Jaysz28

iTrader: (30)

I have been trying to get my charging voltage up in my 91 firebird. I used a 98 ls1/T56 with a 99 harness/ecm to do the swap.

With everything off and the car idling, I would only see 13 volts. Enough yes, but fully loaded (ac, headlights, both cooling fans running) it would drop to 12.4 or so.

So I replaced the alt with a ac delco replacement, upgraded to an 8 gauge wire and added an extra grounding strap to the motor. I also took a few mins to clean all the battery cables and chassis grounds.

Now, I can get 13.7 volts fully loaded

*but*

I notice when the car is up to operating temps, the volts drop to 13.2 volts. It doesn't go any lower than that.

Other things to note

I have a powerbond underdrive pulley. I compensated with a ASP overdrive pulley to get where I am at now.

I thought about using a KG3 (145 amp) alt from a truck. I actually bought one from the boneyard for $45. I went to install it, but I can't get it clocked correctly to where it will clear the factory k-member and allow for me to be able to plug it in.

I am also considering upgrading the battery and charging cables to some 4 gauge or better welding wire..... that probably won't hurt.

Just to be clear, all voltages were measured at both the battery and the alt.

My car has a small amp that drives a single 10" sub.
Headlights are wired with relays off the battery.
I also have a racetronix FP and relay kit.

Am I paranoid or just used to seeing 14+ volts on my old TPI swap?

84 1LE

iTrader: (9)

13.8 to 14.3 @ the battery is the ideal range, that's idling w/ the headlights on.I don't think you're in any danger of running down the battery.

Are you reading that voltage off a DMM or gauge?

Jaysz28

iTrader: (30)

Good point

All voltages are read with my DMM or DVM

I use the gauge as a reference point

and I agree, 13.8 to 14.3 is the ideal range, but I am not there once the car is up to operating temps (195* or so)

Just wondering if one of those 145 amp alts I see for sale would be a better choice....

I have the old alt that I pulled off. If I could find a rebuild kit with a higher rated rectifier bridge that could bump the output up I would be happy to try that. Yes, I already checked to see if I could stuff the guts of the truck alt in to the ls1 case. The answer is no.... lol

bigal55

iTrader: (1)

Where is the S wire on the alternator connector hooked to? The S or sense connection tells the alternator what the system voltage is by referencing the voltage at the point of connection. The alternator will try to maintain 13.7 volts at that wire and vary the voltage output accordingly. Your load hasn't changed when you did the LS swap, or did it, so your alternators amperage should not be the problem. If you are truly seeing 12.4 volts at the battery when running, try connecting the S terminal directly to the battery positive terminal and see what voltage you get then. I should also ask, does the voltage rise if you rev the engine? That would be a sign of low amperage output due to your RPM of the alternator being low. If this is the case, I don't know if I would worry too much unless you do a lot of idling with all your accessories on.

LiquidBlue

voltage is impacted by heat. that may be what you are seeing.

Jaysz28

iTrader: (30)

I do understand that heat effects voltage, and is a cause of voltage drop......

The voltage does not seem to change with revs, and is steady.

LS motors/alts use a 10v exciter wire that is controlled by the pcm as far as I know.... perhaps someone else could confirm or correct me on this. When the car is warm, it has 13.2v with the car fully loaded. I would like it to be 14.x fully loaded.

I am in Florida, so I spend a decent amount of time idling in traffic with the ac on. When the voltage drops, you can definitely notice it in the blower motor. You can hear/feel it slow down.....

I just think of how the voltage drop must affect the ignition, fuel pump, etc....

84 1LE

iTrader: (9)

Use a DMM, at the battery.The gauge isnt that accurate,plus the voltage drops from the alt to the gauge.

92BLKL98

This info is copied from Powermaster Performance.

http://www.powermastermotorsports.co...pulleys_a.html


For most people, the only alternator rating they are familiar with is the amperage rating. Alternators are spoken of as a 65 amp or 100 amp alternator. When replacing the alternator on the family car, this is probably the only information that is necessary. After all, all one needs is an alternator that matches the original.

When building a custom car from the ground up however, a deeper understanding of the power curve of an alternator is required. Usually a custom pulley or so-called "power pulley" set is used with a performance alternator. A mismatched pulley ratio and alternator will spell trouble, especially at idle speeds where alternator performance is critical. To avoid this, it is important to understand the alternator's capability at slow speeds.

An alternator's output is dependent on speed, but this can be deceiving because this output is not linear. Instead, it follows a curve. Each alternator has a unique curve, and at idle small changes in the alternator's speed can make a big difference in its output capacity.

Because of the preceding, pulley ratios are very important, especially when using high amperage alternators. The pulley that are supplied with the alternator are matched to the winding and power curve. It is important that any dress up pulley sets do not deviate from this ratio. Typically, a street driven car should have a pulley ratio of at least 3:1. If the vehicle has an automatic transmission with a low idle and the vehicle spends a lot of time cruising, then a higher pulley ratio - perhaps 3.5:1 - should be used. Alternators can take high speeds up to 20,000 RPMs for short periods, so overdriving the unit is not a problem.

The output of high amp alternators can drop off substantially under 2400 rotor RPMs. Therefore, Powermaster does not recommend power pulleys with high amp alternators.

Powermaster uses a state-of-the-art computer alternator dyno to measure the performance of each alternator we manufacture. Output curves, engine idle speeds, and alternator pulley ratios are carefully considered to assure good drivability at idle and slow cruising speeds.



How to Determine Ratio and Rotor Speed

The alternator rotor RPM is not necessarily the same as engine RPM. To calculate the actual alternator RPM, determine the ratio between the two pulley diameters.

Ratio = Crankshaft Pulley Diameter/ Alternator Pulley Diameter

Now that we know the ratio, we can now determine the rotor speed:

Rotor RPM = Pulley Ratio x Engine Speed
(example; 2.1 x 870 = 1827 Rotor RPM)

Once I understood alternator performance in automotive use I trashed the U/D pulley kit. On the street they are a waste of time and for the claimed HP gain are a waste of your long green. To start I would go back to the factory pulleys and call the spent money a lesson learned. Then I would always buy the best alternator your money can buy, stay away from the bargain shop alternators. If your problem continues then at least you aren't chasing problems you added to the equation.

bigal55

iTrader: (1)

OP states voltage does not go up with revs so probably not the problem. The voltage sense is not correct I would guess but I am not familiar with the LS charging system and I suppose it could be controlled by the PCM. I would have to ask why GM would add cost to have the charging system controlled by the PCM when a reference (sense wire) right at the PCM supply could be used.

bigal55

iTrader: (1)

Ok, I searched a little and found some info. The exciter wire is for the PCM to control the Field of the alternator, thereby turning it off and on, not regulating the voltage output level. As stated above, the S terminal or Sense terminal regulates (or helps to) the voltage level. If you are concerned about the PCM having good voltage at idle, connect the S terminal to the PCM power at the PCM. You need to remove the wire from where ever it is going now and connect it to the PCM, can't have it connected to 2 different places. Here's a drawing. HTH!


The D terminal in the drawing is the Sense wire, shown going back to the starter.

Jaysz28

iTrader: (30)

As stated previously, all numbers were obtained using a DMM. the gauge is just a reference

I read that article and agree that U/D pulleys are worthless. However since the pulley and balancer are one piece and it was on my rotating assembly when it was balanced (a 'gift' from the engine builder) I am kind of stuck with it.... That is was prompted me to try a couple of different O/D alt pulleys, like the one I bought from ASP. It was what brought my idle voltage into the 13's to begin with.....

I appreciate the diagram, and I prolly need to follow that gray wire to make certain there is not an issue somewhere in the harness. I only have two wires going to my alt, the charging wiring and the gray wire.

I've had this swap done for a few years now, and not until I was out getting a street tune for my new injectors did I really know I had an issue. My tuner pointed out that my Alt must be dying as the voltage the PCM was showing was low. He actually told me to get it fixed and come back to see him. That is what started this whole debacle

Thank you all for your ideas.... I will try to do some more digging myself

TTOP350

iTrader: (8)

Something I just did on one of my cars that had been sitting for 10 yrs b4 I got it.
When I got it running last yr, the ALT was showing low voltage on the dash gauge but at the batt and and alt it was normal.
On a whim, I removed the cluster, all the gauge screws (1 at a time), cleaned the backs of the screws an where they contact the copper on the printed circuit then cleaned the main part of the printed circuit where it plugs in the dash harness.
Now it shows over 14volts with a underdrive and lights on.