jrichker where are ya? Need some help over here.

Blown331LX · Oct 11, 2007

**UPDATED**jrichker where are ya? Need some ALTERNATOR help over here someone

I'm looking for the alternator guru for some diagnostic help. I installed the 3G on my 1990 GT. I used the stock pigtail and harness from a 1994 GT that was a donner car. The car is a basic as they come, the only acessory Im running is the alternator. The entire EFI harness was stripped out before I got it. I installed a battery in the rear the 0 gauge Taylor power wire running from the starter solenoid back to the battery. My MSD box, and electric fan along with the one 2gauge black and orange wire that goes to the 2 grey and then ring terminal from the alternator are all hooked to the battery side of the starter solenoid. The yellow wire is the 20 amp fuse power to the alternator also running to this location. The LG/RD wire I wired directly to the charging circuit keyed hot from the instrument cluster. I have a thick ground cable going from the lower subframe to the motor mount bolt. Everything with MY wiring seems just like it should be and wired just like the factory and Alldata diagram, and looks OEM with loom and all. My problem is that the bastard isnt charging. I have been building this car for 2 years so the only test driving Ive done has been in the past 2 days in my neighborhood. At first the ALT seemed like it was charging just only at 2,000 rpm or higher, which I know can be typical for a under drive pulley like I have on my setup. Well now it's not coming off 12 Volts from my autometer gauge, and I even tested for the needed 14V with a DVOM at the starter solenoid battery cable and still got 12 V. The altenator I recently rebuilt using my painted and clear coated case and new internals from a donnor alternator. I have rebuilt them before so I know what Im doing but is it possible this bitch is already bad or do I have something backwards?????????? WTF someone help I want to drive.......

jrichker · Oct 11, 2007

Diagram courtesy of Tmoss & Stang&2birds

Note: all voltage measurements use the chassis as the ground. When you make a voltage measurement, it is assumed that one voltmeter lead is connected to ground.

Notice the ignition switch in the lower RH corner of the diagram. It has a red/lt green wire that goes to the alternator warning light. It should have 12 volts when the ignition switch is in the run position. No 12 volts and the ignition switch is bad. This is the signal that tells the regulator to turn on and supply current to the alternator field windings.

Turn the ignition switch off & disconnect the small plug on the alternator. You will find 3 wires: white/black, yellow/white & lt green/red. The yellow/white wire should have 12 volts on it. No 12 volts and the fuse link shown in the diagram is open.

Next make sure the ignition switch is on and 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 is bad.

Turn the ignition switch off, and measure the resistance between the lt green wire and the red/lt green wire on the ignition switch. You should see 500 ohms or less. More than the 500 ohms indicates bad wiring or a bad connection.

The next paragraph is only for a car with the stock 65 amp alternator
Next disconnect the plug with the 2 black/orange wires and the white/black wire. Measure the resistance between the white/black wire on the big plug with the black/orange wires and the white/ black wire smaller plug with the lt green/red, yellow/white wires. It should be less than 1 ohm. More than that indicates bad wiring or bad connection in the white/black wire.

Blown331LX · Oct 11, 2007

My wiring is identical to this diagram and the Yellow has 12 V B+ hot all times and the Green has 12 V B+ keyed hot, and the ground wire is connected. When your saying to check the resistance between the LG/RD and RD are we speaking across the instrument cluster?? Arent there 2 screws on the back of the reg that are used for testing ? Can I ground and or power one to verify the alternator ? Oh and read the post about the 5.0 miata, I made myself laugh.....

HISSIN50 · Oct 11, 2007

If not mentioned, ensure your powdercoated alt is achieving a ground pathway. You might have to scuff it at the mounting bosses to make sure.

Good luck.

jrichker · Oct 11, 2007

If you have 12 volts on the lt green/red wire at the alternator with the ignition switch in the run position,
that circuit is OK.

A test lamp may be more useful from this point on since it draws enough current to fail to light brightly if
there is a high resistance circuit.

Look for 12 volts at the alternator output with the ignition switch off. No 12 volts and the fuse or fuse link
between the alternator output and the battery has blown or open circuited.

If there is 12 volts at the atenator output and you have followed HISSIN50's advice about the ground,
the alternator is suspect. Pull it and have the local auto parts store check it for you.

Blown331LX · Oct 12, 2007

All of the input was great, all of the power signals are right where they should be. I thought the likely culprit to be the ground so i came home tonight and checked the ground to it. So heres the verdict, the alternator shaft and pulley have a good ground, BUT the alternator case DOES NOT. Does the case have to also be grounded ?? If not I'm going to pull it out, have it bench tested and buy a new one.

HISSIN50 · Oct 12, 2007

fabfordtech said:
All of the input was great, all of the power signals are right where they should be. I thought the likely culprit to be the ground so i came home tonight and checked the ground to it. So heres the verdict, the alternator shaft and pulley have a good ground, BUT the alternator case DOES NOT. Does the case have to also be grounded ?? If not I'm going to pull it out, have it bench tested and buy a new one.

This is what we were discussing before. You'd remove the alt and sand off the powdercoat around the mounting holes on the alternator. The alternator is grounded via its connection to the mounting bracket, and the engine itself is grounded. This is the OEM ground pathway. Now you see why Jrichker's engine ground upgrade is so important.

If you have any shiny metal on the alt (someplace else - not near the charge cable connection, )you can use a jumper cable to test this hypothesis. Ground the jumper cable to the chassis and then connect it to the shiny metal on the alternator. This provides a remote ground pathway. If the alt works, you need to start sanding/grinding around the alt bosses.

Good luck.

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