Engine 1990 GT Alternator confusion

Jan 5, 2019
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North Carolina
So I went about the 3g alternator upgrade by stripping an alternator and wiring harness from the junkyard. Slim Pickens and I ended up with a 105amp motorcraft reman. I thought I had the 04 turns out it was from an 05. Not sure if that makes a difference or not. Anyways the project car I’m working on did not have any wires or power cables so I ran the alternator wires from assumptions. What did I do wrong? What’s missing? When I check voltage from batt,alt and starter relay I get 12.4v. Any suggestions would be greatly appreciated. Oh I almost forgot the important part. Car won’t crank after it’s been warmed up. Have to let it sit or disconnect negative and reconnect for it to start up. Seems like some wiring routed out of place on my part. Yellow and green wire I attached together and run 1 single wire to relay positive. Black and orange also wired to relay positive.
 

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billison

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Can you post a better picture? Can’t tell what you have or what you did.

But the wiring is pretty simple on those.

Green wire from the alt goes to the green wire in the car, white wire goes to stator plug on the alt and the yellow goes to where you hook up your power wire, then the power wire.

How do you confuse a 04 and a 05? That was a completely different modle.
 

jrichker

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http://www.stangnet.com/mustang-forums/threads/3g-alternator-install-a-how-to.646825/#post-6673702
Alternator wiring.





Starter solenoid wiring 86-91 model cars.

Connect the fused 4 gauge wire to the alternator and the battery side of the starter solenoid.


Starter solenoid wiring 92-93 model cars.



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.

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
 
Jan 5, 2019
8
1
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North Carolina
This is the alternator that I grabbed. Red post has 2 8-gauge wires which are both grey in color no stripes just solid grey. The connecting pigtail has 2 wires. Yellow/white and green/red. I ran the yellow wire to the starter, I spliced and ran the two 8-gauge fused to the starter relay as well. I ran the green/red wire to a 12v hot. I’ve read the diagrams but I think my brain is fried from the rats nest of wires I’ve had to comb through on this project.
 

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Reddevil91

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So I ran my 3g upgraded unit like lmr states to set it up in their video by running the 4g cable and still using the two black/orange cables, spliced in the new plugs etc, then upgraded both my positive / neg cables to 4g as well. my buddy also bought one and took his to get installed somewhere and they installed it differently. They removed the two black and orange cables completely, ran only the 4g cable from the back of the unit to the solenoid, then did this set up with the plug harness. Is this legit? The car seems to run fine
A8BCF4A3-80C0-4782-BEDC-0B3519AE6B09.jpeg
 

jrichker

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So I ran my 3g upgraded unit like lmr states to set it up in their video by running the 4g cable and still using the two black/orange cables, spliced in the new plugs etc, then upgraded both my positive / neg cables to 4g as well. my buddy also bought one and took his to get installed somewhere and they installed it differently. They removed the two black and orange cables completely, ran only the 4g cable from the back of the unit to the solenoid, then did this set up with the plug harness. Is this legit? The car seems to run fine
A8BCF4A3-80C0-4782-BEDC-0B3519AE6B09.jpeg
How do you like electrical fires? connecting both the 4 gauge and 2 black orange wires to the alternator is a good way to have your very own electrical bonfire.

Here's why:
Here is the reasoning behind using only a single 4 gauge fused power feed to the alternator. If you use the two 10 gauge black/orange wires in addition to the 4 gauge wire, you have two fused power feed paths. The total current capacity of the wiring is the sum of the fused paths. The 4 gauge path is fused for 125 or amps whatever the fuse is rated for, and the two 10 gauge wires are fused for 60 amps. That is a total of 185 amps, which exceeds the capacity of the alternator. Overload can occur without the fuses blowing, damaging the alternator.

The worst case scenario is that the alternator develops an internal short to ground resulting in a catastrophic failure. The initial short circuit surge current is limited by the resistance of the wiring. The current in a parallel circuit divides up according to the resistance of the branches. If the 4 gauge fuse opens up first, the two 10 gauge black/orange wires will be carrying the short circuit surge current. Depending on the time lag of the fuse links, they may open up before a fire starts or they may not.
 

Reddevil91

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How do you like electrical fires? connecting both the 4 gauge and 2 black orange wires to the alternator is a good way to have your very own electrical bonfire.

Here's why:
Here is the reasoning behind using only a single 4 gauge fused power feed to the alternator. If you use the two 10 gauge black/orange wires in addition to the 4 gauge wire, you have two fused power feed paths. The total current capacity of the wiring is the sum of the fused paths. The 4 gauge path is fused for 125 or amps whatever the fuse is rated for, and the two 10 gauge wires are fused for 60 amps. That is a total of 185 amps, which exceeds the capacity of the alternator. Overload can occur without the fuses blowing, damaging the alternator.

The worst case scenario is that the alternator develops an internal short to ground resulting in a catastrophic failure. The initial short circuit surge current is limited by the resistance of the wiring. The current in a parallel circuit divides up according to the resistance of the branches. If the 4 gauge fuse opens up first, the two 10 gauge black/orange wires will be carrying the short circuit surge current. Depending on the time lag of the fuse links, they may open up before a fire starts or they may not.
Hmm nope I don’t want no fires!! Wonder why they would say to run it this way then on their website install video. Appreciate it
 

jrichker

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Hmm nope I don’t want no fires!! Wonder why they would say to run it this way then on their website install video. Appreciate it
I have a 2 year degree in electronics and 40+ years experience fixing computer & electronic equipment. That experience has led me to make the following statement:

There is a whole world full of people who can only spell electricity on a good day. LMR evidently is one of those groups of people...
 
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Reddevil91

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I have a 2 year degree in electronics and 40+ years experience fixing computer & electronic equipment. That experience has led me to make the following statement:

There is a whole world full of people who can only spell electricity on a good day. LMR evidently is one of those groups of people...
Ha ha true true which is why I don’t try and act like I know crap about wiring bc it scares me in the fact that it can quickly turn bad. So can I just fully pull the orange and black wires out? I know the guys did on my friends car just to get the wiring out of there. Off topic, on my new gauge cluster set up I asked you about I did the jumper on the wiring harness from classic dash plug and play vs doing it on the factory plug harness which is what Florida 50 said to do. It solved my problem with the alternator not being excited. I got a cool wired up resistor set up from blown88gt that he sent me. Do you think I should plug that in as well or just run it how I got with the red 14g jumper wire? Sorry didn’t mean to take this off topic. The jumper is from pin 14 the green/red alternator wire and pin 2 the large red with green stripe, but like I said I just used the P&p harnes to do it bc I’d rather if I mess it up it be that, than the actual factory harnes side. Basically mimicked the factory cluster set up where the resistor connected pin 14 and 2
CAF7AFD8-508B-4B5E-AF28-7144ABD8AA7C.jpeg
 

jrichker

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See my post on Weds; it has the proper way to do the wiring.

If you remove the black/ orange wires, that disconnects the voltage sense for the alternator wiring - not a good thing.
 
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Would it be possible to wire the voltage sense similar to the '94-'95 setup to eliminate the blk/or wires? I need to do this upgrade to my 1990 Mercury and would like to eliminate those wires if I can. The current setup is identical to the Mustang with longer wires.
 
Jan 5, 2019
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North Carolina
So it appears I have only 1 black and orange wire but it is possibly 10gauge. I’m guessing for my application it is my main power wire to the distributor relay? I only have the large black and orange, R/G and the Y/W. I’m apologize for beating this old dog to death but it seems my setup is different, or I’m an imbecile. I’d say the latter.
 
Last edited:
Jan 5, 2019
8
1
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35
North Carolina
I’m guessing I need to Replace the black/orange and run a fused 4 gauge in its place? The yellow gets fused and runs to the same spot the 4 gauge is attaching? Green/red wire gets capped off or do I wire it to the green/red from the stock pigtail?
 
Jan 5, 2019
8
1
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North Carolina
See my post on Weds; it has the proper way to do the wiring.

If you remove the black/ orange wires, that disconnects the voltage sense for the alternator wiring - not a good thing.
See my post on Weds; it has the proper way to do the wiring.

If you remove the black/ orange wires, that disconnects the voltage sense for the alternator wiring - not a good thing.
.
Big thank you to jrichker for the diagrams and all the help I have received from you guys here on the forum. I’ll admit I feel somewhat embarrassed that such a simple wiring diagram had me as confused as I was. Staring at several diagrams and reading dozens of paragraphs on what works and what doesn’t while working on the car will drive one overboard. Once I sat down and studied the diagrams it all clicked and made perfect sense... Ol girl is running strong now thanks to everyone’s support you guys rock.
 

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