clear mind.

[markandson]

There is another possibility here that no one has mentioned. One of the two relays in the headlight system could have a set of bad contacts inside and that would cause the problem also. I have a set of 10' long red and black leads that I made with a small battery clamp on one end and alligator clips on the other. I used them extensively when rebuilding my entire electrical system. Look at the schematic and then pull the bulb feed wires off the relay one at a time and put 12 volts on them right from the battery, if you get nice bright lights I would tear into the relays. You can also use the long negative lead to bring ground directly to those heavy brown ground wires near the MC which would eliminate some of the questions. You could also have corrosion between the wire and the ring terminals at that same grounding point which can only be fixed by cutting the old ring terminals off and soldering new ones on. It is a very simple circuit so just start at the bulb and work your way back in the wiring until you find the culprit. Leave no stone unturned, question every connection point including the ones inside the bulb connector and try to keep the circuit isolated which means taking those ground points apart and also replacing those old rusty sheet metal screws with a nice 6mm bolt right through the fender housing with a nut on the inside and then put the rings back on and put on another nut to capture the whole mess. Don't forget to clean the area down to bare metal before you put the bolt through and use washers on both sides to increase the contact area.

[namba209]

Have you tried switching the lamps yet? That would isolate or confirm the bulbs being good or bad, also would confirm a wiring problem. One suggestion, based on another post's query, check the lamp that is dim to see if both filaments are glowing. If so, then you have the dreaded insulation failure between the White and Yellow wires to that headlight, and a complete rewire of the front harness may be needed.

[wrench459]

Brohate
I know your flustered over the lack of the headlights working properly.
If you could please try to get princeable behind the voltage drop test. Believe me once you master it there is nothing on the old girl you wont be able to fix. Be it a relay,switch,alt.,starter or ground.
I hesitated to mention what I call "functional diagnostics" ie jumper wires.
Yes I've got several from a 3 inch to a 30 foot and a few lenghts in between.
One word of advice if you use a jumper....inline fuse it!!!
Another word of advice dont even try this on a computer controlled car unless your willing to buy a PCM,BCM, or any other control module acronym from A-Z.

[Dennis Texas]

Brohate, like you I just got tired of the checking (my harness was flaky all the way to the harness for drivers side light it had to be repaired anyway ) I just pulled back my wires at the harness in engine compartment to the left of the ignition coil on the top of inner fender well, one is a white and the other yellow wires look like 12 gauge stranded, I ran three new wires to each headlight (high, low beam and ground wire)also 12 gauge stranded and just rewired them into the existing harness try to solder or make a permanet connection there and you can wrap it back up and hardly tell it has been reworked.
The voltage drop method is good for making sure you have wire issues or lights but it really won't tell you whether it is wiring or relays that are the culprit.
If your old wiring is brittle it will need to be changed anyway just cut at the harness ( only the white and yellow the rest of the wiring stays) add new ground, wire in the spliced ends and then if problem persist start at the relays under dash and work your way back.
Personally I have never seen a bad headlight lamp they either work or they don't in most cases theres always an exception I guess. I would just rewire the headlights and go from there. With new wires and ground you will eliminate the wiring. (I still think yours are miswired or shorted together running both filaments instead of high or low) The reason I think that is that the drivers side light is good and it uses the same ground so it is either wires or the receptacle for light is wired wrong they both use the same 12 volts from relays as well so that is not the problem, it is something with the wiring for the passenger lamp..

[wrench459]

The voltage drop method is good for making sure you have wire issues or lights but it really won't tell you whether it is wiring or relays that are the culprit.


I humbly disagree
If you look at the above links..you'll notice that there checking the field coil and the contact points of a starter..aint that the same as a relay?