Converting 24volt to 12 volt M1009 CUCV

[OL AG '89]

Leave it at 24 volts and take all the 12volt power you could ever want from the terminal strip on the firewall, just make sure you apply proper fuses to protect the rest of the systems......
If you ever have a wiring problem down the road the TM's will be useless... and you will be stuck again.
IMHO the 24 volts lives a separate life and won't - don't - interfere with any other operation of the truck.

 

[airmech]

I converted mine to 12v for a few reasons and am quite happy with it. The only part of the truck that is 24v is the starter, everything else is 12v. There is a common misperception that the 24v starter is better, it is not. Both starters are rated at the same torque and rpm at their given voltage. The advantage of 24v is it performs the same amount of work at half the amperage. In a military environment where everything is run on 24v, it was necessary to be able be slave started and run other equipment. In a civilian world of12v, it is not practical.
The conversion can be done simply without hacking the system to pieces, and returned to stock just as easily.Rather than going the Rosco route, I ran the alts in parallel to allow a standbye system. I found it to be much less complicated.

Your firewall resistor bank has already been bypassed so you don't need to worry about that, just confirm that glowplug relay is running on 12v.
Remove the RED wire and condensor from the passenger alt ground terminal, insulate and tie back to the harness. Install a ground wire from that terminal to the engine.
Locate the RED wire that goes from the diamond shape 12v junction on the firewall to the front battery. Cut the wire near the terminal strip behind the rear battery with enough excess to attatch to the positive terminal strip with a ring connector.
Re-wire your batteries in parallel using #2 cable, or larger.
Replace the volt gauge in the dash with a 12v unit.
Replace the starter with a 12v.
If you want to return to 24v, you only need a new military battery cable. If you wanted , you could remove/save the orig cable and just run a new wire from the 12v junction to the positive terminal strip. I have a new cable in my part stash so I cut the orig.
The 2 alts will only both put out under heavy loads, under light loads the one with higher voltage will act as the primary. If one fails, the other will carry the full load giving you time to make the repair at your liesure.

 

[watkinssr]

A lot of work and money for no gain that I can see. There is plenty of 12 volt available in it's stock configuration.

 

[transman]

IF IT AIN'T BROKE DON'T FIX IT !!!

 

[NMC_EXP]

I'm going to be the devil's advocate here and ask why you want to convert it to straight 12V? You have the unique ability to run both 12V and 24V stuff, a far superior voltage for demanding starts.

Double devil's advocate....if a person will not be running 24V military radios and does not own other military vehicles to jump start or be jump started from, is there a big advantage to the stock 24V setup?

In the case of the CUCV the 24V starting circut is something which was grafted onto an existing system to meet the DoD's specs. Absent the need to run 24V radios or slave start/be started with 24V it only adds complexity (one more alternator and more wiring/connections) and at least one obsolete part (the resistor bank).

Of course I may be just whistling in the dark - I just did the Roscommon 12V conversion after my 2nd 24V starter failed.

Took it for a test drive afterwards, came home and the fuel lift pump was puking all over the driveway.....wondered why that moron in the pearl white Escalade who was briefly tailgating me turned on his wipers and backed off to a respectable difference.

Soon to be another non-original item on this truck - an electric fuel pump.

Regards

Jim

 

[watkinssr]

I'm going to be the devil's advocate here and ask why you want to convert it to straight 12V? You have the unique ability to run both 12V and 24V stuff, a far superior voltage for demanding starts.

Double devil's advocate....if a person will not be running 24V military radios and does not own other military vehicles to jump start or be jump started from, is there a big advantage to the stock 24V setup?

In the case of the CUCV the 24V starting circut is something which was grafted onto an existing system to meet the DoD's specs. Absent the need to run 24V radios or slave start/be started with 24V it only adds complexity (one more alternator and more wiring/connections) and at least one obsolete part (the resistor bank).

Of course I may be just whistling in the dark - I just did the Roscommon 12V conversion after my 2nd 24V starter failed.

Took it for a test drive afterwards, came home and the fuel lift pump was puking all over the driveway.....wondered why that moron in the pearl white Escalade who was briefly tailgating me turned on his wipers and backed off to a respectable difference.

Soon to be another non-original item on this truck - an electric fuel pump.

Regards

Jim

Well, the question is, what is to be gained.

If you have a truck with a bad starter and have a 12 volt one laying around, maybe. But why spend a bunch of time and money to re-engineer something that isn't broken?

The stock setup works, and you still have more than enough 12 volt power to run anything you need.

I don't have any military radios, but I do have cordless power tools I want to run off the slave plug. They make inverters in 24 volt, so I will probably wire up one of those some day. I have more possibilities by keeping it stock. What advantages are there to switching it over?

 

[1stSarge]

....if a person will not be running 24V military radios and does not own other military vehicles to jump start or be jump started from, is there a big advantage to the stock 24V setup?

I wanted one good hard working pickup. I bought an 1008. Now, two months later, I’m looking at 109’s, 105’s 101’s 1009’s and of course, at least one plain Jane deuce.

 

[NMC_EXP]

Well, the question is, what is to be gained.

If you have a truck with a bad starter and have a 12 volt one laying around, maybe. But why spend a bunch of time and money to re-engineer something that isn't broken?

The stock setup works, and you still have more than enough 12 volt power to run anything you need.

I don't have any military radios, but I do have cordless power tools I want to run off the slave plug. They make inverters in 24 volt, so I will probably wire up one of those some day. I have more possibilities by keeping it stock. What advantages are there to switching it over?

These was no cost to convert as I needed a new starter anyway. As for time,mine is not worth much anymore.

12V inverters are available and probably cheaper.

The resistor bank is obsolete - if it fails....

I now have one less alternator/alternator belt to fail way out in the boonies.

I now have a "free" spare alternator.

I can now check my power steering fluid without dislocating my wrist to get under the #1 alternator.

I now have the potential to mount a York air compressor where #1 alt used to live (and maybe make it impossible to check the PF fluid).

I want to be able to jump start my 12V junk, er stuff from this truck and vice versa using the slave receptacle with custon cables with alligator clamp ends.

The only downside I see is that it ain't original. But it'll never be entered in an MVPA show so no problem.

Regards

Jim

 

[NMC_EXP]

I wanted one good hard working pickup. I bought an 1008. Now, two months later, I’m looking at 109’s, 105’s 101’s 1009’s and of course, at least one plain Jane deuce.

Used to have a 1962 M37-B1 w/w but the building it was stored in burned down.

My banker/accountant/bookeeper/wife won't let me buy an M35 or M109.

"Really babe, the M35 is just a big pickup truck and the 109 would make a great camper".

Sheesh...have to settle for my now bastardized M1009.

Regards

Jim

 

[ryan77]

Leave it 24 volts make a custom slave plug to run your dewalt power tools.....

 

[blazerman]

Thanks for all the input!!! The long and short is, I have no need for 24 volt power, it simplifies my electrical system to
convert it and better suits my needs. I am not going to be taking the M1009 to shows or rallys....... I need it to function
in my application. Thanks again for all the great advice, I'll post pictures when i'm done!!!

 

[m16ty]

Thanks for all the input!!! The long and short is, I have no need for 24 volt power, it simplifies my electrical system to
convert it and better suits my needs. I am not going to be taking the M1009 to shows or rallys....... I need it to function
in my application. Thanks again for all the great advice, I'll post pictures when i'm done!!!

IMO you need the 24v to start the truck. Sure, 12v will get you by but 24v will turn the starter much better (especially in winter). With what you're wanting to do, I have to disagree that it will simplify your system.

Sounds like you've already made up your mind to go 12v but I really wish you would read and understand about the 12/24v system in these trucks before you do away it. I think it's exactly what you need for your needs. Besides, if your system is working now it doesn't cost a dime to leave it like it is.

 

[watkinssr]

Yeah, not really sure where shows or rallys come into it

 

[NMC_EXP]

24v will turn the starter much better

If comparing 24 vs 12 volt starting, with the same pair of batteries and same engine (starter torque/power required to crank the engine), does Ohm's Law back up that assertion?

Regards

Jim

 

[watkinssr]

24v will turn the starter much better

If comparing 24 vs 12 volt starting, with the same pair of batteries and same engine (starter torque/power required to crank the engine), does Ohm's Law back up that assertion?

Regards

Jim

It's not as simple as ohms law when dealing induction and electric motors. Once coils and magnets get into the equation, the math gets more complicated.

 

[Recovry4x4]

Even Ohms law does apply, Higher voltage means less amperage and the gauge of wire comes into play. The statement of not really needing 24V I understand but I also compare it to "not needing insurance." I'm not planning any crashes now or in the future. One day 24V can come in mighty handy. Also note that the truck is already 12V..............and 24. For those who think this is just a bastardization of an existing 12V harness, you couldn't be farther than the truth. The engineers did a brilliant job at creating this. How many folks are trying to add 12V to their big trucks just to get to where the CUCV already is. The M880 series added a stand alone system with 2 extra 6TLs and an extra alt. Thats adding quite a bit of weight just to operate radios. Sorry, I think the CUCV system is ingenious just as it is. To the OP, by golly it's your truck, do as you see fit.

 

[m16ty]

24v will turn the starter much better

If comparing 24 vs 12 volt starting, with the same pair of batteries and same engine (starter torque/power required to crank the engine), does Ohm's Law back up that assertion?

Regards

Jim

All I know is that with the 6.2 engines I've been around, The 24v starters turn the engine over faster than the 12v do. I could be wrong but that has been my experience.

My personal theory is that the voltage drop in the main starter wire is not as much on a 24v system. You're not having to try and "push" as many amps on 24v as you do with 12v to get the same amount of work done.

 

[watkinssr]

According to Delcoremy, the manufacturer of the starter (talking about the gear reduction version here)
12volt pulls between 125-190 amps

24 volt pulls between 75-90 amps

Bench test readings. no load

RPM of the 12 volt is 3000-5600
RPM of the 24 volt is 3600-5400

So the 24 volt version draws a LOT less current, and probably, under load, will spin the motor faster.

 

[NMC_EXP]

According to Delcoremy, the manufacturer of the starter (talking about the gear reduction version here)
12volt pulls between 125-190 amps

24 volt pulls between 75-90 amps

Bench test readings. no load

RPM of the 12 volt is 3000-5600
RPM of the 24 volt is 3600-5400

So the 24 volt version draws a LOT less current, and probably, under load, will spin the motor faster.

Thanks for the Delco Remy data. According to Delco for a gear reduction unit the 12V starter uses from 40% to 111% more amps than the 24V. The 12V is anywhere from 20% slower to 4% faster than the 24V. Wonder if the same is true for direct drive starters? I assume the DD starters have to work harder.

As I recall with batteries in series (stock CUCV) the voltage is doubled but the available amperage is unchanged. Also if one of the batteries is weak, that is the amperage which you will get.

With batteries in parallel (CUCV with two battery 12V conversion), voltage is constant but the amperage doubles.

Seems to me that when using an electric motor to start an engine what matters is Power (watts). And watts = volts x amps. I do not know how many watts are required to spool a 6.2 up to starting speed.

In a 12V system with two 800 amp series 31 batteries the theoretical power is:

12V x (800A x 2) = 19,200 watts = 19.2 kW.
​

With the same two batteries in the 24V system you have:

24V x 800 = 19,200 watts = 19.2 kW.
​

I suppose a guy would have to measure the total resistance between the batteries and starter to know how much power the starter was actually getting.

I cannot see the 24V system having an advantage unless it requires 19kW to start the engine and there is enough resistance in the circut to really knock down the higher amperage requirement of the 12V.

Of course I had electric/electronics shop class in 1968 so this stuff is getting a little fuzzy. I blame it on boozheimers.

Regards

Jim

 

[watkinssr]

Thanks for the Delco Remy data. According to Delco for a gear reduction unit the 12V starter uses from 40% to 111% more amps than the 24V. The 12V is anywhere from 20% slower to 4% faster than the 24V. Wonder if the same is true for direct drive starters? I assume the DD starters have to work harder.

As I recall with batteries in series (stock CUCV) the voltage is doubled but the available amperage is unchanged. Also if one of the batteries is weak, that is the amperage which you will get.

With batteries in parallel (CUCV with two battery 12V conversion), voltage is constant but the amperage doubles.

Seems to me that when using an electric motor to start an engine what matters is Power (watts). And watts = volts x amps. I do not know how many watts are required to spool a 6.2 up to starting speed.

In a 12V system with two 800 amp series 31 batteries the theoretical power is:

12V x (800A x 2) = 19,200 watts = 19.2 kW.
​

With the same two batteries in the 24V system you have:

24V x 800 = 19,200 watts = 19.2 kW.
​

I suppose a guy would have to measure the total resistance between the batteries and starter to know how much power the starter was actually getting.

I cannot see the 24V system having an advantage unless it requires 19kW to start the engine and there is enough resistance in the circut to really knock down the higher amperage requirement of the 12V.

Of course I had electric/electronics shop class in 1968 so this stuff is getting a little fuzzy. I blame it on boozheimers.

Regards

Jim

well, it gets a bit more complicated than that. You're dealing with coils of wire(inductors) not straight resistance. You are dealing with an increased voltage/current drop across the cables with a lower voltage. It's been more than 20 years since I was an electronic engineering major, so I'd have to look everything up. I'm more than rusty on the formulas. The load an inductor, such as the coil in starter, isn't just straight resistance IIRC.

Where it is going to make a difference is when the batterys get a little long in the tooth, the weather is cold, and you have to crank it for longer periods of time. Higher current draw is harder on all of those situations.