Dimming Lights at Startup

[allenpf]

When I start my 2003 Dodge RAM 2500 the voltage gauge reads below 14 and the lights dim go normal and dim for a couple of minutes and then the voltage needle moves to the right of 14 and the dimming discontinues. I took it to a the dealer who said that is normal in cold weather. My question is it? seems odd to me.

 

[nickleinonen]

normal... them grid heaters draw lots of current. they will draw the battery voltage down enough to make the lights dim, the blower fan slow down... it is completely normal

 

[Pawpaw]

That is correct. The owner's manual tells about this. Because the grid heaters draw so much current, the computer does not allow the alternator to work until either the engine temp rises above a certain point or road speed goes above 18 MPH.



It's to keep the grid heaters from killing the alternator.



Bob

 

[jwduke]

Yep, just the grid heaters doing their job. Turn your blower on high, then it is REALLY noticable.

 

[Snoxracer]

Where does it say that the alternator is shut of buy the computer? The alternator is working on my truck all the time the motor is turning, unhook it sometime before you start your truck and watch the voltage drop when it is cold and just running on the batteries. It will drop about twice as far it does when the alternator is hooked up.



Snoxracer

 

[Cattletrkr]

My battery gauge is almost always to the right of "14". Very seldom, even in sub-zero temps, does it drop to 12 or 13.

 

[Pawpaw]

Well, I thought it was in the owner's or service manual that the alternator didn't operate during grid heater operations, but I can't find it in either one.



However, I still believe it is true. The service manual states that the grid heaters are each supplied by a cable that will deliver approximately 95 amps. It doesn't take rocket science to figure out what could happen to a 136 amp alternater when a 190 amp load is applied. If the batteries are good, probably nothing. If one or both are extremely weak, the alternator could easily go up in smoke.



As for the voltmeter reading, here is what the '04 service manual says on page 8J-43 under "System Voltage Low (Charge Fail) Message"



"On vehicles equipped with the optional diesel engine, the ECM is programmed to restrict the voltage gauge needle to a position above the graduation on the far left end of the gauge scale and suppress the check engine indicator operation until after the engine intake manifold air heater has completed a pre-heat or post-heat cycle. "



Disconnecting the alternator would be a different matter, because the ECM would know it's not there.



Bob

 

[Dieselnerd]

The alternator does not shut off when the heaters are on. It didn't do it on any earlier generation Dodge RAM and it doesn't do it now. If the load exceeds the maximum output it doesn't try to put out more and then burn up, it just puts out all it can. Don't confuse it with a stand alone generator that shuts itself off when you exceed the rated load. Think of the batteries as a water tank with the alternator keeping the tank full. When you draw "water" faster than the alternator can "pump", the "water" level goes down slowly since the alternator is putting out all it can and as soon as the excess load is gone the batteries can be "filled" again.

 

[Snoxracer]

It is not true, paw at least not on mine. Thanks for explaining it a little better dieselnerd.



Snoxracer

 

[Pawpaw]

Well, I think I've got it figured out. Doing a few well-placed searches, it seems a lot of people interpreted the lower reading on the voltmeter to mean the alternator was not charging while the grid heaters are working. More than one person stated it as a fact, just like I did.



What I now believe (reading my previous quote from the service manual), is that the ECM "sticks" the voltmeter on approximately 12 volts (YMMV) until the grids stop cycling. This apparently lead some to believe the alternator was not charging.



FWIW, according to the manual they will cycle until the intake air temperature reaches 66 degrees F or vehicle speed reaches 18 MPH, whichever comes first.



Thanks to everyone who took the time to point out my error. We've got a lot of sharp guys on here, but you still can't believe everything you read.



Bob

 

[Snoxracer]

You are right pawpaw, what they do is fool the ecm while the grid heater is on so you don't get a low voltage code, because the heating grid takes so much amperage.



Thanks guy!



Snoxracer

 

[DaveHess]

One thing I did not notice from any of the posts. The 2003 will do that for 150 seconds. I don't move mine until after that time.

 

[Pawpaw]

Why? If you've got oil pressure, it won't hurt anything to drive it... just don't hammer it. I keep it down to 2,000 RPM until the temp gauge gets above 140.



Others may have different opinions and you should do what makes you happy. Also, I don't live where it gets very cold. It's rare for us to see single-digit temps.



Bob

 

[GWBourne]

it is even worse when you have the interior lights on... . kinda freaked me out the first time, but i knew what was going on. just made me use my brain for a second.

Grant

 

[nickleinonen]

It's rare for us to see single-digit temps.

if all i ever saw was single digits [negitive or positive] i'd be happy, but when you can see double digit negitives... i hate the cold

 

[betterthanstock]

Where does it say that the alternator is shut of buy the computer? The alternator is working on my truck all the time the motor is turning, unhook it sometime before you start your truck and watch the voltage drop when it is cold and just running on the batteries. It will drop about twice as far it does when the alternator is hooked up.



Snoxracer

First, don't try unhooking the alternator. Except for some marine alternators, most will fry the regulator when running with no load.



I looked at page 8w-20-3 (diesel wiring diagram) and the alternator field is controlled directly by ECM. Unless you have positive proof that the alternator works during the warm up, I believe it does not. Had my truck at a body shop in excess of one month, and the guy repeatedly idled all vehicles there several times a week. Nearly killed my batteries, they had black charge indicators (less than 70%, it's bad for non-deep cylcle to go below 90%), and I asked him to hook up the charger. I'm sure they will die a year sooner now because of that.

 

[Dieselnerd]

Of course the batteries will not be fully charged if you start and idle several times a week. You take a lot out of them starting the motor and running the heater grids and it takes a long time to recharge the batteries, more than just a short period of idling.



Positive proof? Simple to do, just use a meter in the circuit. These 3rd gens take a lot of juice just sitting turned off. In the manual it says if you won't be running the truck for 21 days or more to disconnect the batteries. Mine often stands for a week or so and I do put a charger on it. It takes several hours to recharge after just sitting for several days. Believe whatever you want but the alternator is not shut off when the heaters are on.

 

[DaveHess]

Pawpaw: I start mine in a heated garage (45 degrees) and will pull it out to idle after getting the highest oil pressure reading. We've had a mild winter here with only a couple of days below zero but last year and the year prior were tough. If you've never started a CTD in sub-zero weather without the benefit of a plug-in for your block heater after 9 hours, you're in for a real treat. Shake, rattle and roll is an understatement. Following the owners manual, I'll get her started and then slowly raise the RPM to about 1,000 RPM and then slowly release it to it's normal idle. But it is still a belligerant truck. So, I do let it idle through the 150 seconds and light pedal her the 9 miles home. Even after that 9 miles, the temp gauge doesn't move and the air from the heater is luke warm if you're lucky. I don't run a cold front on her so my fix was to buy a $100 Chevy caprice Classic 4-door "Please let some moron broadside me" special. That way Rammy stays nice and warm and only comes out once in while and only when the street are dry. Bet that makes some of you guys' skin curl. What can I say, I love my truck!