im planning to do a dual alternator setup on my 02 dmax, the only thing im unsure about is the wiring for the 2nd alt. can anyone post instructions or wiring diagrams, thanx

Big wire on the back goes to the red box above the ps pump (+ junction box).

Splice into the brown wire on the primary alt (connector on the side) and run it to the same terminal on the secondary alt. Both come on at the same time, all is well.

Big wire on the back goes to the red box above the ps pump (+ junction box).

Splice into the brown wire on the primary alt (connector on the side) and run it to the same terminal on the secondary alt. Both come on at the same time, all is well.

If they both come on at the same time, don't you risk over charging? I thought that's why the factory dual unit came on as demand increased.

If they both come on at the same time, don't you risk over charging? I thought that's why the factory dual unit came on as demand increased.

The charge is controlled by the internal regulator -- which controls the output. when the voltage is dropping down by using accessories--(lights, AC and etc) the regulator senses a voltage drop and turns on the alternator to charge more with amps. No danger to overcharge because when the voltage starts to go high, it reduces the amps. Just the supply and demand! Both alternators work the same way, even though they are connected together. However if one goes bad, the system will still work, but with reduced amps.:D

shouldnt the 2nd alt be connected to the pcm? and be controlled by the computer?

It would be by splicing into the brown wire as Rttoys stated, the PCM would be controlling both.

Big wire on the back goes to the red box above the ps pump (+ junction box).

Splice into the brown wire on the primary alt (connector on the side) and run it to the same terminal on the secondary alt. Both come on at the same time, all is well.

shouldnt the 2nd alt be connected to the pcm? and be controlled by the computer?

From the factory, the dual alternators are controlled independently by the ECM.
If connecting both alternators to the same ECM wire, you'd think both alternators would share the load, (and heat), but GM didn't wire it this way, maybe for a reason.
Here's some info, but no wiring:
http://www.dieselplace.com/forum/showthread.php?t=677&highlight=dual+alternator

Post 19 below has a schematic for the 2003, which I thought has a different ECM than the 01/02's.
I compared this 03 schematic to my Helms 02 schematic, and the Primary Gen left, (passenger side), and Aux Gen Right, (driverside), connections to ECM, wire numbers and colors are the same.
http://www.dieselplace.com/forum/showthread.php?t=49349&highlight=dual+alternator+schematic&page=2

The charge is controlled by the internal regulator -- which controls the output. when the voltage is dropping down by using accessories--(lights, AC and etc) the regulator senses a voltage drop and turns on the alternator to charge more with amps. No danger to overcharge because when the voltage starts to go high, it reduces the amps. Just the supply and demand! Both alternators work the same way, even though they are connected together. However if one goes bad, the system will still work, but with reduced amps.:D

As wired by the factory, the regulator of each alternator controls the output of it's own alternator, but the regulators are controlled independantly by the ECM.

GM likes to make things overly complicated. The simple way to add the second alt is to do it the way I described. Both gens have internal regulators, so you won't overcharge anything, it only adjusts according to load. Mine's been running this way for 3 years without failure or any overcharging.

Unfortunately when 2 alternators are wired in parallel, 1 takes the majority of the load because their voltage regulators can not be set exactly the same. The regulator with the highest setting will maintain the load & cause the other one to essentially quit charging.

Lets say 1 is trying to maintain 13.5V & the other 13.6V. One will work to achieve 13.6 & if it can, the other one stops charging at 13.5 because it is "over" voltage. You can get an amp probe for a multimeter & measure the actual current from each alternator while running the truck with all accessories turned on. This will show the portion of load that each is carrying.

The only way both are really working is when the first one cannot maintain voltage on its own. If the system is at 13.4V in the above example, then both will be working, but this means the battery is very discharged or the charging system is drawing more than 1 alternator can provide.

It works, but not as good as running them the factory way.

I had the task of fixing quite a few ambulances & big stereo equipped cars that had alternators wired in parallel, that would fail when people swore 2 alternators should never have a problem charging them.

I am looking at a dual alternator setup and I see that at idle the first thing to remember is that the alternator is not producing very high power output. It is not going to be able to support a heavy load at idle and the voltage will begin to drop. The battery will cover for a time period, but the voltage is lower. This dims the big lighting rack some. Putting a second alternator on the system will brighten the lights and bring the voltage back up. It will only bring the voltage up to the max of the regulators on the alternators.
The lower max rated regulator voltage will shut down it's alternator output as the RPMs increase and the other alternator pushes the voltage past its rated max. That is the function of the regulator and it is a good thing. Now most of this has been said before, but here is the trick to the system: The cables from the alternators to the battery have only a little resistance and at low currents don't drop much voltage across themselves. But when the current is high and they heat up a little, that little resistance will create a voltage higher then the difference of the two max ratings on the alternators. What this is saying is that both will supply current to make the drop across their wires and the voltage on the battery terminal the same as their max regulated voltages.

The tricky part is when the load is low and the RPM is high. This is when there is a lot of voltage on the alternator and the regulators are shutting off the current. Only one of the two will source current to the system in this case. The other will be held off. There will not be enough drop in the cable to allow the second alternator to source any current to the system.

What does this mean to the user?
You will notice the idle conditions are much improved for system power. you will notice max current draw to be almost double under super high loads which require higher RPM. Only in the case of high RPM and low system power output will one of the alternators be shut off.

For the real tech. nuts the voltage drop across the cable from the held off alternator is added to the drop required by the difference in the max regulator voltages. That is the lower max voltage alternator must see the drop across it's cable added to the battery voltage being below it's max regulator setting to source any current.

Hope this helps,
Mark

I installed a second alternator and used a switch with 12 volts to turn the alternator on when needed. When not needed it just spins free. works great.

Big wire on the back goes to the red box above the ps pump (+ junction box).

Splice into the brown wire on the primary alt (connector on the side) and run it to the same terminal on the secondary alt. Both come on at the same time, all is well.

Nice and simple way to add a second alternator to charge a second pair of batteries (creating a seperate electrical system) for the stereo.:cool: Later this summer that jam box is going from 360 watts to well over 1000 watts!
John

interesting post guys you know there is a company that makes a 220amp alt the fits into the stock location..

Here you go High Amp Alternators (http://www.nationsautoelectric.com/ad244.html)

www.mean-green.com also