M915a1 voltage question

Hooty481

New member
707
3
0
Location
Russell County Kentucky
Voltage meter reads 14 volts when truck is idling and driving at 55 mph. When I start turning electronics on it keeps dropping voltage. With the head lights and parking lights on it reads about 13 volts. Then when I turn the heater on it drops to what's looks like is 12 volts. This is what the volt meter in the dash says I have not checked with a volt meter but I would say there won't be any difference. Any ideas?
 

goodguyzy

Active member
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Location
medford oregon
My M916 does the same thing but worse, I just installed new batteries thinking it was two bad ones that I had but still does the same thing. Every thing seems to work fine though.
 

Ferroequinologist

Resident railroad expert
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Liberty Hill, SC
My truck also does this. After driving back from the GA rally this year, with my lightbar running, the truck barely had enough power to start after shut off. I believe my alt is not working correctly. I am going to change out the regulator, and see if that fixes it, if not, it is total alt replacement time. I'm afraid of what one might cost, due to the slow trickle of parts for them coming out of surplus at the moment.

I've thought about putting a 12v/24v HMMVEE alt I have on, but the 12v circut is only 15 amps I think, which won't work because the M915 is mostly 12v.
 

Hooty481

New member
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Location
Russell County Kentucky
I'm just afraid mine will get me out and get me stranded. No idea on the humvee alt. Haven't had any experience with those. I really don't need 24volts on my truck but wouldn't mind just for the military trailers.

If you have any more ideas feel free to let me know
 

Danl

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Location
Lyman Maine
My 915 runs the same way , I've been working a night sift 6 pm to 6am on a gas pipe jobfor 6 weeks now and the last four weeks or so I've been running the heater . I did fear a shortage of battery power , but all is working fine. I do shut off the heater a few miles befor i get home in the morning , thinking that I'm helping the power, I feel better doing this ., but the truck starts every night, I say run it till its stops, don;t worry be happy, You are luckey to have a well over built truck ,
 
491
1
16
Location
Carrizozo, NM
My two M915A1's did the same thing. I have been replacing all bulbs in them with LEDs. I have yet to break the bank and get led headlights but with just the "parking / marker lights " turned on the needles on the volt meters don't even move off of 14 volts. When I turn on the headlights it goes down alittle, but not like before the led bulbs. Even coupled to my M871 trailers (also converted to led) the volt meter barely moves at all.
 

Ferroequinologist

Resident railroad expert
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Location
Liberty Hill, SC
That's the only reason I've gotten away with it barely charging all these years, is that all my trailers are LED.

I've got to get it fixed now though, as the batteries no longer hold enough charge to start it due to the low charge voltage for so long.
 

greenmonster

Member
119
4
18
Location
cody, wyoming
I think part of the problem is the alt are such a small amperage in these trucks, With the amount of amp hours the battery's have I guess they figured there was no need to have a high amperage alt. So with blower and headlights and trailer lights more than likely it just can't replenish enough amps to run everything for ever. Led headlamps would probably do enough to not have to worry about it anymore. Weak battery's would compound this affect. With 100amps going out and 95 amps coming in the battery's, they act as capacitors or the reserve for the system. More going out than in is no good, so it is possible to have a fully functioning system as far as voltage regulator and alt. and battery's but just have more draw than the system is capable of handling.
 
491
1
16
Location
Carrizozo, NM
I was driving one of my M915A1's home when I purchased it. I picked up one of my M871 trailers in Oklahoma. After driving several hours that night I turned on the heater blower and within minutes all the lights went out! In pitch black darkness I got to the side of the road shut off the blower and applied the hazards. In acouple min. The lights came back on and I continued on my way to a better place to stay the night. This was prior to led light conversion .
 
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Hooty481

New member
707
3
0
Location
Russell County Kentucky
I had this to happen to me as well. I hook to one of my goose neck trailers and in just a few minutes all the lights went out.

Something that I just thought of...since the original light switch will not handle the load of the marker lights would this add to the effects since it creates more heat and sometimes melts the pig tail on the switch?
 

Ferroequinologist

Resident railroad expert
4,797
631
113
Location
Liberty Hill, SC
I had all the wires burn off the back of my stock plastic light switch. I've put LED headlights in now but prior to that, I moved all the circuits to a relay and put a new light switch in that only controls the relay. Never had melted wires again.
 
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