Joined: Oct 13, 2011 Posts: 14 Location: Kagel Canyon, CA
Posted: Tue Oct 18, 2011 12:10 am Post subject: M38 Headlight Problems
I recently purchased a 1951 M38 in very decent condition. I have begun the process of restoring it. One of the issues so far is that the headlights don't work. I've tested the bulbs and they are good. The switch seems to be original, and turns on the tail lights, and gauge cluster but no dice on the headlights.. an auto/electric mechanic looked it over and blames the voltage regulator. Anyone have any advice on this? (24V system)
Joined: Feb 28, 2011 Posts: 148 Location: Escondido, CA
Posted: Tue Oct 18, 2011 2:17 am Post subject:
There is a color-coded wiring diagram available in the downloads section of this website under the M38 M38A1 M170 M606, Electrical link . That is a good starting point for troubleshooting. The headlights are fed by wires 17 & 18 (high and low beam, respectively) from the dimmer switch , which is fed by wire 16 from the light switch. The main power feed for the light switch is wire 11. Since the power for all the lights is routed through the light switch, and some of your lights do work, it appears the cause of the problem has to be the switch, the wiring between the switch and the headlights, or a bad ground for both headlights. Could be the dimmer switch, a broken wire 16, a bad connection, or a bad light switch. Simple continuity and voltage tests, with wiring diagram in hand, should reveal the root cause of your problem.
You say "the switch seems to be original, and turns on the tail lights". Do you still have blackout tail lights, blackout stop light, service tail light, and service stop light? If so, which "tail lights" come on, and in what switch position?
Find a new auto/electric mechanic - that guy has no clue. _________________ Jim M.
1952 M38 son-father project
Discovering more worn out parts, one assembly at a time
If the bulbs are really good then there is no way the alternator is the problem. If the bulbs are really already blown (burned out) then the alternator is usually the culprit because when it over charges the higher voltage burns the lamps out. Did the auto/elect mech say the alternator was overcharging? Did he say the lamps were burned out. When you say alternator does that mean 60 amp military 24 volt alternator or civvy 12 volt alternator? If 12 volt then suspect the wiring and perhaps the 12 volt headlamps are on a separate switch! At any rate we need to know a lot more about what you actually have before we can offer any troubleshooting guidance that will truly help. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: Oct 13, 2011 Posts: 14 Location: Kagel Canyon, CA
Posted: Wed Oct 19, 2011 12:17 am Post subject:
Thanks for the replies! I do not believe that it has the original blackout taillights.. (At least not yet) The light switch has been painted over, so I can't really read which of the 3 levers controls what.. One turns on the taillights and gauge lights, and one seems to dim them. I agree that the auto/electric guy is guessing, and hopefully will be able to send back the voltage regulator he suggested I replace.. I checked out the color diagram, and it's great.. That with a decent mechanic should help track down the real issue.
Regards,
David
Joined: Feb 28, 2011 Posts: 148 Location: Escondido, CA
Posted: Wed Oct 19, 2011 2:01 am Post subject:
The original light switch has three levers: one top center, one lower left and one lower right. The LR is an interlock that keeps you (a soldier) from inadvertently turning on lights: it must be raised to the UNLOCK position to allow either of the other levers to be moved and will spring back when released. The TC lever has five positions (from left to right): BO DRIVE, BO MARKER, OFF, (service) STOP LIGHT (only), and SERVICE DRIVE (headlights and service tail light). The LL lever has four positions: PANEL BRIGHT, PANEL DIM, OFF, and PARK (left rear tail light only). The TC lever has to be in something other than OFF for any lights to be on. More info here: http://olive-drab.com/od_mseries_lightswitch.php
You may not have this arrangement even though you could have the original switch. It is conceivable that someone rewired things for more convenience for civilian driving. Not too much call for blackout driving in civvy life unless you're up to no good. With the military arrangement, you don't even have any stop light without setting the light switch. _________________ Jim M.
1952 M38 son-father project
Discovering more worn out parts, one assembly at a time
Joined: Apr 14, 2005 Posts: 196 Location: Van Buren, Arkansas
Posted: Wed Oct 19, 2011 7:57 am Post subject:
The blackout marker lights display several different
'shapes' depending on your distance from them. Your
buddy would watch the shapes on the vehicle ahead
of you, and would warn you if you were getting too
close. The blackout light on the left fender shines
a pale beam on the ground so you can see the edge
of the road. This is why lots of trees in the ETO were
painted white at the bottom, which became a style
over here after the War. It continues to this day even
though the original purpose has been long forgotten.
The lower right lever is the lock lever and must be pushed to unlock so you can move the top lever to any of the svc (service) positions. Service drive position turn on the normal civvy street lighting functions.
This is called a "Pinout" and illustrates what the switch does internally and what each wire is for and which pin # or letter is used for which wire.
Another switch operation explanation sheet.
This is the BO driving lamp that puts a dim light on the ground immefiately ahead of the jeep. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: Oct 13, 2011 Posts: 14 Location: Kagel Canyon, CA
Posted: Wed Oct 19, 2011 10:19 pm Post subject:
Thanks for all the incredibly helpful info!! I do have that switch in my jeep, and have no idea yet if it has been rewired. The diagrams will be undoubtedly useful in trouble shooting the problem! I will let you know what the solution was when it's solved.
Regards,
David
Joined: Oct 13, 2011 Posts: 14 Location: Kagel Canyon, CA
Posted: Sat Oct 22, 2011 12:01 pm Post subject:
Just wanted to update all of you that tried to help me with my non-working headlights.. after studying the data and hints you sent, the one photo of the switch that showed one of the 3 levers to be a "lock" was the answer.. mine was painted over. I moved the lever to unlock, and tried the headlights.. yup. they work perfectly, including the hi/low dimmer. Thanks for solving it!
Joined: Feb 28, 2011 Posts: 148 Location: Escondido, CA
Posted: Sat Oct 22, 2011 11:44 pm Post subject:
Good deal - glad to hear they are working for you. A bit confusing, though, because in your first post you said the taillights were working. In the stock configuration, NO lights will come on without moving the UNLOCK lever (Note 4 on the explanation page Wes posted). I wonder why you had taillights but no headlights? _________________ Jim M.
1952 M38 son-father project
Discovering more worn out parts, one assembly at a time
Joined: Oct 13, 2011 Posts: 14 Location: Kagel Canyon, CA
Posted: Sun Oct 23, 2011 12:05 am Post subject:
As for the taillights ..my guess is that that either the switch wiring has been modified, or I never tried the unlock switch while moving the headlight switch simultaneously. I did notice that though the taillights work, the brake lights do not.. Is the original system wired with taillights? It seems that it does have the original taillights, but the lenses have been changed from black-out to a one piece red lens.
Study the posted illustrations closely. All answers are right there.
Jim is not quite spot on above. With everything turned off and the lock lever locked you can always select "BO marker" with the top switch and the 4 marker lights will come on. You can also select "Parking", "Off", "Panel Lights Dim" , "Panel Lights Bright" or "Headlights"with the left switch. The left switch will move and control panel lights if the BO Marker position is selected with the top switch, but no front or rear lights will come on unless you have the top switch in "Stop Light" or "Service Drive" You'll get used to it.
Original lighting for the M38 included the two main headlights, a pair of front Marker Lights (like front civvy parking lights but they only work in BO use), a pair of tail light assemblies that have the BO marker function (same as civvy tail lamps but only in BO driving) on both sides, a red lens stop and tail lamps on the left only (these work in "Syop Light" & "Service Drive" positions only) and a BO Stop lamp in the right light assembly (that only works in the BO driving positions of the top switch). _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: Oct 13, 2011 Posts: 14 Location: Kagel Canyon, CA
Posted: Sun Oct 23, 2011 11:18 am Post subject:
Thanks Wes! With all the different choices in positions, I'm wishing my switch wasn't so painted over.. what a bad idea someone had.. Another question.. I have a "new" rebuilt fuel pump on the way to replace the after-market non-working electronic one that is inline before the old pump. I noticed that the current fuel lines are rubber all the way from the tank. Should I go ahead and purchase a fuel line replacement kit to go along with the new pump? I have a fellow M38 owning friend coming over to install it with me, and I would like to have all the right parts when he gets here..
It's up to you. If you are not comfortable fabricating them from steel tube and double flating them then replacement tube kit is in order. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: Aug 15, 2011 Posts: 40 Location: Upstate SC
Posted: Mon Dec 12, 2011 7:48 am Post subject:
wesk wrote:
Another switch operation explanation sheet.
This is a great graphic! It never ceases to amaze me the amount of info available on this site.
What manual/publication was this graphic from? My wife and I were testing lights last night and trying to record what operated in the various positions.
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With the military arrangement, you don't even have any stop light without setting the light switch.
