Strange discharge/charge behavior

[SilverFox]

Last night I started my engine while I was programing my Navigation Radio for a trip I planned to take today and noticed the interior lights get dim and then bright and go back and forth like this about every 4 seconds. This interior lights flickered like this for a minute or two. The voltmeter showed a substantial discharge during the whole episode which lasted about 5-6 minutes. During this time the voltmeter never moved, showed continuous discharge. Changing the engine RPM didn't change the discharge rate. Nothing changed it, lights, heater fan, etc. Then for no good reason it started to charge at about 3 times the normal rate.



I drove the truck about 225 miles today and when I started the truck this morning I got the same discharge for about 5 minutes followed by a strong charge until I stopped the engine again. (Each time I started the engine I drove from 20 to 90 miles). The discharge time got less with each start until the last time I started it there was hardly any discharge and the charge rate was back to normal.



I have a feeling I'm going to get the same discharge in the morning followed by a high rate of charge.



Would anybody like to venture a guess as to what is causing this strange discharge/charge behavior?

 

[ttreibel]

Grid heater..... it draws like 70 amps it is starting to get cold.

 

[SilverFox]

I thought it might be the grid heater but the discharge rate lasted for about 5 minutes and the cycling of the interior lights only lasted a minute or two. It was chilly last night, about 60* and this morning about 40* when I left. It got up to 79* this afternoon and was still doing it but only lasted a minute or two. Do you still think it is the grid heater?



Edit: I think my "On board air" Oasis draws about 150 amps and it doesn't show a discharge like I had last night and today.

 

[rbattelle]

I agree... grid heaters. I thought their draw was closer to 100 amps?

 

[SilverFox]

Ok if you guys think it's my grid heater I guess it is. This morning it was 56* when I starter the truck and the discharge only lasted for a couple of minutes. I've owned diesel tractors and trucks since '64 and never seen glow plugs or grid heaters drop the volt meter like this Cummins. If it's normal I'm satisfied. Thanks for the information.

 

[rbattelle]

The grid heaters begin cycling below 66*. There's a huge chart in the service manual that describes when they come on and for how long based on intake temperature.

 

[RonWeiss]

GRID HEATERSOo.



Ron W.

 

[Prairie Dog]

I think they draw well over 100 amps because of the way they can suck down your voltmeter even at 2k RPM's.

 

[wsteffes]

I experienced this last fall. Not liking the response "It's normal they all do it. " I dug a little deeper.

The dimming of the lights you have is the grid heater cycling on and off when the alt. is not charging.

The reason for the alt. not charging is due to the ECM programming. The voltage regular is part of the ECM. It uses voltage input (battery charge level) and temperture (sensor mounted under battery) to determine when to charge and how much. A cold charged battery's voltage is less than a hot charged battery. So in cooler weather you will see a longer delay until the ECM will allow the battery to be charged.

I was given two different reasons why it's set up this way.

1. Some discharge of a cold battery will help it warm up and it is better for the battery to be charged then instead of when cold.

2. Prevents additional load on engine when cold and helps exhaust emissions during cold idling at start up.

True or false don't know but it's better than "It's normal they all do it. "

 

[SilverFox]

wsteffes,

Great answer Oo.

I was convinced it must be the grid heaters but it sure is nice to find out exactly what's happening. Thanks to everyone, I feel much better now.

 

[Vaughn MacKenzie]

Mine don't come on below 66, it has to be at least below 50 before they kick in. I usually start driving off within 30-60 seconds after startup and as soon as I hit 15mph (or is it 18?) they turn off.



It was 79 here today but we're supposed to finally break out the ice scrapers this weekend. The grid heaters will get some exercise for once



Vaughn

 

[rbattelle]

I think the cancel reheat happens at 20. Or at least 18. Anyway, I've noticed mine don't always come on when ambient is below 66. Frequently, ambient has to be lower (closer to 55 or so). Probably because the grid heaters are controlled by intake temp, which is likely a little higher than ambient.