M915 headlight switch problem. Odd. Clues below:

[JH1]

I have a M915 that someone messed with a little. The original headlight switch is long gone, and it looks like they wired in a Ford Pickup switch in it's place. It was working until a while back when it got hot and quit. All headlight current (10A) goes through it, as opposed to using a relay to carry the high current. Bad design...
I'll put a relay in there, but there is yet another issue. There is an output pin on the switch that sends out 1 amp in the running-light position, and then falls to zero when the switch is moved into the full headlight position. So here's the question: What system (lights?) gets turned off whenever the headlights are turned on? Blackout lights, maybe? In general, when you go to full headlight position, that should just add current in one circuit and leave everything else alone, shouldn't it?
Thanks,
JH1

P.S. From reading the posts, the M915-M920 series has a design flaw because they chose to run all 10A of headlight current through the switch. This odd reduction in current is not related, so far as I can tell. I'm using an inductive DC clamp meter to watch current flow. Handy little device. $60 at Sears.

 

[Bighurt]

I thought the marker and blackout had their own switches respectively, and that the headlight switch just controls running and headlights. I'd have to walk out and look to be sure.

I agree though it is a bad design and I stop using the original "jeep" part awhile back.

 

[wreckerman893]

The original M915's (AM General cabs) used a commercial switch and they were a constant source of annoyance in the military and to those of us who purchased them from Uncle. On the Freightliner cabs they use the military switch we are all familiar with from the deuce and 5 tons.
The old switches have been known to leave you sitting on the side of the road in the dark (ask me how I know) and it wasn't uncommon for those of us who drove them in the military to have a few unauthorized spares in the OVM box.
I don't think you could adapt the military switch to the older cabs without a lot of wiring hack but if someone wants to try it and is successful it would be a great mod thread. I'm electrically impaired so I will stick with carrying spares should I ever get another older M915.

 

[Bolkbich]

The light switch on my M916 was getting hot and popping the breaker. I wired the switch so it powered a relay for the headlights instead of the headlights themselves. Works great. I'm wondering what the military did all these years ?

 

[Scott88M]

I drove 915s over the road in a line haul company or of Fort Campbell and we always had to turn the 12v dash fans down to point down into the light switch panel

 

[JH1]

Wow. Ok, I'll use a relay. I wonder if the blackout circuit automatically disengages if the headlights are turned on? Obviously, there's no need for them with headlights on. I'll go flip some switches and report back. Thanks, guys.

 

[Castle Bravo]

Does anybody have a part number for a better headlight switch that is a drop in item?

Wow. Ok, I'll use a relay. I wonder if the blackout circuit automatically disengages if the headlights are turned on? Obviously, there's no need for them with headlights on. I'll go flip some switches and report back. Thanks, guys.

It works the other direction - the headlights do not work with the blackout switch on. The headlight switch activates the blackout lights and blackout headlight when the blackout switch is on.

 

[JH1]

One thing that is easy to do and might help is to run the dash light knob all the way to full bright, instead of dim or half dim. That will eliminate the heat generated by the circular spring-like resistor. By setting it to full bright, the resistor is not in the circuit, and therefore will not generate any heat. That might not fix the overall problem, but it sure reduces the heat nearby. Had the dash open and that thing gets incredibly hot.

There is a drop-in item that is available at Carquest. I don't know the number, but it's listed in a thread somewhere on this board. Saw it yesterday. Good luck.
JH1

 

[Artisan]

Does anybody have a part number for a better headlight switch that is a drop in item?
snip

Matt CLICK here for that part number but read the thread, look at what
Dueceman is saying, I am so poor on Electrical I might follow your
lead on this mod. Please include me if you can if you contact deuceman
about his MWO, if he has a kit we might get two ehh?

My M916 is surely the same yes? That headlight switch looks like the switch
in my 79 J10 if I remember right.

 

[goodguyzy]

Funny, when I recovered mine the first thing that fell to pieces was the switch luckily it was day and we had time to stop and get it temped together. I replaced it with one it got at a auto parts store.

 

[JH1]

Another really odd thing happened. Can't believe it, but here's the scoop: There are 4 small 12v bulbs illuminating the 16 speed gear selector, 3 of which were burned out. The 4th one would blink. Blink? Really? Book doesn't say squat about a blinking light there. Inserted 3 new bulbs (#1831 from Oreilly Auto Parts for $2.50 ea) and they all worked fine. No blinking. Only the original one blinked. Didn't matter where I plugged it in, either. Strange. Replaced it with a new one and no blink. Eh?? Took the bulb into the shop and put it on a 12v power supply by itself. It blinks. ??? Looked at it under a microscope and lo and behold, the filament is old and twisted around so it sorta touches one of the internal terminals of the bulb. As it heats up, it apparently moves the filament and disconnects. Then cools and wanders back and touches again. Exactly like a commercial flasher unit for cars. I've never seen this in a bulb before. Blinks at about a 2 second rate. I'm gonna keep this and show it to fellow tech-nerds. Stuff like this really helps when you're trying to diagnose a problem with undocumented hacks. Not....

 

[Artisan]

FWIW here is a pic of the CARQUEST number 53-22424 replacement
switch which is probably just like the OE switch.



Standard Motor Products DS-216 Headlight Switch = SAME SWITCH

Standard Motor Products S-720 Headlight Switch Connector ( I am 99% sure this is it )

 

[JH1]

Good input, Artisan! I'm going to scrounge up a couple of 30A automotive relays and use them to send the heavy currents to the running and headlight circuits. From what I can deduce, there are two independent circuits inside that switch. One for headlights and one for running lights. The dash light output pulls from the running light circuit. Those two circuits are independently fed from two breakers. The headlights are fed by a high current breaker and the running lights from a lower current breaker. I'm going to lift the wires from the switch and put them on the relay, and then use the switch output to also power the relay. I'll gin up a wiring diagram when done. Probably put the two relays right next to the light switch, since it's close and nothing else goes there.
JH1

 

[JH1]

Success! To recap, what I did was reestablish a working headlight switch. My truck was hacked before, so my solution is not universal. However, assuming you start with a standard issue setup, take the switch outputs that normally go straight to the headlights and running lights, and instead, run them to relay coils that will in turn, route power to the lights. That takes the heavy currents out of the actual headlight switch and puts them out in suitably heavy relays.The relays should be in sockets (commonly available. I got mine from All Electronics.) so that if they ever do go bad, it's a 30 second fix. You should still pick 12v off of the breaker in the dash instead of running straight from the batteries for electrical safety.

As a further tip to prolong the life of the headlight switches, run the dash lights at full brightness. That takes the variable resistor out of the circuit. That resistor generates heat, and lots of it, when you run mid-range brightness on the dash lights.

JH1

 

[R Racing]

I switched to LED headlights to draw less current from the switch.

 

[JH1]

That's an excellent idea! How much current do they draw? Incandescent headlights are 70 watts (max on high) and 50 watts (max on low), so the current on high is 140W/13.8V = 10.15 Amps for both headlights.
JH1

 

[Artisan]

Be forewarned that LED headlights do not get hot enough to melt ice and snow
and it can build up on them causing issues. Truck-Lite does have an LED that
has a built in automatic heater that comes on at low temps but I have not
found them reasonable priced, yet.

 

[Castle Bravo]

ice and what?

 

[jamboly]

Well it is time again to replace the headlight switch in my M920. After the first one melted the connector that goes to it, I added the relay fix and haven't had any heating issues. I replaced it with the CarQuest 53-22424 switch, which worked fine. Current problem is some sort of internal switch issue that causes the instrument lights to stay on when switch is off unless you wiggle it just right. When looking for another Carquest switch, seems they are no longer available since Advanced Auto bought Carquest. Doing an internet search, it "appears" that it is the same switch that goes in a 1979 CJ5. My local O'Rielly auto parts dealer system calls out a Borg Warner S440 or S450 switch. The only difference listed was that the S450 was stamped 5751098. They both looked the same. After a little more research, I found that Standard Motor Products makes a DS198 (same as S440) and DS216 (same as S450), listing the 5751098 stamping as the difference. The only thing I could find out about the difference between the DS198 and DS216 was a review comment for a purchase that stated the DS198 has an internal strap that the DS216 does not (didn't specify which terminals). Atrisan posted in this thread to use the DS216, which I think is wrong. There is another link in SS (search Headlight switch for M915a1) that says to use the S440 (same as DS198, which I believe is correct.
Now, can someone tell me the right connector for the switch???

 

[WillWagner]

Auto parts store should be able to look up the pigtail needed