Engine 92 5.0 Never Seen A Rev Problem Like This Before Have You ????? With Video

The instrument cluster has the alternator warning light in it. The warning light also supplies a signal to the alternator to turn on the field windings to make the alternator produce power. The warning light has a 500 ohm resistor in parallel with it so that if the bulb burns out, the alternator will continue to work.

Bottom line: without the instrument cluster properly installed, the alternator will not work without rewiring it.


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 15 April 2012 to add simple check for regulator failure in Engine off ignition on, battery fully charged section, item 2.

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.




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.

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

knr1313 said:

yes i used that check list all the time i know it very well and it saved me on my 87 i have BUT my 92 all checks out good the only thing thats missing on the 92 is the 500ohm light/resistor on the back of the cluster like on my 87 has do 92s have them to??
only thing i can think of is that dang Alt but it checked out at autozone and OReillys .
when i unplug it and it runs like a champ pulls nice and hard and revs smooth

Click to expand...

Look at my post for alternator testing - it says 87-93 on the diagram. What is on the diagram is what that is on all the years it says it covers.

knr1313 said:

yes i used that check list all the time i know it very well and it saved me on my 87 i have BUT my 92 all checks out good the only thing thats missing on the 92 is the 500ohm light/resistor on the back of the cluster like on my 87 has do 92s have them to??
only thing i can think of is that dang Alt but it checked out at autozone and OReillys .
when i unplug it and it runs like a champ pulls nice and hard and revs smooth

Click to expand...

Any luck so far, I have similar problems sputters like yours around same rpm

knr1313 said:

nothing yet bro
nobody can figure this out
going to check wiring for the 10th time this weekend and also try a dif Alt as well

Click to expand...

I know it sucks man been like 4 months since Ive drove it normally without sputter, Could It possibly be starter? Same problems as yours but something is causing my starter solenoid to arch bad, other day car on its own revved all the way to 4k outta no where any idea? thanks

knr1313 said:

nothing yet bro
nobody can figure this out
going to check wiring for the 10th time this weekend and also try a dif Alt as well

Click to expand...

885.0h said:

I know it sucks man been like 4 months since Ive drove it normally without sputter, Could It possibly be starter? Same problems as yours but something is causing my starter solenoid to arch bad, other day car on its own revved all the way to 4k outta no where any idea? thanks

Click to expand...

Have you gone through ALL the Alternator Test Path I posed?

If the alternator checked good, then have you ever thought to dump the computer codes?

Think guys; I know that thinking is much harder than swapping parts, but it is necessary...

Dump codes sticky.
Look at the top of the 5.0 Tech forum where the sticky threads are posted. One of them is how to dump the computer codes. I highly suggest that you read it and follow the instructions to dump the codes.

Di you get a code 11 when you dumped the codes? Did you get the engine running codes as well?

If you got code 11 and no other codes for both code dumps, I would suspect the TFI module or the PIP sensor