where to take HMMWV ignition +

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privatevince

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My HMMWV came with USShift Quick 2 transmission module hooked up to one of the 2 batteries, and manually switched on and off before and after every drive. found out: the Quick 2 must be switched, otherwise it drains the battery flat in less than one day. aua

i thought that's not a good way, so i bought a 24to12 (10Amps) converter and plan to automatically have it powered in the run position.
(the converter with nothing hooked to it is "warm", so i assume it is using power all the time, not only when supporting certain devices.)

where can i take ignition + (or as we call it on the other side of the pond: switched +)
do i need to put a relay in the circuit of the run/start switch or is there a better place to take this from?
 

WillWagner

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You could still use it like it is hooked up now if you used a relay switched with 24 to pull 12 from the second battery. Need a drawing?

Keep in mind that it isn't good to use 1 battery to supply a high amp draw device, but if it isn't used for hours on end, it will be fine.
 

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privatevince

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thanks Will,
i want to use the (already bought, $ 17) converter just to draw equally off the 24V system. also, don't want extra small cables screwed to the big battery cables or terminals which is always a mess.

maybe someone knows a good spot where to steal 12V on ignition. otherwise i'll use a set of in line connectors to take it from the run position... but that's cheap and dirty imo
 

papakb

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The #54 wire to the top of the IP is powered up with the ignition switch and would be an easy place to access a switched 24 volt line for your converter. If you want to drop the instrument cluster you could pick up the +24v signal to the gauges. Tee into one of the ribbed connectors on the gauges.
 

papakb

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There's no need to complicate things with a relay when it's a simple matter to feed a converter directly from a line switched by the ignition switch. When it comes to electrical circuitry the KISS principle works for non-electricians.
 

privatevince

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i like that principle.
but the converter (with the module) takes around 7.5 to 10A max.
don't know if that ignition curcuit is capable of handling the extra load.
safe before simple
 

MarcusOReallyus

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You've got the right idea, Vince. Not just the obvious safety factor for you, but for the vehicle. These things are not well engineered, and they are not new. A relay will protect your investment. If you need help with wiring it, give me a holler.
 

MarcusOReallyus

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There's no need to complicate things with a relay when it's a simple matter to feed a converter directly from a line switched by the ignition switch. When it comes to electrical circuitry the KISS principle works for non-electricians.


  1. You're just flat wrong, for the reason Vince pointed out. Try to pull that amperage from that circuit without a relay and you are going to be playing with fire, literally. Just because it's "simple" doesn't mean it's good.
  2. I don't mean this as an insult, but seriously, a relay is a very simple device. If electricity is such a mystery to you that a relay seems complicated, you just should not mess with anything electrical, and you certainly should not give advice on the subject. That's simple.
 
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Wire Fox

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There's no need to complicate things with a relay when it's a simple matter to feed a converter directly from a line switched by the ignition switch. When it comes to electrical circuitry the KISS principle works for non-electricians.
I think this might be the first time I've ever disagreed with you on a matter. Like Marcus was saying, I think that the current draw is just too high to trust the extra load on this switched circuit. Just about everything in the vehicle runs through one of two circuit breakers. One is just the heater's blower motor. The other one is pretty much everything else. I'd be concerned with splitting all the lights, gauges, and then that converter on the one circuit. It's probably just within the margin to work, but it seems like it's just asking for trouble when someone could just drop on a small relay that can easily be rated to pass 15A of current to handle that converter with no sweat.
 

papakb

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Sorry gents, I hadn't considered the high current loading you might get with a voltage converter. Your right, in this application a relay is the best way to go. My solutions would work well for low current loads like LED lighting. As an engineer I've seen too many cobbled together nightmares and we know the dangers of bad wiring so I tend to tell people to keep it simple whenever possible. When it comes to power, Bubba is not your friend.
 

privatevince

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Hey people,
it's great that there's debate! and good at last we all agree about the too high current.
to narrow things down i (great artist) drew two circuits...
tell me what you think (also, Marcus, throw some other in here, if you have a good idea, please!)
Scan11062019_2.jpg
 

Wire Fox

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Figure 1 doesn't look electrically accurate. Figure 2 does. The negative on the relay magnet will go to a ground point. The positive is a branch off of the run switch. That way when you switch to run, it has power to move the armature and complete the circuit. The switching side of that relay is just one side of the circuit-preferrably the positive side. It just interrupts whether there is positive voltage going to the voltage converter or not. The converter is constantly grounded, just as the relay is constantly grounded.

Sent from my Nokia 6.1 using Tapatalk
 

papakb

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Keep in mind that the body of the HMMWV is not a reliable ground because of it's construction. It's not like your regular car. Your relay return line ( - ) should tie back to one of the vehicle ground points. There's one behind the heater controls or you could tie into the ground point on the rear of the left cylinder head where the supplemental harness attaches.
 

papakb

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HMMWV dash ground image:

HMMWV dash ground highlited.jpg

This ground is a known problem point and is typically the cause of a lot of strange gauge readings when it loosens up or gets corroded. What I did with mine was to replace the screw with a 1" long stainless setscrew with a star washer and nut on both sides. This way I could tie in grounds in the engine compartment and the dash areas without disturbing the ground itself. It won't hurt to use some anti-oxidant grease on them either.
 

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pjwest03

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My favorite item for that is NO-OX-ID. I learned about it as a favorite of antenna maintenance crews. I also use it in the Packard connectors.
 

privatevince

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that's great advice
i can easily check behind my instrument panel, the guys from the shop bolted it with a single bolt :roll:

pj, you can't even buy stuff like that over here. our car aftermarket simply doesn't know "dielectric grease". what we have is battery post grease...
gonna bye no-ox-id overseas!
 
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