alternator strangeness

Got a '97 2500 diesel. When I start it, the alternator gauge swings from lowest acceptable position on the gauge to normal reading. It generally cycles from lo to high every 4 or 5 seconds. This normally goes on for 30
seconds or so.
When the meter drops to the lowest position, the idle rpms fall way down. When the raises to normal position, the idle goes to normal rpms. What's going on here? Is the alternator getting ready to blow?
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What you are seeing is the grid heater operating. When it cycles on, it draws a huge amount of current (100A IIRC) and that will cause your gauge to swing to the low side. Supplying this level of current does not come for free and when the heater is drawing this power the alternator will put a large load on the engine trying to supply it and that will cause your idle speed to drop. It is all normal operations and I wouldn't worry to much about it. OTOH, you might want to get your batteries load tested as it appears that they no longer have the reserve capacity they should have and the vehicle is relying far more on the alternator alone to supply the required current needed during the warm-up phase.
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TBone wrote:

What does the grid heater do? Both of the batteries were just replaced a few months ago with Auto Zone units -- about $80 each IIRC. I cleaned the battery connectors with a battery cleaning tool when they were changed out.
After installing the new batteries, the starter spins at an unreal speed -- maybe twice the speed before. Cold weather does seem to zap the batteries pretty fast tho. Should I have these tested somehow? If yes, what procedure?
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gauge
for
idle
and
out.
speed --

The grid heater warms the air entering the engine when it is cold and replaces the glow plugs that some other diesel engines use. If the batteries are that new they should be fine but if you want to test them, have them load tested but I don't think that it will be necessary. As I said before, during warm up (on a cool or cold day) the operation that you are seeing with the diesel engine in your truck is normal.
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This poster has more experience with diesels than I do. Believe him, not me.
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Christopher A. Young
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TonyD wrote:

As the other guy explained this is the intake air heater draw. My 1996 3500 diesel does the exact same thing.
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Sounds like the alternator is responding to the idle. My last couple Dodges, the alternator didn't put out if the engine was running slow.
I suggest treat the engine speed, and the alternator will be OK.
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