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Headlight Voltage Weirdness
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sped372
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Joined: March 30, 2006
Posts: 653
Location: Waunakee, WI
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PostPosted: Mon Oct 08, 2012 5:49 pm    Post subject: Headlight Voltage Weirdness Reply with quote

I was driving the Ghia the other day when I noticed the bundle of wires under the dash (from the column up into the trunk) was hot! I turned off the lights and the bundle cooled back down, slowly. We rarely drive the car at night (or with the lights on) so I presume this has been going on since we've owned it. I started digging into the electrical system this evening and noticed some peculiarities. The system looks pretty much unmolested from inside the trunk. I will end up cutting open the bundle that was hot (it has a very nice electrical wrap job on it over the entire length of the bundle) but I have not gotten that far yet. I wanted to do a preliminary investigation first.

Lights all work, so do the blinkers. They have always seemed somewhat dim, but not much moreso than on the Bus or my old Beetle. Hi/Low relay works. Oil & Gen lights in the speedo are bright. Blinker indicator typically signals once, and then doesn't (the actual blinker lights continue to blink).

As I started tracing things my major observation is a voltage disparity between the front and back of the car, depending on whether the headlights are on or not. It doesn't seem correct to me.

Engine off, ignition key off, headlight switch off
Voltage directly at battery: 12.5
Voltage at + side of voltage regulator: 12.5
Voltage at 'always hot' terminal of fuse box: 12.5
Voltage at left headlight fuse: 0

Engine off, ignition key on, headlight switch off
Voltage directly at battery: 12.3
Voltage at + side of voltage regulator: 12.3
Voltage at 'always hot' terminal of fuse box: 12.3
Voltage at left headlight fuse: 0

Engine off, ignition key on, headlight switch on
Voltage directly at battery: 12.3
Voltage at + side of voltage regulator: 12.2
Voltage at 'always hot' terminal of fuse box: 9.5
Voltage at left headlight fuse: 8.7

I checked a few other points up front (at the headlight switch, at the hi/low relay, at the flasher relay) and they all get 'pulled down' whenever the headlights are on. I didn't start the engine but it didn't seem like it should make a difference since I have essentially full voltage at the battery regardless of whether the lights are on or not.

I think my next check will be to either unplug the headlights to see if eliminating the draw allows the voltage to 'stay up' or else dig into the bundle under the column. Probably the headlights first since it doesn't require surgery.

If anyone has experienced this or has any thoughts I'd love to hear them.
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1971 Karmann Ghia - 1600 DP
1984 Westfalia - 1.9 WBX
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e&m_ghia
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Joined: April 04, 2011
Posts: 418
Location: NoVA
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PostPosted: Mon Oct 08, 2012 8:00 pm    Post subject: Reply with quote

When I was sorting out the wiring of our newly-bought 73 Ghia last year, I traced out all the circuits before connecting them. (PO did the cable harness, but a lot of it was connected incorrectly, but mostly not connected at all.) I was deeply suspicious of the resistance drop in the ignition switch & cable harness (seemed like it might heat up too much), so I ended up wiring the headlights through some extra relays, and only ran the low-power stuff & relay coils through the ignition switch. From the "front end" of the car, only the headlights should consume a lot of power.

I didn't record the readings that made me suspicious.

What you describe could be an impedance drop, or an excess load. You might want to check the connections (clean & solid) & resistances, to see if there's a drop to the "always hot" fuses. Sounds more like an excess load, though. Maybe check if the fuses are all nominal ratings, if the situation has persisted since you got the car. Maybe also unplug the lamps and check resistance to ground on the circuits, see if there are any shorts in the headlight or tail-light circuits, or any of the other lamps that run out of those circuits. A lot of those wires run out independently from the spaghetti mess around the back of the trunk, so it's not actually too difficult to check.
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Volfandt
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Joined: March 15, 2012
Posts: 500
Location: Knox County, East TN
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PostPosted: Tue Oct 09, 2012 4:50 am    Post subject: Reply with quote

I'm somewhat surprised at your "Engine off, ignition key on, headlight switch on" test that shows practically no voltage drop at the battery.
Headlights are quite a load and especially when coupled w/the tail lights and parking lights(if they also light up w/your headlights) all being on.
I would expect a voltage drop. at the battery. I may test mine tonight to see if I get similar results.

My recommendation would be not to rewire anything just yet. Trace each circuit out 1st.
My 72 had a very strange electrical problem that involved my headlights & turn sigs.
The right side would work perfectly w/headlights on and off but the left side turn sigs wouldn't work w/the headlights on and the left front headlight would be dim.
Ended up the left side wasn't grounded. I never found where the brown ground wire was open but according to the schematic in the Bentley it should have shared the same ground as the right side.
I made up new separate grounds for both sides and the problem went away.
I had one other problem later as I drove the car more. The right side taillight and tag lights quit working. I couldn't find where the "hot" feed wire from the left side lights was open on it either so I ran a new feed from the left side tail lights and that also fixed the problem.
Check your grounds and each circuit individually up front to the headlights, you either have an open ground and/or one of the headlight feeds may be shorting w/another circuit in the bundle running from the fuses to the headlights.
Good luck
Dave
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1972 Karmann Ghia, the Dragon slayer....
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sped372
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Location: Waunakee, WI
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PostPosted: Tue Oct 09, 2012 5:17 am    Post subject: Reply with quote

Volfandt wrote:
I'm somewhat surprised at your "Engine off, ignition key on, headlight switch on" test that shows practically no voltage drop at the battery.

I was surprised too. It did drop, just not very much. I definitely plan to continue tracing everything first, there's no sense in redoing it all since everything I've checked so far looks good.
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1971 Karmann Ghia - 1600 DP
1984 Westfalia - 1.9 WBX
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