I removed my smog pump and placed an extra HO alternator in it's place to help replace the amperage drain on my batteries from my stereo system. I moved the locations of the idler pulley and tensioner and since I have yet to get the belt to stop slipping during start-up. Things i've thought would be causing the slippage is either the location of the tensioner being between the top alternator(which is still in it's factory location) and the a/c compressor, and the tensioner i'm using is alot stiffer than my factory tensioner and doesn't fluctuate so easily during acceleration. If someone could help me figure this out I would be greatly appreciated. I've tried smaller belts until i was barely able to get them on but they still slip. The tensioner i'm using is from a 2001 dodge dakota with a V6. I can't twist this with my hand but i was able to twist my factory tensioner. Anyone have any ideas?

 

Just some thoughts.

Torque transfer on a belt drive is dependent on friction coefficient, number of ribs and rib/pulley groove design, belt tension, pulley diameter, and degrees of belt wrap around the pulley.
With your moving things around, have you changed (like lowered) the degrees of wrap around the original Alternator's pulley?
How about the degrees of wrap around the second alternator, is it at least what the original alternator had before any modification?

The armature of an alternator has a lot of mass, a lot more than the rotor of an air pump. On startup, that mass resists quick acceleration from zero. And now you have two of them to resist. It could be possible that even if you have both alternators wrapped sufficiently, the slippage could then come from the crankshaft pulley, that is, the angular acceleration load of the extra alternator added on to the loads already there exceeds the dynamic torque transfer of the crankshaft pulley to belt. That one would be hard to solve, to try to get more wrap on the crankshaft pulley has obvious problems.

Some background - Alternator armature inertia has caused problems in cylinders-on-demand schemes. General Motors ran into trouble with cylinder deactivation, as the "dead spot" in torque caused by a shut-off cylinder causes less drive on the belt. This instantaneous slowing of the belt allowed the rotating mass of the alternator, which wanted to remain turning at the same speed, to power the belt. In effect, the alternator's rotating mass was causing bursts of belt slack right after the alternator. This was causing vibration problems all over the engine and belt drive system.
They cured it by coming out with an alternator pulley that has a one-way clutch in it. The belt can drive the alternator directly, but the alternator cannot drive the belt, it free-wheels instead.
Accessory drives are not as simple as they first appear.

 

I did notice that while running the belt is tight except after the harmonic balencer. There is maybe 1 to 1.5 inches of slack and the belt flops in that location. This area is between the harminic balencer and lower alternator. My engine runs fine and the alt is charging fine. I only have 1 alt connected, the other is free spinning with the belt on the pulley. Should I swap the idler pulley and tensioner back to their original position or add another tensioner? There is actually more belt wrap on the pulleys than before

 

I looked at my Bronco today to refresh my memory.

Like all engines, as viewed from the front, the crankshaft turns CW, so think of the crank/balancer pulley as pulling belt from the right and shoving belt out to the left (as viewed from the front). So the loosest part of the belt when running is after (to the left) of the crank pulley.

After the crank pulley, the belt would go over to the air pump, which is a low-load and low-inertia device. After the air pump, it would go on to the tensioner, which would pick up any slack dynamically, and wrap the belt tightly around the original alternator. After that, on to a fixed idler that completes the tight wrap around the alternator, then on to the A/C clutch pulley.

With the stock setup, if someone were to swap the positions of the tensioner and the fixed idler after the alternator, I would suspect that the belt could slip trying to spin the alternator up (and maybe slip in heavy electrical load situations also), as the belt would not be tightly wrapped around the Alt. due to the slack in the belt not being taken up previously (because the tensioner would be AFTER the Alt. pulley if they were swapped).

Then you introduced a second Alt. where the air pump was. And it is being fed by a slack(er) belt coming right off of the crank pulley.

I think I would put the tensioner and idler pulleys back where they were. I don't know what this does to belt wrap for your second Alt.

Adding a Second Tensioner - At first look, if you could get a second tensioner ahead of the second Alt., that would pick up the slack after the big pull of the crank pulley, and help keep it tight around the second Alternator's pulley.
So on an individual basis, that sounds like a good solution, though the mounting mechanics will be tough for that second tensioner.

But I really don't know what will happen dynamically with TWO TENSIONERS in the system, and close together at that.
Interactions... will they play nicely together? maybe, or...
Will they oscillate between them at certain speeds or loads, either creating bad vibrations or harmonic motions that could build up and create enough momentary slack to throw the belt?
Will vibrations or harmonic motions start acting like a pile-driver on all of the accessory drive bearings?
Don't know. That's what expensive heavy-duty mechanical simulation software is for. With two tensioners, I think that it is becoming a complex higher-order system.

Another idea, though the mounting details will be bad, would be to put a fixed idler where the stock location tensioner is, such that belt wrap around original Alt. is the same as stock. Then put the tensioner AHEAD of the second Alt., taking up the slack after the crank pulley. And the tensioner positioned so it gives a good wrap around the second Alternator's pulley. That may work, though like I said, the mechanics of mounting will not be trivial.

 

That makes a lot of sense. Even though its going to be a pain taking everything back off its definately worth it to get rid of the belt slip and voltage fluctuation. I'll get to work on it first thing in the morning. Thanks a lot for all the help guys and I'll update with the outcome.