In quotes because I'm hoping it's not huge.

1966 F-100 CustomCab LB
351w, power steer/brakes, C6 auto

For two days now my alternator light has come on and stayed on with driving. It's on at ACC key position and does not turn off. Before this stayed on (while at a stop a week ago) the idle was rough and she took a nap; started up fine after and no problems. Today while driving, at a stop died again, started up, ran super sluggish (like starting in 3rd), then fired up and did fine the next 20 miles (with many stops).

Observations:
1.) The blinker sounds like it blinks slower now (might be me).
2.) Alternator light does slightly dim when blinker chimes/clicks.
3.) Horn barely blows now (discovered randomly).
4.) Lights work.

Battery? Loose belt? Dead alternator?

Thanks in advance guys.

P.S. - Thanks to many on the site I changed out door cores and found/replaced missing hardware so I can now lock the doors. Oh the small things...

 

For sure your battery is weak, for whatever reason it's not charging, or getting charged enough? Charge up the battery with an outboard charger while you figure out what's what. If it will accept and hold a charge...

Measure the voltage at the battery posts at idle and at 2500 rpm. Clean grounds and cable ends, alternator connections and grounds otherwise it may be alternator going south.

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Thanks Tedster. I'm going to swing by Autozone and have them test the battery and see where to go from there.

 

you need to charge the battery before you test it or it will test bad.

 

Battery checked low, but solid at Autozone. Charged and horn, lights etc. now work like normal.

The problem is she won't start. I re-checked all connections and despite multiple tries I get nada. There's no gas smell either, so I checked fuel filter (looked like a small copper top hat covered with I'm guessing rust). Cleaned with fresh gas and still nada.

Any suggestions are appreciated. Seems odd to just not work now.

 

If it won't start, how did you get to autozone? (Not trying to sound snarky)

What is the charging voltage when at idle and say, 2500 rpm?

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I pulled the battery yesterday and took my daily driver. (The battery is reinstalled correctly.)

I don't have a way to check battery levels at my home yet.

 

When you say won't start. Turns over?

You say lights work. Turn headlights on. Try to start engine. Do the lights stay brightly lit and the engine turns over quickly or, do the headlights dim and maybe the starter speed slow down?

If you have or can borrow a charger, it pays to keep battery on it while getting a no-start situation figured out. Give the starter itself plenty of breaks between tries too. In just a few seconds of cranking the windings will get smoked, the idea here is to not make a situation worse.

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Agreed. I'll work on getting a charger. Any good ones?

I'll check back on the lights after dinner and post again.

 

So I turned lights on (turned on indoor too for extra challenge) and, yeah, super dim interior lights, mild dimming of headlights and dash, little to no change in "rrrrrrrrr" engine start sounds. If it helps, just turning to ACC caused a subtle dimming also.

Thanks again Tedster.

 

Yes. The engine turns over...fan spins, etc. But no love...

 

Ok. Now the thing is, the battery has to be charged back up again, to replace the current lost when testing and other stuff. They are expensive, for me anyway. Chargers are cheap. Pick one up at random. If the battery is more than 5 years old though, replace it with a fresh one.

So, the ALT light is no longer illuminated, (but it won't start - turns/cranks, but won't run?)

We did discover though, in our test there are some electrons being lost or diverted somewhere. If everything was electrically clean and tight (and charged up) the lights should barely flicker when trying to start. Even better, it would start. They'll dim some, but not much when healthy. It "sounds" like maybe bad connections - grounds or cables, but because they can mimic other things, and the opposite is true.. It's tough to guess.. When a starter is going out, they will eventually just choke and refuse to turn over very fast. A bad cable will do exactly the same symptom though, and a bad battery will choke in much the same way..

This is where it helps to kind of have a history of the truck and maybe narrow things down. "Hey, that battery is old as dirt, I wonder..." "Hey, that green stuff growing out of the battery terminal...hm." What I like to do is first eliminate common faults that don't involve spending money versus replacing parts (that wasn't necessary and didn't fix the problem.) a little time investigating can save money even if taking it to the pros is necessary. Better a $10 loose cable than a $180 alternator that was just fine. Hope this helps.

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The truck is new to me (this year) so history is limited. The battery is at least three years old, likely older. The PO is not helpful, but did say the battery was replaced before he got it (~2 years ago). It charged to 100% at Autozone though, so hopefully it's OK.

The alternator light is on at ACC and since it won't fully start up, I don't know if it stays on or not yet.

I guess it's time to try following wires. With all this attempted starting, I figured at some point I'd flood it and smell gas. Since I haven't, is it wrong to suspect a fuel system issue? It seems like after the battery was checked I have spark and air, just not necessarily fuel.

 

It really depends whether you want to spend time or money remedying the problem. If it's not too bad $$$ by all means let the pros do it now and then. If you want to DIY ya hafta be methodical and be sure of what you are seeing. The reason being is that when somebody else is doing the troubleshooting, they are using your wallet, no way around it.

When I was in the .mil, there was a weekend off (my weekend off!) in jeapardy due to some tasking of maintenance and somebody (who naturally isn't going to do the work) had diagnosed a problem and just had it all figured out what was the show stopper here and that's that, of course the solution involves my replacing some expensive phq-1137 whatsit box, (and an assload of maintenance checks required)

I knew that wasn't the problem, but throwing parts at things is very easy to do when somebody else is paying for it. In that particular instance it was in my interest to troubleshoot the actual problem, not that I was paying for the parts, or saving taxpayer dollars *cough* - and I did.

Learned a valuable lesson on what motivates a troubleshooting tree, however. There's not much to like about crawling around underneath a truck sometimes so either saving money or making money is it.

So yes, you can do anything if you are motivated, tho if u can find a good mechanic shop they can knock that stuff out quick. Actually getting someone who is able and willing to work on vintage iron is maybe part of the problem. Sent from my iPhone using IB AutoGroup

 

Agreed.

I'm motivated, but lack knowledge or any discernable skill with engines. I'm not afraid to tinker and can usually fix things, so I'm going to give it a go. It's that or have it towed to an unknown location/shop.

Thanks again.