Convert M920 to all 12v, or all 24v?

[CBR Metalworks]

My M920 doesn't seem to be charging the batteries like it should, I have not looked into it too much but I do know the alternators in these have problems with the 24v side. I had thought about converting to 12v, but then thought it might make more sense to convert to 24v.

To convert to 24v I would have to change all the gauges to 24v, which I would kind of like to do anyways since some of the gauges don't seem to be working properly. I could also run an off-the-shelf military alternator, and leave the original starter in the truck. Obviously I would have to change all the light bulbs, I think my headlights will work on 12-30v so they should be good.

What else would need to be changed?

 

[73m819]

I would go 24v, you can ALWAYS step down if needed but can not step up.

 

[CBR Metalworks]

I also thought about using 12v gauges and just running them off a single battery (with the rest of the system being 24v). Other than the blower motor for the heater (and lights), what else would actually run off of 12v? Since I have so many military trucks keeping the 24v option would be nice so that slave cables will still work.

 

[CBR Metalworks]

any ideas?

 

[CBR Metalworks]

I am also thinking this would be a good time to replace the 'jumpy' tachometer/speedometer combo with separate units. Anyone know of any gauges that would work? Obviously I know I would have to make a new gauge panel.

My oil pressure gauge doesn't act right, always reads max pressure (and about 40psi when truck is not running)
voltage gauge reads low (which it may actually be low voltage)
water temp gauge reads about 170 as well, not sure if it is right or maybe the truck needs a thermostat

 

[Artisan]

Did you try to find a resolve via the TM's yet?
I am a proud new owner of an M916 so in the club I am now,
well, they took all my money and I await an EUC .

Looks like the poop starts at and in TM-9-2320-273-20,
PDF Page 90 (Document section 2-41).



ENGINE RUN SWITCH. Supplies 12-volt power from batteries through switch, circuit breaker, to voltmeter.

VOLTMETER. 10- to 16-volt gage indicates voltage provided by battery pack, and alternator.

ClRCUIT BREAKER (CB-6). Protects electrical components of volt meter circuit by
opening when load exceeds 20 amps. Automatically recycles until overload is removed.

BATTERY PACK. Comprised of four 12-volt, maintenance-free batteries. Two batteries
wired in paralIel in each set. Each set connected in series for 24-volt output. Battery
pack is used for both 1.2- and 24-volt output.

AC GENERATOR (ALTERNATOR). Generates voltage for recharging battery pack.
Has external 24-volt transformer rectifier.

Here we go, in the;
Composite Troubleshooting Symptom Index there is a link to a repair flow section




CHARGINGCIRCUIT.
Batteries are being undercharged or overcharged. 5-5

If you need the TM I can put it on my server for you to DL, just ask.

 

[Hawssie]

I also used to think you cant go up, but I have had good luck using 12VDC to 24VDC converters i bought on EBAY. They are from China (of course) but seem to be built pretty good. If your load isn't too large they can be a big help. I would post some pictures but I dont have any here right now. They have different sizes, the ones I use are good for 12 amps of 24 volt power which is quite a bit. I also have some that are 24v to 12v but most people know about those. That size runs around $35 direct from the Middle Kingdom. They take around 8-10 days to get here.

 

[CBR Metalworks]

Artisan,

The TM would be great. I hate having the TM's on the computer and not in a book, but I guess that is better than nothing. Computer issues are nice because they don't take up much space, but navigating through a book is a lot easier.

I think the 'upgrade' to 24v would be better, the 12v wires on the truck will handle 24v easily (better than 12v). My main fear of converting to 12v is I would not have a useable 24v slave cable plug, and I have come to find those are VERY handy. Plus, 24v alternator's are easily found, cheaper than having to buy a 12v starter AND 12v alternator.

 

[Artisan]

M915, M916 AND M920 TM's

I paid a guy on Ebay recently for what is very close to a full set of OE TM's for
the M916. There are more.

There is a huge advantage to having them on the computer because
you can do a search for keywords or NSN's etc and find a LOT VERY quick.

I find having it both ways PLUS a copy of the TM's on an IPAD in the truck
pretty much covers all the bases.


M915, M916 AND M920 TM'S ETC

OPERATORS MANUAL -10

HAND RECEIPT MANUAL -HR

DIRECT AND GENERAL SUPPORT MAINTENANCE

UNIT DIRECT & GENERAL SUPORT MAINTENANCE RPSTL

ORGANIZATIONAL MAINTENANCE

LUBRICATION ORDER

That should keep you busy for a while...

 

[CBR Metalworks]

Ughhh. After messing with the 920 today I have come to the conclusion that it needs ALOT of wiring work. Not very hard to believe since it is 35 years old, but I found quite a few wires with bare sections, broken wires, removed and 'hanging' wires, etc. The wire loom is also rotted. I think I will just go through it a little at a time and re-wire a harness at a time. The biggest pain will be trying to reuse the original connectors, since they are a pain to get apart, they would need all new pins as well. I am going to try and source somewhere to get new pins for the original connectors, but if they are too expensive I may change to Weatherpack connectors or something similar.

 

[cornrichard]

I had the 24volt side of my alternator go bad on my 915a1. I removed that p.o.s. component and had the alternator rebuilt. It is a standard 12 volt unit after you pull the junk off the back. I then installed a 12 to 24 volt duvac in the battery box. You need one that turns off with the ignition. The only 24 volt systems that I can think of on the 915-920 trucks are the blackout lights, military plug trailer lights and the starter. I only use the starter so the 5 amps charging my duvac puts out keeps me going fine. The only trick is finding a duvac cheap.

 

[CBR Metalworks]

I bought some alternators from OKC today, 28v 450amp units off MRAP's. I am debating putting one of those on the 920, then just running the lights off of one of the batteries instead of both, I don't see why this would be much of an issue since I am converting to LED's on most of the lights, so there won't be much draw. If it would be a problem to run all the 12v stuff off one battery then I guess I could get a 24v>12v converter. I don't really MIND the dual voltage, I just am having charging problems so I am trying to decide how best to fix it. The 450amp alternator would be a very nice upgrade, the only problem is making it FIT, as it is much longer/larger than the original unit.

 

[Artisan]

SNIP The 450amp alternator would be a very nice upgrade, the only problem is making it FIT, as it is much longer/larger than the original unit.

Perhaps find the bracket that is was used with originally,
you might find that 50% of the work is already done!

 

[CBR Metalworks]

Perhaps find the bracket that is was used with originally,
you might find that 50% of the work is already done!

That might work if I was putting it on a similar engine to what came in the MRAP's The MRAP alternators look nearly identical (just longer) to an alternator in a M939 series 5-ton, even the alternator on a 5-ton would be a tight squeeze (lengthwise) in the space where the 920 alternator is.

I am thinking you got a newer M916(A1?) so it probably has a different setup than that on the 920 (Detroit vs cummins)

 

[cornrichard]

There is almost no 24 volt load on these trucks. The 12 volt side runs the headlights, markers, flashers, fuel shut off, heater motor, and everything except the starter. I love the 24 volt starter in cold weather. The 12v side is what you should consider the primary side. I would be pumped for 450 amps of 24 volt power. Unfortunately there is no extra room to pull the stock alternator and you will start smoking batteries when the 12 v side is doing all the work.

 

[Artisan]

snip

I am thinking you got a newer M916(A1?) so it probably has a different setup than that on the 920 (Detroit vs cummins)

Nope, I purposely went for the M915 (no suffix) Cummins, Cat 7155

The A1's 2'3 and 3's have all been going for over 25K.
I guess for the wrap around cockpit and A/C.The Detroit
engine is known for HUGE money when repairs are needed,
so is my CAT 7155 but I have learned a LOT about them. I
found a CAT 7155 TOOL REPAIR KIT I acquired
a spare tranny, and I may need it more sooner than later.

I "think" some idiot burned out the cluthes... Oh well, if
that is the only big problem I am doing OK, I got the new tranny
for a song on GL.


I am looking for a doner M915 to rob the A/C off of it.

Maybe a few other parts as well.

Chains, I need snow chains too.