Jumpy Battery Voltage

[unreal808]

Get a copy of the circuits for your car, and a multi-function digital voltmeter, and go at it. I don't think it is your battery or alternator at this point. :goodluck:

[unreal808]

But if you want to test your Alternator your self


Alternator Diagnosis Worksheet
1. Visual Inspection – First check the Drive Belt. Is the V-belt or S-belt that turns the alternator tight enough. With the engine off, can you turn the alternator by hand? (try pushing the external fan blades on the alternator to get it to move while the belt stays still.)

And, does the belt feel tight enough? Don’t skip this step, loose belts are real common. OK ____ Not OK ____

Next, check the wire and cable connections to the alternator. Are all the wires and cables secure? Can you move the nut by hand? If you pull off the connection, is there black burning that tells you the electrical terminal is loose and heating up. (It can’t carry the amps then.) OK ____ Not OK ____

Don’t forget loose nuts and bolts. They cause a lot of noise problems. Are they tight enough? OK ____ Not OK ____ Worn bearings can also cause noise problems. Check here: Bad bearings __ And stator problems on the inside can also cause extra noise. For all this alternator testing to be accurate, the battery must be good and at full state of charge.

2. Check Voltage at Battery – Hook up voltmeter, and measure the voltage at the battery with engine idling. Record here: ***________. Raise engine RPM to about 1500 and measure voltage again. Record here: ________. (With most accessories off, these two should be about the same.) If alternator, battery and cables are good, this is the voltage level set by the voltage regulator. Should be about 13.8 – 14.5 volts.

3. Prepare for Amp Measurement: Hook up amp pickup, or whatever will measure your amp output now, so it’s ready to use later. Don’t’ turn the carbon pile yet. Are you measuring around the wire or cable coming from the alternator? (this is best) Yes ___ Or are you measuring at the battery where some amps go into the battery, and some amps go out to run the car? Yes ___ Record type of tester ________________

Check off which method you are using: Load down the battery to 12.0v with a carbon pile tester __ Disable ignition and crank engine for 15 seconds before testing __ Leave on headlights for 3 minutes to lower battery voltage __ Battery is already low on charge __ Full field alternator by grounding the right bolt, hole or wire __ Turn on every accessory in the car you can __ Other method __ (describe ________________________ )

4. Alternator Amps Output – Raise RPM to 1500 get maximum output and record the amp output here: _____ Also, record the output specification (it may vary from 40 to 130 amps): _____ Amp output is OK ___ Not OK ___

5. Diode or Stator Check – This checks the evenness of the alternator output. Voltage jumping up and down from a bad diode or stator winding can confuse computers on the car and cause engines to stall. It also limits amperage output to about 65% of specification. This test must also be done when the alternator is putting out maximum amperage, or it won’t be as accurate. Use one of the following: On a VAT40 or similar tester, just dial in the test as you load down the battery. OK __ Not OK __ With a voltmeter set to AC, on a small scale, measure AC volts at the alternator output cable or wire. Over 0.5v AC is bad. OK __ Not OK __ Using a lab scope, watch output voltage waveform. (Nice, even humps are normal, but drastic up and down jumps are bad.) OK __ Not OK __

6. Voltage Drop on Alternator B+ Output Cable/Wire – High resistance on the output cable can limit amperage, cause the alternator to overwork, or even cause a fire if the resistance is too much when the alternator is putting out a lot of amperage. Make the alternator put out high amperage, place the red digital voltmeter lead on the output cable or wire at the alternator. Place the black lead on the battery positive post. And read the voltage on a low scale. (Should be 0.3 v. or less) Voltage reading: ____ OK __ Not OK __.

7. Voltage Drop on Alternator Ground Cable (if it has one) – Like above, make the alternator put out high amperage, and measure voltage drop from alternator body to battery negative post. Place black lead of voltmeter on alternator body and red lead on battery negative post. And read the voltage on a low scale. (Should be 0.3 v. or less) Voltage reading: ____ OK __ Not OK __.

8. Voltage Regulator Check – This is optional, but can show you how the system works or help you diagnose a bad regulator. The voltage regulator is regulating the amount of amperage flow through the rotor field coil, to make the alternator charge more or less. You can sense the rotor creating a magnetic field by putting your pocket screwdriver tip near the back of the alternator, in the center where the bearing would be. When the alternator is charging, you should feel magnetism. Yes __ NO __.

Next, you locate the regulated part of the field circuit with a wiring diagram or manual. One end of the coil always gets power or ground (usually power). And the other end is regulated to control magnetic strength (usually grounded a small or large percentage of the time). For example, on a Ford, you often see a bolt that says “ground here to test”. You can ground this to make maximum magnetism and greatest alternator output, called “full fielding”. Some Toyotas, and older GMs have a hole you can ground to full field. Others have a wire someplace you can ground (or power). Ground (or power) this and record amp output: ___________. If you had to manually make the field work, then the regulator or wiring is probably bad.

You can also measure the voltage at this point to watch the regulator work. If the alternator is charging a lot, record this voltage ______. And as the alternator has charged the battery and charges less, record this voltage _______. This second voltage is usually a lot higher, because the regulator is grounding the field circuit less. This shows the regulator is working properly.

A good regulator will make the alternator charge less amps as the battery gets fully charged, and you should see this if you are watching the charging amps. Record the charging amps when the alternator is charging a lot because the battery is low: ______. Then when the alternator is charging less, record the charging amps again: _____. The second should be a lot less than the first.

9. For Intermittent problems – Optional. There are a couple things you can test. Tap lightly where the brush area of the alternator is. Watch the alternator charge more or less. This can make the brushes work or not work. (Brushes wearing out is a common problem) OK __ Not OK __.

Also, check the wire connections at the back of the alternator. Often an intermittent problem is these connections are loose or bad, and so the alternator doesn’t always charge. OK __ Not OK __.