2018 Ram 3500 Laramie 4 wheel drive long bed 6 l diesel with dead dash won't start batteries good

[62Blazer]

The engine supplies most of the draw, the fuse or relay obviously couldn't supply all the power it takes to run an AC, which is why it doesn't. I don't know the amps, but it's a lot. Thus why you lose up to 10% of each gallon of gas when it's running.

The AC draws nowhere close to 200 amps. Maybe on an all electric vehicle, but definitely not on a normal gas powered engine. Just look at the size of any wire going to the AC compressor and it's not going to be big enough to handle 200 amps. The only electrical power it needs is enough to lock up the clutch on the compressor which is probably only 3-5 amps. After the clutch is locked up the engine is providing all the power to spin the compressor. There is more electrical draw but that would just be if you had the HVAC fans inside the truck on. That's not directly related to the AC as you don't have to have them on, but would guess if you wanted the AC on idling you probably had them cranked up. I would suspect the fans to use upwards of 30 amps if on high.

 

[Burla]

Maybe on an electric vehicle it does? Boom moment? Energy is energy, if it takes 200 amps to run an AC unit in an EV, it also takes 200 amps to run it in a gas engine (give or take), it just isn't run directly off the battery. Now I know it isnt 200 amps which is why I said less, it is adding every item together to run an AC system and yes I added some, it was lazy math didn't expect 20 guys to die on this hill. I believe EV ac's are over 100 amps mostly (up to 150), which means I also know the hemi ac is close to the same.

 

[Burla]

So the question is, if the alt puts out less then the 150 amps plus any other item he has on such as radio etc at idle, where does the power come from? These are the numbers our alts put out, mind you at rpm not idle see below... Question remains, where does that power come from? If the amps alt's put out at idle is less then what is being put to the electrical system demands, it comes from one place, the battery. As far as an ECM kicking up idle, have you ever seen a ECM kick up idle to peak alt rpm's? Me neither, it is only a small amount of air and fuel and then the ECM lets the battery take over. These trucks are not designed to run ac at idle with an aging battery. it they were, the ECM would kick the idle to 2k rpm's and that would be that, but that doesn't happen.

• Standard Alternators:
  • Hemi V8: Often equipped with a 180-amp alternator.
    
    
  • 6.7L Diesel: Standard alternators typically start at 180 amps, but can be 220 amps or higher on Laramie, Longhorn, and Limited trims.

 

[Ritchie_Rich]

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No problem, I can help you with that. Notice the pulley, that is what supplies the lion share of the power to run that item. That is a ram A/C compressor mostly powered by the pulley not going through a relay. That item takes about 2 gallons of fuel to run per tank, good luck converting that to amps, it's complicated. Now if that battery is the original battery, or in the case 2, from 2017 when the batteries were put in a 2018 ram that lives in new mexico, then there is very little chance this issue isn't the battery.

Plus the electrical draw from clutches and blowers, etc. Those do go through relay.

WOW
Man, you are way out in left field.
I’m talking about the electrical requirements of the a/c system, which is what affects the batteries and alternator. The physical aspect of the engine running the compressor doesn’t even factor in. OMG

 

[Ritchie_Rich]

With respect, he asked me a question on an item before I said I wasn't going to comment, so I respectfully explained the pic.

A= W / V, thus in a 12v system. Example- 100-watt bulb in a 12-volt system will draw 8.3 amps. Now compare that to what it takes to run an AC a compressor versus how much power an alt can generate at idle. Its science.

Oh boy.
As I mentioned earlier, the a/c compressor will have a fuse for the clutch, which is what transfers power from the rotating pulley to the compressor shaft. The clutch in reality draws ~8-10amps, IIRC.
Any modern vehicle with a good battery and alternator can easily handle that at idle.

 

[Ritchie_Rich]

Okay. I’m done here.
There’s no reasoning with someone who doesn’t understand how an automotive a/c system works.

 

[Docwagon1776]

With respect, he asked me a question on an item before I said I wasn't going to comment, so I respectfully explained the pic.

A= W / V, thus in a 12v system. Example- 100-watt bulb in a 12-volt system will draw 8.3 amps. Now compare that to what it takes to run an AC a compressor versus how much power an alt can generate at idle. Its science.

You're combining two things and muddying your own understanding.

The compressor isn't being ran by electricity from the alternator/battery. It's being ran like a water pump or power steering pump, a belt drive from the crank. The only electrical draw the compressor has is the clutch to cause it to engage and disengage. Then you have the fan running electrically, and any cycling of vents is some combination of electric and vacuum, depending on vehicle.

So the question is, if the alt puts out less then the 150 amps plus any other item he has on such as radio etc at idle, where does the power come from?

*THE MOTOR*. That's what the serpentine belt is doing, transferring power from the rotation of the crank to the various accessories that are being driven, including the AC compressor. AC existed in cars back when 55-60 amp alternators were the norm.

*edit* Think about it for a second. If you used a serpentine belt that bypassed the AC compressor but left it wired up - would you have AC? No. Right. It is not using the power from the power cable to spin and compress the refrigerant, just to engage the clutch. No power from the motor, no spinning, no compressing...

I had a 1975 GMC 3/4 ton with an aftermarket AC unit. I didn't need to upgrade the alternator, just change brackets to put a compressor in.

 

[Burla]

Then why would the ECM open air valve and send gas at idle to run ac? Point being, it only sends so much air and fuel to change engine rpm's a bump over the 800, because the idle rpm of a hemi (maybe a ******) is not enough to power the AC, it's a fact that is why the ECM sends more fuel and air. Then there is actual current needed versus what the ECM sent in boosted idle. It takes that extra from the battery. It still powers what is send through the battery, and also compensates what isn't coming from the pulley. The battery compensates in general if needed, it is sacrificial to the act of turning off the engine, the alt and battery and engine work together to keep the engine on. The battery remains in the circuit with alt, the demand of the entire system at idle is too much and now add a bad battery to this balance.

Once the engine is running, the alt takes over, providing electricity and charging the battery. The battery also acts as a voltage stabilizer for the electrical system, acts as a reservoir for power demands. In this role it can deplete itself especially if capacity is low from age on battery.

Hopefully OP can clear up the question on battery.

 

[Docwagon1776]

Then why would the ECM open air valve and send gas at idle to run ac? Point being, it only sends so much air and fuel to change engine rpm's a bump over the 800, because the idle rpm of a hemi (maybe a ******) is not enough to power the AC, it's a fact that is why the ECM sends more fuel and air. Then there is actual current needed versus what the ECM sent in boosted idle. It takes that extra from the battery. It still powers what is send through the battery, and also compensates what isn't coming from the pulley. The battery compensates in general if needed, it is sacrificial to the act of turning off the engine, the alt and battery and engine work together to keep the engine on. The battery remains in the circuit with alt, the demand of the entire system at idle is too much and now add a bad battery to this balance.

Once the engine is running, the alt takes over, providing electricity and charging the battery. The battery also acts as a voltage stabilizer for the electrical system, acts as a reservoir for power demands. In this role it can deplete itself especially if capacity is low from age on battery.

Hopefully OP can clear up the question on battery.

You've never done troubleshooting on a vehicle's AC and it shows. The compressor is mechanically driven. There is no electric motor inside of it. Take the belt off and apply power to the compressor and you will hear the clutch engage but nothing will ever spin. It's impossible, there's no mechanism inside it to convert electricity to motion.

Then why would the ECM open air valve and send gas at idle to run ac?

Because energy isn't free and drawing power from the crank requires fuel and air no matter if the power is going to the accessories or the driveline.


The electric component is the clutch that engages and disengages the drive. If the AC could run off the alternator and battery it would not stop cooling the vehicle when stop/start cuts the vehicle off at a standstill. The clutch could disengage the drive pulley so the stopped accessory belt wouldn't interfere and your imaginary internal electric motor could power it, but that's not what happens.

Maybe this will help you understand, as he takes one apart and uses his hand to make the compressor run.

How an A/C Compressor Works

Here's what's inside of an automotive air condition compressor and how it works to cool the interior of your car.The A/C compressor is powered by the crank p...

www.youtube.com

The only electrical component is the clutch assembly, which uses an electromagnet. The compressor itself is a *mechanically driven* pump. Zero electricity used or required, same as your water pump, the crank drives it *mechanically*.

 

[Burla]

Not talking about conversion of electricity. When the clutch engages what happens, it draws power from the engine, a lot of it. I'm guessing you think at warm idle this mechanical power is enough to power the entire AC system plus any other draw needed, which would be generally true, except when you have a battery on the outs. It is in the circuit with alt and power from engine it is one, it isn't separate. What happens with the mechanical power isn't enough for the needed output?

What happens when the required output power exceeds the available mechanical power? Don't get hung up on mechanical power, but rather what role electrical power has to that mechanical power. Then accept battery is on the circuit with alt, it matters not what item comes from mechanical power if there is not enough mechanical power needed.

 

[Docwagon1776]

Not talking about conversion of electricity. When the clutch engages what happens, it draws power from the engine, a lot of it. I'm guessing you think at warm idle this mechanical power is enough to power the entire AC system plus any other draw needed, which would be generally true, except when you have a battery on the outs. It is in the circuit with alt and power from engine it is one, it isn't separate. What happens with the mechanical power isn't enough for the needed output?

What happens when the required output power exceeds the available mechanical power? Don't get hung up on mechanical power, but rather what role electrical power has to that mechanical power. Then accept battery is on the circuit with alt, it matters not what item comes from mechanical power if there is not enough mechanical power needed.

If you're not talking about conversion of electricity into motion, then what do you think electricity is doing? There is nothing but mechanical power driving the compressor. Obviously the crank provides enough power or it wouldn't maintain RPMs and would stall.

Show me how electricity from any source can help spin the compressor. Show me a single component that can power the pump. Did you watch the video? You can run power through it until it melts but there's nothing in there that can do anything useful with the electricity.

I think you're also vastly overestimating the power required from the crank. Even at max, AC is costing about 10hp top end on dynos pulls.

 

[LouM]

The ECM will control the engine to maintain the desired idle rpm, the engine runs the AC compressor it has no concern of the load. Even since ECM's came out they control the engine rpm and output.
Once an engine is started and running a small 12 volt battery could suffice to keep everything in the vehicle operating.

 

[mtnrider]

This thread has gone so far out into left field and I doubt the op will even come back.....

I'm sure he has figured out he has a bad battery, case closed/

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[RamDiver]

This thread has gone so far out into left field and I doubt the op will even come back.....

I'm sure he has figured out he has a bad battery, case closed/

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The OP hasn't returned since starting this thread and hasn't seen any of the replies yet.

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[mtnrider]

The OP hasn't returned since starting this thread and hasn't seen any of the replies yet.

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Yeah, pretty typical of these type posts. Guy has a problem, panics and joins every forum out there, then realizes it was an easy fix and dissapears. Either that or they are just trolls.


.

 

[62Blazer]

Maybe on an electric vehicle it does? Boom moment? Energy is energy, if it takes 200 amps to run an AC unit in an EV, it also takes 200 amps to run it in a gas engine (give or take), it just isn't run directly off the battery. Now I know it isnt 200 amps which is why I said less, it is adding every item together to run an AC system and yes I added some, it was lazy math didn't expect 20 guys to die on this hill. I believe EV ac's are over 100 amps mostly (up to 150), which means I also know the hemi ac is close to the same.

Are you talking about the total "power" or "energy" to run the AC on a gas engine vs. EV vehicle? If that is what you mean, then yes both systems probably require about the same amount of total power. However if you use the term "amps" that is specific to electrical power. In the context of this post and responses talking about loads on the battery and alternator, the power source between a gas engine and EV to run the AC are completely different. Since amps is specific to electrical you can't say a gas engine AC system draws 200 amps......because it simply doesn't. It does draw power from the engine (such as horsepower), but it not drawing significant electrical power from the battery or alternator.

 

[62Blazer]

The OP hasn't returned since starting this thread and hasn't seen any of the replies yet.

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Doesn't surprise me. As mentioned by several people it was probably just a dead battery.
Looking at the alternator output curves even the big 220A optional alternator is probably only putting out 15 amps max at idle. Even with big dual batteries the reserve capacity at 25 amps of those is only a few hours. Between having the AC on, fan cranked, other electrical draw....and maybe some subpar batteries it's quite feasible that you are just slowly draining the batteries.

 

[RamDiver]

Looking at the alternator output curves even the big 220A optional alternator is probably only putting out 15 amps max at idle.

The graph heading indicates the rpm is at the alternator, not the engine.

From a short review at several sites, the consensus appears to be that the alternator rpm is typically twice the engine rpm.

Using a conservative estimation of 1500 rpm at the alternator, the available current is 110-130A.

This does sound higher than I would have expected, but most of my alternator experience is more than a few decades old.

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[62Blazer]

The graph heading indicates the rpm is at the alternator, not the engine.

From a short review at several sites, the consensus appears to be that the alternator rpm is typically twice the engine rpm.

Using a conservative estimation of 1500 rpm at the alternator, the available current is 110-130A.

This does sound higher than I would have expected, but most of my alternator experience is more than a few decades old.

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After looking at it again, I believe you are correct....and it does make a little more sense.

On the Ramtrucks.com website there is an alternator output curve. For the 220A version is shows 19.5 amps at 1,250 alternator rpm and jumps up to 90.6 amps at 1,600 alternator rpm.

That should be plenty of power to run the AC and HVAC fans at idle assuming the battery is not bad and drawing a lot of power. But I could still see it causing an issue if it sat for a long period of time at idle, especially with a subpar battery.

 

[RamDiver]

After looking at it again, I believe you are correct....and it does make a little more sense.

On the Ramtrucks.com website there is an alternator output curve. For the 220A version is shows 19.5 amps at 1,250 alternator rpm and jumps up to 90.6 amps at 1,600 alternator rpm.

That should be plenty of power to run the AC and HVAC fans at idle assuming the battery is not bad and drawing a lot of power. But I could still see it causing an issue if it sat for a long period of time at idle, especially with a subpar battery.

I would say that until the OP can provide definitive evidence of the battery's condition, we're just spinning our wheels in the muck.

As you say, any subpar battery is highly suspect under these conditions.

Hopefully, the OP or someone else will find some useful tips here.

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