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I give up - question on lights

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I've asked everyone that I can and have not yet been able to get a clear answer to my question / issue.

I have a 1974 2002, manual 4 speed. The car underwent a custom resto and I was the guilty party doing the re-assembly.

Here's one of my remaining issues: I only have 10 volts at my headlights. That is with the low beams on (not high beams), engine running. I show 12 volts at the relay and I've swapped relays as well as replaced them.

Result is my lights are very dim, hardly useable for night driving. High beams help some.

Can someone tell me what voltage I should get at the lights (I would assume 12 volts). If it is 12, any thoughts on how to track down my issue?

Thanks!

Len

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Stock wiring.

As far as I know, all grounds are sound. Exception, I see a ground in the wiring diag that comes off the harness near the front firewall / passenger side and I'm not sure I recall seeing that on my harness at all.

That said, what grounds specifically are you talking about? Are there light specific grounds?

Thanks!

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Measure the voltage from the battery ground terminal to the relay input, then output, then to the headlight connector. They should all be over 12V and be about the same. When you find the point that has a lower voltage, you have found your bad connection. If these are all good, measure from the battery ground to the common terminal on the headlight connector, if this is about 2V your problem is in the ground side of the circuit. You can use the voltmeter across all of the junctions/wires to find your voltage drop, everything should be near 0V. A common problem on all older cars is a bad ground from the battery to the chassis or the battery to the engine. The high current through this connection tends to corrode faster and the vibration loosening the connection doesn't help.

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12 + at relay (two terminals - assumed to be input and output)

Driver side lamp = 11.61 volts

Pass side lamp < 10 volts

volts at common < 1 volt

Alternator new, charging battery, assumed to be OK

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From what I can tell looking at a 75 harness on the table (I have no wiring diagrams for the later cars) and assuming the 74 and 75 are the same --

The relay output feeds both lowbeams from the same point. If L & R are different and both lower than the relay terminal, I would guess there is corrosion on the relay terminal where the wires are crimped. You can pierce the insulation of the wire with a pin and measure from the terminal to the pin. There should be no voltage drop. The other option is to remove the terminal from the relay connector (the terminal with 2 wires) and visually inspect for corrosion on the wires or crimped part of the terminal. If the wire is bad, pry open the crimp, remove the wires and splice on new ends, clean the terminal and recrimp and solder.

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Now that I have a wiring diagram (Thanks Jerzy!) I can see that the '74 is a bit different than the '75. You have a low beam relay and no high beam relay, correct? The relay is BEFORE the fuses. The output of the relay feeds 2 fuses, . #9 for the left and #10 for the right. Since the voltage at the relay is good and different for both lights, the problem must be at the fusebox. Check for corrosion at the fuse connections and the terminals on the back of the fuse box. If it all looks OK, you can use your voltmeter across the fuse mounts to see if the fuse to terminal connections are bad, and across the wire terminal to the fuse mount to see if the push on terminals to the back of the fuse box are bad.

Bill

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Well, maybe it's as good as it will get.

Engine off, ignition on, trickle charger on the battery, light on low: If I pull the connector off the front bulbs, I get 11.5 to 12 volts. When I plug the connector on the bulb, the voltage at the bulb drops to about 10.4 volts.

Motor running at idle (900rmp) the voltage at the bulb (connector on) is about 10.7, with engine off idle 2,000 rpm +, the voltage goes to about 11.4.

I'm thinking I should mabe go to different bulbs and or move on to other issues.

Thanks for everyone's help!

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The bulbs have nothing to do with it, they are just drawing the current that causes the voltage drop. You have a high resistance connection somewhere that is dropping at least 1V under load. A resistance of .2 ohms is all it will take to do this at 5A. If you measure the voltage across each junction with the lights on, you will find it. I would still bet it is at the fusebox.

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Something that might help...

use the voltmeter ACROSS components and wires.

So measure the voltage drop across the fuses. Then move

'up' the wire to include connections, wires, the relay, etc.

When you see a big jump, there's a potential culprit.

You're looking for small voltages- anything over .05v is a

big deal for an individual component/junction,etc).

(And if you're running 100watt bulbs, that WILL cause trouble-

they'll try to draw too much current, causing a voltage drop)

All that said, running a short fused wire on a separate relay

using heavy (10 ga or so) wire with a big fuse, and then using the

stock wiring to control that is common in the early Datsun world-

the wire was just too small...

fwiw,

t

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hrg88 and TobyB give good advice. It's the wiring or fuses, not the bulbs. Try these tests with key on, headlight switch on, and engine stopped.

It's very common for the connections between the fuses and the fusebox contacts to weaken, even when the fuses look fine. It happened to me last week, so I did a complete fusebox service, and the problems went away. Do this quick check on your fuses 9 and 10.

1. Left side: Attach voltmeter from battery positive post to yellow/black wire at left headlight connector. If reading is more than 0.2 volts, you have a weak Fuse 9 connection. Short Fuse 9 contacts with a screwdriver; if voltage drops below 0.2, a weak fuse connection is confirmed.

2. Right side: Attach voltmeter from battery positive post to yellow/blue wire at right headlight connector. If reading is more than 0.2 volts, you have a weak Fuse 10 connection. Short Fuse 10 contacts with a screwdriver; if voltage drops below 0.2, a weak fuse connection is confirmed.

If you have fuse problems, do this Fuse Box Service:

1. Disconnect the battery ground cable.

2. Remove and discard the fuses.

3. Polish the fuse contact surfaces on the fuse box with a miniature wire brush (a Dremel tool is perfect for this) or Scotchbrite pad.

5. Bend the fuse box contacts as needed so they're tight against the fuses.

6. Blow the crud out of the fuse box.

7. Install new fuses.

8. Reconnect battery.

If the problem is not the fuses, check the wires and wire connections as already posted.

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