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Old 01-31-2011, 02:02 PM   #1
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Red face 1995 Alternator won,t charge

I bought my kid a 1995 F150 4x4 with 5.0.It has been a slow starter since we got it,but always started.It had a new battery and alternator on it when i bought it.Recently it has been not starting when hot and i checked the timing whitch i had to adjust.I was thinking about a new starter until recently when i checked the battery and found it was not holding a charge,so i replaced it wih a new battery.It worked fine for a couple of days and then wouldn,t start.I took off the alternator and had it tested,working fine at 14.5-15.0 vdc.Checked wiring and i am pretty sure it's good.Does anyone know where to get a schematic so i can check voltages at regulator.It has a three wire connector with one of the wires going to a single connector and a larger cable to the battery.
Thanks in advance
Rider2
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Old 01-31-2011, 03:53 PM   #2
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There is a schematic in the back of the Haynes manual. My alternator isn't working on my Mustang, the the problem is complicated. The big wire charges the system, and that's pretty obvious. There are one or two fusable links on this main wire before it goes into the junction or to the battery. Not exactly sure how it is on a 95' F150. On the Mustang it junctions on the side of the fuse box before going to the battery. Check for wire continuity to make sure those fusable links haven't blown. The little white wire that goes back to the alternator controls the field current. I assume you had the alternator tested on a bench, which means the field current wire is working. The other two wires are usually yellow and green. The yellow one is usually slight larger, if the colors are not exact. Anyway, the yellow one has 12 volt power from the main fuse box in the engine compartment. That wire should have 12V power regardless of key position. If you aren't getting 12V power to that wire, check the alternator fuse in the main fuse box.

Now the green wire is the real MF and far more complicated than the rest of the wiring. The green wire goes to a circuit that travels through the instrument cluster, and the charge fail light. This is the circuit that is messed up on my Mustang. The circuit starts at the ignition switch which provides 12V power to the circuit. The power travels through one of the circuits in the instrument cluster, and parralel circuited through the charge fail light, and a resistor that is there in case the charge fail lightbulb blows. It then travels through the instrument cluster back into the harness and too the alternator. The circuit provides power to the alternator to tell it to turn on. Once the alternator is on, the positive voltage should be equal on both sids of the charge fail light and therefore it won't illuminate. If the Alternator fails, then the voltage will drop to 0 on the alternator side, and power will travel to the alternator illuminating the bulb. It also works for an over voltage condition on the alternator with electricity flowing the other way. Anyway. With the key in the on position, there should be about half battery voltage at the green wire due to the resistor and light bulb. So, about 6 volts at the green wire.

There is some sort of short in my Mustang and it keeps frying the circuit in the instrument cluster, causing the alternator to not turn on.

Kurt
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Old 02-01-2011, 08:27 PM   #3
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Kurt
Thanks for the info,your pretty right on with the wiring,I am going to check it out tonight to see if there is something going on there.
Thanks
Riders2
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Old 02-02-2011, 09:27 AM   #4
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I made a bad assumption, and that was that they field current wire would be working. If you had the alternator tested on a bench, then the field current wire would be on the bench, not on the alternator. So just give the white wire a good continuity test.

Kurt
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Old 02-02-2011, 09:27 AM
 
 
 
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