willysmjeeps.com

1952 M38. One plastic headlamp, one glass.
Glass headlamp dim.
Confirmed correct attachment of #17 and #18 wires to lamp.
Confirmed continuity from #91 ground wire to lamp.
Cleaned and reattached #91 wire to ground. Still dim.
Grounded #91 wire directly to battery negative terminal. Still dim.
Now the weird part. When removing #91 ground wire, I expected the headlamp to turn off.
Instead, it got even dimmer (with "no ground").
What am I missing?

Hi Jeep Daddy,
It's hard to offer long distance advice about electrical issues without seeing exactly what you have and what you've done.
My crystal balls are out being washed so I can't see (the mystery).

You don't say what voltage does you jeep operate on? 24volt? 12 volt?

Some questions come to mind: did these bulbs ever work correctly, or did this problem just show up?
If the problem just surfaced, what was the last thing you did?

What voltage ratings are these bulbs? Is the plastic lens bulb marked for 24 volts? 12 volts? How about the glass bulb?
No marking means you don't know what you have, and operating a bulb with the wrong voltage usually doesn't work good for long.

I assume you have good wiring and inspected it --- nothing painted, frayed, crumbling, dirty, corroded, or loose?
Batteries fully charged?

Same dim bulb problem whether the engine is running or not?

Same dim bulb problem only on low beam? Only on high beam? Both?

Unless you already tried it, a simple test would be to swap the bulbs around and see if the symptom ("dim bulb") moves with the bulb or stays where it was.

If the dim bulb moves to the other side (dim bulb stays dim) I'd suggest buying a new bulb and see if that fixes it.
Sometimes they go dim with age or just before failing entirely.
If the new bulb has the same problem, then you know you have a wiring or other problem.

Internal problem in either the main light switch or high-low beam switch come to mind...

It may, or may not be, a "ground issue".

Good luck!

Thanks for the reply RonD2.
24 volt mostly original M38.
First headlight was dim so purchased a second from MWM believing it was simply a bad lamp.
However, new headlight has the same problem. Dim on both high beam and low beam, with engine running and engine off.
But why does the light stay on (but becomes even dimmer) when the ground is removed?

Have you bench tested the bulbs?

Have you removed the Hi-beam wire while the ground wire is off and the bulb is still dim...did it go out?

Mike B

"Why does the light stay on (but becomes even dimmer) when the ground is removed?"

Because there's a path somewhere to ground for current to flow. No path for a complete circuit = no light at all.
I wouldn't get fixated on that (yet).

With two known good bulbs, you have a bad connection somewhere. Could be a hot wire, could be a ground.

Did you swap bulbs side-to-side and both bulbs stay dim on one side only? Which side (for clarity)?
The answer to this question is important.

It's time to take a few voltage checks on the dim side.
Any voltage reading less than battery voltage is suspect.

Probably a good idea to take apart the Douglas connectors for closer inspection, and double-check every connection is clean and tight and wires are good.
Sometimes wire (under the insulation) can look good from the outside but be rotted inside.
Do you have old or new wiring?

Sometimes the internal lead can be shorted to the Douglas metal connector housing and in turn the housing could be touching a body ground. If this happened with either wire # 17 or 18 in either side Douglass connectors the light would stay on dim.

Wires #17 & 18 can also find their way to ground inside the main light switch plug and at the dimmer switch.

I’ve seen a lot of dimmer switches with really bad wiring for what it’s worth. But both lamps would behave similarly?

Still waiting to hear if the bulbs traded to the other side behaved the same.

I’m with Wes. It’s grounding, even partially so, somewhere.

I wonder how it turned out.

Was the mystery solved?