I noticed it driving to work in the dark. My perpetually lit EMISSIONS light pulses too. They never go all the way out, just dim a bit. It's a pretty regular pattern, makes me think it's rotational (alternator).

Battery checks out.. Alternator output hovers around 14.1 V. Floor mounted high beam switch and the floor around it isn't rusty or corroded. The actual headlight wiring seems like it's fine, I can't provoke a short by moving it around. I haven't done any checking of the regulator yet sooo... maybe.

Or does this all come down to the usual suspect... ground(s)?

ETA: This looks like a good place to start...
https://www.ford-trucks.com/forums/1710355-pulsing-lights-and-loss-of-power.html

 

Does the frequency change with engine RPM? With a bad alternator, typically the pulsing is more pronounced at low RPM because there's more time between the spikes. At higher RPM, the pulses tend to run together.

Quick and dirty troubleshooting:

1) Charge your poor battery. This is important before starting any electrical troubleshooting, so please don't skip this vital step. If your alternator is indeed wigging out, the pulses may trick the voltage regulator into thinking the battery is properly charged when it really is low.

2) Disconnect the drive belt to the alternator. With your fully charged battery, run the engine again and see if the pulsing is gone, which I suspect it will be. Your battery will be the only source of electrical power, but if fully charged (how many times is that?), you should be fine for a quick test drive around the neighborhood.

3) Take the truck to a place that sells batteries. Ask them to check the charging system for AC ripple. Most auto parts stores will do this for free. If the kid doesn't understand the question, go to the next place. It's possible to check at home with a regular multimeter, but not all models can catch the rapid voltage spikes of a failing alternator. It's best to use a special meter designed for this purpose, which an auto parts store should have.

 

Most auto parts stores will be able to charge your battery for free. I suggest removing it from your truck and dropping it off in the morning.

 

I had the battery tested a few mos back and the counter guy (who should have been trying to sell me a new battery) said it tested fine. I assumed the standard amperage-under-load, CCA type of testing but didn't ask (or even know) about AC ripple test mentioned by @kr98664... never heard of it. The good news was that l didn't have to buy a new battery. The bad news was that it's something deeper. The battery shows 12+ V w the engine off but always seemed weak... that's why l had it checked.

I'm going to have to spend some more time on this, get a little more in depth, run the tests outlined above. Maybe try to get the batterytested more thoroughl.. Even if l don't figure it out, l'll at least be able to provide more information to someone here who can.

 

Pulsing lights usually comes from the charging system. You need to check the connectors on the alternator. Pull the plug off and check for corrosion. You didn't say what year truck you are working on, the older ones had a regulator mounted on the inner fender. Unplug that also and check for bad wiring and corrosion. And then check if the regulator has a grounding wire under one of the mounting bolts. Take it out and check it also. It's best to undo it and put it back incase there is a bad connection there.

After all this unplugging, checking, and plugging back in, don't be surprised if the problem goes away by just you messing with the connections.

 

You running head light relays?
Dave ----

 

Sometimes l actually count on that happening... go in without a plan, take stuff apart, wiggle this, bang on that, and hope it fixes itself cuz I'm clueless.

 

Nope. But I'm starting to think l should.

 

I had the same problem on my 86 Bronco. I had a guy swap it to a one wire alternator. If you've got the time and money you can swap to the next generation up. It's a much better system.

 

Any progress? If interested in testing the alternator yourself, consider a tester like the Ancel BA101:

https://www.ancel.com/products/ancel-ba101



This is an affordable version of a tester an auto parts store will use. I have one and got it for about $40 on Amazon. Look around, as there are other brand names for what appears to be the same product.

For what I paid, this thing is slicker than snot. It will test a battery without the need to have it fully charged. It will also test for AC ripple from the alternator.

 

had the same issue with a mid 80s 250. upgraded to a G3 Taurus alternator. problem solved. the faint woop woop sound coming from the original 55 or 65 amp alternator and the headlights going from dim to bright told me it was warn out.

 

I've been on the road and haven't gotten back to it yet.
l don't know how accurate it is, but my ammeter shows a constant, high rate of charging from the alternator and my multimeter shows that it's charging normally.
Even so...
I've been looking into the 3G upgrade and, since all of the figgerin'-out has been done for already, it seems like a doable project for me. I'll probably buy the alternator new but the wiring harness will probably require a trip to the junkyard.

 

Cue the Twilight Zone intro music. We don't hear very often about an ammeter that actually works.

If accurate, this makes me think something ain't right. With a fully charged battery (Criminy, is that all he ever talks about...), you shouldn't see much deflection on the ammeter. Normal behavior is a moderate rate of charge for a few minutes after engine start, followed by slow taper off to the neutral position on the gauge. A continuing high rate of charge points towards a partially discharged battery or a voltage regulation (overcharging) problem. A bad diode in the alternator could cause all of these issues, including the pulsing lights. Please note I didn't say to rush out and replace the alternator, but do please test it, specifically for AC ripple.



What alternator do you have now? The 1G? That's what I have on my truck and it is perfectly thoroughly adequate. I work the electrical system hard. We carry a camper, with a second coach battery, AC in penguin smuggling mode, 12V fridge, and my stereo blasting The BeeGees. (Blame them for disco and in a heartbeat I will corner you in some dark alley, swinging a tube sock full of soap bars...) Sure, 3G is better, but why stop there? Ford must be up to the 17G by now, or something like that. From everything you've described, your situation sounds like a garden variety alternator problem. Unless you absolutely, positively need the increased output of a 3G, I would discourage you from doing a "just because" upgrade. You could finish your troubleshooting, quickly fix the problem, and be on your merry way.

All bets are off with the 2G, as the connector was a known problem. But the 1G? They will give reliable service.

 

I didn't have much time to spend w the truck this weekend. Replaced a block-to-body ground that didn't seem to help this issue much. Made a short video showing the tell-tale emissions light and bouncing ammeter needle under various electrical loads and engine speeds but it's apparently too big to post here. If l figure out how to link it, I'll put it here:

 

You may have reached the "paralysis by analysis" stage of problem solving. Sometimes you just have to try a stupid and futile gesture, and see what happens. Most of the clues are pointing to a bad alternator, with the less likely possibility of a bad battery. Lots of good troubleshooting tips so far in this thread.

If still inclined to test something, have the alternator tested for AC ripple. This will only take a few minutes at the auto parts store.

If you just want to throw a part at it, the alternator is your likely culprit.