Electricall Charging Question....maybe I am a dummy...

[DHerzog]

I just noticed that when I start my truck, the voltmeter shows about 10-12 volts until I actually start driving... then it comes up to 14+ volts.



It may have done this all the time and may be normal... my steering wheel cuts across the gauge, so I don't generally look at it or notice where it is at.



I started watching it the other night when it was cold (7 degrees) and the headlights would flutter in brightness until I finally got going... . I did not want to race the engine to build rpm's due to the temperature.



It this normal and I just have not noticed, or am I headed for some electrical work??? HELP :{ :{

 

[TSpecht]

It's supposed to do that. I believe it runs off the batteries for the 2 minutes or so the grid heaters are cycling. When you exceed 18mph it shuts off the grids and starts running with the alternator doing it's thing...

 

[Scrappy]

it is because your grid heaters are cycling on and off until you go over 15-20 mph. it is normal operation

 

[redram]

I just noticed that when I start my truck, the voltmeter shows about 10-12 volts until I actually start driving... then it comes up to 14+ volts.



It may have done this all the time and may be normal... my steering wheel cuts across the gauge, so I don't generally look at it or notice where it is at.



I started watching it the other night when it was cold (7 degrees) and the headlights would flutter in brightness until I finally got going... . I did not want to race the engine to build rpm's due to the temperature.



It this normal and I just have not noticed, or am I headed for some electrical work??? HELP :{ :{

Sounds normal - the lower voltage is in response to the cold weather requiring the 'grid heaters' to operate when below 45 F.

You have read your owner's manual that should have a section on this - right?

If you had a first gen truck, they used to have an Amp Meter and it would really swing as the grid heaters pull about 30 amps each and there are multiples.

My 96 will do similar except that in Houston - we don't have to deal with sub freezing weather too often. Oo.



I believe that the Common Rails have a similar setup to the older first and second gens with grid heaters- just with a volt meter like the 94-02 trucks.

 

[betterthanstock]

They don't allow the alternator to begin charging until the grid heaters are done cycling, to prevent frying the alternator. (I think they are 2x50 amp) That means, when you idle a cold engine for ten minutes with the headlights on and the a/c on defrost, the electric seat warmers on, radio, mirror defrost, etc it all comes out of the batteries.

SO if you have weak batteries and you shut down your truck after a ten minute cold idle, IT MAY NOT be able to crank the engine.

As soon as you hit about 18 mph is when it turns on the alternator and disconnects the heaters. The next time you idle at a stop light it will usually keep on charging.



I have never idled it long enough to find out when it starts charging at idle, but I had my truck in the body shop for a few weeks, and I pointed out that the battery indicators were black. The guy told me that they regularly idle all cars for about an hour. He either lied or it takes a LONG time at idle to begin charging.

 

[TSpecht]

My grids shut off after about 2-3 minutes of idle, at which time the volt meter comes up to a hair over 14. I have never had the grids come back on after the engine has been running just from coming to a stop. The only thing I've had happen at that point was it bumping itself up to 1000 RPM's, but the grids don't activate when that happens that I can tell.

 

[KWentling]

The grid heaters can draw up to 95 amps each according to the service manual on a '98 12-valve. I expect that hasn't changed.



Kim

 

[Kirwin]

Alternator operation

I don't think that the alternator is not turned on after start up, I think its more that the alternator cannot charge at a sufficient rate to overcome the nearly 200 amp draw from just the grid heaters. I can tell you that the volt meter is severely dampened. Thus what we see is the volt meter hangs out below 14 volts and comes up after the grid heaters cycle off and the alternator restores the energy used to start the truck and cycle the heaters. Ken Irwin

 

[TowPro]

Kirwin's got it. The alt is charging, but the grid heaters draw more AMPS then the alt puts out, thus it drops the voltage.



I have also noticed (at least on the 2nd gen's) that you can drive for a couple hours on the highway, pull off and stop for a couple minutes while you go in and eat. When you restart the grid heaters will start to cycle again for a couple minutes.

 

[DHerzog]

Thanks for all the great responses... . this is my first diesel and I learn something new almost every day... . thanks for the TDR