Re: [DML] battery voltage loss
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Re: [DML] battery voltage loss

A little 101 on alternator basics.

 A lot of people might think that power comes from the battery,  But 
that is not true,the source for all of that energy is the gas tank. 
Yep. And the link from the gas tank to the battery is that mysterious 
thing called the alternator. It takes mechanical power from the 
crankshaft, transmits it via a "fan" belt, (it used to run the 
cooling fan as well,but On the Deloreans,runs the Water pump and alt. 
only) (or serpentine belt as it is called in most of the newer cars), 
and turns the alternator. So, the main function of the alternator is 
to convert power from the gasoline engine that drives your Delorean, 
along the road, to electrical energy to keep the battery in tip-top 

So, what happens when your alternator goes bad? Well, at first, 
nothing. That is, because the battery has some reserve power in it, 
enough to keep the engine running for quite some time, (with the "D" 
orignal alt. charging it, maybe to the nearest safe spot to pull 
over) others,many many miles in fact. So a bad alternator doesn't 
necessarily mean a tow truck should be called right away. As long as 
energy is conserved elsewhere, like turning off the blower motor, the 
rear window de-fogger, the stereo and the headlights (if possible), 
you could make it for some distance(again NOT with the orignal "D" 
alt.) on just the battery reserve alone.

One major problem which will finally occur as the battery loses its 
charge is that there will not be sufficient voltage to keep the 
engine running well. What might and possibly WILL happen down the 
road,is that the catalytic converter Could/would be glowing white hot 
and flames coming from beneath the car.(You Don't want that, with a 
oil based Fiberglass body!!)  What had/would happen is the alternator 
quits, the battery runs down, the engine is not firing on all 
cylinders and the unburned fuel is being burned in the catalytic 
converter! Yikes!!

So, how do you know when your alternator is going bad? Most of the 
time the alternator fails in stages. A little techie talk here. The 
alternator gets its name from the fact that it generates alternating 
current (AC).  Well the battery can't use alternating current so the 
alternator output is fed into what are called diodes, which convert 
the AC into DC. The alternator has a unique feature in that it is 
able to generate a relatively high voltage while the engine is at 
idle.  The alternator can do this since it is really three 
alternators in one body. Each of the three sections of the alternator 
generates its voltage out of phase with the other two sections. Since 
the complete cycle (one revolution) of the alternator is 360 degrees, 
each phase is shifted by 120 degrees from the next phase. So in one 
revolution of the alternator it puts out three separate 
voltages.What.. You didn't know that?

OK, back to the failure mode. Each of the three phases has its own 
windings in the alternator and each of the windings has its own pair 
of diodes. Each of these windings and/or diodes can fail, one set at 
a time. If this happens the alternator can still charge the battery, 
but only with a limited current, approximately 2/3 of its original 
capacity if one system fails. If two systems fail, then it puts out 
only 1/3 of its rated capacity. What that means to you is that you 
can go a long time on a limping alternator. Chances are if you don't 
need headlights or air conditioning or other high current using 
accessories, you would never know that the alternator was in the 
process of failing! The time you will find out is when it is 10 below 
zero and you wear down the battery by cranking the starter, then put 
the fan on high for heat, and then drive in the dark.

In your case I would think,you have one or more, bad Diodes,causing 
your battery to discharge!!

So, how can you tell if your alternator is failing without taking it 
apart and doing some measuring inside the alternator? It's really 
pretty simple. You will need a simple voltmeter. You can get one at 
Radio Shack for under ten dollars or thereabouts. Here's what you do -
 start the car, make sure all the accessories are off and rev up the 
motor to a fast idle. Set the Voltmeter to the DC scale (not AC or 
Ohms). Measure the voltage across the battery terminals - red lead of 
the voltmeter on the positive terminal, black on the negative (ground 
in most cars). The voltage should, and probably will, read around 14 
volts. If it reads less than 12 volts you may indeed have a failed 
alternator, and you can skip the next step. Next, turn on the heater, 
the rear window de-fogger, the radio, the headlights and anything 
else that draws power. Now rev up the motor and watch the voltmeter.
(DON'T USE the Delorean Voltmeter to test) It should still be reading 
around 14 volts. If it reads lower than 13 volts the chances are that 
your alternator is not up to snuff.(Time to switch to the improved 
Vendor Alternators)

One last failure mode is of course noise. The rotor inside the 
alternator rotates on bearings, normally very high precision needle 
bearings, and these can fail. When they do you will hear a loud 
grinding noise associated with the alternator. To isolate the noise 
take a length of tubing, heater hose will do fine, put one end to 
your ear and move the other around in the vicinity of the alternator. 
The noise will be much louder when you point it at the alternator if 
that is the culprit. Other possibilities are the water pump, which 
are also driven by the engine belt. To further isolate the noise 
disconnect the drive belt and spin the alternator by hand. If you 
hear a rumble or grinding noise then the bearings are shot. If you 
don't hear a noise the problem may still be in the alternator since 
the bearing might be quiet without the loading of the drive belt 
tension. Check for side play in the pulley. If you are pretty certain 
the noise came from the alternator it is a relatively simple task to 
take it apart and visually inspect the bearings, or else swap it in 
for a improved Vendor(GM) alternator. 

Also make sure to check the connections at the battery terminals and 
also check to see that the voltage is the same at the alternator 
terminal (the big fat one with the heavy wire attached) Check to make 
sure the belts are tight and not slipping. Replace them if they are 
cracked or shiny on the side that faces the alternator pulley.

One final thing to check - the field voltage. In order for the 
alternator to generate electricity it must be supplied with a field 
voltage. If you know which wire is the one that supplies the field 
(normally labeled 'F') then simply check with a voltmeter to see if 
there is 12 volts at the field. Another check is to use a hacksaw 
blade or a lightweight screwdriver , anything magnetic, and hold it 
near the side of the alternator with the ignition switch turned in 
the on position. If there is a field voltage present then the metal 
will be attracted magnetically to the side of the alternator, not 
very strongly, but you will feel it pull the metal to the side of the 

Also you could install a cutoff on the battery,or disconnect the 
battery,and check the battery 24hrs.later for voltage loss.

Hope this helps


--- In dmcnews@xxxxxxxxxxxxxxx, "John Hervey" <john@xxxx> wrote:
> Brian, How is the alternator doing. If there is a problem in it, 
that would
> be one place.
> John Hervey
> -----Original Message-----
> From: dmcnews@xxxxxxxxxxxxxxx [mailto:dmcnews@xxxxxxxxxxxxxxx] On 
Behalf Of
> Brian Davis
> Sent: Thursday, September 22, 2005 8:07 PM
> To: dmcnews@xxxxxxxxxxxxxxx
> Subject: [DML] battery voltage loss
> If I leave my car parked for a few days my battery
> draws all the way down. I originally thought it was my
> door light switch as the light was staying on if I did
> not get the door closed really solid. I adjusted the
> switch and the light was going out. Problem is my
> battery still has a slow draw somewhere.

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