Battery Light Stays On

[JD1964]

The battery light stays on constantly whenever the car is running. Good battery, good alternator. Charging 14.5v at idle.

Any idea on what might cause this? No electrical codes present that could be directly related.

I’m not terribly worried because I can always pull the bulb. But, I do like the idiot function when the system works as it should. I know there’s a voltage gauge but who stares at that always?

 

[killer5.0]

3g alt? Stator wire making a good connection?

 

[JD1964]

No 3g. Stock alt

 

[General karthief]

My idiot light is always on

 

[imp]

@John Dirks Jr
The light is directly connected with the voltage regulator within the alternator. If the regulator is screwing up, and the alternator is putting out adequate voltage, but maybe all the time, you just may overcharge your battery. If it's using water quickly, that's a good sign of overcharging. imp

 

[jrichker]

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. .

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.

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

 

[billison]

Just a stab in the dark, but did you swap the dash lights with led’s?

 

[mikestang63]

Just a stab in the dark, but did you swap the dash lights with led’s?

yep. not enough resistance in the LED bulb

 

[JD1964]

Just a stab in the dark, but did you swap the dash lights with led’s?

I never put an LED in the batt position within the cluster. This constant on condition was present ever since this car has been in the family which is 3 yrs or more. It was lit constantly with the standard 194 bulb in place.

 

[mikestang63]

John, first there is a fusible link between the alternator and the starter solenoid that can fail over time- I think its the yellow wire .check that and all the grounds.

 

[JD1964]

I will check the fusible link you mention Mike. But also, are there any diodes in the harness anywhere that may cause issues?

In my other postings I talk about seeing 2.66v on the ignition wire that runs from the ignition switch to the solenoid. This is the red with blue stripe wire that connects to the starter solenoid. Normally you would only see voltage (12v or there about) when the key is pushed to the start position. And I do get that voltage when I push the key to start. However that same wire also shows trace voltage (2.66v) when the key is in the run position. But, it's only measurable when the wire is not connected to the solenoid. If connected and tested at the solenoid post, it shows no voltage. But there is definitely trace voiltage of 2.66 present on that wire when disconnected.

I wonder if this trace voltage on this wire is also related to the battery light remaining on constantly.

 

[mikestang63]

I will check the fusible link you mention Mike. But also, are there any diodes in the harness anywhere that may cause issues?

In my other postings I talk about seeing 2.66v on the ignition wire that runs from the ignition switch to the solenoid. This is the red with blue stripe wire that connects to the starter solenoid. Normally you would only see voltage (12v or there about) when the key is pushed to the start position. And I do get that voltage when I push the key to start. However that same wire also shows trace voltage (2.66v) when the key is in the run position. But, it's only measurable when the wire is not connected to the solenoid. If connected and tested at the solenoid post, it shows no voltage. But there is definitely trace voiltage of 2.66 present on that wire when disconnected.

I wonder if this trace voltage on this wire is also related to the battery light remaining on constantly.

diode would be in the alternator
there is a resistor inline from the ignition switch to the battery dash light that mightbe burnt
check the ignition switch on the column. they are prone to failing.

 

[JD1964]

I already checked the ignition switch. No voltage bleeding over at that point.

 

[MFE92]

Like the OP, the battery light in my 3G-converted 92 LX 5.0 stays on even though the alternator charges just fine, and the car starts and runs just fine. All other gauges and accessories function normally, with one exception. Perhaps coincidentally, my A/C compressor won't activate, whereas it worked fine before the battery light situation.

Searching this condition has led me to this thread where @jrichker mentions a secondary ground between the intake and firewall that can affect both these items. I'm not familiar with this, and the only ground I can find is the strap between the cylinder head and firewall.

I don't know if it's related but this all started when during road course work, my new power steering pump started puking fluid all over everything including all the wiring nest on the drivers side of the engine. About that exact same time, the alternator light flickered for several seconds, and then stayed on for good. And since then, the A/C doesn't work either.

I have not had time to do the usual troubleshooting but this combination of batt light and A/C inop is too coincidental to ignore, and I'm wondering what you think.

Thanks for your time and help.

 

[General karthief]

In that glob of wiring behind the ps pump is several connections including the wires for a ground that has a black cylindrical connector from the fenderwell ground bolt and also includes wires for the ac with a fuseable link. is the strap from the fire wall to the back of the head on the drivers side stock? If so, you may want to up that wire a couple sizes.

 

[jrichker]

Like the OP, the battery light in my 3G-converted 92 LX 5.0 stays on even though the alternator charges just fine, and the car starts and runs just fine. All other gauges and accessories function normally, with one exception. Perhaps coincidentally, my A/C compressor won't activate, whereas it worked fine before the battery light situation.

Searching this condition has led me to this thread where @jrichker mentions a secondary ground between the intake and firewall that can affect both these items. I'm not familiar with this, and the only ground I can find is the strap between the cylinder head and firewall.

I don't know if it's related but this all started when during road course work, my new power steering pump started puking fluid all over everything including all the wiring nest on the drivers side of the engine. About that exact same time, the alternator light flickered for several seconds, and then stayed on for good. And since then, the A/C doesn't work either.

I have not had time to do the usual troubleshooting but this combination of batt light and A/C inop is too coincidental to ignore, and I'm wondering what you think.

Thanks for your time and help.

In that glob of wiring behind the ps pump is several connections including the wires for a ground that has a black cylindrical connector from the fenderwell ground bolt and also includes wires for the ac with a fuseable link. is the strap from the firewall to the back of the head on the drivers side stock? If so, you may want to up that wire a couple sizes.

karthief has described your secondary power ground well enough that you shouldn't have any problems finding it...

 

[Blown88GT]

1. Test the alternator diodes.

View: https://www.youtube.com/watch?v=xgikeXt91vM

2. Check the fusible links.

 

[MFE92]

I’m done for the night. I cleaned all the connections buried under the coil. I’ll check diodes and fusible links another night, but first,

Do diodes like the one in the AC clutch switch line go bad, and how are they tested?

And, what is this gray connector? I don’t see it in the diagrams and I can’t for the life of me see what it connects to, but it must be close by. Both the wires that run to it are black with a white stripe. The plastic part had come off and was under the battery, while the female terminal itself was just open to air.

 

[Boostedpimp]

And, what is this gray connector? I don’t see it in the diagrams and I can’t for the life of me see what it connects to, but it must be close by. Both the wires that run to it are black with a white stripe. The plastic part had come off and was under the battery, while the female terminal itself was just open to air.

That gray connector you speak of is the windshield washer pump test port.. I know because I like you was tracing out a problem in the past and couldn't figure out what it was for but here's the schematic

 

[Blown88GT]

...Do diodes like the one in the AC clutch switch line go bad, and how are they tested?....

It's a "flyback diode". Almost never go bad & not the cause of your problem. They only conduct when the switch is opened, bleeding off the residual energy stored in the inductor (L), i.e. compressor clutch.
A flyback diode (sometimes called a kickback diode, snubber diode, commutating diode, freewheeling diode, suppressor diode, suppression diode, clamp diode, or catch diode is a diode connected across an inductor used to eliminate flyback, which is the sudden voltage spike seen across an inductive load when its supply current is suddenly reduced or interrupted. It is used in circuits in which inductive loads are controlled by switches, and in switching power supplies and inverters.