HMMWV Ground Shunt

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steelsoldiers

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So, I am really close to hooking up some batteries to my M998 to see if it will start and run, but I noticed that the ground shunt has been swiped from my battery box. Everything that I have read on the net and in the TM's suggests that it is only really used by the STE/ICE wiring for ammeter purposes. Do you guys think it would be OK to bypass the shunt and run the negative from the battery to a stud, which would have the starter ground bolted to the other end? The wiring schematic doesn't really present any issues with that plan, but I figured it would be good to ask some of you more experienced HMMWV owners. Thoughts?
 

cranetruck

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Chris, the shunt is a low ohm resistor used to measure current indirectly by monitoring the voltage drop across it. If you are not interested in the current, it can safely be bypassed or removed. Typical voltage drop is 25 or 50 mV and won't have any effect on the electrical systems on the vehicle.
 

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steelsoldiers

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Thanks for the feedback guys. I kinda forgot about the shunt after encountering some more difficult engine issues. I will post a full report soon.
 

AOR

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I am no expert and I have never researched a ground shunt but I would think it may act like a fuseable link if something were to overload the system or if there was a short large enough to break the metal bars like a fuse. I am pretty sure I have seen something similar used a slow blow fuse. if it were me I would replace it with the correct one I am pretty sure its there for a reason
 
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patracy

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Well, the shunt shouldn't be smoking. And like AOR pointed out, that became a "fuse" if so. It certainly needs replacement. But further checking needs to happen to find out the short in the system causing the issue.
 

Guyfang

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One of the other nice reasons to have the shunt in the truck, is because it limits the amount of Amprage allowed to go to things like battery charging meters and such devices. Why? Well, you don't need the full Amprage available to be in your control panel, when something shorts or you make an "involuntary" connection with a screwdriver or wrench.
 

Coralsniper

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On my way home today in the m998 I noticed my alternator guage almost in the red, I got home and pulled my battery cover went to start and all kinds of sparks came flying out of the shunt area, checked the voltage and with engine off I was at 26v, looks like my new alternator(replaced 1k miles ago) is overcharging, weird for it to suddenly start doing that. Just thought I would chime in that my "shunt" was freaking out about all the extra voltage.
 

98G

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On my way home today in the m998 I noticed my alternator guage almost in the red, I got home and pulled my battery cover went to start and all kinds of sparks came flying out of the shunt area, checked the voltage and with engine off I was at 26v, looks like my new alternator(replaced 1k miles ago) is overcharging, weird for it to suddenly start doing that. Just thought I would chime in that my "shunt" was freaking out about all the extra voltage.
What kind of batteries do you have?

Fully charged yellowtops are frequently 13.2v each.
 

papakb

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The current shunt is an extremely low value resistor designed to generate a signal in the millivolt range so it's really not a current limiter at all. Unless you have badly corroded high resistance contacts you should never see much more than a volt or two across it. It's sole purpose is to send a signal to the STE/ICE test set for monitoring current draw to the system but primarily starter current.
 

Coralsniper

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I have two 6TL batteries replaced new 6 months ago, I was thinking about that and 13v should be fine I think your right. I think I'll take the cap of my 60amp alternator tomorrow and measure the voltage coming out of it, should be about 28v correct?
As for my shunt could it just be dirty contacts? The sparks were coming from the forward of the two bolts that hold the shunt in place. I attached a picture of it and small burns are visible.
 

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Coug

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Batteries during charging = 28-29V
Batteries after shutdown = 26-28V
Batteries after resting, 100% charge = 25.2V (minimum) or higher, usually above 26V for several days if properly charged.

A fully charged 12V battery will read 12.6V At 12.2V it is considered to be fully discharged, trying to crank an engine on this battery will both shorten the life of the battery as well as the starter, as lower voltage will cause the starter to draw more amperage in order to turn.

Alternator/generator voltage has to be higher than the intended battery resting voltage, or it will never charge up.
 

Coug

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if you are seeing any sparks, then there is a bad connection, usually just a loose wire.
Typically the only time you'll get sparks is when the electricity is trying to jump a gap. The shunt should just be a bunch of metal bars going from one point to another point.

The burns in your picture are very typical of electrical arcing.

Disconnect the ground terminal on the battery, then go through all the bolts on the shunt (and the rest of the electrical connections while you are in there) to make sure all of them are tight. Any connection that looks like it might have any corrosion take it apart and clean thoroughly with a wire brush, then reconnect and torque down (I believe the TM will have torque specs)

Will probably be easier if you pull the batteries out completely to make sure you get everything properly, can be difficult in tight space with the batteries in.

You might already know this part, but I'll add it just in case someone who doesn't comes across this thread:
Since you mentioned the 60 amp generator, if you haven't done it recently (like the past month or two) now would be a good time to switch the battery positions, so the front battery to the rear, rear to the front. There's nothing to keep the batteries equalized, and the forward battery tends to get more power than the rearward, so rotating position every month or two will help balance out the charge.
 

Coralsniper

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Great info, I will do all the above tomorrow! I had no idea about the un equalizing aspect of the 60amp alternator, I was planning to buy a battery equalizer (from big rig setup) as a way of pulling 12v power evenly for my accessories, this may also help with the 60amp and its favoritism for the front.
 

Milcommoguy

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You will NEVER see a volt or two across the shunt meter points in a HMMWV, UNLESS the shunt elements have detached or corroded away from the end poles. ( Can't afford those kind of voltage drops to run a HMMWV)

It is NOT a fuse or a regulator. In the case of a HMMWV, IF it blows open.... YOU GOT BIGGGGERRR problems.

This shunt is rated at 1000 Amps = 100 millivolts or mV. Basically a precision resistor. Adding the word shunt, usually invokes a need to measure a higher current than a standard wattage resistors using a voltmeter. OHMS LAW shows up again in the HumV.rofl

In the case of the HMMWV at 1000 Amps of current flowing one would expect to see 100 millivolts or 0.100 volts. Very VERY small voltage.

So 0.050 Volts = 50 millivolts or 500 Amps. (that's a LOT of Amps) I weld with 10 to 250 Amps average 90 or less.

With a good starting engine, 75 degrees, average starter current 180 AMPS, you would measure 18 millivolts or .018 Volts

IMG_8010.jpg
MY TRUCK STARTER RUNNING ONLY. INJECTOR SOLENOID DISCONNECTED. NO run condition. 180 Amps thru the shunt = 0.018 DC Volts.

One or two volt... I would run🏃‍♂️ and plug my ears 🙊and wait for the fire department.🔥🚒

The picture IMO could use a little PM. The left connection bolt is easy to figure out. The right one is a feedthrough to the other side with lugged ground wire to starter frame ( your first ground to the rest of the truck).

It gets loose or mechanics fail to work the other side and properly tighten the whole assembly up. It's easy to do... just have to do it. More or less the same for the positive feedthrough too. Careful not to crack / bust the insulator on the HOT one.

Wire brush clean and properly tighten.... AND something to check on a new auction rig. Your picture looks to have been sparking and arcing, MG, My Guess.

The HMMWV shunt can be a useful diagnostic tool for charging, starting and glow plug current measurements, but for light loads like... lights, horn, heater motor, IP solenoids, etc. best to get in line with the circuit.

Always something to tighten, clean, check, wipe down, learn, etc, CAMO

AND don't forget to disconnect batteries when working.
 
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Guyfang

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You will NEVER see a volt or two across the shunt meter points in a HMMWV, UNLESS the shunt elements have detached or corroded away from the end poles. ( Can't afford those kind of voltage drops to run a HMMWV)

It is NOT a fuse or a regulator. In the case of a HMMWV, IF it blows open.... YOU GOT BIGGGGERRR problems.

This shunt is rated at 1000 Amps = 100 millivolts or mV. Basically a precision resistor. Adding the word shunt, usually invokes a need to measure a higher current than a standard wattage resistors using a voltmeter. OHMS LAW shows up again in the HumV.rofl

In the case of the HMMWV at 1000 Amps of current flowing one would expect to see 100 millivolts or 0.100 volts. Very VERY small voltage.

So 0.050 Volts = 50 millivolts or 500 Amps. (that's a LOT of Amps) I weld with 10 to 250 Amps average 90 or less.

With a good starting engine, 75 degrees, average starter current 180 AMPS, you would measure 18 millivolts or .018 Volts

View attachment 800799
MY TRUCK STARTER RUNNING ONLY. INJECTOR SOLENOID DISCONNECTED. NO run condition. 180 Amps thru the shunt = 0.018 DC Volts.

One or two volt... I would run🏃‍♂️ and plug my ears 🙊and wait for the fire department.🔥🚒

The picture IMO could use a little PM. The left connection bolt is easy to figure out. The right one is a feedthrough to the other side with lugged ground wire to starter frame ( your first ground to the rest of the truck).

It gets loose or mechanics fail to work the other side and properly tighten the whole assembly up. It's easy to do... just have to do it. More or less the same for the positive feedthrough too. Careful not to crack / bust the insulator on the HOT one.

Wire brush clean and properly tighten.... AND something to check on a new auction rig. Your picture looks to have been sparking and arcing, MG, My Guess.

The HMMWV shunt can be a useful diagnostic tool for charging, starting and glow plug current measurements, but for light loads like... lights, horn, heater motor, IP solenoids, etc. best to get in line with the circuit.

Always something to tighten, clean, check, wipe down, learn, etc, CAMO

AND don't forget to disconnect batteries when working.
AND don't forget to disconnect batteries when working.

The above warning is ignored at your own risk. For 25-30 years I carried an old 1/2 inch, open end, box end wrench around in my tool box. It was melted half way through the grip. I never used it, it was just there to remind me about what can happen when I didn't at least remove the main Negative cable from the battery's.
 
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