Alternator output

I put a new alternator on my '01 GMC fullsize. The alternator has a 14.8 vset regulator in it, but I'm only getting 13.7 hot/14.1 cold. I've been told that the ecu regulates the voltage. If so, How can I get around this?
Someone said something about a resistor in the harness, but I don't know where or what size resistor, anyone know anything about this?

mechman has a gm voltage regulator pigtail that goes inline with the harness and alternator. From the looks of it, it looks like a pot with heat shrink, but i talked with the guy at mechman and he says its a pot with "protection device", meaning there is probably just a diode in there also, but i do not know the specific values.

Maybe i'm a little lost here or not following you, but what do you mean 13.7 hot / 14.1 cold?

That's the voltage output at the alternator when its cold (14.1v when you first crank it up) and hot (13.7-13.8v after it runs about 5-10 minutes). When it should be 14.8 cold and around 14.4 hot. Probably a little less at the battery. The alternator has a 4 pin plug on it with 2 wires. A ignition (+) wire which turns the regulator on and a sense wire that controls the output of the regulator. Seems like I remember being able to put a resistor on the sense wire to trick the ecu so it charges higher and keeps the battery light off in the cluster. I guess you could put a diode on the battery wire to prevent a surge and save a couple fuses, but I don't have that to worry about right now, LoL

HallMarc wrote:

That's the voltage output at the alternator when its cold (14.1v when you first crank it up) and hot (13.7-13.8v after it runs about 5-10 minutes). When it should be 14.8 cold and around 14.4 hot. Probably a little less at the battery. The alternator has a 4 pin plug on it with 2 wires. A ignition (+) wire which turns the regulator on and a sense wire that controls the output of the regulator. Seems like I remember being able to put a resistor on the sense wire to trick the ecu so it charges higher and keeps the battery light off in the cluster. I guess you could put a diode on the battery wire to prevent a surge and save a couple fuses, but I don't have that to worry about right now, LoL

ok, engine temperature, you are referring to voltage output while the IACV (or whatever GM calls it) is doing its job and then closed. I don't know about anyone else, but almost every vehicle i have ever seen had an alt output (stock) of 13.8 at idle.

Not sure about GM, but Ford uses 3-pin harness. All analog.
send - Unswitched battery voltage
send - regulator control
return - load indicator

If you can probe the regulator control signal... and map out a few points to make:
ALT Volts = f(REG CON Volts)

You could easily boost or drop the signal voltage to match your needs.

Barret is most likely correct, the only protection you would want is fusing and diode, so a spike won't mess with your computer.

BTW, IACV is usually used to denote idle air control valve signal. Not sure what it means in GM-speak, but I'm guess it's the regulator control signal.

twentyhurtz wrote:

ok, engine temperature, you are referring to voltage output while the IACV (or whatever GM calls it) is doing its job and then closed. I don't know about anyone else, but almost every vehicle i have ever seen had an alt output (stock) of 13.8 at idle.

I'm not referring to engine temperature, I'm referring to the alternator itself. Your alternator gets hot as it works. And its not a stock alternator.

Which is precisely why people put ice packs on their alternators and heat blankets around their batteries. Temperature plays a big factor. My DC alt keeps 14.6-14.7 hot, cold, low load, high load. Best alt I've ever seen, though there are alot quality alts.

well when i was referring to IACV, i know on a honda, its a physical valve that remains open to allow air to bypass the TB into the manifold, causing the engine to idle higher (more voltage output from alt), then once the coolant absorbs enough heat from the engine and becomes hot, the IACV, by sensor closes and shuts off the airflow which was bypassing the TB, allowing a stock idle.
If your directly referring to the alt being hot or cold, well heat reduces efficiency if that's what you are referring to is an energy loss.

Either way concerning the matter, yes i'm sure you can find a diagram online. i am on gmfullsize.com and there are not tons of electronically inclined people on there, but there maybe a schematic floating around. Making your own will be better than paying a bill for a Pot and diode any day. I know i will be doing the same soon.