81 280zx Turbo Alternator

[81 ZXT]

When I drive my car with the headlights on, the alternator light glows very dimly. I have not had any problems starting and this has been going on for months. I have checked the voltage at the battery with car running and I get about 13.4 or so. Belt is tight, connections cleaned. Is my alternator going bad and if it is, do I have a 60 amp or a 70 amp alternator?

[sblake01]

The voltage should read in the 14+ range. The internal regulator part of the alternator is likeky malfunctioning so you'd have to replace the entire alternator. 'Back in the day' I used to have a source for the i.c. regulator module that goes in it but I haven't seen those for sale anywhere in years. 81 280ZX's all had a 60 amp alternator, turbo or NA.

[81 ZXT]

Thanks Stephen, I could not think of a reason the turbo model would need a higher amp alternator, but I had read on a different thread that it was 70 amp. Thanks again for clearing that up.

[sblake01]

According to the factory service manuals, 81 only got the 60 amp, NA or Turbo but in 82 and 83, the NA got the 60 amp while the Turbo did the get the 70 amp.

[Pilgrim]

You don't have to replace the alternator. You can remove it, take it to a shop that rebuilds starters and alternators and they can rebuild it.

[81 ZXT]

OK, It's been two years since I first posted the alternator light question. The other day, my battery died and so did car. I charged the battery on my "smart" charger and tested the alternator. It does'nt give voltage readings but said the output was out of normal limts. I took the alternator off and had my local NAPA check it. It was putting out 14.9V but the test concluded with: Alt: BAD. They had ordered a new alternator and tested it. It tested at 15.3V but also concluded with Alt: BAD Is my alternator bad or do I have a wiring problem. I installed the old alternator back yesterday and cleaned all the connections including the battery connections. Again, my smart charger said the alternator was out of normal limits. During all of this the alternator light may have glowed slightly but never actually came on.

[Mikes Z car]

I love electrical gremlins, fun to troubleshoot (laughs maniacally). I found the following test (a brief excerpt from the much longer article) by typing into a search engine "alternator light slightly glowing". It strikes me that the same test could be done for the ground connection by connecting the meter between the negative post of the battery and the frame of the alternator using extreme care to stay away from any rotating parts of the engine:

charging_system_woes

So what’s the deal when the light just glows dimly? The reason the light glows dimly is the there is only a slight difference in voltage potential. This occurs, when for instance, there are 12 volts at one side of the bulb and only 9 volts at the other side of the bulb. The 3-volt difference creates the low voltage glow.
But where did that missing 3 volts go? It got lost in a bad connection somewhere in the electrical system. Somewhere is the operative word and where a great deal of time can be lost in repairing this problem. Fortunately you do not need a big fancy (See expensive) piece of diagnostic equipment to locate the problem. All you need is a $12 Digital Multi Meter (DMM) from Radio Shack and a spool of 16-gauge wire to extend the leads as necessary.

To test the wiring from the alternator to the battery you would hook one meter test lead to the battery positive and one up to the alternator B+ connection. (This is a parallel voltage test) The electrons leaving the alternator that are being blocked at a bad connection (a burnt alternator to starter wire for instance) will see the voltmeter as a easier way to the battery and travel through the voltmeter. The reading at the voltmeter indicates how much extra resistance is in this electrical circuit.

************** my comment:
Back in the old days I tested my alternator at a hobby shop with a waveform tester that showed bad diodes without taking the alternator apart. I don't know if that kind of tester is still available anywhere. Not everyone would be familiar with the operation of that kind of tester limiting its usefulness. Can't help wonder if an oscilloscope could be used.

[81 ZXT]

Thanks for the response. I will go home today and start checking things with my meter. I will let you know what I find.

[madkaw]

Might take the alt to another store and get a second opinion. Also take the battery and get it tested.

[Wade Nelson]

It might help if you understood how alternator lights work. When you turn the ignition on, there's 12V on one side of the bulb (battery voltage), and since there's nothing coming out of the alternator (yet), there's zero volts on the other side. So the bulb glows.

Once you start the engine, there's 12V on both sides of the bulb, so it goes out.

Now that's not 100% true. There's 13.8V or so coming out of the alternator, and since the alternator is tied directly to the battery (in most cases), the battery voltage ALSO rises to 13.8, so the lamp goes out.

While your engine is running, if your alternator bulb glows dimly, that means you have a voltage DROP.

Instead of having 13.8 on both sides of the bulb, you may have 13.8 on one side and 13.0 on the other side. Just enough to make it glow, dimly.

So how did it get that way? You'll have to get a schematic, and see how voltage gets to both sides of the bulb. Does it come through the ignition switch on one side? If so, the contacts in the ignition switch may be corroded, worn, dirty. If the alternator light is mounted to a circuit board, the power/grounds to that circuit board may be corroded / weak.

What you really need to do is a voltage drop TEST!

Start by measuring the voltage across the battery terminals with the engine running. If the alternator's good, it should read 13.8 or thereabouts.

Now, leave the black test lead on the battery terminal, and hook the red test lead to each side of your alternator light, in turn.

Let's say one side reads 13.75 volts. That's "good enough!" Nothing wrong with THAT side. (A minor voltage drop of .05V isn't significant on a car this old)

But the other side reads 12.5 volts. Something's wrong. YOu've got a voltage DROP on THAT side of the wiring. You've got to trace that wire back, through whatever switches, fuses, splices, relays, whatever path it takes all the way back to either the alternator or battery + terminal. Find the bad connection and repair it.

Testing alternators on the vehicle is tricky. It's a lot easier to take them off, take them down to NAPA or Autozone or wherever and put them on their tester. Usually a diode fails, one out of three, so instead of a 60Amp alternator being CAPABLE of putting out 60A, it's only capable of putting out 40A.

I don't know what kind of tester your guy used, but it obviously had a problem correctly testing a new alternator, on-car.

In general, however, here's how I test vehicle elctrical systems.

I start by checking battery voltage. 12.5-12.6 is 100% charged. Anything above 12.6 is just "surface charge" and can be ignored.

A battery that is 12.3 is 50% discharged. Surprised? No, it's not 6V, but 12.3.

Then I start the vehicle up. With my meter still on the battery the voltage should rise to at least 13.5 Volts, indicating the alternator is running.

Then I turn on every power accessory the vehicle has, fans, AC, defroster, wipers, headlamps...

The voltage should stay steady at 13.5 or so. If it drops, the alternator isn't putting out sufficient current. Time to get it checked.

The first thing you should do on EVERY electrical repair is remove and clean the battery terminals, the main engine ground, chassis ground -- which you'll often
find mounted on top of paint instead of shiny metal. Unless you have good power feeds and grounds, nothing else you do will produce clear results. Just inspecting
them doesn't work. Too often there's corrosion where you can't see it and it'll stop a car dead, prevent it from charging properly, yada yada.

Hope this helps.

[81 ZXT]

Ok, I cleaned every connection that I thought might be involved and then checked my voltage at the battery with the car off. It read 12V. I then started the car and checked at the battery and got 12V. I left the car running and checked the alternator at the back where the connections are and only had 12V. Is there anything that would reduce the reading at the back of the alternator other than a bad alternator (ie broken wire, bad connection, etc)? I really don't think the alternator is putting out anything I am just reading the 12V from my charged up battery.

[81 ZXT]

Well, I bit the bullet and bought the new alternator NAPA had ordered. (50 bucks - not bad) Installed it last night and solved all my problems. I get 14.5V at the battery and the light doesn't "slightly" glow anymore. I think it may run a little smoother also. I'll know about that when I check the gas milage. Thanks for the advice during this.