New to the forums, recently acquired an ‘81 F100, running and driving with a few quirks that I’ve been slowly working on. I’m also fairly new at working at a vehicle from this era, so bear with me - if I sound inexperienced, I am.

When we got the truck, we had a charge light in the dash glowing on and off, as well as headlights dimming intermittently while driving, radio volume going up and down, etc. Changed the very nasty-looking old voltage regulator to a new ACDelco one, problem solved. Steady lights, no charge light on the dash, running better, etc.

Got in the truck after grocery shopping yesterday and noticed a really slow crank. Got home fine, but checked battery voltage and was at about 11.4. Checked again with the truck running, no difference, although very slowly dropping while running - but the truck ran like this for awhile before I shut if off. So it seems like the alternator was still charging, just undercharging.

Working backwards to that voltage regulator replacement, I did notice today that it had loosened from the mounting place, and I imagined it should be solidly grounded. I tightened it up, started the truck (at this point battery voltage is under 11), and this time it ran for less than a minute, dropping voltage, then cut off.

The wiring overall in the truck isn’t pretty, and I’ve been slowly replacing chewed-up plastic connectors, so I’d love to go through and clean terminals, etc., but we’ve got a rainy day here in Kentucky, so... what avenues would you pursue first? I have checked fuses, replaced #18 that I believe was the gauge cluster. I also have not had a charge light at all during these times I’ve had the truck running.

I can throw an alternator in easily enough, not too expensive, and I have suspected it’s going out because I think I can hear some bearing noise. Just wondering if I might be missing something stupid.

 

There's a resistor in the back of the cluster. It runs parallel with the bulb. That way the alternator still charges even if there is no bulb or a blown bulb in the cluster. If the resisor is bad AND the bulb is blown, the alternator won't charge.

Are you getting 12v at the green/red wire? That's a key on wire to tell the alternator to start charging.

I've blown a lot of money replacing parts that worked fine because I didn't diagnose properly in the first place. Unless you just want to replace it anyway, I'd do some diagnosing first. Ensure that you have 12v at the green/red wire when key is in the ON position.

 

Thanks. I'll give that a shot. Would rather do a lightbulb than an alternator. I imagine I have to pull out the instrument cluster to check but easier than getting under the hood.

 

Before you get into the cluster, check that the green/red wire has 12v. If the red/green wire has no voltage, you need to go upstream of it and troubleshoot the other areas. Then is when you go into the cluster.

You should be able to field test the alternator itself by bypassing the regulator. If the red/green wire has voltage (you're verifying that first), go to the regulator. With key off, jumper the plug from A terminal to F terminal. This bypasses the regulator. Start the truck and check voltage. If it's no longer falling and increases with RPM, the alternator itself is good and the regulator is no longer functioning.

That could be a bad regulator or it could be a bad ground, like you mentioned was an issue. If that is the case, you can connect a wire from the negative battery post to the regulator and rule out a bad ground.

 

Maybe you know this already, but if not do yourself a solid and put the battery on a charger and make sure it's charged up thoroughly. This will take several hours or overnight with a regular battery charger.

 

Okay, some potentially good news: went to check the voltage on the red/green wire, and I do have 12v (or at least 11.5, which is all the battery has right now). When I went to unhook the voltage regulator, the red/green connector fell completely out of the pigtail. One of the many plastic connectors that’s just trashed and in need of replacement on this vehicle.

So, I managed to jam it back into the pigtail connector with a screwdriver just enough that it would slide back onto the pin on the regulator. Key on is now giving me a lit charge light (very happy I won’t have to take out the cluster). May be possible that having that wire disconnected was causing my alternator not to charge at all?

Not enough juice in the battery, but we’ll see if I’m back to normal after a charge and after I secure that voltage regulator to ground more tightly. In the meantime, I’d better order a new pigtail.

 

Go look at post #2 again. The green/red brings the alternator online to charge. No green/red wire or no voltage on the green/red wire, no charging. I see you have a light when you first turn the key, but haven't started it yet. That would have been a good thing for you to check earlier, but I didn't think of it till you said something about it.

Here's a new connector that might work for you.

 

So, I do have voltage on the green/red wire. Have it firmly back into the connector, voltage regulator is securely grounded. Got the battery charged up a bit, so now I have, with key to ON, oil light and charge light on. Truck started okay, charge light remains on. It continues running, but battery voltage hovering at around 12.03.

I cut the engine and bypassed the regulator by jumping the A and F terminals as cstephens mentioned. Sure enough, some revs and I’ve got battery voltage up to 14 volts - alternator is charging but it’s making a godawful squealing sound.

:edit: I won’t rule out belt squeal. Is there any reason the previous owner put a belt with teeth onto smooth pulleys??

So, maybe my brand new regulator is a dud and while my alternator will still charge, it’s on its way out?

 

Further update... I removed the bypass wire, took my new regulator out, and put the old one back in. My old problem (flashing charge light, headlights going dim/bright alternately) came back, but the alternator at least charges. So I definitely was sent a dud regulator. Any recommendations on where to find a reliable one?

 

Take it back and get another one. Sometimes it happens.

Some belts have teeth on the inside to make them more flexible to go around small diameter pulleys. Sounds like you need a new belt or tighten the old one. In fact, you might want to tighten the belt with the old regulator and see if some of the blinking goes away.

 

Ford. Motorcraft has available the 1G regulator. PT# GR540B

 

When you ran that jumper wire and got the 14 volts what you did was let the ALT put out its MAX VOLTS
and why it could be making noise. Also because it is putting out max volts the belt could vary well be slipping/ squealing.

If that 1 wire fell out of the connector what you may want to do till you get the new one is install new ends on each wire and plug them onto the regulator that you cleaned the connections on to see if that fixes the light blinking.
Also make sure that regulator is grounded to the fender really good.
You may want to check all grounds.
Battery to block cable, Block to frame cable, block to body wire and you should have a small braided cable from fire wall to hood.
When looking at the battery cables are they old looking? You cant tell if they are good or bad by looking at them as they go bad from the inside out.
If they have the "get you by" bolt on battery ends just replace the cables as you will have a failure someday with them.
When replacing the battery cables get the largest dia. cable you can, some replacements are pretty small.

Good luck
Dave ----

 

Wanted to thank y’all for all the tips. Went and grabbed another regulator, plugged it in and alternator is charging properly, no dash lights, things seem happy altogether. I will be replacing the pigtail to the regulator and probably the wiring to the alternator as well, it’s all seen better days. Battery cables seem newer and pretty thick. I think my squeal with the alternator running wide open is a loose/old belt after all, so that’s on the list as well.

This truck was definitely someone’s project before I owned it, lots of aftermarket parts, but I can tell wiring was not their strong suit - there is a lot of stuff twisted together with electrical tape. Cheap fixes though - engine feels really strong and I don’t have any puddles underneath. It’s a fun and very different learning experience, my daily driver is a ‘99 Explorer with 230,000 miles and I’m a little spoiled by plugging a scan tool into it.

 

For reference, this is when we brought it home... since removed the roll bar!

 

I can tell you from experience ( working as an alternator rebuilder for 40+ years ) that a lose of ground will cause the electronic regulators to fail. I would even tell my customers to bolt them on before plugging in the connector. Just because you replaced it don't search for another problem until you've checked the voltage at the plug. Since it charged when you jumped A to F you know those 2 wires are good. You should have voltage in the I terminal ( unplugged with key on) and if you bypass the regulator with engine running you should get !/2 the charging voltage on the S terminal. If output is at 14 S terminal will be 7. Don't go tearing things apart yet. It would be worth your while to replace the regulator connector since you said a wire fell out. Also they get very corroded over the years and are cheap and readily available.