To begin with..............was it doing this BEFOREthe silly-noid swao out?
Remove solenoid  and sand and clean mounting point. (don't let 'just the mounting screw'  do the grounding)
See what happens and report back....we'll finger-it out
6sally6

That light is what we call in the electrical field as an “idiot light”.

The reason we call it that?

It only lets you know you have a problem with your charging system.

The charging system consists of the alternator (generator on early models), voltage regulator, battery, and wiring in charging circuit.

From your description of hard and slow cranking, I would check battery voltage.

Off should be around 12.5 volts
Running should be about 14 volts

If it is starting okay now like normal, I would look at alternator and voltage regulator and wire connectors.

Start simple by checking all connections are corrosion free.

There is a wire from alternator that has a connection near battery tray that is a common problem too.

Hope this helps ya out.

An old 65 I used to have had the flickering light.  After checking it with a meter, tracing the problem turned out to be the voltage regulator.   

Check your voltage with the engine running.
Check belt for slippage.
Look for any broken/frayed wires between battery and alternator and voltage regulator.
Clean the connections on your alternator.
Check/clean connections on the voltage regulator plug.

Remove alternator have it tested. Replace if necessary.
Replace voltage regulator.

Hope this helps.

Last edited by Greg B (6/21/2021 11:33 AM)

6sally6 wrote:

To begin with..............was it doing this BEFOREthe silly-noid swao out?
Remove solenoid and sand and clean mounting point. (don't let 'just the mounting screw' do the grounding)
See what happens and report back....we'll finger-it out
6sally6

BobE wrote:

Nos681 wrote:

I would check battery voltage.

Off should be around 12.5 volts
Running should be about 14 volts

If it is starting okay now like normal, I would look at alternator and voltage regulator and wire connectors.

Start simple by checking all connections are corrosion free.

There is a wire from alternator that has a connection near battery tray that is a common problem too.

Hope this helps ya out.

If the battery hasn't run down over time, I would guess the alternator is fine and either a connection is intermittant and needs cleaning, or the voltage regulator is the problem.  As stated above the voltage with the engine not running should be about 12.6 volts, with the engine running the voltage should be between 13.7 and 14.3 volts.  On initial shart-up, the voltage will be higher and should drop off slowly (a few minutes) as the battery is replenished. 

Greg B wrote:

An old 65 I used to have had the flickering light. After checking it with a meter, tracing the problem turned out to be the voltage regulator.

Did a bit of troubleshooting, but haven't yet nailed down where the problem is. I did figure out that if I rev the engine to around 3000rpm or higher and keep it up there, the alternator light stops blinking. Maybe that could be a clue...  So far I've done the below:

Checked the battery voltage and it read at the correct levels given above
Sanded and cleaned the firewall mounting points for the solenoid
Double-checked alternator belt tightness - it's tight enough that I can't push the alternator fins to move with my fingers
Cleaned the contacts on the voltage regulator
Alternator wiring harness was replaced a few months back (no issues at the time, the wires were just looking pretty old and stiff), so the wires are now all in good condition. I double-checked that all were tightened down to make good contact
Replaced battery terminals and cables with new ones (upgrading from 4AWG to 2AWG) as they were looking a little worn out and had been on my upgrade list for a while anyways

From all of your responses and other online research, it seems the problem could also be the brushes on the alternator, or else potentially the voltage regulator going bad. Any ideas?


 

Last edited by mustang6518 (6/23/2021 9:33 AM)

​Based on what you've done, change, or adjust, the voltage regulator.
The alternator brushes could be worn down, you would need to identify how many miles is on the unit.  These brushes usually last a very, very long time. 
 

Rufus68 wrote:

Get one of these for your toolbox.  Plug it into the lighter socket anytime you need to keep an eye on the charging system.  With the battery fully charged, you should see 13.1 volts or more when you're driving.  If the battery is being charged, you should see better than 13.7 volts up to 14.3 volts.
https://www.amazon.com/Zeltauto-Cigarette-Lighter-Digital-Voltmeter/dp/B00VL9JZ0K/ref=sr_1_9?dchild=1&keywords=cigarette+lighter+voltmeter&qid=1624473007&sr=8-9

I also have this (although a different brand) and found it works quite well.  I've checked the accuracy with my digital Fluke meter and found it to be "close enough".  And, you can't beat the price!
 

BobE wrote:

​Based on what you've done, change, or adjust, the voltage regulator.
The alternator brushes could be worn down, you would need to identify how many miles is on the unit.  These brushes usually last a very, very long time. 
 

6sally6 wrote:

IF alt is making 13.7 - 14.3 amps that means its do'in its 'thang' sooooo...it must be the voltage regulator (my guess)
Looks like your results shows it won't leave you on the side of the road any time soon(maybe!).
Just eliminate the VR would be my next move.
You don't have points do you?
6sally6

Nope, I don't have points.

Just replaced the old unknown brand voltage regulator this morning with a Motorcraft GR540B and it doesn't seem to have helped. If I rev up and hold it at 2000RPM or above the light stops flickering (used to stop at 3000RPM or above), but otherwise there's no difference from the old voltage regulator. That RPM difference could also just be a fluke. Looks like taking a look at the alternator brushes is the next step?

Would the constant voltage regulator behind the instrument cluster have anything to do with it? I replaced it a couple years ago with one of the new Scott Drake solid state ones, so it should be fine (and the gauges otherwise work well)...

Last edited by mustang6518 (6/26/2021 2:06 PM)

BobE wrote:

mustang6518 wrote:

BobE wrote:

​Based on what you've done, change, or adjust, the voltage regulator.
The alternator brushes could be worn down, you would need to identify how many miles is on the unit.  These brushes usually last a very, very long time. 
 

6sally6 wrote:

IF alt is making 13.7 - 14.3 amps that means its do'in its 'thang' sooooo...it must be the voltage regulator (my guess)
Looks like your results shows it won't leave you on the side of the road any time soon(maybe!).
Just eliminate the VR would be my next move.
You don't have points do you?
6sally6

Nope, I don't have points.

Just replaced the old unknown brand voltage regulator this morning with a Motorcraft GR540B and it doesn't seem to have helped. If I rev up and hold it at 2000RPM or above the light stops flickering (used to stop at 3000RPM or above), but otherwise there's no difference from the old voltage regulator. That RPM difference could also just be a fluke. Looks like taking a look at the alternator brushes is the next step?

Would the constant voltage regulator behind the instrument cluster have anything to do with it? I replaced it a couple years ago with one of the new Scott Drake solid state ones, so it should be fine (and the gauges otherwise work well)...

The CVU will have not have any effect on the Alt light.  The light circuit supplies +12v, from the ignition switch, to the field circuit of the alternator.  This allows to alternator to produce power to the vehicle.  The light goes out when the alternator is producing voltage.  As the voltage is the same on both sides of the light, the light goes out.
The alternator brushes could be the problem.  If you have a rebuilder nearby, they may be able to check the entire system.and pinpoint the issue.

Opened up the alternator today and one of the brushes fell apart in pieces -- guessing that was probably the problem. Ordered some new ones, so hopefully that solves this

Nos681 wrote:

Do you have a condenser (capacitor) at voltage regulator and/or alternator?
Looks similar to one use for points.
There are female bullet connectors in harness.
Those are used to filter (smooth out) the rectified DC voltage from alternator.

What is your voltage at the fuse box?
Engine on and off?
How does it compare at the battery?

Replacing the brushes didn't work, so I replaced the alternator entirely with a rebuilt Motorcraft and I'm still getting the intermittent flickering.

When engine is off, voltage is 12.60 at the battery and 12.55 at the fuse box. With the engine on, the voltage is 14.33 at the battery and 14.28 at the fuse box. 

I don't have the condenser / suppressor at the alternator or at the voltage regulator. I was under the impression those were only for radio noise? Since I currently don't have a radio in the car I haven't replaced either of those. Do they have an effect beyond just the radio?

Last edited by mustang6518 (7/06/2021 4:37 PM)

In addition to the checks recommended by wsinsle, if possible, check the voltage on both sides of the bulb.  This voltage should be equal ... that causes the light to go 'out' when the alternator/regulator is working properly.
I'd check every connector on the voltage regualtor, alternator and the main firewall harness, for a poor, or corroded, connection.  If the battery is staying charged, I suspect the wire from the light bulb to the voltage regulator has an issue.  The 65 Mustang Wiring Diagram that I have does not contain the charging circuit. 
I do have a generic schmetic of this circuit, but it not show the actual wiring.  I can forward this to you, send me your email address.

Is the IVR anywhere in the alt light circuit? Led bulbs have a cut off voltage and may flicker at close to that.
Just a wild guess.

mustang6518 wrote:

Turns out it's something to do with the LED bulbs I have in the instrument cluster. When I swap in an incandescent for the ALT light, there's no more flickering. When I pulled it I also noticed a couple of the other LED bulbs were also doing the same flickering, it's just so faint I didn't notice until the bulb and plastic socket were pulled out of the cluster. Now to figure out how to fix these damn LEDs!

LEDs will not work properly in the alternator light circuit, you need to add a relay (BB has a sketch for this).
I've had issues with LED bulbs in the house flicking slightly, and replaced them, that ended the flickering.  I beleive the flickering is due to poor quality manufacturing of the bulbs.  I've had LEDs from "Super Bright LEDs" for my dash lights for the past couple of years w/o any issues.