M38 dash lights - dim & full

Harleygaz · Post by **Harleygaz** » Sun Oct 20, 2019 6:19 pm

Hi. How did the M38 light switch control dim & full brightness? I don't mean switch position but how did it limit voltage or current to give the dim setting? I checked voltage and on both settings I get the same voltage on the supply to the dash lights.

Post by **wesk** » Sun Oct 20, 2019 7:23 pm

I'll bet you have not looked in my photo gallery: http://www.willysmjeeps.com/v2/modules. ... _album.php

In my sub-album: M Series Light Switch

Then sub-album: Bendix-Scintilla Switches

Then finally: TM 9-1825E Ch 4 Sect 1 Light Switches

Fig 29 on page 54 note item "WW" Look at the index for Fig 29 on page 55. Now go to page 56 and read paragraph 22c (3).

or go to our downloads page and download the PDF copy of: TM 9-1825E Chap 4 Light Switch repair

RonD2 · Post by **RonD2** » Sun Oct 20, 2019 8:42 pm

Hi Gary,
Edited and deleted my original post. I beg your pardon, and my apologies to you and all. I totally mis-read your question.

Thanks Wes. Wise counsel.

Regards,

BBDE · Post by **BBDE** » Mon Oct 21, 2019 12:25 am

Hi Gary,

as Wes has explained, there is a internal resistor inside the lightswitch.
Inside the "BENDIX-SCINTILLA" switch (following the description to the sub-album from Wes) you can see the resistor.

Inside the "newer" Lightswitches, the resistor is more like a cable but the function is the same.
Dim of only the panel lights, dependent the levers position.

Here you can see the "Internal Main Fuse, or better circuit breaker"
and the "Resistor"

If you have trouble with the "late" switch, look to this link:
http://pages.suddenlink.net/randygar/switch/switch.html

With voltage you running your system?

Sincerely yours
Frank

Harleygaz · Post by **Harleygaz** » Mon Oct 21, 2019 7:42 am

Thx Frank, that’s what I expected to find but wanted to be sure. Headlights, service tail & brake, BO marker, drive and brake all work fine. I took the dash lights out and cleaned everything and they both work except for one blown bulb. New bulbs have arrived so hopefully I’ll get it back together in a day or so. I’m running 24 volts.

mdainsd · Post by **mdainsd** » Mon Oct 21, 2019 8:01 am

With only one dash bulb working, the "dimming" effect will only be half of what it would be if both bulbs are working. It is a matter of having a parallel resistance (2 bulbs) in series with a single resistance (the resistor in the light switch).

Note: measuring the voltage at the bulb sockets with no bulbs will read 24V in either full or dim position. Again it has to do with resistances. This is also why the military light switch will not give full and dim when used on a 12V system, the resistance of the resistor in the light switch is calibrated to the resistance of two (sometimes 3) 24V dash light bulbs.

Harleygaz · Post by **Harleygaz** » Mon Oct 21, 2019 9:16 am

Mdainsd, that’s what I was thinking, I just wanted to confirm the resistor in light switch itself. I should have it working tomorrow and I’ll post here to confirm it’s all working correctly.

jnissen · Post by **jnissen** » Mon Oct 21, 2019 10:42 am

For those of us who have gone to the dark side of 12V you can still get the dimming function if you change out the resistor. Just seal up the light switch after the modification.

Harleygaz · Post by **Harleygaz** » Tue Oct 22, 2019 11:19 am

Good news is that my dash lights are working both sides on both dim and full settings and they light up the speedo OK. Bad news is that repro gauges with opaque plastic cases don’t appear to let enough light through but I’ll try again later when it is properly dark.

mdainsd · Post by **mdainsd** » Tue Oct 22, 2019 11:34 am

Get rid of the repo gauges? Also, if you have the correct early dash lights (the large ones), you can pop the red glass out of them and clean them up real well inside and out, it really helps with the light coming out of them.

Harleygaz · Post by **Harleygaz** » Tue Oct 22, 2019 6:33 pm

NOS gauges are too expensive. Budget is already blown and more to do. I cleaned the glass bowls and they look like new. I’ll live with it until I can afford NOS.

Post by **wesk** » Wed Oct 23, 2019 9:53 pm

You can still light those non-original gauges up at night. If they have the provision for internal lamps then add a 24 volt small lamp to the socket that snaps in the back of the gauge and wire it into a junction splice to the #40 wire. If there is no provision for internal snap in lamps then cut a small slot in the side of the case closest to the stock panel lamps to allow their light into the gauge. If you wish you can glue a thin piece of clear plastic over the slot.