92 Mustang LX 5.0 Charging Problem

JDMxJOE

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Feb 12, 2012
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Hey guys im new to the forums and also new to the Mustang Scene..I picked up a Vortech Supercharged lx 5.0 at auction with 80k miles the issue im having is that the alternator is not charging the battery. I replaced the alternator and the cars voltage was reading around 11-12. I thought it to be normal due to it having an aftermarket fuel pump, radio, crane fireball ignition ect..Drove to advance auto where my alternator/battery checked out ok..only issue was the battery's cranking amps were at 412 rather than the 580. Next day I drove the car and the voltage was in the middle around the 14.1/2 range..which I thought was pretty good..on the way home I noticed the volt gauge was low, around the 11-12 range, so I kept driving. I finally noticed it drop to around 10/9 so I stopped at a gas station and it wouldnt not start up. The battery was too weak! My friend came with another battery and I drove it home on battery power. I now notice the battery light on the cluster does not work so I pulled the cluster and replaced the bulb. Still doesnt work. I checked the alternator with the 3 wire plug off and its putting out 11.4v on both prongs isnt that low? The other thick wire on the starter solenoid goes all the way under the car but it doesnt get any voltage..Is that the starter power wire? I am a newbie to the mustangs but im not a newbie to cars. I need help from you guys. And also would like to make some new friends here too.

1. I replaced the alternator with a new one.
2. No charge to battery.
3. Took off alternator to get it checked and it checked out good.
4. Bought a volt meter and checked the 2 black and orange wires coming from the alternator and it read 11.4v.
5. Traced the black and orange wire running up around the radiator probing it every foot and it read 11.4v all the way to the starter solenoid.
6. Battery shows 11.4v while engine is running and off but it drops while running.
7. Probed the brown fusible link on the battery side of the starter solenoid and it read 11.4v
8. Checked the fuses under the dash and all are good.

Im at a loss to what else to check. I cant figure out why the battery is not charging. All the gauges work. None of them are acting funny. All lights work well the turn signals seem a little slow but they work. And the car starts and runs well when the battery is charged. I downloaded the alternator/starter solenoid diagram to help me find me some wires but they're all getting at least 11v. Its frustrating...please help.
 
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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.

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.

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 regulator. This is an actual measurement taken from a car with a working electrical system.

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

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



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.
 

JDMxJOE

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Feb 12, 2012
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Thanks for the diagram and the check list. What would cause the car to have good voltage through out the day and suddenly drop and not charge the battery? All the wires that would charge the battery is reading at least 11v but the battery ends up getting weaker and weaker..
 

Blown88GT

Founding Member
Nov 13, 1999
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...

1. I replaced the alternator with a new one.
2. No charge to battery.
3. Took off alternator to get it checked and it checked out good.
4. Bought a volt meter and checked the 2 black and orange wires coming from the alternator and it read 11.4v.
5. Traced the black and orange wire running up around the radiator probing it every foot and it read 11.4v all the way to the starter solenoid.
6. Battery shows 11.4v while engine is running and off but it drops while running.
7. Probed the brown fusible link on the battery side of the starter solenoid and it read 11.4v
8. Checked the fuses under the dash and all are good.
...
1. Too bad you didn't do the 3G upgrade. 2G alternator is 75A, 3G is 130A.
2. Agreed.
3. They may not load test them.
4. Way too low.
5. Way too low.
6. Should be around 13.5, but with a 2G, should not drop below 12.7.
7. What's the voltage on the other side of the Brown fuse link? I suspect this link is blown.
8. Irrelevant.

EVTM_p012.jpg

EVTM_p013.jpg
 

JDMxJOE

New Member
Feb 12, 2012
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I replaced the fusable link and still no charge..I swapped out the alternator for a remanufactured motorcraft alternator and still no charge.
I also spliced a new alternator harness and still no charge. The new alternator I got is throwing out 0.3/4 volts..is it possible that one is bad? what are the odds?? The other alternator was at least giving 11.4v but this one threw out .3v lol..tested it straight from the alternator too..my Battery warning light does not come on at all..I replaced the bulb and still doesn't come on..thinking about taking this to the ford dealership..I work at the dealership so maybe they'll help me out..but I really want to fix it at home. Please help I'm getting desperate lol
 

Blown88GT

Founding Member
Nov 13, 1999
2,179
525
164
Palm Beach Gardens, FL
Ignition Switch. There was a factory recall. Look up the VIN at your dealership. If it was never done, they will do it for free.
95S28 NOV 95 Safety Recall 95S28 - Ignition Switch Replacement
 

txstang84

15 Year Member
May 21, 2005
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Tuscola, tx
Did you ever check if there was battery voltage getting to the field (yellow/white) wire as noted in jrichker's post? If you don't have volts to the alternator, then it won't charge the battery.
 

jrichker

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Do you want to fix this bad enough to folllow this instructions I posted? If so you can find and fix the problem.

Good troubleshooting takes time. Everyone seems to be looking for the magic tip that says replace this $13 part and your problems are cured. You don't have to think, dig or diagnose, just buy the part and install it. Unfortunately, it doesn’t work that way 99% of the time.
Thinking is hard work and educated thinking requires much more effort than simple parts swapping. The complexity and age of these 5.0 Mustangs works against that, as well as the number of hands that have “modified” the original design. Sometimes that modification is well done with good workmanship and other times it is mechanical road kill. It is very easy to become the victim of the previous owner’s efforts.

I spend a great deal of time trying to communicate the methodology of structured troubleshooting principles. For those who haven’t been beaten with this stick before, here they are again:

1.) Understand the system. That involves reading some books to get an overall picture of how it works. For 5.0 Mustangs, that list starts with the Chilton shop manual and the Probst book, Ford Fuel Injection & Electronic Engine Control 1988-1993 by Charles Probst :ISBN 0-8376-0301-3. It's about $30-$40 from Borders.com, see
http://www.amazon.com. Select books and then select search. Use the ISBN number (without dashes or spaces) to do a search. Use the ISBN number and your local library can get you a loaner copy for free for 2 weeks or so.
For free automotive electrical training, see http://www.autoshop101.com/ . I have personally reviewed the material and it is very good.
Another resource is the following website by Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring: http://www.veryuseful.com/mustang/tech/engine. Study the diagrams and tech articles. There is an amazing amount of good stuff in one place for FREE.

2.) Isolate the problem. You need to be able to break the system down into functional blocks or subsystems. Once you know what subsystem has failed, it is much easier to troubleshoot. Group problem symptoms according to what system controls them. Don’t look for a fix for ignition problems by adjusting the fuel pressure. Have an organized, step by step, approach to work your way through a subsystem before you quit and go off wildly chasing rabbits. You have three tools to use in your efforts: measure, observe and think!. Use these three tools to narrow down the list of possible failure points into a small, easily managed list of items.

3.) Install the fix. Now that you have a short list of suspected villains, plan your repair efforts so that you do the easy things first. If an item from the easy list didn’t fix your problem, then look at the high failure rate items. Spend some time here on Stangnet and you’ll get an idea of what the most commonly replaced parts are, things like TFI modules, ignition switches and fuel pumps.

If you make a change and it doesn’t relieve the problem symptoms, put it back the way it was. I have a stack of parts from where I swapped a part and it didn’t fix the problem. I put the old part back on the car and the new part went on the shelf. Someday I will either use them or trade them for something else.