Educate me on diodes

[rabler]

All the wires that are hot with the engine off are hot with the engine running. There is just 1.5 volts more running than not running.

One possible issue is that when the high beams are not on, that wire could be acting like an antenna. Basically an antenna is just that, a wire that is only connected at one end, to the "receiver". If the high beam indicator triggers at some ridiculously low voltage, then when on low beams, the high beam power wire, acting as an antenna, may pick up enough signal from the engine ignition to turn the high beam indicator on.

The other noise source is the alternator, which puts out significant ripple especially when the headlights are on. The battery should help reduce that ripple, but again, electrical noise isolation is complex. Since the high beam and low beam wire run in parallel, the amount of cross talk between them could be significant.

Check that high beam indicator wire with the engine running, to see how much voltage is on it, when the lights are on low. Check both AC and DC volts.

 

[Truckmannorth]

I would also check the grounds at your Headlights. A bad ground would allow power to back feed through the headlight bulbs. Probably enough to light the indicator.

 

[alloy]

To me that's the only plausible explanation I can see . The idea of a diode fixing this doesn't make any sense. Power is going to the led not from it.

So how would a diode keeping the power from flowing back from the led into the harness help? Apparently excess power is getting to the diode, not from it.

Edit: Would a resistor that would lower the voltage going to the led work?

 

[Flyinfool]

You posted page 3 of the wiring, can you post the other pages so that we can see where all the wires actually go?
There is a LT Green wire coming from the engine that is connected to the HI indicator. But don't know what on the engine it is.

Set your voltmeter to AC to see if there may be a bad diode in the rectifier. If there is some AC getting mixed in the system it can cause all kinds of weird things.

 

[mksj]

I think Rabbler may be correct. I would start out with using a regular diode as they suggest, but can't see that it would change anything according to the wiring. I have used zerers in reverse to drop the voltage and may filter out some of the hash, they are also used to clamp the voltage so that any spikes above a reference go to ground. I am more of a hack on these things, so more trial and error method. They can be used to take out voltage spikes and decrease the hash picked up from other sources.

One option to try is to run a temporary wire from the instrument 12V power directly from the battery terminal to see if this has any effect. LED's/circuitry often have very low current turn so the voltage could be induced at very low current. Try the diode and the other option is use something like a 2V 500mA-3A zener diode in reverse with a standard diode in the high beam LED indicator line, this will drop the voltage and may have some filtering from rectification effects and dropping the voltage by 2V. I have diodes but not Zener's in that range.

 

[pdentrem]

Looking at the wiring diagram, I tend to agree that a bad ground could be the issue.
I would likely start with a meter and disconnect the instrument panel from the rest of the harness, to make sure 12v on the light green wire coming from the switch, is there only in high beam at the switch. The wire to the IP should have no voltage with it disconnected, hook at a temporary ground for the IP.
Digging deeper, remove one headlamp at a time. This could show power being transferred from low to high wire at the bulbs instead of going to ground.
Headlights are usually grounded to rad support and the instrument panel to the dash support. Should have ground for the rear lights somewhere inside the trunk. Chassis to body, body to engine and engine to negative battery post or large gauge wire for remote mounted battery.
One of my friends bought an new harness for his 1969 AMX. There was one crossed wire in the steering column connector. Took us a while to figure it out.
At the garage, we hated wiring/electrical issues! Quick fix or hours of head scratching.
Pierre

 

[tq60]

Others beat me to it, I suspect bad ground someplace.

The headlight has 2 filaments with common ground connection.

Please draw this as we go, place 2 lights in row and draw a ground between them.

Now add the high beam indicator and switch to 12 volts to one, be sure indicator is on light side of switch, and just 12 volts on the other.

With the high beam switch off and 12 lights on, current flows through bulb to ground.

Now, open the ground connection.

The power travels through BOTH bulbs then to the indicator.

The current path is clear.

You say the lights work?

Get a length of wire, connect one end to the battery negative, AT THE BATTERY.

Route the wire to near the headlights.

Use the new ground wire for the negative connection for voltmeter.

Measure voltage on all pins of the lights in all conditions.

I will guess you may measure some voltage on the ground connection.

Any voltage measured here is due to a poor connection to battery negative.

DO NOT DISTURB AND CONNECTORS BEFORE MAKING THESE MEASUREMENTS.

You may see some voltage on the high beam bulb, this is due to floating ground, your indicator only needs a hint of energy to come on.

Sent from my SM-G781V using Tapatalk

 

[alloy]

If I hadn't wired the car myself I would agree the very first thing I always look for in wiring problems are bad grounds. I went overboard with grounds on this one especially with the add on fuel injection. Intake has a ground wire to buss bar, heads are both grounded to the buss, block in grounded to the buss, buss is grounded the the battery with a 10 gauge, body is grounded at two places, frame is grounded to the battery in the trunk, front lights are grounded with a 10 gauge wire to the buss bar. My fuel pump and fuel gauge are grounded to a buss bar beside the battery. I've had zero problem with other lights, gauges, ect. Everything works perfectly except the high beam led is on with the low beam only when engine is running.

I can't start the car at the moment to check more waiting for a new part to come in. I already checked the dash ground first thing.

I hate chasing electrical issues also, so I make sure there aren't any problems by going the extra mile when I put the harness is. Learned the hard way on that one.

I think if it were a ground problem, it would happen with the engine on or off.

 

[Bone Head]

Curious. Where's your fuse box located?

 

[alloy]

It's under the dash behind the firewall on the left side.

A pain for an old guy to get to with the front seat in.