Why is my voltage so high??

[HeatRaveR]

After a day trip through the mountains, I noticed that the Charge warning light was lit on my '82 280ZX Turbo. I discovered the wire, where it connects to the alternator, was just fried. The ring terminal on the end of the wire was literally crisped and crumbled. Nothing else was singed or arced in the area, so I'm guessing too much power was sent through the wire and melted it down. Later (after getting towed home), I started the car on battery power alone, and tested the voltage at the alternator terminal and it read 17 volts (though someone told me it only reads that because it had no battery wire connected to it). The alternator was only 2years/14k miles old, and purchased from MSA.

 

So, I replaced the alternator with an appropriate one from Napa and replaced the battery wire with 10ga cable. When I started the car, the voltage crept up to 15.1V. The alternator's instructions said that the voltage should read between 13.5-15.1V, so it was at the upper limit of the acceptable range.. However, I was noticing the wiring and fusible link were getting awfully warm... So I replaced the wiring with 8ga wiring and a high-amp circuit breaker. The good news is that the wires no longer heat up. The bad news is that the voltage is now reading even higher, at 15.6V!

 

So my question to you all is, what could cause the alternator(s) to be "over-volting"? Could one of the wires in that little 2-wire plug that goes to the alternator cause this?

 

Thanks guys!

Edited August 30, 2010 by HeatRaveR

[MikeBZ]

I would start by having the Alternator tested by itself and make sure its not faulty. Then I would break apart the harness by the alternator and inspect / replace any sketchy wires. Something might be shorted out inside the harness..

[Challenger]

Do not keep replacing wires with heavier gauge wires. Thats not going to fix overcharging, its just going to fry other things, possibly multiple things.

 

Its not likely since youve changed alternators but could it be your voltage regulator? AS mentioned might want to take the harness apart a little to see if there is a short nearby.

 

Did you try measuring voltage at the battery and other spots on the car?

[NewZed]

Where and how are you measuring voltage?

 

As I understand the internally regulated alternator, the S wire (one of the two wires at the little plug) determines how much voltage that the alternator puts out. It tries to hold it at ~14.8 at the S wire. So you could have 14.8 at the end of the S wire (often attached at the battery terminal or the main lug on the starter), but actually higher at the alternator itself, depending on where you have the S wire connected, and the quality of the connections. If you have a dirty connection the alternator will pump out more voltage to get the S wire up to 14.8. Connections at both ends of the S wire need to be good and clean.

 

My limited understanding of the charging system...

[bjhines]

When you see a connection that has burned, You are looking at a bad connection. The wire will usually melt at about the same current rating the connector melts at. The hottest point has the most resistance. Your connection, or the internal crimp on the terminal was bad and is now very bad.

 

NewZed is correct, The sense(S) wire is how the regulator determines what voltage is correct. If the "S" wire is not in order then the system will read too high voltage. The alternator can spike voltages quite high. It can also maintain 16-18V if the sensing wire is not connected properly. In addition, a bad main connection(Your problem) will cause the system to over voltage.

[HeatRaveR]

Hey guys, thanks for the replies. I tested the voltage using both an onboard voltmeter (which takes its reading via a switched relay that eventually leads back to the battery) and with a multimeter probe at the battery itself.

 

I pulled off that 2-wire plug, turned the key to the on position, and found that the thinner, blue, wire on that plug was giving a voltage reading about .3V less than reading at the battery itself. Could that slightly lower reading be enough to cause the alternators to charge so high? If so, where does that blue wire lead to/get its reading from? And what does the thicker, white, wire on that plug do?

[NewZed]

The thin blue wire would be the L (Lamp) wire. It supplies power from the battery to the windings in the alternator so that it will start charging immediately. It is .3 V lower because it runs through the Charge light (Lamp) in the dash voltage gauge.

 

The white wire is the S(ense) wire. The other end is probably connected to your starter positive lug or your battery or the fusible links. You could track down the connections and make sure that they're clean or you could run a temporary jumper from S to the battery positive to see if the S wire connections are your problem. Rev the engine over 1500 and see what the stable reading is.

 

If that crud in your first picture is corrosion (you live in the Bay area, I see) and not frying from a short, you might just have more corrosion to clean up.

[HeatRaveR]

So if I'm understanding correctly, the white "S" wire should be getting +12V constantly, right? I unplugged the connector and stuck a multimeter probe in the white wire's spade connector, and am only get a few millivolts, both with the key off, and in the On position..

[NewZed]

From the end of the white S wire to ground (the body or the battery negative terminal) should read battery voltage minus any connection losses, constantly, key on or off.

 

Don't measure across the end of the wire to the alternator connection, you won't get the right reading. Measure from the wire to ground.

 

By the way, it's possible that if you tried to test your new alternator by disconnecting the battery, or the main charging wire, it may have been damaged. Apparently the alternator circuitry is sensitive. Something to consider. Your bad connections might have damaged something also.

 

Anyway, check that S wire for power from the positive terminal and go from there. If it's bad, run a jumper from S terminal on the alternator to battery to see if your alternator is still good.

[bjhines]

That is not corrosion in the picture on the main power wire. A crusty crimp caused the wire terminal to overheat which is the problem. I realize that the "S" wire is important in setting voltage. But this problem is SIMPLY a bad connection on the main power wire terminal. This would also cause over-voltage because the bad main connection is throwing the entire system off.

[HeatRaveR]

That is not corrosion in the picture on the main power wire. A crusty crimp caused the wire terminal to overheat which is the problem. I realize that the "S" wire is important in setting voltage. But this problem is SIMPLY a bad connection on the main power wire terminal. This would also cause over-voltage because the bad main connection is throwing the entire system off.

 

At this point, it's not an issue of a bad main connection, because that wire in the picture is no longer there; since it was so burned, I replaced it with brand new wire, with a gold-plated ring terminal that has been both crimped AND soldered on! Haha, I think that's about as good a connection as I can do! So it's overvolting even with that new, good connection.

 

So yeah, that Sense wire seems to be the culprit since it's not getting any juice (I have yet to test jumper it). But that wire doesn't seem to go up to the fuse box/battery. Any idea where it's pulling its voltage signal from? (somewhere from the interior?)

Edited September 1, 2010 by HeatRaveR

[NewZed]

I agree, that does look like localized overheating. Just suggested corrosion as another possibility since there was rust next to the bubbled insulation. Who knows, maybe it's corrosion-induced overheating?

 

Anyway, the S wire is probably crimped on to one of the wires in the harness going to the fusible links. I would just run a new one to the positive lug on the starter, at least to test it out. That's where I put mine after a swap to an internally regulated alternator and it gives proper charging and a good steady reading on the volt gauge in the car. I had it on the battery terminal for a while and the gauge was kind of jumpy.

 

You can even peel back the rubber on the plug and stick a small male spade end in the back of the S wire connection temporarily if you don't want to cut wires. Just leave the old dead S wire there for a while.

 

You can always dig in to the harness later after you determine for sure what the problem is.

[bjhines]

My point was that the "S" wire is not the problem because the system was not properly connected at the main terminal. The volts are high because the main terminal was bad.

[HeatRaveR]

Today I finally did some further testing by temporarily replacing the S-wire with a jumper that led to the fuse box. Sure enough, the voltage stabilized to what it should be! So now I have to pull apart the loom wrap to find out where the original S-wire leads to, and why it suddenly stopped carrying voltage.

[NewZed]

bjhines, I went to the S wire focus because HeatRaveR said he had replaced that bad wire and connection and still had the problem. Looks like he has other bad connections.

 

HeatRaveR, congratulations on finding the cause of your problem. You might check the wiring diagram for your car first, to see where the S sire goes, before tearing things apart. There should be one at the back of the FSM. http://www.xenonz31.com/s130/reference.html

 

Careful with those wires or you'll end with a "where's that smoke coming from?" issue next. Like your avatar...

[HeatRaveR]

Well guys, I put everything back together yesterday, and drove it about 70 miles today. So far, so good! Thanks for the help guys!