95sev6

After many years of trouble free service my '95 pickup has developed some electrical problems. While coming home from work the other night, I noticed the wipers seemed a little slower than normal; which along with dim dash lights finally caused me to check the volt meter I installed years ago. Stuck on 11.5 volts. The idiot light hadnt' come on. I spent several hours checking the system today. Had the panel out and checked the light, no problem there, the bulb works if it gets power. Also, checked the circuit board on the back of the panel; had power to the correct circuits at the correct times. And checked the voltage to the alternator; everything seemed right and all the relays I could find. Problem is the charging system stays on 11.5-12 volts and the idiot light doesn't work, won't even come on when you just turn the key on. All the other warning lights work. I pulled the alt. (which was a rebuild about 5 years ago) and will get it tested tomorrow.

Any ideas regarding the "stupid" idiot light that won't work?

TIA!

NPath

Only thing I can think of that you didn't mention is the battery.

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NPath

95 Pathfinder SE
06 Pathfinder SE

95sev6

Don't think it's the battery. It's a 3 yr. old Optima that's been holding a charge just fine, no starting problems.

Also I think I've had this problem intermittently for awhile. A few months ago I noticed that the voltmeter would be on 11.5-12 volts intermittently. Wasn't too often and just about the time I'd start to get worried it would pop up to 14 volts and everything would work normally for a few more weeks.

Xeno

Light maybe burnt out... Have you checked your voltage regulator and or alt?

95sev6

The light bulb isn't burned out and that part of the circuit seems to test OK; its just that the light didn't come on when the alternator failed and it doesn't come on with the rest of the idiot lights when you first turn the key on.

The rebuilder called late this afternoon and said the voltage regulator in the alternator had failed. So, I'll install the rebuilt alt. tomorrow night. Which should take care of the problem.

Now if I could just figure out why the stupid idiot light didn't come on when the alternator failed...

95sev6

Ok, the truck is up and running, charging with the voltmeter reading about 14 volts. The rebuilder replaced the voltage reg., rectifiers and bearings. And the idiot light works now; comes on when you turn the key on and then goes out when the engine starts. Everything is as it should be.

Moral of the story:
Be aware that there are certain scenarios where the charging system can fail and you will have no indication that anything is wrong. The idiot light will NOT come on and you'll be fat, dumb and happy until the engine just quits. I know that from experience. The first time it stranded us in the outback about 30 mi. from Cheyenne, Wy. This time it didn't get that bad because I had added a voltmeter after the first problem. I think Nissan designed a faulty system here and the best way to protect yourself is to install a voltmeter.

Hope this helps somebody!

spat

The whole reason the term idiot light came about is you have to be an idiot not to have noticed a problem by the time they come on. Examples: gas light comes on right about the time you cant make it to a gas station , the oil light comes on when the oil pressure is to low to push oil to the crank but the top has been starving forever , I suspect the charge light comes on only when battery voltage is detected but no charging voltage is present . Pull the charge wire off the alt. and I bet the light will come on. Bottom line, you found the problem befor the light came on so your not an idiot!!!

__________________
It's much easier to travel through a maze when you have a map!

Spat
Former Nissan Tech
14yrs ASE all but AT it's a bitXX!
New career - HandyMan/Electrician - Freedom at last!!

86.5 Nissan HB D21 234K Miles
94 Nissan Altima GXE ? Miles
97 Dodge Grand Caravan 3.8L 132K Miles
64 Chrysler 300-K 2-door Convertible 63K Miles

95sev6

Interesting take on the idiot lights; guess I just got lucky this time.

I still maintain that there is a design fault in this charging system. Now that its working properly I too think that if I pulled the charge wire off of the alt. the idiot light would come on. But have the voltage reg. fail and guess what; no charge voltage, no output from the alt. and NO idiot light.

KLL

only idiot is not the bulb, but the nissan as well...

yesterday i removed the dashboard to replace bulbs, and i found that there are two missing bulbs behind. guess which ones are they?

yeah correct, one is for battery sign, other one is the overheating indicator.

Greybeard

Here’s a 3-minute on the idiot light:
Regardless of the design of an alternator or generator, the windings need some kind of external current to get the "flux line" in motion so that electrons can begin to flow.
In about 25% of all alternator designs, a small permanent magnet generator provides the excitation to energize the field windings so that they can develop sufficient electrical current to charge a battery.
The other 75% of designs, including most Nipondenso, sees field excitation current flowing directly through the idiot light when the ignition switch is selected to the "On" position, then into the alternator field winding (rotor) and regulator circuit to ground.
This small current in the field produces a very weak magnetic field, just enough to get the alternator to start generating when the engine is spooling up. As long as the alternator isn’t producing a charge, the regulator’s “ground plane” is closer charge-wise to the negative terminal than to the positive terminal, so the light comes on. This excitation current is also "regulating" the regulator to keep it from going directly into max conduction. Otherwise, with battery voltage lowest right after start, the alt would try to go into max charge as soon as it started cranking over, and the alternator would burn up before long.
As the generator spins, that weak magnetic field from the bulb current produces the initial output from the alternator. As the rotor speed increases, so does the excitation current that is being fed back to the field. Now, up to a point, the alternator is producing slightly less than battery volts, and the regulator is continuously seeing more internally produced voltage and less "excitation" voltage During this time the bulb continues to get dimmer and dimmer until alternator output exactly matches the battery voltage, at which time the bulb goes out completely.
As long as the alternator's regulated output remains above battery voltage, the light will stay off. With no accessories on, and the battery now recharged after start, the regulator voltage falls, and only rises to match the demand on the battery from the ignition system before dropping back to only slightly above battery voltage. During these low-voltage points, the bulb is actually feeling some current, but not enough to see with the naked eye.
Depending on what’s happening inside the regulator, the bulb can be “on” while the alternator is putting out correct voltage, or “out” while the battery is getting no charge at all, yet come on with the key turned to “ignition” before start. For example, a cracked diode or dirty brush contacts can result in a low output with dim light and a slowly dying battery.
And, in a correctly charging system with clean connections, a good battery, and properly working alternator with (clean brushes and slip rings with no leaking diodes) alternator speeds above about 500 rpm should have little or no bearing on the output, and blipping the throttle should hardly hange the intensity of the headlamps or fan speed.

95sev6

Thanks for the explanation.

So, if you really want to know what's going on in your charging system install a voltmeter. You cannot rely on the idiot light, cause if the regulator fails it wont' work.