alt. wont charge 500 ohm dash resistor?

The resistor is a probably a 1/2 watt part soldered to the instrument panel circuit board. It would have
green, black and brown bands, and a 4th band that would be gold or silver.

It is there as a backup for the alternator warning light. If the warning light burns out, the resistor takes over
and supplies the regulator with a turn on signal.

The following should be a repeat of the proceedure you already have...

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.

Alternator troubleshooting for 86-95 5.0 Mustangs:
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 voltage measured across the battery is 15.25 volts, you should see 14.50 volts

Diagram courtesy of Tmoss & Stang&2birds


See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Engine Information Everyone should bookmark this site.

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

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

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 & pinout
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

Because of the resistance in the circuit, the voltage reading can get pulled down.

Does the battery light go out once the car is started?

I have had the parts store guys tell me an alternator is good, when it fact it was bad.

If you ran JR's tests and all was ok, did you check the continuity in the stator circuit (white/blk wire)? I skimmed JR's post and am not sure if a test was in there (it was probably and I missed it).

Disconnect both ends of the circuit and check continuity. You should see less than 1 ohm of resistance.

andyctgt said:

all tested well till i came to testing the grn red wire with ignition on. it only had 4 volts. so now i have to chase it under the dash and see if that conection is good?

Click to expand...

The fact that the light comes on and stays on [because the alt circuit has an issue] indicates that the excite circuit is ok.

Also, for all other tests to pass, the excite wiring pathways would have to be ok.

the exciting circuit is the green/red wire. If you have 12v from the ignition switch to the dash, then I'd check the connector to the dash circuit so that will require pulling the dash panel in front of the driver. If you have 12v to the bulb and resistor then maybe the circuit has a bad connection between there and the voltage regulator.

Current in the circuit with a light out (not yuour case) would be I = E/R = 12/500 = 24 milliamps. If you pull the p[lug from the Alternator with the green/red wire and hook a multimeter up to measure 24 millamps and turn the ignition switch on with the bulb out, you should get 24 milliamps - anything much less and you've got a problem with a connection - could be the plug itself, so check that out too.

No, you won't get 12v on the other side of the bulb when things are working right. I f we ignore the resistance of the wire in circuit, the 12 volts will be "dropped" across the loads in the circuit. If you have two 12v bulbs in series (one connected to the other) from 12v to ground then there will be 6v "dropped" across each bulb. Since the charge indicating light is in series with the voltage regulator, I'm not sure what the correct voltage is on the other side of the bulb.

The fact you have voltage on the other side seems to indicate that the problem may be between that point and the regulator plug - like I said, it may be the plug itself where the grteen/red wire plugs into the regulator. If you connect a multimeter in series from the plug green/red wire through a multimeter to ground on a current setting that can measure ~24 milliamps, you should be able to pass at least 20 milliamps to be sure the circuit has the ability to pass enough current to get the regulator to work. If it does flow 20-24 milliamps. then the connector or regulator is suspect.

andy, check your ground strap on the block, and run the negative from the trunk to the engine bay.