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Going to tag @Minuit here since he’s one of our electrical gurus (I’m sure there are others as well). 
 

Wanted to discuss the actual current draw and charging system of the HO MJ’s/XJ’s. Not sure how much they have in common with the renix era stuff but the principles are the same. 
 

Alternator: when it comes to the current output, alternators are typically rated at a specific engine rpm (~2500) and a specific voltage. I’m no expert in this field and am mostly self taught from tinkering with electronics. But from what I have learned, as voltage decreases amp draw increases to maintain the same accessory output (eg electric fan). 
So let’s say a 90a alternator is only putting out somewhere around ~50a when idling. 
 

The HO charging system runs from the alternator to the pdc on an 8awg cable that has a splice to 2 8wag cables that enter the pdc. It then goes to 2 different 60a fuses and on to the fuse buss bar. The battery is connected to the opposite end of the pdc via the two stud posts. This is how the battery is charged on the stock system. And how it provides power to the pdc when the engine isn’t running. 
 

Now on to my issues (and I suspect many of you have had or will have similar issues). I run a FF dynamics triple 10 fan setup with shrouds. This is an overpriced crap system. Amp draw for each fan is 5a while running but startup is about 35-40a. Max cfm is 900 cfm but that’s at 14.4v and you will rarely see that kind of voltage at idle with that much current draw. So let’s tally this up (and this is where I need your expertise)

 

Fuel Pump draw: ?

Ecu draw: ?

Lighting draw: ?

Fan draw: ? 
TCU draw: ?

AC clutch draw: ?

HVAC blower draw: ?

 

I'm sure there are many other items. My point being, I don’t think that 50a at idle is nearly enough to power everything. Thoughts?

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my basic understanding of automotive electrical systems is that your battery provides the power and your alternator recharges the battery.  so the 50 amps might be just fine because your battery has more than enough reserve power to fire up the fans (and then the alternator will replace it while the fans are running). 

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51 minutes ago, Pete M said:

my basic understanding of automotive electrical systems is that your battery provides the power and your alternator recharges the battery.  so the 50 amps might be just fine because your battery has more than enough reserve power to fire up the fans (and then the alternator will replace it while the fans are running). 


If that’s the case then no accessories would run at full alternator voltage (~14.25a). The alternator needs to be able to handle the full load of all accessories. That’s why it goes directly to the PDC I believe but perhaps I am mistaken

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Alternators aren't built to handle full draw on idle. That is why many police packages have a high idle option to spin the alternator faster. The biggest draw in the electrical system is the blower motor about 13A. Lights draw around 12A if not LED. Stock electric fan is about 12A, Everything else is pretty small probably totals around 5A. BTW the jeep stock fan produces 2200 CFM. Pretty impressive. Full load should be 30-40A.

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14 minutes ago, Limeyjeeper said:

Alternators aren't built to handle full draw on idle. That is why many police packages have a high idle option to spin the alternator faster. The biggest draw in the electrical system is the blower motor about 13A. Lights draw around 12A if not LED. Stock electric fan is about 12A, Everything else is pretty small probably totals around 5A. BTW the jeep stock fan produces 2200 CFM. Pretty impressive. Full load should be 30-40A.

That makes sense. I would think the fuel pump would be at least 5a also. Ac clutch has got to be a bit of a draw also. AC on idling in traffic and there is definitely a significant voltage drop on a 90a alternator. Those FF dynamics fans need a solid 14+ volts to keep temps in check. I have a singer 270a alternator I’m sending in for a rebuild but I seriously considering switching back to a mech fan setup. The ff system was a big investment to just dump though. 

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Sorry to disappoint, but I don't have good numbers on how much the various accessories draw. Won't stop me from guessing though. And it also reveals a shame I have to admit: surrounded by electrical test gear I may be, I don't actually own a DC clamp meter. I literally don't have the ability to measure DC currents over 10 amps. So, as a result of this, these figures are a combination of intuitive guessing based on the wire gauge and fuse rating, what the FSM says, what I found on google, and what I pulled out of my @$$. So take everything you read here with a grain of salt and a considerable tolerance. I'm a semi-pro vehicle electronics person and I hold no formal credentials in this area but have spent quite a bit of time studying.

 

Not all of this is a direct response to you @ghetdjc320, but I had some extra stuff I wanted to add that might be useful to someone, and might end up in a compilation post someday.

 

TCU: Probably well under 5 amps, the highest draw almost certainly being from the solenoid driver transistors. The solenoids are specced at 11-15 ohms; they'd draw about an amp each. Highest draw would be in second gear. Just sitting in park, I don't think any of the solenoids are active. Probably contributes nearly nothing in an idle condition.

 

A/C clutch: Roughly 5 amps or so, maybe a little less seems reasonable to me. Couldn't find a spec in the FSM. UAC, a popular aftermarket supplier of remanufactured paperweights A/C parts gives 3-5 ohms as a typical resistance for a clutch coil.

 

HVAC fan: I honestly have no idea. This circuit is fused at 25A on HO models and fed by a 12AWG wire. 15-20A on full blast sounds about right to me, with each lower position subtracting about 5 amps. An aged blower motor with dried out bearings could cause this number to creep up. I recommend replacing these pre-emptively with the Four Seasons version that comes with the wheel attached since this seems to be such a common problem. Calculating current draw of motors is actually somewhat more complicated than the usual V=IR that we try to keep things to around here, but moving air does take quite a bit of effort.

 

Fuel pump: Fed by a 14AWG wire on the HO models, although it does run from the PDC to the fuel tank. The factory fuel pump doesn't strike me as being an especially ballsy unit and the ballast resistor will drop that circuit's current draw by a bit, but I'm not really up to date on my fuel pump amp draw numbers. I'll give it 10A, but I really think it's probably lower than that.

 

Aux fan: Not even going to guess. I'm also not going to blow the fuse on my meter to go find out. Big startup spike, less big steady state draw. As you can tell, the power consumption of electric fans is a nontrivial issue. I like my clutch fan, to be honest.

 

ECU: Coordinating six fuel injectors going pew-pew and doing all of the other stuff the ECU does draw a bit of power, but not THAT much. I'd say a couple of amps on a continuous basis tops. As far as things that draw a significant amount of current, the ECU is responsible for switching a few relay coils (150mA or so each) and pew-pewing the 14-ohm fuel injectors for a few milliseconds at a time.

 

Lighting: Let's say you've got two 55W low beams, and a bunch of 2057 bulbs with their 7W dim filaments on only. Add in the license plate lights and that's about 12-14A worth of lighting in total. Add a little more for instrument panel lighting and dome lights and stuff like that if you really want to be particular.

 

Since I know someone will ask, a radio playing at normal listening volume will add less than an amp. Much of that will be the light bulbs on the radio. Amplified systems are of course more, but their draw is transient. Of course, a big enough sound system will create a need to install a bigger alternator.

 

I went out this morning and tested things out on my truck. At idle, I turned absolutely every electrical load on I could except the A/C clutch - it's evacuated right this minute. I pulled the plug for the coolant temp sensor to make the aux fan run. Windshield wipers, fog lights, blower, everything. I didn't see a voltage below 14V at the battery terminals, and that's with my original 90A alternator. For what it's worth, I have a new aftermarket 97+ style auxiliary fan on my truck.

 

The factory alternator is sufficient for the factory setup, and maybe a little more if you switch out some halogen bulbs with LEDs or something like that. Jeep wouldn't design a system that lets you run the battery down by idling with accessories on. But once you start adding much of anything to that system, it's time to consider a new alternator. But "upgrading" to the biggest alternator you can stuff under the hood is no good if the smaller unit is up to the task anyway.

 

I wonder how the police package "extended idle" option is integrated into the ECU. Probably just grounds out a pin on the ECU plug or something.

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Regarding the initial observation, Pete and Limey are actually right - while the alternator may not be able to fully satisfy the needs of the system, the battery can more than compensate.  This means you're not running at full charging voltage, of course, but systems on cars are not designed to only operate at charging voltage.  Regarding cooling capacity, marginal fan output is cubically related to fan power, and the energy dissipated by the radiator is not a constant function of temperature, either.  I would expect that the ~2V difference would not impact your actual cooling rate. Without expending the absurd effort of figuring all the flow rates and fan speeds, you should really only concern yourself with two outcomes: if your truck overheats at idle, and if your truck's battery discharges an unhealthy amount for any reasonable length of idle.  And even if you're continously short 10 Amps, the battery still won't discharge an appreciable amount in less than an hour.  This is something I recommend testing rather than trying to achieve via theory.

 

If you're at all concerned about the alternator output, I've heard it's supposedly fairly easy to install a 136A alternator from late-90s Dodges.  You can also swap to a smaller pulley to increase the speed at the expense of alternator lifespan.

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2 hours ago, scaleless said:

Regarding the initial observation, Pete and Limey are actually right - while the alternator may not be able to fully satisfy the needs of the system, the battery can more than compensate.  This means you're not running at full charging voltage, of course, but systems on cars are not designed, of course, to only operate at charging voltage.  Regarding cooling capacity, marginal fan output is cubically related to fan power, and the energy dissipated by the radiator is not a constant function of temperature, either.  I would expect that the ~2V difference would not impact your actual cooling rate. Without expending the absurd effort of figuring all the flow rates and fan speeds, you should really only concern yourself with two outcomes: if your truck overheats at idle, and if your truck's battery discharges an unhealthy amount for any reasonable length of idle.  And even if you're continously short 10 Amps, the battery still won't discharge an appreciable amount in less than an hour.  This is something I recommend testing rather than trying to achieve via theory.

 

If you're at all concerned about the alternator output, I've heard it's supposedly fairly easy to install a 136A alternator from late-90s Dodges.  You can also swap to a smaller pulley to increase the speed at the expense of alternator lifespan.


I have a burnt out Singer 270a alternator I’m getting rebuilt. The ultimate issue is these fans. They are marginal at 14v and the cfm is too low at 12.5v. All the added electronics (HID lighting, arb compressor, triple fans, audio system) I’ve integrated are just too much to maintain a good voltage. 

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4 hours ago, scaleless said:

the energy dissipated by the radiator is not a constant function of temperature


Please elaborate. The the only energy dissipated by the radiator is thermal unless you are referring to a disruption of flow??? 

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Very interesting discussion, but I am lost as to what the issue is here? Is it that your triple fan setup is crap?

I've got a 137A ZJ alternator in my '92....only because I've got a big ol' amp and 2 10" subs behind the seat. Did I actually NEED it? Prolly not. But I don't ever have issues with my charging circuit, even at idle. My plan is to put in a winch up front and hopefully this should be able to keep up with that over intermittent use.

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4 hours ago, Tex06 said:

Very interesting discussion, but I am lost as to what the issue is here? Is it that your triple fan setup is crap?

I've got a 137A ZJ alternator in my '92....only because I've got a big ol' amp and 2 10" subs behind the seat. Did I actually NEED it? Prolly not. But I don't ever have issues with my charging circuit, even at idle. My plan is to put in a winch up front and hopefully this should be able to keep up with that over intermittent use.


It’s more of a brainstorming session regarding the HO charging system. I’m also running a Warn 10k winch along with all the other electrical add ons. The electric cooling fans have always been my number 1 issue. They have a real performance difference between 12.5 and 14.25v. My Singer alternator needs a rebuild so I sent it off and installed a new Napa 90a. While reviewing the spec sheet that came with it it got me thinking about what is actually happening with the voltage in my MJ. I just got done completely revamping and relooming the entire under hood harness. Replaced connectors, repairs and broken insulation and tested every single FI and charging system circuit. Alternator output is 14.25 at idle but once I turn on ac, fans and lights I drop down to about 13v. Not a big if it wasn’t for the fact that the e fan output drops quite a bit with that 1.25v difference. Enough that it can’t keep up with the cooling demands in traffic at idle. 

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So I monitored my voltage at the pdc, alternator and battery (optima yellow top approx 2 years old). With no load at all I have 12.95v from the battery which is right on. When idling i have 14.28 until I turn on AC, fans and headlights. It drops to 14.17 and hold steady for about 1 minute. Over the next 2-3 minutes the voltage steadily drops to about 13. This 1 volt of difference is enough to drop my fan output below what it needs to keep up with this engine (fairly heavily built). 
 

 I sent off my Singer 270a to get rebuilt and plan on putting it back on either way but am seriously debating where to go for cooling, the factory fans didn’t cut it when I had that setup. Kind of back to the drawing board on this one. Again, idle over temp is my only issue. 

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