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1991 Chevy Silverado Low Voltage/No voltage from Altenator

Tiny
MIGHTY_MAX
  • MEMBER
  • 1991 CHEVROLET SILVERADO
  • V8
  • 2WD
  • AUTOMATIC
  • 250,000 MILES
Hello,
Here is the problem.
My voltage guage on the dash sits around 10 - 12 volts and of course battery light comes on (it seems no
voltage is getting from altenator to battery).
I put a volt meter on the battery (pos and neg posts) and it shows about 12 volts.
I put the negative probe on the negative post of the battery and the positive probe on the wire attached
to back of altenator (engine off), and it shows around 12 volts. So that leads me to believe the wires
are fine between battery and altenator.
But of course once I turn the engine on all it will show is voltage of the battery. Tests confirm no
voltage from altenator (my battery tester and Advance Autos tester).
Now some times the system will correct itself, like if I make a left turn (not right turns), or last night
when I was working on the truck in my driveway (slight incline) he was fine until I moved him back a
little bit in the driveway.
It is not the altenator, I had put a refab in him, but the problem persisted so I installed a new one
last night and the problem still continues. So 3 altenators have been in the truck (the original, the
refab and now the new one).
So this leads me to believe there is a wire somewhere that is swinging against the chasis.
But I have been looking at the wires and all appear to have their insulation on them or are in
electrical tape.
These wires I have inspected are in the engine compartment area.
So:
-- Are their areas elsewhere I can check? Would wires in the dashboard cause a similar problem?
-- I saw something talking about the wires going to the starter. I have only glanced at those
wires. They appear to be in good shape and not touching any metal. Is there something else
I should be looking at when inspecting the starter?
-- Is there anything else I am not thinking of or looking at that I should?

I have disconnected the relay that connects to the fog lamps to rule that out. (They are after market)

I have noticed that when I push the gas, the initial incline of the front end will correct the issue and once the truck levels out from that, the problem starts again.
So it seems like a swinging wire, or something making/not making contact.

Can a relay switch cause this behavior?
Or does a relay switch do something else entirely different?
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Monday, June 21st, 2010 AT 11:55 AM

11 Replies

Tiny
CARADIODOC
  • MECHANIC
  • 33,910 POSTS
Hi Mighty_Max. Welcome to the forum. I have to do this from memory of a '98 model that I bugged for students to troubleshoot, but I think the circuit is the same. The confusing part is why the battery light is coming on.

Look at the plug on the side of the generator. There should be a brown or red wire, (I can't remember), but it should have near battery voltage when the system is working. It will have around 1 or 2 volts when it's not charging. That circuit comes from the battery light in the instrument cluster. If you find 0 volts on that wire, and the battery light is on, unplug the connector. If the light stays on, that wire is grounded somewhere between the cluster and the generator. The tickle of current that comes through the bulb is what tells the internal voltage regulator to turn on. If that wire is grounded, the regulator will see 0 volts, just as if the ignition switch was turned off, but there would be a complete circuit for the battery light to turn on. If that wire was open someplace, the regulator would again see 0 volts and not turn on, but the battery light would not turn on either.

A common place to find an intermittent connection, at least on the '98 models, is on the flexible circuit on the back of the instrument cluster. Pushing on the cluster might wiggle the connector which is a sore point to start with. Also find the battery light socket and follow both copper circuits in the flexible plastic to see if you can find a hairline break in one of them. It may be necessary to use an ohm meter to check the two circuits. The bulb itself will not cause a no-charge because there is a resistor in parallel with it to pass enough current to wake up the regulator.

Caradiodoc
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Monday, June 21st, 2010 AT 4:22 PM
Tiny
MIGHTY_MAX
  • MEMBER
  • 7 POSTS
Hey caradiodoc,

Last night I did have the truck on and that 1 wire connected to the Voltage Regulator unplugged.
I believe that when I did that (I had forgotten I unplugged him when I turned on the truck) there was NO battery light, and the Volt meter on the dash was around 14.

I did plug in the connector while the truck was on, and the issue appeared. So should I be looking at this single wire as the culprit?
And from reading what you posted, that wire runs from the regulator to the dash light (just to confirm)?

Also, I looked on Auto Zone's website for that part, and the one they showed for my truck had 4 wires coming off it. Would that be a problem?

I did charge the battery and tested voltages again last night. The voltage at the battery posts was 12.25V and the voltage at the neg. Post and the red wire on the alternator was sitting at 12.26V (truck off). So that would lead me to believe there is no grounding on that side of the circuit, correct?
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Tuesday, June 22nd, 2010 AT 6:10 AM
Tiny
CARADIODOC
  • MECHANIC
  • 33,910 POSTS
Boy, everything points to a defective generator, ... Again. Current goes through the dash light to that small wire on the generator to "wake up" the voltage regulator. When it starts to charge, the regulator sends 12 volts out on that small wire to oppose the battery voltage on the other side of the bulb. That makes the bulb go out since it has roughly the same voltage on both sides. Unplugging the small wire after the engine is running creates an open circuit so the bulb will not turn on but the generator could continue working until the engine is stopped. System voltage is monitored internally. What doesn't make sense is why it appears to start charging properly when you unplug the small wire.

The other unused pins in the connector are for sensing system voltage from a remote location and to run a tachometer with a diesel engine. The remote sensing is typically used with a digital dash because it is very important for the voltage in the cluster to remain stable to prevent flickering.

Caradiodoc
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Wednesday, June 23rd, 2010 AT 2:54 PM
Tiny
MIGHTY_MAX
  • MEMBER
  • 7 POSTS
Ok, I was digging around today and the wire that runs from the pos. Post of the battery to the fuse block was melted and showing exposed wire.

So I cut him back to where it was good wire and shortened him up a bit so he would not come near the hot engine.

Still same issue, light on, low voltage.

I stuck my Volt Meter on him.
I would read 12.44 V at the battery posts.
12.36 at the fuse block where the battery wire connects.
Same voltage at the fuse block for the alternator wire.
And then also the same 12.36 at the back of the alternator itself.

So in short, the battery post Voltage was higher than the Alternator post.

But still light on.

I took apart the dash last night.
The wires all run to a connector that plug into a connector on the dash board which is a PCB.
So if one thing was having issue, I would think other guages and lights would be doing similar.

I can't believe that 3 alternators are bad.

Especially when they all exhibit the same behavior.
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Wednesday, June 23rd, 2010 AT 4:02 PM
Tiny
CARADIODOC
  • MECHANIC
  • 33,910 POSTS
You're absolutely right about three units not all being bad. Again from memory of a '98 model, the "gauges" fuse feeds the battery light along with some of the gauges, but a single wire leaves the cluster to go to the generator.

I assume the voltages you listed were measured while the engine was running. Since the dash light circuit is working, the system should be working The only thing I can suggest is to check the smaller negative wire that goes from the battery to the fender and any ground straps between the engine and body. They "shouldn't" be an issue because there is the large negative cable going to the engine, but check them anyhow.

This might be stupid, but is it possible the connector is turned around and plugged backwards?

Caradiodoc
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Thursday, June 24th, 2010 AT 1:58 AM
Tiny
MIGHTY_MAX
  • MEMBER
  • 7 POSTS
I will check the negative cables. But previous inspection of them seem to have a good connection to the body.

My voltage checks were with the engine running.

The battery connectors are in the same position they have been since I got this truck and before this problem surfaced. I have inspected them to make sure their is no corrosion.

Could a bad battery be causing this problem? I would think a bad battery would cause the alternator to produce more voltage vs. Less though.

I took the alternator down to AAP last night and they benched it. Worked perfect and held a steady 14V.

This is such a simple circuit.I just don't get why it is acting this way.

Driving in this morning to work hard left turns seemed to correct the issue (not always). Right turns never corrected the issue.

Man this thing is bugging the devil out of me.
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Thursday, June 24th, 2010 AT 6:31 AM
Tiny
MIGHTY_MAX
  • MEMBER
  • 7 POSTS
I disconnected the single brown wire that goes to the regulator.
The voltage guage and battery light still stayed on.

W/ this single wire setup, he really doesn't do anything, right?

The alternator internally does all the sensing and regulation, right?
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Thursday, June 24th, 2010 AT 6:35 AM
Tiny
CARADIODOC
  • MECHANIC
  • 33,910 POSTS
OK. Hold the phone. Things are starting to make sense. If you unplugged the small wire and the battery light stayed on, that wire is grounding out someplace. That wire serves two functions. First it gives a tickle of current to the regulator to turn it on so it will do its thing. That current flow is what initially turns the light on. Once the generator is up and running, the regulator sends battery voltage back up that wire to turn the light off.

When that wire becomes grounded, the light will turn on, but instead of the regulator seeing that tickle of current, it will see the 0 volts and 0 current. To him it will appear the same as if the ignition switch was turned off. The secret to finding this is to unplug the connector, verify the light is still on, then move the harness around until the light goes out. It will be helpful to have a helper watch the light for you. An alternative is to use a voltmeter to measure the voltage on the wire while it is unplugged. When the wire is grounded, it will read 0 volts. When you remove the short, you will read 12 volts.

Caradiodoc
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Thursday, June 24th, 2010 AT 4:07 PM
Tiny
MIGHTY_MAX
  • MEMBER
  • 7 POSTS
Yep, I think you are right.
Cause I was playing with it earlier today. And I don't really understand why.
The wires coming from the alternator (red & brwn) are bundled with a bunch of other wires and then electrical tape is wound around the bundle very tight.
It runs by the TBI (passenger side) towards the firewall, and then the wires go to their various spots.

So today, grins and giggles, I took a zip tie and pulled the bundle of wires away from the TBI towards the accumulator.
And that seems to have corrected the issue.

So I guess I will need to undo the tape and start inspecting the wires.
But like I said, I don't really understand why it happened since all the wires were bundled very tightly in this electrical tape and was not contacting any part of the engine.
But hey, I won't look a gift horse in the mouth at this point.

Thanks for the help on this.
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Thursday, June 24th, 2010 AT 6:33 PM
Tiny
CARADIODOC
  • MECHANIC
  • 33,910 POSTS
Holler back when you find the cause. I like mysteries. Well, except when my stupid computer won't work right!

Caradiodoc
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Friday, June 25th, 2010 AT 3:14 AM
Tiny
REDSKINSFANAM
  • MEMBER
  • 1 POST
Did you ever figured out what it was. My truck has the exact same problem and has been sitting in a various shops for a month now since no one can figure it out
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Saturday, October 10th, 2015 AT 8:43 PM

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