1952 - 1968, M38A1, M170, questions, discussions, regarding anything related to the M38a1 series. Show mikesjeepG-Sergeant First Class  jumping a 24 volt systemJust a thought. Also has anyone ever seen or have a inter vehicle slave cable that connects one slave on a jeep to another? So that you can jump the jeep this way? Does the slave connect to other items such as search lights, generators etc? Thanks in advance
Rich SaylorG-Major General jumpingPost by Rich Saylor » Wed Jun 14, 2006 6:16 pm To only jump one battery, you'd connect the external battery ground to the vehicle battery's ground (either one), then positive to positive. For only one, don't connect to vehicle ground, just directly to whichever battery's ground you're jumping to. This may not be enough to start the vehicle if both batteries are low. Another, more effective way would be to get 2 jumper (12 volt) batteries, and put them in line just like the 2 in the Jeep, that is connect the external battery ground of the first battery to vehicle ground (at the under hood battery) then jump from the first external battery's positive post to the second external battery's ground, then from the second external battery's positive post to the cowl battery's positive post. Make sure that all the connections are corrosion-free, as that's often the cause of low or non existent cranking or vehicle power. As for built in jumper connections, that's what the M38's hood cutout is for- a jumper outlet hooked up to the 2 batteries as previously described, with a pin connection (normally sealed with a water-resistant screw-on cap) that a heavy duty accessory jumper cable is plugged into, one end going to each vehicle, or whatever you have a matching pin connector for that uses 24 volts. The wavy hood cutout provides a space for not only the jumper plug mounted on the passenger side fender, but also a mounting point for the snorkel hose (that goes up the right side of the windscreen) for deep water fording. Both systems were supplied as field-installed accessories, to the best of my knowledge. Rich Wes KG-Lieutenant General Post by Wes K » Wed Jun 14, 2006 8:05 pm Jumping only half of your batteries is not a good a plan. If you know for a fact that one of your two batteries is bad then remove it and sub the one 12 volt you have. As a general rule when a dual 12 volt powered 24 volt system is too low to start both batteries are low. By jumping a good 12 volt battery to only one of the two will quickly take your fresh 12 volt down to the level of the other 24 volt sytem's battery. The best choice is to buy a 24 volt system battery charger. Wes K
Re: jumping a 24 volt systemPost by nf6x » Thu Jun 15, 2006 3:23 pm
I've never tried to jump start between two vehicles with different electrical system voltages, and I don't see any reason why I'd want to try it unless somebody was shooting at me and I had to get the vehicle moving Right Now. Unless it's a life-or-death situation, I'd rather burn the money on a tow or taxi ride than risk burning one of the vehicles. Or myself, if I managed to burst a battery and splash acid around.
Yes. I have one, and have used it dozens of times. I have the new-style "1 pin" NATO cable (there's still two conductors, but the plugs are coaxial with one pin in the middle) and a pair of adapters to plug into the older two-pin plugs used on older vehicles like the M38, M38A1, etc. The new cable with the adapters can be used with either style of connector in any combination, as long as both vehicles run at the same voltage (which should be 24V for almost any post-WWII US or NATO MV unless it's been hacked up into a 12V configuration by a civilian owner, and I don't condone that practice at all).
I've seen things such as electric impact wrenches that can plug in there. I think they're most commonly just used for jump starting, but they're somtimes used for electrical accessories, and I don't see any problem with using them for that purpose as long as reasonable precautions are taken (because it's a direct, un-fused connection straight to the battery). The slave connectors and cables may seem kinda pointless if you only have one 24V MV, but if you have access more than one then it's very handy and makes jump-starting a lot easier and safer. If you or a nearby friend have multiple vehicles, it's handy to have the slave cable around. (Edited to add:) For recharging my batteries without jumping them from another vehicle, I use a pair of identical 12V battery chargers that I bought at Target. I disconnect both batteries from the vehicle and each other before I clip on the chargers. You can charge one battery at a time if you only have one charger, but that will take twice as long, of course. One of these days I'd like to buy or build a 24V charger and rig it up to plug straight into the slave cable connector, but using a pair of cheapy chargers on the disconnected batteries works well enough in the mean time. Wes KG-Lieutenant
General Post by Wes K » Thu Jun 15, 2006 9:54 pm You're working too hard on the battery charging thing. If you're going to disconnect the jeeps batteries then just charge both at the same time with one charger by connecting the two batteries parallel. 24 volt charges are not that expensive. From $75 for the small 5 amp aircraft specialty shop units to $300 for the commercial truck fleet 6-12-24 types. The small surplus MEP series generator sets with a slave cable work great for charging and jump starting. Wes K
mikesjeepG-Sergeant First Class 24 volt inter vehicle cablePost by mikesjeep » Tue Oct 17, 2006 4:12 pm Where did you get the inter vehicle
cable. one that connects from one jeep to another via the slave. M38A1 MP JeepG-Colonel Post by M38A1 MP Jeep » Wed Oct 18, 2006 6:52 am A couple questions come to mind. 1. What is the correct intervehicular cable length on the early M100 trailer to M38A1? I have several of different lengths 2. What is the correct routing of the cables from the slave connector up through the firewall to the battery box? 3. I assume that the positive cable goes to one of the positive posts on one 12 volt battery and the negative post on the second 12 volt battery. Is this correct? 4. What is the minimum size 24 volt charger one would need to charge both batteries? Wes outlined the low and high ends, but was is a realistic requirement for one? Thanks Jon I am seeking information on M38A1's configured for MP use from 1953-?. I am also interested in equipment and uniforms worn and carried by MP's during this timeframe. Post by petesilfven » Wed Oct 18, 2006 7:14 am Jon, if you're not in a hurry to get the charge done, just go to www.batterytender.com and get a 24 volt Battery Tender. I have one permanently installed on my firewall, and I just plug it in when the vehicle is not to be driven for awhile. MVPA 21473, GLMVPA
M38A1 MP JeepG-Colonel Post by M38A1 MP Jeep » Wed Oct 18, 2006 7:26 am Pete Thanks... I think a battery tender would be prefered over something used to try to put a full charge in a hurry Thanks Jon I am seeking information on M38A1's configured for MP use from 1953-?. I am also interested in equipment and uniforms worn and carried by MP's during this timeframe.
Wes KG-Lieutenant General Post by Wes K » Wed Oct 18, 2006 7:54 am
This has been well researched for the M38 but not the A1. Most M38's left Willys with the cables installed but the very late ones did not. The M38A1's did not leave Willys with the cables. They were installed in the field when needed. The ground cable length would be adequate to get from the receptacle to the frame straight down below the floor below the tool box area where the radio ground is normally connected or to the voltage regulator tray. The positive cable should actually go to the battery cable terminal on the starter switch not to the battery itself.
The exact
length ORD 9 lists the entire cable as DKI# 73-1656 ORD# 7728811 and the cable only as DKI# 73-1651 ORD# 15-C-2164-750 or 7056686. TM 9-2330-201-14 only lists the cable assembly as FSN 2590-855-9304 and ORD# 8722864. If you can't find a cable marked with one of these numbers then use the cable that will allow free left and right swibg without stretching the cable and yet the cable doesn't hang below the "A" frame drawbar.
It depends on waht you want the charger to do for you. As Pete said a 1 or 2 amp maintainer works fine for keeping the battery charged when not in use but will not give your dead battery a quick charge to get you on the road quickly. A 5 amp (which is the common middle size) will get you charged up in about 1 hour. An industrial 6-12-24 charger with start boost will get you charged up in an hour or less and can also give you a heavy boost to start up right away with a low battery. Wes K
M38A1 MP JeepG-Colonel Post by M38A1 MP Jeep » Wed Oct 18, 2006 8:53 am Wes Thank you. You make an interesting point, backed up by many period M38A1 pictures I have - the slave cable plug not being installed. Instead, there is the plate. I have seen the plate on many restorations and always thought that this was done after it was released by the military by a civilian owner. You made the comment "The ground cable length would be adequate to get from the receptacle to the frame straight down below the floor below the tool box area where the radio ground is normally connected or to the voltage regulator tray." Question: Where does the radio ground normally connect? Thanks Jon I am seeking information on M38A1's configured for MP use from 1953-?. I am also interested in equipment and uniforms worn and carried by MP's during this timeframe.
Wes KG-Lieutenant General Post by Wes K » Wed Oct 18, 2006 12:27 pm The M38 and A1 are under the tool box on the frame if they followed the service manual directions. These photos are from TM 9-8015-2 C3 (Sep 66)
Last edited by Wes K on Wed Oct 18, 2006 3:04 pm, edited 1 time in total. Wes K Post by petesilfven » Wed Oct 18, 2006 2:18 pm Best way to start a dead one is the good ol' tow strap. Only takes a minute. I haven't had to jump mine in over 5 years, because I keep the batteries up to begin with. Usually rolling down the drive and popping the clutch will get things going. MVPA 21473, GLMVPA Bob P.LT-USN,
Retired Post by Bob P. » Sat Oct 21, 2006 2:48 pm When I installed my Aux Power Recp. I spoke with George Baxter @ AJP because I could not understand TM 9-8012, pg 191, b. installation, para (3). He told me the ground is attached to the starter front support using the bolt that secures the commutator end head to the support. after I read it about 20 times I realized that is what it said. Other end goes to starter terminel. Hope this helps. Wes, where can one get the surplus MEP series generators w/slave cable? Thanks, Bob Perrett 52 M38 (Ugene) Wes KG-Lieutenant General Post by Wes K » Sat Oct 21, 2006 3:16 pm The M38 uses the genny front mount for a ground but the M38A1 does not. The reason I listed the M38A1 routing was because "M38A1 MP Jeep" (Jon) was the one who asked and he has an M38A1. You'll have to hunt for a Generator set Bob but I don't think you'll find one with the cable. I had to pick up a cable and adapt it. Wes K Return to “M38a1 Technical Knowledge Base” Jump to
Who is onlineUsers browsing this forum: No registered users and 1 guest Can you jump start a 24 volt system with a 12 volt battery?No, not normally. Attaching the 24V battery to the 12V battery could easily cause the jumper wires to spark, melt, and probably explode the 12V battery. It's very dangerous. You'd need 2 X 12V batteries connected in series to match the trucks 24V system safely and start the truck safely.
How do you jump 24V with 2 12V?Two or more 12-volt batteries wired in parallel—positive to positive, negative to negative—is still a 12-volt system. Two or more 12-volt batteries wired in series—the positive terminal of one battery connected to the negative terminal of a second battery—develops 24 volts, but amperage doesn't change.
How many volts does a 24 volt system need to start?The target voltage for a 24 volt charger for AGM or some flooded batteries is 2.4 to 2.45 volts per cell, which is 28.8 to 29.4 volts.
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How do you jump a 24 volt system
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