Alternator/Voltage Rectifier

Sorry, just a quick reply as I'm buried in work at the moment. Let me add that I don't have a 't', but a QV.

The DC readings look good.

In my experience, most DMMs will correctly read an AC voltage in the presence of DC, so that 30 VAC reading is of concern. Alternators do generate a lot of ripple, it may simply be over-responding to that. But it sounds like an open or shorted diode to me.

If your alt. light is the traditional tungsten lamp wired in the conventional way, it is wired to the battery so that excitation current from the battery to the field coil is provided through it. Normally it doesn't illuminate because the voltage difference is small, and a large current doesn't flow. If the voltage at the field coil (actually at the diode common) is either substantially higher or lower than the battery voltage, a larger current flows -- sufficient to light the lamp. (Note that it's insensitive to the direction of current flow.) I'll also add that many (most?) alternators today have an internal diode, and they are self-exciting. Even an old alternator that requires current to the field coil with often become self-exciting once it reaches a certain speed due to magnetization of its parts.

My feeling is that you may have a failed diode in the diode pack... the alt. is still generating voltage and rectifying it from some of the windings, but one of the windings may be inactive due to a defective diode. It think it could be either open or shorted, though a shorted diode will usually show up as a good parasitic drain on the battery when the car isn't running. Also, a shorted diode in the pack will really put a limit on the alternator's ability to produce voltage under load. (it's not a direct short to ground in this case, and there's still a winding connected to it with an induced voltage on it, so it will "sort of" work even with a shorted diode in my experience.)

If you have an oscilloscope, you can measure the ripple. If you see, for example, a low or nonexistent ripple peak consistently every third peak, then this would suggest a failed diode or winding. (the former being much more likely).

I can't imagine replacing a diode pack with the alt. still in the car, though I suppose it's possible in some cases depending on construction and access. I would pull it for sure.

Long term prognosis is not good IME, I'd service it now... or at least test it. Again, a 'scope will tend to confirm this rather quickly. If pulling the alt. in a 't' is not easy because of the engine location, then I'd try to perform some more tests.

It would be interesting to see what the alt. puts out with lots of electrical loads switched on.

 

Excellent info here.

 

With the age of these vehicles now it can do no harm to do some voltage drop tests on the positive and negative side battery and alternator wiring. These are best done with the engine on and at idle with some load on the system.

For the positive side tests the DVM (voltmeter) should have one test lead directly connected to the battery positive terminal POST and the other test lead connected to the alternator B+ terminal. There should be a very small reading on the DVM (<0.2v would be nice). Anything higher means you need to investigate the wiring and connections further.

For the negative side tests the DVM should have one test lead directly connected to the battery negative terminal POST and the other test lead to the body of the alternator. Again look for readings as above.

Any bad connections or wiring in the alternator/battery circuit can cause current to flow where is shouldn't and lead to overheating, diode failures, etc.

Another check is to dis-connect the ground side of the battery warning light and see if it goes off. This means is may be the alternator side at fault. If remains on then it may be the wiring side at fault.

Whatever you do with these test make sure you are happy doing them and stay safe.

 

I bought an 85 QV in January of this year.

For the first couple of months, the battery would die, if I let the car sit for more than a week. I attributed it to an old battery, and in April, put a new battery in the car.

One evening in May, I was out for a drive and I noticed the radio turning on and off (cycling about every 5 seconds). My first thought was - crappy old radio, no big deal. Then, I noticed that my turn signal indicators weren't lighting up. Uh-oh - bigger problems here. Then I tried the headlights. Only one raised up. Tried to raise the window - no luck as it wouldn't move. At this point, the light bulb in my head went off - no power, no electricity - you've got an alternator issue.

The big question - what to do? I'm about 10 miles from home. How long can I drive? To make a long story short - I made it home, through traffic (including stop lights) without the car dieing on me. Once I turned the engine off, everything electrical was dead. No lights, no radio, no windows, no interior lights, etc. Dead as a door-nail. Frankly, I was amazed at how far the car lasted.

We (my son and I) removed the alternator that weekend, and had a local alternator/generator shop refurbish it for about $120. They even buffed/polished the exterior. Re-installed it, charged the battery, and have driven the car problem free since then. Total down time was less than a week.


My only complaint in this entire episode was the placement of the alternator. Whom in their right mind thought it would be a good idea to put an alternator at the bottom of an engine, nestled in between the headers and the exhaust? Other than dragging it on the road, I can't think of a worse location.

Sid