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Discussion Starter · #1 ·
The kind woman down the street just gave me her old '97 Taurus GL wagon with 430,000 kms on the clock. It has been sitting for about 6 months. Was running when she parked it but needed outer tie-rod end (no problem). Battery was totally dead so I took it home and recharged it. Started right up and runs well but charging system is not working, rear steel brake line blew on the way home, and some body hardware not working well (rear exterior door handle mechanism busted and rear hatch siezed shut).

My biggest challenge now is figuring out how the alternator/rectifier/regulator are wiring and how to diagnose which component has failed. I have the Haynes manual but it is rather vague in its descriptions. Sure I could simply buy a rebuilt alt for $150 or a used one for $75 and my charging problem would be solved. But maybe not. I have heard rumblings that the alt may have to receive a wakeup signal from the computer system. I suspect that the current is fed to the field coil through the brushes and that the charging current is induced in stationary windings before being rectified/regulated and exiting through the heavy red cable and grounded by the alt casing. According to the wiring diag the field current enters the alt from the battery through the org/lt blue wire. I think there should be continuity between the heavy red cable and the alt casing (I have none). I think there should be continuity between the org/lt blue wire tab on the brush holder and the pos brush screw (I have none). Does anyone know how this alt actually works???
 

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The orange and blue wire should have 12V+, there is a fuse for it in the underhood fuse box.

There should be 12V+ on the red cable too that is being back fed (when the car isn't running) from the battery. This goes through the large mega fuse on the under hood fuse box.

There should be no continuity between the cable and the alternator case as the case is grounded.
 

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Discussion Starter · #3 ·
:notworthy: Thankyou SHOZ123. That diagram is exactly what I was hoping for. Where did you get it? You da man! However, I think you misunderstood what I meant by continuity through the alternator. I am talking about continuity through the alternator itself (with the cables and wires off). I know about the 175 amp mega fuse and the 30 amp field coil fuse, and they are both OK, but the alternator does not charge the battery. Battery light on dash stays on and voltage across battery posts drops to 12.2V with vehicle running compared to 12.5V with igniton off. There is something wrong with the alternator and I am trying to determine which component is failing. When I said there should be continuity between the heavy red cable and the alt casing I was not talking about the heavy red cable itself, but the POS DC post on the alt that the red cable attaches to. I should be able to detect continuity between that post and the alt casing by attaching the black lead of the ohm meter to the POS post of the alt and the red lead of the ohm meter to the casing of the alt. The current from the ohm meter should pass in that direction through the alt, although it will not pass through if the leads of the ohm meter are reversed, because the diodes of the rectifier prevent it. There should also be continuity from the O/BL connector tab on the brush holder, thought the field, to the ground on the alt casing unless the transistor in the regulator stops it without a switching signal from the LG/R wire. What you think? Sound possible?
 

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Take the voltage regulator off the back of the alternator. On my 94 it's held on with 4 torx screws. The brushes are part of this assembly, and when they wear down, the alternator will not work anymore. I'm not sure if they are replaceable, but in my volvo they are. If you take the assembly to an alternator repair shop, they should be able to replace the brushes for you if they are worn down, and with well over 100k miles, if the alt is original, they will be very worn.
 

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Discussion Starter · #5 ·
Take the voltage regulator off the back of the alternator
I have already done that. The brushes appear to be OK and there is continuity between the two brush hold down screws on the brush holder when the armature is stationary). That means to me that the current IS passing through the field coil (although the brush contact points on the armature do appear to have deep grooves worn in them). So, I think the brushes are OK, unless continuity is lost when the armature spins (I will test that later today). I suspect the problem is in the stator rectifier circuit or in the voltage regulator. Testing those should not be too difficult, thanks to SHOZ123, who was kind enough to supply the wiring diagram.
 

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Discussion Starter · #7 ·
:chili:

Wahoo. I managed to solve my charging problem, although it took a bit of cogitating and some wire tracing to do it. My tests on the alternator itself showed that the alt was OK, so I redid the voltage tests on the POS cable and the O/LB wire. Voltage through the O/LB wire from the battery was only 6.7V (not good). Voltage at the battery side of the 30 amp field coil fuse was 12.68V so there was some interruption between the fuse and the alt. The O/LB wire does not take the short route from the underhood fuse panel to the engine, but takes a rather tortuous route through the wire loom around the battery and behind the engine. Turns out the O/LB wire was rubbing on the loom connector mount under the corregated air intake hose, had worn through the rubber wire coating and shorted out the wire. I soldered in a new section of wire, insulated that section of the loom with an old piece of inner tube, and no more charging problem. Many thanks to the contributors of the diagnosis.

Cal
 
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