Electrical Did Tune Up, Had Horrible Idle, Fixed Idle Issue...now My Battery Light Is Coming On....

loosenut02GT

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May 17, 2004
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It is now coming on anytime the engine is below 2000 rpm..... All happened after tune up.... It has a horrible idle...found out it was a vacuum leak...fixed the leak, idle was fixed...But now the battery light is on anytime engine drops below 2k rpms...any ideas..coincidence?? All connectors are good...nothing was/ is broken...just don't get it....
 
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Moosee1955

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Sometimes we work on one thing on a vehicle, and something totally unrelated pops up. Look at the bright side, it's not a customers car. Check around alternator to make sure you didn't knock a wire or plug loose on or near alternator. Put a volt meter across battery terminals with motor running. See what you voltage is. Needs to be 12.5 or higher motor off. 13.8 to 15.2 running. What year is your car? You said it charges above 2000 rpm? Check fan belt and pulley. I personally think you blew out a diode in alternator. This is why you always disconnect negative on battery every time your wrenching on vehicle. Check to be sure ground from battery is clean and tight, on both ends. Same with positive leads. Beyond that you need to pull alternator and have it tested.


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loosenut02GT

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Pulling the alt now to have it tested. I think with the surging idle, it blew the alt somehow...I've been doing this almost 20 years, and this is a first for me... Just got the car, and doing basic maintenance, for now. It's a 94 Cobra with 33k miles on it...the plus, wires, cap and rotor were all factory installed parts, 22 years old....
 
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loosenut02GT

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Just had alt tested, passed three tests in a row.
Bought a volt meter, going to reinstall and check a few things. Any pointers?
 

Moosee1955

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From here your going to have to do some tests. I'm sure one of the other guys can shoot you a diagnostic procedure to follow. Does your 94 have a separate voltage regulator. Don't feel bad, I've worked cars/trucks and motorcycles 45 years, and have weird things happen that shouldn't. I'm working just with an I phone and can't get you the schematics and troubleshooting flow charts you'll need. Can you read and understand wiring schematics? Keep us posted with results.


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loosenut02GT

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From here your going to have to do some tests. I'm sure one of the other guys can shoot you a diagnostic procedure to follow. Does your 94 have a separate voltage regulator. Don't feel bad, I've worked cars/trucks and motorcycles 45 years, and have weird things happen that shouldn't. I'm working just with an I phone and can't get you the schematics and troubleshooting flow charts you'll need. Can you read and understand wiring schematics? Keep us posted with results.


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I can read and understand them, yes. The voltage reg, is on the back of the alt. It's like 60$.
It's just super weird... Charges at idle...rev it up, volts drop..At 2k then pick back up around 2500.... Then sometimes it'll do the drop and not start charging again till it hits idle...it shows at the alt, and battery, 13.8-14, till drop then it hits battery voltage 12.6 or so....
 

Moosee1955

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If all the wiring is good, and the alternator checked out ok. I'm sure I'm going to hear from the other guys, how this is wrong, but your next step is replace the voltage regulator. Your connections to alternator clean and snug. Check the wires and pins going into voltage regulator. I've been following your post, and I'm surprised one of our members didn't shoot you the electrical schematic, with step by step procedures. Sorry I can't on my end. Just got off phone with oreillys in my town. I explained about possible voltage regulator problem. She said it's returnable even after install. Just keep receipt and packaging. Just some food for thought on an easy fix.


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loosenut02GT

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If all the wiring is good, and the alternator checked out ok. I'm sure I'm going to hear from the other guys, how this is wrong, but your next step is replace the voltage regulator. Your connections to alternator clean and snug. Check the wires and pins going into voltage regulator. I've been following your post, and I'm surprised one of our members didn't shoot you the electrical schematic, with step by step procedures. Sorry I can't on my end. Just got off phone with oreillys in my town. I explained about possible voltage regulator problem. She said it's returnable even after install. Just keep receipt and packaging. Just some food for thought on an easy fix.


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It's all good!! Thank you!! Yeah, that was my next step, someone else had eluded to that as well.... 60$ is better than 180$ lol
Soon as I get it installed, I'll update progress.
 

Moosee1955

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Just rembered something for you to check on your Mustang. Make sure the fusible link is in good shape. I was reading about another member, that had a charging issue that shop couldn't figure out. Turned out to be fusible link.


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loosenut02GT

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Just rembered something for you to check on your Mustang. Make sure the fusible link is in good shape. I was reading about another member, that had a charging issue that shop couldn't figure out. Turned out to be fusible link.


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New reg, same issue...
Where's the fusable link located?
 

loosenut02GT

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Also, it charges..like I said, but...if you rev it, it has that [email protected] Now, after the new regulator, it's almost as if it's worse..... It's losing volts on the single white wire, when it stops charging... It shows 6-7 volts when it is charging...
 

Moosee1955

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It's possible you got a bad voltage regulator. Put your old one back on, just to see if your charging returns. I would check plug and wires going to regulator. Shouldn't have gotten worse. I don't have a DVD or book in front of me to tell you where fusible link is. Maybe one of the other members knows.


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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:
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 dash. 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. .

attachment.php?attachmentid=64167&stc=1&d=1286329941.gif

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.


attachment.php?attachmentid=64898&stc=1&d=1292685364.gif



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
 
Last edited:

loosenut02GT

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May 17, 2004
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[QUOTE="jrichker, post:

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.
[/QUOTE]

I've done this whole thing.
When I get to this point.... I have 1.5v
It's also intermittent, like right now, it's charging, soon as I rev, it drops charging for a bit, then it comes back...
 

jrichker

StangNet's favorite TOOL
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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.

I've done this whole thing.
When I get to this point.... I have 1.5v
It's also intermittent, like right now, it's charging, soon as I rev, it drops charging for a bit, then it comes back...
Take a 5 amp inline fuse and connect it to the green/red wire in the ignition coil and then connect it to the lt green/red wire on the alternator. Then start the car and check the voltage.
 

loosenut02GT

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May 17, 2004
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Take a 5 amp inline fuse and connect it to the green/red wire in the ignition coil and then connect it to the lt green/red wire on the alternator. Then start the car and check the voltage.
Didn't get this test completed. I took the alt to an alternator shop.... They found the intermittent charging issue as well......
Turned out, had a cracked rotor. It was flexing on centrifugal force, hence why it was intermittent. Having it rebuilt as we speak.
 

Moosee1955

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Sep 30, 2016
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Glad you figured it out. Sure saved a lot of diagnostic time. Weird how working on car triggered this. Could have been on it's way out, who knows. Moose


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