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Old 11-18-2012, 09:37 PM   #1 (permalink)
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Default Replacement Alternator Heat Issues

Hi,

I came across these Forums recently when I was replacing and Alternator in my 97 Mercury Sable DOHC Duratec engine. I was driving home one day and noticed smoke coming from the engine, I pulled over, stopped the car and lost power.

When I checked it out, I discovered that the back of the alternator was completely melted, all of the cabling around the alternator was melted and shorted and the positive battery cable was melted. There is a picture of the bad alternator below.

I replaced all of the bad cabling, and the Alternator. Now everything seems to be running ok, The voltage is ok at about 13.8 volts.

My problem now is that the cable going from the Alternator to the battery is extremely hot, it seems like it's just going to short out again if I leave it running too long. I'm wondering if there is still some underlying problem causing this. The rectifier and regulator are built in to the Alternator, and the whole thing has been replaced.

I disconnected the alternator from the battery and checked it's output voltage with a voltage meter - one end at the battery negative terminal, and the other end at the alternator cable disconnected from the megafuse. The output measured was 40 V DC which seems way too high, but it's a new alternator.. could there be a grounding issue ?

The only other thing that could possibly be related is that I replaced the Battery 2 months ago, it has 36R72 - previous battery was a UL-58R.

Any help or suggestions are greatly appreciated.

Thanks

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Old 11-18-2012, 10:02 PM   #2 (permalink)
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Just FYI, you should never run the engine with the battery disconnected. The high voltage can damage engine components.

If the positive alternator cable is warm, it means it has a high resistance or has a high amount of current flowing through it. The proper way to test this is to do a voltage drop test (not by disconnecting the battery). With the engine running at 2000 RPM, put one probe of your voltmeter on the terminal on the back of the alternator, and put the other one on the positive post of the battery. The voltage drop should be less than 0.2 volts. If it's higher, you need to replace the cable.

If you just replaced this cable and the connections are all good, it's possible you used too small of a cable. If the battery was completely dead or you have a high electrical load on the system the cable might get hot due to high current flow. If you have an inductive clamp ammeter, measure the alternator current.

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Old 11-19-2012, 10:09 AM   #3 (permalink)
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Thanks for the Reply, you might be onto something there. It's possible there is a Grounding issue with the Engine, because the Voltage from the Battery Negative across the Alternator + is 40V which is way off.

Maybe I should check voltage from alternator physical ground and alternator + to check voltage, and hopefully I get ~ 14DC.

I didn't run the car with the battery disconnected, I ran it with the Alternator + disconnected from the Battery, so I could check the Voltage before it hits the Battery.
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Old 11-19-2012, 01:56 PM   #4 (permalink)
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Is there anything grounding out the "F" ternimal on the alternator. In the diagram for the 3rd Generations, if that terminal or screw (can't remember which) it will run the field at full voltage which will cause the alternator to max itself out,.
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Old 11-19-2012, 02:38 PM   #5 (permalink)
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13.8V is not normal. Maybe your battery has shorted elements inside and pulls "down" the 14.2V from alternator. That would amount to high currents and damage the new alternator too...
Go to Advance Auto parts - they have free testing for battery, alternator - on car.
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Old 11-19-2012, 03:09 PM   #6 (permalink)
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Quote:
Originally Posted by the_intimidator_02 View Post
Is there anything grounding out the "F" ternimal on the alternator. In the diagram for the 3rd Generations, if that terminal or screw (can't remember which) it will run the field at full voltage which will cause the alternator to max itself out,.
Thanks for the tip. There is nothing shorting out the F screw, It's a copper colored screw, I think it's this screw that makes sure there is a good ground between Engine and Alternator.
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Old 11-19-2012, 06:34 PM   #7 (permalink)
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Originally Posted by martinamw View Post
Thanks for the tip. There is nothing shorting out the F screw, It's a copper colored screw, I think it's this screw that makes sure there is a good ground between Engine and Alternator.
If the F screw is grounded (touching anything metal), the alternator will be at full output all the time and will overheat. It is NOT a ground screw.

When the alternator is disconnected, the voltage regulator can't sense the voltage (it's not seeing any charging) so you'll get a high voltage output. Don't do this again because the high voltage can damage the alternator if it starts arcing internally.
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Old 11-19-2012, 06:53 PM   #8 (permalink)
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Originally Posted by behlinla View Post
If the F screw is grounded (touching anything metal), the alternator will be at full output all the time and will overheat. It is NOT a ground screw.

When the alternator is disconnected, the voltage regulator can't sense the voltage (it's not seeing any charging) so you'll get a high voltage output. Don't do this again because the high voltage can damage the alternator if it starts arcing internally.
behlinla: I was mistaken about the F screw, I was referring to just one of the bolts that hole the alternator in (the one at the top).

The high voltage while disconnected makes perfect sense, I'm glad I have an answer for that, it's been bugging me for a while, so thanks for that.

My original issue that got me suspecting a problem was that the alternator output cable was hot. This was simply because I didn't have it bolted down tight. I had it loose so I could quickly disconnect in case of a problem during testing.

So now, everything appears to be running well, I would feel better if I knew how the bad alternator got to this state in the first place, so if anyone has ideas on that, please let me know.

Thanks for all responses.
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Old 11-19-2012, 06:59 PM   #9 (permalink)
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Quote:
Originally Posted by martinamw View Post
My original issue that got me suspecting a problem was that the alternator output cable was hot. This was simply because I didn't have it bolted down tight. I had it loose so I could quickly disconnect in case of a problem during testing.

So now, everything appears to be running well, I would feel better if I knew how the bad alternator got to this state in the first place, so if anyone has ideas on that, please let me know.
Yes, the alternator bolts and the flanges that touch are important for the alternator grounding. They should be clean and tight.

And yes, a loose cable will get fairly warm because of the added resistance. You're dealing with a lot of current so you always want tight connections.

My guess is that the diodes in the alternator went bad, which would basically cause a short circuit in the alternator. You would have a lot of current flowing from the battery into it, so it would get very hot (and that's probably why your cables were fried to).
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Old 11-19-2012, 07:22 PM   #10 (permalink)
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Quote:
Originally Posted by behlinla View Post
Yes, the alternator bolts and the flanges that touch are important for the alternator grounding. They should be clean and tight.

And yes, a loose cable will get fairly warm because of the added resistance. You're dealing with a lot of current so you always want tight connections.

My guess is that the diodes in the alternator went bad, which would basically cause a short circuit in the alternator. You would have a lot of current flowing from the battery into it, so it would get very hot (and that's probably why your cables were fried to).
Ok, thanks for your help. The only other thing I need to figure out is the alternator mega fuse. It's a 175 Amp fuse. The cable goes from the battery +, to the mega fuse, then to the alternator.

The fuse didn't blow, but the cable from the battery to the megafuse was fried, and so was the actual alternator.. I could be thinking about it wrong, but shouldn't the fuse have blown ?
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