Strange amperage draw

This isn't a fix-it thread as I don't think anything is wrong, just a curiosity.

You know how you might be idling in traffic or at a red light and the electric fans kick on?  You notice it first because your HVAC fan slows down a bit for a second as the fans draw startup amperage.  Same thing might happen in your diesel pickup as the glow plugs cycle a few times.  Following so far?

My bone stock 06 Express van does this sometimes, but it's not when fans kick on, (I don't even think it has electric fans) and it's not a diesel.  Usually happens within the first few blocks of leaving the house.  For about 1 second, the headlights dim, HVAC fan slows down, dash lights dim.  Not a huge dimming like something's wrong, but some kind of amperage draw that doesn't seem to coincide with the usual suspects.  What draw in my van might be causing this?  It's predictable enough that I think it's intentional, just have no idea what appliance/accessory I have that predictably draws large amperage.

I keep trying to watch the voltage gauge when it happens but it's usually quick enough that I miss it... and the gauge is pretty slow to react anyway so it might miss it.

Just a guess.  Are you running the defroster?  It may be the AC compressor kicking on.  Just spit balling.

The voltage gauge on that is probably pretty heavily buffered anyway.

My 2000 (GMT400) Tahoe does this also. Every once in a while, everything dims for a split second and then comes right back. The voltmeter on the dash will just slightly show it as well. I usually notice it at night because of the headlights dimming. It is about as simple a vehicle as there is these days, and there isn't much in the way of electrical draw aside from HVAC and seat heaters. Engine RPM doesn't change, does it with the AC off, etc. I did replace the (original, 270k mile) alternator due to failing bearings....come to think of it, I'm not sure I've noticed it happening since then. Hmm.

Maybe pull the A/C fuse (or unplug the clutch) one day and see if it still happens? It may be cycling the compressor to check function, or just to  keep things moving freely. 

In reply to Dirtydog (Forum Supporter) :

Doesn't matter if the compressor is in use or not.  When the compressor kicks on you can hear it but it doesn't draw enough to dim lights or slow the blower fan.  It also only seems to happen once per trip, almost like the ECM is self-checking something but not sure what.

No Time said:

Maybe pull the A/C fuse (or unplug the clutch) one day and see if it still happens? It may be cycling the compressor to check function, or just to  keep things moving freely. 

I could try that, but for the first year there was no freon in it so it never kicked on. (well, there was about 10psi)  It's also just usually once per trip, not cycling like the compressor would.  The compressor kicking on doesn't cause this kind of draw, nor does it coincide with compressor use.

gearheadE30 said:

The voltage gauge on that is probably pretty heavily buffered anyway.

My 2000 (GMT400) Tahoe does this also. Every once in a while, everything dims for a split second and then comes right back. The voltmeter on the dash will just slightly show it as well. I usually notice it at night because of the headlights dimming. It is about as simple a vehicle as there is these days, and there isn't much in the way of electrical draw aside from HVAC and seat heaters. Engine RPM doesn't change, does it with the AC off, etc. I did replace the (original, 270k mile) alternator due to failing bearings....come to think of it, I'm not sure I've noticed it happening since then. Hmm.

Yeah, this one is almost like it's trying to start up 6 radiator fans at once.  I'll try a graphic representation of what I'm getting.  The blower motor slows to maybe 2/3 of its original speed.  It seems to be a big draw... if I were to venture a guess based on ballparks, if an AC compressor takes 10-15A, a starter takes 175A, and lights take 10A, I'm guessing this is the equivalent of a 40-50A draw at least given how the lights dim and the blower motor slows down.

In reply to Curtis73 (Forum Supporter) :

Mine is the square wave as well; I probably didn't describe it well. I can probably do a "one one thousand" kind of count and then it jumps back up.

gearheadE30 said:

In reply to Curtis73 (Forum Supporter) :

Mine is the square wave as well; I probably didn't describe it well. I can probably do a "one one thousand" kind of count and then it jumps back up.

Same exact here.

After reading this I went and did some googling.

It looks like GM went to a variable output approach on charging around 2005 and based on load and other criteria it will  adjust the alternator output. According to the info I read, if you have a 2 wire plug on the alternator then you have a variable output alternator that uses PWM control. 

Alternator mode link indicates the dip is a transition from a "starting" to the "charge" mode.
 

No Time said:

After reading this I went and did some googling.

It looks like GM went to a variable output approach on charging around 2005 and based on load and other criteria it will  adjust the alternator output. According to the info I read, if you have a 2 wire plug on the alternator then you have a variable output alternator that uses PWM control. 

Alternator mode link indicates the dip is a transition from a "starting" to the "charge" mode.
 

 

Well, that explains a lot.

Thanks!