Voltmeter fluctuating at drive through ???
 

For about the past month I have noticed when at stop lights or in a drive through, the headlights going dim and the voltmeter dropping from 14V and then back up, drop, then back up. It never seems to do it under power, just when idling. The truck has less than 10k miles on it. Any ideas?

If it's cold outside, you are probably seeing the air intake heater cycleing. It's normal.

read the "Voltage Drop / Lights dimming on cold start" thread under 3rd Gen Engine and Drive Train.

ya my does the same thing in the morning when its cold, its a normal thing.

if yours is doing it in the middle of the day in texas it's probably some other kind of issue though

mine has been doing it also here in texas this week if I kill it in a drive thru and the restart, but its been in the lower 30s all week till today,and it was almost 75

It is only supposed to do that after a start or restart. If you've been driving it around and all of a sudden it does it then something is wrong.

Despite what people think on the interweb, the grid heater is not what pulls the voltage down. The ECM controls the field on the alternator, which means it can "tell" the alternator when to charge and when to just freewheel. On startup and restart it freewheels. It will do that after you've been driving and restart it, but pulling up to a stop sign should not!

It could be the ECM (perhaps a reflash?), wiring, or that alternator itself.

Andy

once above 15 mph the grid heaters wont cycle anymore

brett

It is true that the field is turned off when the grid heaters are on and cause a voltage drop, but the grid heaters do cause a major voltage drop as well. If you believe the voltmeter in the truck the grid heaters will drop the voltage down to about 8 volts or so.

However if you were to just turn the ignition switch on and not fire the engine (so there is no generation) without the grid heaters being on (so there is no massive current flow), then your unloaded battery voltage will be a good bit higher than 8 volts, usually a little over 12 volts.

Mine even gives the wait to start light sometimes if its up into the mid 60's. It's odd, after it hit 20K miles it doesn't seem to do it as much though at those warmer temps.

Sometimes it won't give the wait to start light if it's in the upper 40's is what suprises me... Probably heat soak, I can drive it to lunch then 4-5 hours later come out and it won't give me the wait to start light.

The grid heater also cycles if the motor is completely warm even if the wait to start light doesn't come on... Like at a drive through or whatever depending on the outside temp.