4108 Alternator Questions

[richard5933]

Two quick questions about my alternator:

1) Finally got the 24v to 12v battery-to-battery charger ordered and it arrived today. It's designed to run only when the alternator is charging so that it won't drain the chassis batteries, and there is a variable cut-in voltage settings. The GM service manual shows 27.5v as the regulator setting, but I suspect that this is the upper limit. What would be the lower voltage limit I should use as a cut-in voltage for the 24v to 12v charger? I'm not concerned with it running at idle but want it to run whenever we're heading down the highway.

2) I've got a tachometer from our 4106 which was never installed. It is a VDO 333-962 and from what I see in the instructions is can be set to run from an output on the alternator. The 4108 does not have a tach so I thought it might work out that I could install the VDO tach in it. Any one have a clue if it will run from the 24v alternator in the 4108 and if so, to which terminal on the alternator would I connect it?

Thanks and all the best to everyone for a healthy and happy new year!

[j.m.jackson]

I'm running a VDO tach off my 12v Delco 50 alternator. The R terminal has unrectified AC voltage on it, which should drive the tach without an issue. You'll just need a cheap optical tach to calibrate the VDO with. <20 bucks for the optical tach on amazon.

[richard5933]

If the R terminal has ac voltage on it, then it shouldn't matter if it's a 12v or 24v alternator - the VDO seems to have a pretty wide range of adjustment.

Now I just need to figure out what the voltage is supposed to read when the alternator is charging so I can set the cut-in limit on my battery-to-battery charger.

[bevans6]

The R terminal usually has un-rectifed AC at half nominal voltage and I found a document that says the 50DN alternator R terminal frequency is 1/10 the alternator RPM.  So you would use this equation:  engine RPM X pulley ratio/10 = Rfreq.  The alternator normally is fully capable of charging at idle rpm (the 50DN 24 volt can put out 140 odd amps at typical idle) so you should set your voltage cut in to anything higher than fully charged battery voltage (12.7 or 25.4).  I would probably set it at 13.2 or 26.4.  I might make it manually switchable - with my 50DN connected to both my house and start banks, if I do a cold start and both are drawn down a bit the belts slip and squeal from the alternator drawing too much power at idle.  I do a cold start, let the alternator charge the start batteries for a minute or two, then add in the house bank.

Brian