YOU can not take a useful reading on VOLTS DC
with the load...(as outline two posts back) The only thing you will see is buss voltage and the fact that somewhere in the system , there exists a proper ground through a device..
If you place a DC VOLT meter between the ground on the lamp and hard ground..then turn the lamp on..your meter will read 12 volts, but the lamp won't light..Leave it on for a month..will the battery go dead? ..No..No load..or useful work is implied on the battery..(lamp is not lit EVEN though it says 12 volts in series..)
DC VOLTAGE READINGS
are always taken in PARALLEL
, Or across the LOAD..or the battery..+ and -..To check your state of charge..put the meter ACROSS the battery, engine at rest..it should read around 12 volts if it is charged..much below , could indicate a bad cell..
NOW, Start the car..let it warm and the high current surface discharge about 5 minutes to recover..then measure AGAIN, ACROSS
the battery..It should read between 13.95 and 14.4 VOLTS DC...If the alternator has an output.
NEXT, to measure AMPS..ALL READINGS
are taken in SERIES
with the circuit..To Do that, remove all load sources..(Domes, hood lamps trunk lamps, anything that will draw power while you test) Remove the battery cable from the battery, CONFIGURE
your DVOM for AMPS
, and set it for the HIGHEST
scale, (usually 10 amps).
Place one Probe on the battery terminal, the other on the Battery CABLE, note your reading as you do..If it pegs you have a major problem that your meter will not be able to handle..stop immediately , or you'll destroy the meter..get a higher AMP Capable meter..or seek help from a local auto electrical shop..If it reads low on the scale..reduce the scale until it "Just" tops out..That will be your actual reading..
On a modern vehicle with Computer, CD player with memory presets, Alarm systems, and the like, it should read between 0.3 and 0.8 (less than 1 AMP) to be good..any higher..you have a problem.
If that's the case..The first logical step , because it is the most common offender, Is disconnect the Alternator complete from the system, and recheck the AMPS reading in SERIES with the battery..If it falls back into Spec, replace the Alternator, OR you can get a diode/Regulator and rebuild kit, for around $15..and an hour or so of your time..
If no change, on the readings, If your Fuse buss is Well marked, pull ALL the fuses and relays from it and recheck the AMP draw again..If it is within spec, or lower, one of the branch circuits are the culprit..
While you have them out check ALL your fuses for CONTINUITY across the fuse, (it is not your cause of the problem, but you'd be surprised how many are found bad..and how much "stuff" begins working after) ..replace any bad ones..Don't eyeball them, the filament can open at or near an end or "blade foot" and you'll never spot it..use a test lamp or DVOM set for Continuity, or Ohms R X 1, calibrated 000.
Replace the fuses back in the buss one by one, watching your Current draw in AMPS..when it jumps up (above an amp) that is your offender..Troubleshoot that branch circuit. Repair or disable as needed. Continue until all are back in the fuse buss.
The Sender you have (numbers wise) should be the right one..Try your test again with just the sender hooked up to your meter, set it for OHMS , R X 1 Scale, Calibrated to 000, hook one probe to the center terminal and the other to the sender body, and carefully retest your sender..with / against your lab thermo..note the ohm readings as it ascends / declines and be sure it is the sender advertised on the box (they mix them up some times..) If it is..then I'm thinking you have a gauge linearity problem.. If the sender test "True" post back, I can give you a circuit you can build that you add in line to "Gain " linearity over the range..
Also, there may be a problem with draw through the dropping resistor to the gauge, try putting an instrument lamp on the power side of the gauge..this will keep the draw constant through the resistor as the gauge resistance to ground is changing..If that works, you'll need to wire the lamp up to the gauge + and ground , permanently, or install a 6 volt instrument regulator ..
The Alternator sounds like it's wired correct. The main output from it to the battery via Fuse link or maxi fuse, the red terminal jumpered to it, and the brown wire to the Idiot lamp, bypass resistor and fuse into the switch.