Since so many of you have asked so nicely, and I'm (frankly) tired of finding it to copypasta, here's my "primary electrical" testing writeup.
Mods - you may want to stick this. Everyone else - if you can't search this up easily, please turn yourselves in for a vigorous brain-scrub and a remedial Google-Fu session. It's about as easy as possible...
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- A multimeter
- A notepad
- A pen
- A helper (very useful, but not necessary. You'll need longer DMM leads if you don't have a helper)
- A hydrometer (if your battery has removable caps)
- Some distilled or R/O filtered water (ditto.)
- If you are doing this because of suspect voltage readings at your Instrument Panel (IP,) VERIFY WITH DMM. The IP voltmeter is a notorious liar, particularly on earlier (1990-back) models!
- If the battery itself is suspect, check the electrolyte level (topping off with distilled/RO water PRN.) Put on a slow charge overnight (<5A charge rate.) Do this BEFORE YOU DO ANY OTHER TESTS
- If the alternator is suspect, charge your battery for at least TWO FULL HOURS at a medium charge rate (2-15A) before doing any other tests. Also, check the electrolyte level as above (if the alternator is undercharging, the battery will simply be discharged. If the alternator is overcharging, it's possible that water has boiled off in the battery, and the electrolyte will be low.)
- If the starter motor is suspect, check: the mounting screws (it grounds through them,) the stud nuts for the electrical contacts (loose connections increase resistance,) and that where the starter motor contacts the engine/trasnmission assembly is clean (screws with the ground.) Also, charge the battery - min. 1 hour at min. 10A.
(Yep - all of thes involve topping off the battery charge. And for good reason. If the battery is partially discharged, it WILL fail test!)
Now that you've got the basic checks out of the way:
1) If the battery caps are removable, pop them off (WATCH YOUR EYES! I'll usually put whatever I'm prying with in the notch under the cap, put my hand flat over the cap, and then turn away while I'm prying the cap off - and I wear spectacles! You can't be TOO careful with your eyes...)
2) Look into the holes - use a flashlight if you have to. The electrolyte level, ideally, will be even with the bottom of the hole. However, it MUST cover the plates entirely! Top off with distilled/RO water PRN.
2a) If you had to top off, put the caps back on and rock the vehicle at the bumper - you're trying to mix the water in so you don't get a "false discharge" reading.
3) Remove the caps again (see (1), above,) and get out your hydrometer. Draw a sample up into the tube, read the float, and note the reading. Squeeze the bulb and put the sample back in the cell. Repeat for the other five.
4) There are two things that should be checked with the hydrometer (Specific Gravity, or "s.g." readings. This measures the concentration of the acid in the cell, and therefore the state-of-charge of the cell.)
- If you have any values that are "obviously out of line," stop. You've got a cell going out on you. If not, take an average of all six values and then compare the individual values to the average. Individual values should not vary by more than +/- 0.025 from the average.
- Absolute readings - any cell that is OVER 1.225 s.g. is sufficiently charged. Anything 1.224 s.g. AND LOWER is not. Observe the following values (should be corrected for temperature to 72*F, but not all hydrometers do that. The variation is nominal
5) If the battery is fully charged, or after you have charged it, note the "resting voltage" (battery installed, engine OFF, key OFF, doing nothing.) Use a DMM at the battery terminals.
6) Verify that battery voltage exists at the large terminal on your starter motor (should not be more than 0.5VDC below battery voltage at the terminals) and at the large cable on the back of your alternator (likewise.) The primary distribution point may also be tested, but variation there will be less due to lead length (figure it should not be more than 0.25VDC below.) Examine, clean, and/or service any conductor showing excessive loss. If you don't get ANY voltage reading at any of those three points, FIND OUT WHY.
7) Disable your ignition.
8) Have a helper crank the starter motor while you monitor battery voltage. It will drop sharply then recover - it should recover to something above 9.60VDC (more is better.) DO NOT HOLD THE KEY IN THE START POSITION FOR MORE THAN FIFTEEN SECONDS, and ALLOW AT LEAST TEN MINUTES BETWEEN REPEATS IF YOU DECIDE TO REPEAT THIS TEST (don't do it too much tho - or you'll drain your battery.) Note the voltage you read for later.
9) Reconect your ignition. Recheck and note your resting voltage after one full minute - it will probably be lower.
10) Start your engine. Check voltage at the battery terminals. It should be higher than the resting voltage - if it is not, check for output at the back of the alternator. If you have higher voltage at the alternator output post, you have a failure in the conductor in between (they're fused from the factory - check the fuse link or fuses in the distribution box.) If you do not, you may have a failed alternator or a failed regulator.
11) Have your helper run the engine up to 1200-1300rpm and hold it there. Voltage should increase a bit more, especially if it was a bit low.
12) While at 1200-1300rpm at the crankshaft, begin turning on electrical accessories while monitoring system voltage. You should see a slight drop with each new accessory, but the alternator should recover. Unless you've gotten silly with add-ons, and you don't turn your winch on (which will ALWAYS have a draw in excess of your alternator's rated output!) you should see the system voltage stay above the battery resting voltage.
13) Turn all of your accessories off, and let your engine idle for 10-15 minutes. This will do two things - 1) it will charge your battery up from any remaining discharge from starting (typicaly starter motor draw runs 130-250A; depending on displacement, compression, and engine condition.) 2) It will heat up your alternator and regulator circuit.
14) With the alternator warmed up, repeat the tests and make sure you don't lose voltage above what you got "cold" (many automotive electrical/electronic failures are heat-related...) Losing a little with heat is normal - but all values should still be above the values in the chart below.
15) Shut down. Check battery voltage - should stay above the "full charge" value.
16) Check battery voltage again in one hour - it should still be about the value given in (15). If it is not, the battery is self-discharging (because a cell isn't dead yet - you'd have caught that earlier, but it's going...) and you should keep an eye on it.
12.60VDC Battery, Fully Charged (min.)
09.60VDC Battery, Cranking Starter Motor (min.)
13.20VDC Battery, Engine RUNNING, Alternator CHARGING (min.)
15.00VDC Battery, Engine RUNNING, Alternator CHARGING (max.)
-- 1.265 FULL CHARGE
-- 1.225 75% Charge
-- 1.190 50% Charge
-- 1.155 25% Charge
-- 1.120 DISCHARGED (1.120 is a max value - may be rather lower. Compare to computed average to determine if you have a dead cell or not.)
A battery should not be load-tested if the s.g. of the electrolyte is below 1.225 - charge first.