View single post by Mark Rosenbaum
 Posted: 10-16-2005 03:50 pm
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Mark Rosenbaum

Joined: 03-12-2005
Location: Kingman, Arizona USA
Posts: 532
My car has a Delco unit, and I've never observed the behavior of a Lucas alternator, so my comments apply only to the former.  I would expect the Delco and Lucas alternators to behave a bit differently in detail, but not in general. 

"1.  Upon engine start, the guage, at idle, will read 15 volts, just at the edge of the red."
*  So does mine, once the alternator starts providing a charge.  In my case, it takes 10-15 seconds for the gauge pointer to move from the bottom of the white band, to the top.  I reckon a third to a half of this is due to the response time of the gauge itself.

"2.  Then, on the road going through the gears with no headlights or other power users on it will then drop to somewhere in the 14-15 volt range."
*  So does mine, over the course of 10-20 minutes of driving.  I've been presuming that this occurs as the battery gets recharged. 

"3.  When I turn on the headlights while on the road, the voltage drops a little, to 13-14 volts."
*  I see a slight drop but much less than a full volt's worth.  I wouldn't think this is a problem.

"4.  But at idle, with the lights on, the voltage drops to 10.5 volts." 
*  Not normal.  Sounds like the alternator stops charging for some reason.  For Delco alternators, I'd suspect dirty wiring connectors on the sense lines.  For Lucas alternators, I'd suspect a bad internal regulator.
*  Is the 'Ignition' warning light on when this occurs?  If not, the battery may have a bad cell in addition to any alternator problems.  Also, some alternator types may not charge under some or all circumstances, if the warning light is burned out, or is an incorrect type.

"5.  If I then raise the engine rpm to about 2000 it comes back up to 13-14 volts very slowly."
*  It sounds like the alternator once again provides a charge at that time.  This might be the normal response after your Case 4, depending on what you meant by 'very slowly.'  I'd also check the battery terminals and cable ends to ensure bright metal to metal contact there.

Whether Lucas or Delco, the alternator should be able to keep the battery charged under any plausible scenario.  However, if one adds accessories that consume electrical power, the alternator's capabilities can easily be exceeded.  If for example the car has a 60 watt sound system, you would need an additional 60 to 120 watts in additional alternator capacity -- that's 5 to 10 amps more.  With a sound system that rattles windows on neighboring houses, you could be talking about needing an additional 40 amps or more.

In theory, adding a couple of typical automotive relays to operate the headlights might make the difference between having some tiny current left over to charge the battery, and being slightly deficient, but in practice, relays don't use sufficient current to be a concern.

For reference, I've attached the schematic of a typical Delco alternator (not necessarily the one used in the JH).

Attachment: Delco SI schematic.jpg (Downloaded 102 times)