Headlight puzzle

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Steven Unterman
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Headlight puzzle

#1 Post by Steven Unterman » Sat Sep 07, 2019 1:15 pm

Hi folks,

I have a 1965 356C. Years ago I installed one of Joe Leoni's headlight relays and a couple of years ago I installed a pair of Jeff Fellman's SMD headlights. For what it's worth the car is the original 6 volts A few months ago I noticed that one of the headlights seemed to flicker a bit. Figured it might just be a poor connection but then a month or so later both headlights just quit working and wouldn't come on when I was driving the car. What seems odd to me is that a couple of weeks ago I noticed that with the car off and no key in the ignition I can pull the light switch on and both headlights turn on just fine.

Any thoughts among those of you who know these cars better than I do? Any help would really be appreciated.

Thanks,
Steve

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David Jones
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Re: Headlight puzzle

#2 Post by David Jones » Sat Sep 07, 2019 2:44 pm

Rather a strange problem and I would think that checking the voltage of the generator is where I would start. If you had an AC component on the generator output I guess in theory that would cause a flickering LED. Reverse polarity would put them out but if that occurred then the battery would not charge. Set your voltmeter on AC and see if there is an AC component on the DC value. Not enough voltage to trigger the SMD while running will also do it.
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Martin Benade
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Re: Headlight puzzle

#3 Post by Martin Benade » Sat Sep 07, 2019 9:16 pm

Is your running voltage too low (below 6 v) for them to light but the car still manages to start?

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Re: Headlight puzzle

#4 Post by Harlan Halsey » Sun Sep 08, 2019 2:46 am

I would check your grounds.

Steven Unterman
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Re: Headlight puzzle

#5 Post by Steven Unterman » Sun Sep 08, 2019 12:29 pm

Really appreciate the feedback. Sounds like this might be a low voltage problem. Unfortunately I'm electrically challenged. It's my understanding that to check this I need to have the car running. Do I touch one lead of the voltmeter to the D+ terminal of the generator and the other lead to any random ground on the chassis? Do I use the ground on the regulator or do I use the D+ terminal on the regulator and any random ground?

Thanks in advance for any help.

Steve

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David Jones
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Re: Headlight puzzle

#6 Post by David Jones » Sun Sep 08, 2019 12:38 pm

Check at the battery across + and - terminals with and without engine running then with engine running at at least 2K rpm check at the regulator across Bat+ to gen ground then with engine at idle check same. With engine running at revs you should see the voltage rise to a max of 7.3 volts. That may not occur until you reach 3K rpm but you are looking for at least the voltage to rise by at least 0.5 volts above battery voltage with engine off. If all looks well the proceed to check voltage at the headlight fuses with engine running at 2K or above rpm. With a digital voltmeter it does not matter which way round the terminals are as long as they are across battery positive and a good ground.
If I had known I would live this long I would have pushed the envelope a little harder.
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Steven Unterman
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Re: Headlight puzzle

#7 Post by Steven Unterman » Sun Sep 08, 2019 1:08 pm

Thanks, David. If I recall you helped me trouble shoot some problems I was having when I got my 356 years ago. When you talk about checking at the regulator "across Bat+ to gen ground" I'm assuming that's the terminal on the upper right of the regulator to the ground terminal on the top of the generator. Is that correct?

For what it's worth I also have a lead from the B+ regulator terminal that goes to a relay for a third brake light, but I'm thinking this shouldn't make any difference.

Appreciate your time with this. Will let the group know how things turn out.

Steve

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Re: Headlight puzzle

#8 Post by David Jones » Sun Sep 08, 2019 2:23 pm

Steve, I usually put my ground lead on the gen case as that should be as good a ground as you get anywhere. The Bat+ on the reg is better than going to the Gen + as that will show the battery voltage until the regulator relay closes then you will see it rise higher than battery voltage though either will work in reality.
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Re: Headlight puzzle

#9 Post by Steven Unterman » Tue Sep 10, 2019 8:50 pm

OK - the plot thickens. Although the headlights haven't come on the last few times I tried, when I drove the car yesterday they came on. At idle the passenger side seems dimmer than the driver's side and sporadically flickers. When I rev the car both lights appear equally brighter and there is no flickering. There are 6.19 volts across the battery terminals when the car is off. This goes up to 6.76 volts at 2000 rpm. At idle (around 700 rpm) regulator Bat+ terminal to generator ground is 6.04 volts. At 2000 rpm this increases to 7.1 volts. Any thoughts? Help is certainly appreciated.

Thanks,
Steve

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David Jones
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Re: Headlight puzzle

#10 Post by David Jones » Tue Sep 10, 2019 10:03 pm

Check the headlight ground connections Steve. They are in the front trunk on either side. They come through the tube for the horn mount/fender brace at the inner fender wall and must be clean and make a good ground.
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Re: Headlight puzzle

#11 Post by Greg Bryan » Wed Sep 11, 2019 12:30 pm

Steve - you might try substituting the conventional headlight lamps for the SMD headlights to see if the problem goes away or persists. It sounds like the SMD headlights are just on the threshold of their operating voltage, and sometimes they light and sometimes they don't. I seem to remember other threads from the past with similar issues.
Of course, you have received lots of good advice above regarding checking connections and grounds. You may be fighting a number of voltage drops across various components although the Leoni kit should eliminate most of the common ones.
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