Alternator tested good, not charging

Trentman2004

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Aug 6, 2018
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I have a 1990 5.0 and I recently have had the battery and alternator tested at auto zone and both tested good. However, I charged my battery with a charger overnight. Reinstalled in car, it started and ran fine for a dew minutes, took it for a ten minute drive- no problems.

Over the last few day I have started it once yesterday to pull it out to wash and today I was working on the ac system and started and shutdown several times

Finally I ran it to charge the ac system over about 15 minutes and then I went to drive it and my headlights were dim and the car kind of stalled. I turned off the engine and now it won’t even turn over. All lights inside and out are on and battery voltage reads 11.5 volts

Could my alternator still be bad even though the auto zone test showed good?

Ps the auto zone test was done off the car
 
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nickyb

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Apr 3, 2009
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Your battery could also be on its way out,not able to hold a charge.
jump it and use a dvm to check alt. Output.
 

Trentman2004

Member
Aug 6, 2018
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so this morning the battery read 12.6 volts and with the car running it read 12.5 volts. Can someone help me find the a wiring diagram for the wires coming out of the alternator? Also I have a stock set up so I don't know if thats 2G or 3G etc. I was going to buy a new alternator and then see what the output is then. Otherwise I have a bad ground or something.
 

jrichker

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Alternator troubleshooting for 86-93 5.0 Mustangs:

Never, never disconnect an alternator from the battery with the engine running. The resulting voltage spike can damage the car's electronics including the alternator.



Revised 16-Oct-2016 to add 94-95 alternator wiring diagram

Red color text applies to cars with a 3G alternator.

Do all of these tests in sequence. Do not skip around. The results of each test depend on the results of the previous tests for correct interpretation.

Simple first step: Remove the alternator and take it to your local auto parts store. They can bench test it for free.


Use a safety pin to pierce and probe the insulated connectors from the rear when doing tests with the connector plugged into its' mating connector.

Engine off, ignition off, battery fully charged.
1.) Look for 12 volts at the alternator output. No 12 volts and the dark green fuse link between the orange/black wires and the battery side of the starter solenoid has open circuited.
3G alternator: Look for 12 volts at the stud on the back of the alternator where the 4 gauge power feed wire is bolted.
No voltage and the fuse for the 4 gauge power feed wire is open or there are some loose connections.

2.) Look for 12 volts on the yellow/white wire that is the power feed to the regulator. No 12 volts, and the fuse link for the yellow/white wire has open circuited.

Engine off, ignition on, battery fully charged:
The instrument cluster must be in place and working properly for the alternator to charge. The indicator light is an essential part of the charging circuit.

1.) Alternator warning light should glow. No glow, bulb has burned out or there is a break in the wiring between the regulator plug and the instrument cluster. The warning light supplies an exciter voltage that tells the regulator to turn on. There is a 500 ohm resistor in parallel with the warning light so that if the bulb burns out, the regulator still gets the exciter voltage.
Disconnect the D connector with the 3 wires (yellow/white, white/black and green/red) from the voltage regulator.
Measure the voltage on the Lt green/red wire. It should be 12 volts. No 12 volts and the wire is broken, or the 500 ohm resistor and dash indicator lamp are bad. If the 12 volts is missing, replace the warning lamp. If after replacing the warning lamp, the test fails again, the wiring between the warning lamp and the alternator is faulty. The warning lamp circuit is part of the instrument panel and contains some connectors that may cause problems.

2.) Reconnect the D plug to the alternator
Probe the green/red wire from the rear of the connector and use the battery negative post as a ground. You should see 2.4-2.6 volts. No voltage and the previous tests passed, you have a failed voltage regulator. This is an actual measurement taken from a car with a working electrical system. If you see full or almost full12 volts, the regulator has failed.

Engine on, Ignition on, battery fully charged:
Probe the green/red wire from the rear of the connector and use the battery negative post as a ground. You should see battery voltage minus .25 to 1.0 volt. If the battery measured across the battery is 15.25 volts, you should see 14.50 volts

Familiarize yourself with the following application note from Fluke: See http://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. .



You will need to do some voltage drop testing of several of the wires.

Start looking for these things:
1.) Bad diode(s) in the alternator - one or more diodes have open circuited and are causing the voltage to drop off as load increases. Remove the alternator and bench test it to confirm or deny this as being the problem.

2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. Do the voltage drop test as shown in the Fluke tech note link. Measure the voltage drop between the alternator frame and the battery negative post. Watch for an increase in drop as the load increases. Use the Fluke voltage drop figures as guidelines for your decisions.

3.) Bad regulator that does not increase field current as load increases. Remove the alternator and bench test it to confirm or deny this as being the problem.

4.) Bad sense wire - open circuit in sense wiring or high resistance. The yellow/white wire is the voltage sense and power for the field. There is a fuse link embedded in the wiring where it connects to the black/orange wiring that can open up and cause problems. Disconnect the battery negative cable from the battery: this will keep you from making sparks when you do the next step. Then disconnect the yellow/white wire at the alternator and the green fuse link at the starter solenoid/starter relay. Measure the resistance between the alternator end of the yellow/white wire and the green fuse link: you should see less than 1 ohm. Reconnect all the wires when you have completed this step.

5.) Bad power feed wiring from the alternator. Use caution in the next step, since you will need to do it with everything powered up and the engine running. You are going to do the Fluke voltage drop tests on the power feed wiring, fuse links and associated parts. Connect one DMM lead to the battery side of the starter solenoid/starter relay. Carefully probe the backside of the black/orange wire connector where it plugs into the alternator. With the engine off, you should see very little voltage. Start the engine and increase the load on the electrical system. Watch for an increase in drop as the load increases. Use the Fluke voltage drop figures as guidelines for your decisions.

Alternator wiring diagram for 87-93 Mustangs.



The following are diagrams courtesy of Tmoss & Stang&2birds

Alternator wiring diagram for 94-95 Mustangs.


Voltage drops should not exceed the following:
200 mV Wire or cable
300 mV Switch
100 mV Ground
0 mV to <50 mV Sensor Connections
0.0V bolt together connections

Alternator wiring circuit
Notice the green wire connects to a switched power source. The circuit contains a 500 ohm resistor in series between the switched power and the alternator. Connecting it to switched power keeps the regulator from drawing current when the engine is not running. The resistor limits the current flowing through the wire so that a fuse isn't needed if the wire shorts to ground.

Also notice the sense wire connects to the starter solenoid and it is fused. It connects to the starter solenoid so that it can "sense" the voltage drop across the output wiring from the alternator.


Fuse link for 86-93 Mustangs
Replacement parts:
14 gauge fuse link for stock alternator.

Bussman BP/FL14 Fusible link
AutoZone

Dorman - Conduct-Tite 14 Gauge Fusible Link Wire Part No. 85620
Advance auto parts #85620
Pep Boys - SKU #8637594




See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring; http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

94-95 Mustang wiring diagrams
http://www.veryuseful.com/mustang/tech/engine/#95-95Diagrams

Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/91-93_5.0_EEC_Wiring_Diagram.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

O2 sensor wiring harness
http://www.veryuseful.com/mustang/tech/engine/images/mustangO2Harness.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pin out
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

Mustang 5.0 Lights and Radio schematic, by TMoss:
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxLights-Radio_diag.gif

87-92 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang87-92 PowerWindowWiring.gif

93 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang93PowerWindows.gif

T5 Cutaway showing T5 internal parts
http://www.veryuseful.com/mustang/tech/engine/images/5_Speed_Cutaway_Illustrated.jpg

Visual comparison of the Ford Fuel Injectors, picture by TMoss:
http://www.veryuseful.com/mustang/tech/engine/images/Ford_Injector_Guide.jpg

Convertible top motor wiring http://www.veryuseful.com/mustang/tech/engine/images/mustang88VertTopMotorCkt.gif

Engine mounted fuel injector harness
http://www.veryuseful.com/mustang/tech/engine/images/mustangEngineHarness.gif

Location of the TPS, IAB, and the 10-pin connectors on a 5.0, picture by TMoss:
http://www.veryuseful.com/mustang/tech/engine/images/TPS_IAB_Pic.jpg

Starter circuit
http://forums.stangnet.com/attachment.php?attachmentid=21328&d=1080916057

Alternator diagram for 94-95 Mustangs.
http://www.veryuseful.com/mustang/tech/engine/images/Mustang-94-95-Alt.gif
 
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Mustang5L5

Mod Dude
Feb 18, 2001
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Check to see that the wire from the instrument panel is sending power to the alternator. That's what signals the alt to actually turn on. It's connected to the BATT light in the dash
 

Trentman2004

Member
Aug 6, 2018
26
9
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37
North Carolina
So I bought anew alternator and same problem not charging. You might be right Mustang5L5, cause I don’t have a battery light on. I recently converted the illumination bulbs to LED, but the battery light wasn’t one of them. I did check the the connectivity for the alternator positives and grounds and all checked out

I will check out the cluster light tomorrow

Thanks to all for your help and I will keep you posted
 

Mustang5L5

Mod Dude
Feb 18, 2001
30,988
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Massachusetts
There is a way to test it but I think it may be to apply 12V to one of the wires on the alternator to "activate" it.

Did this issue come up after changing the cluster bulbs? I realize you said you didn't change the batt light. Does it come on if you turn the key to on but don't start the car?
 

Trentman2004

Member
Aug 6, 2018
26
9
13
37
North Carolina
Mustang5L5. It does seem happen AFTER i changed the bulbs. I also accidently shorted out the part of the cluster circuit because of that crappy circuit film on the back of the cluster. The battery light does NOT come on when I turn the key on. I have to also check the fuses as well. Could be that simple.
 

Mustang5L5

Mod Dude
Feb 18, 2001
30,988
7,356
224
Massachusetts
I think it's related then.

These are the wiring diagrams for an '88 but I think 90 is the same here. I can check later.

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The Light Green with red tracer wire is the wire coming from the batt light in the dash. Wire 904 on the diagram. It appears it's tied in directly to the ignition circuit without a fuse in between.
 

Mustang5L5

Mod Dude
Feb 18, 2001
30,988
7,356
224
Massachusetts
Here's the wiring diagrams for a 1990. I don't see a fuse. Goes from batt light to ignition and then back to the starter solenoid yellow wire through fusible link A

IMG_4858.JPG

IMG_4859.JPG

IMG_4860.JPG
 

jrichker

StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
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@Trentman2004

Solder some 22 gauge copper wire to the places where the traces have burned up or use some of the material from the Rear Window defrost repair kits to repair the damaged copper traces.
Use gel type super glue to stick the copper foil back to the plastic sheet. The super glue will withstand the heat of soldering as long as you don't apply heat for longer than it takes to melt the solder with a clean tip on the soldering pen.
 

Olivethefet

Slap me as well as point and laugh
May 17, 2018
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I am glad I read this. I would not have thought that the bat light going out, or something on the instrument cluster would cause the charging system to not work. Glad you got it fixed!