Alt light stays on

[68fe360]

Did a search, and did not find anything helpful to my situation. Truck info at end of post.

My alt turned on and wont turn off. Basic volt test of the was good. Checked voltage after starting it, and was reading 14.8 volts. But my electric was on as the engine was still warm enough. So if I'm not mistaken that is a good voltage reading when the VR kicks in the alt to charge the battery.

I could use some tips on trouble shooting why the alt light wont go out.
Thank you!

1968 F100
360 FE w/ 3 speed manual

[390FE1972]

Here are some older threads where members had problems with their alternator light staying on, hope these help!

http://www.fordification.com/forum/view ... =8&t=54556
http://www.fordification.com/forum/view ... =8&t=53980
http://www.fordification.com/forum/view ... =8&t=59765
http://www.fordification.com/forum/view ... =8&t=68665
http://www.fordification.com/forum/view ... =8&t=71737

Let us know what you do to fix it.

Regards

[68fe360]

Thank you. All I could find was people dealing with a hacked wiring.

I decided to clean the ground at VR. Then decided the body of the VR and connectors were crusty, and got a new VR. Still have the same issue. I'm going to try unplugging the VR. Then key in the run position to see if stays on in the morning. I don't want to just throw parts at it. So need to track down what's causing it to light up, but still charge the battery fine.

[68fe360]

So I tested to make sure the wire between the light and VR was not shorted to ground. So I'm good there, and it's narrowed down to an issue in the wiring to the alt, the alt, or possibly a bad new VR. I'm less likely to believe the last one. While yes it's possible, but it's the difference between the possible, and the likely. It's possible aliens infiltrated or goverment, but not likely. Lol

Also made sure my alt belt was tight. Good there.

The light cam on after I losened the water hose clamp on the back side of my 360's water pump so I could get the hose clamp adjuster out of the way of the distributor. Come on about half a day later. Some collant came out while doing this. Not a massive amount. Maybe 1/4 of a cup in total. The alt is on the lower left. So it could have gotten some extra greasy gunk washed down on to it 4 or 5 days ago.

Being the original engine it has grease every ware, and I notice some parts of the backside of the alt has a light layer as well. Not sure if that could be the source of the problem or not. The wiring diagram shows ware the wires go, but does little to explain what they do. So I have no clue if any of those wires on the alt could be part of the issue.

The alt is deffinetly working. Check battery voltage at 12.43 prior to starting. Fired it up, and checked again, 14.47 volts. If I am correct the exact voltage will very depending on the amount of charge needed and rpm, but above 14 volts is what I should be looking for. 15 and above is considered to high, and a bad VR correct?

[68fe360]

Mistaken double post. Dumb slow connection...

[68fe360]

I'm starting to suspect that the alt is just not putting out the voltage that VR is expecting to see. These non single wire alts are far better at sensing the voltage then more modern single wire alts. It's one of the advantages to the old method.

I replaced the alt back in 2014 or 2015 in nowareville Rawlings Wyoming with a tornado baring down on us in the AutoZone parking lot. I was driving cross country from the Central Valley of CA to Central NY. Finished up right as the outer bands started hitting our location. Put my cat back in the cab, and hit the gas!

I replaced the timing chain & water pump before leaving, and should have replaced the alt as well. It had some play in the bearing. But dummy said, "I think it will make it, and will replace it when I get there" So it might be time for a new alt. It's been what 5 or 6 years.

[68fe360]

The more I studied the circuit diagram of the VRM I bet the alt is the culprit. Yes there is sufficent charging voltage, but you also need the correct charging current (amprage) Without enough amprage, but enough voltage the circuit should still ground the alt light.

Alts don't all ways fail all at once. They can go out over time with varring degrees of performance. 14.6 volts with 3 amps will charge the battery..... eventualy. So a low current output from the alt can cause issues that will leave you scratching your head if you only check for the correct charging voltage.

We will see in the next day or so. I just got back from picking up a new alt. I just need to install now.

[390FE1972]

Thanks for keeping us informed of your progress, hopefully that replacement alternator does the trick!

[68fe360]

Replacing the alt fixed it, but not because it was bad. Confused???.

Turns out the wire going to the STA lug on the back of the alt broke off of the ring terminal, but was being held sort of in place. Mostly due to the rubber connector that contains the ring terminals for the Battery, Field, and Stator connections. Which caused as I was starting to suspect, the charge amprage was to low. Due to the slight connection creating a high resistence junction.

So hopefuly if others stumble accross this post they don't just assume the VRM (old or a replacement) is bad if they have the correct charge voltage, but the alt light is on. It's important to verrify that the charge current is correct as well. On the QA tag that came with the new alt showed a minimum charge current of 20 amps at a warm idle speed.

Disconnecting the VRM connector will verrify if there is a short to ground in the wiring between the light and VRM. But it will not indicate a high resistance junction between the VRM and alt, or a problem with the diode in the alt.

I only found the broken wire after removing the alt so i could remove the wires. I touched the wire and it slide out of the connector. I crimped & soldered (correctly) a new ring terminal on the wire. As well as freshening up the ground wire going from the alt to the engine. With new ring terminals, and added a few extra inches of wire. Again crimping & soldering it. Since the stock wire is just barely long enough to reach, and makes servicing the alt a pain. It was far easier to install the alt and connect the ground with the extra length.

-----A word on troubleshooting these types of issues.

A bad replacement VRM is possible, but often not likely. You can reduce this likely hood by inspecting it at the time of purchase. Does it appear to have scratches, smuges, etc. That can indicate it was installed then returned? If yes, point this out & ask for a different one.

Many people do not know that unless you disconnect the battery before disconnecting or reconnecting the VRM you can damage it. One wire has full battery power at all times. The spark created as it is unplugged and plugged in can damage the internal diode, and transistors. They then assume they recieved a deffective unit, and return it. Under paid store staff can at times roll thier eyes and put it back on the shelf assuming the customer doesn't know what they are doing/talking about.

Most VRM replacements are electronic replacemts for both mechanical and electronic versions. The switching devices are not ment to handle the high amp sparking of a sliding connector. Specialy if the VRM's body is not grounded.

If your VRM connector has visable corrosion on it's internal connectors, Advancded Auto and a few other parts stores have a replacement one. If you can't wait, use female spade connectors and a good quality marine grade heatshrink to cover the spade connectors. This reduce the chance of a short between connectors.

The VRM will most often fail in 2 ways. #1 will not excite the alt to charge. This can be checked by simply checking battery voltage right after starting the truck. Voltage shoud be between 14 and 14.9 volts. #2 it will cause an over charge. Check is preformed as before. Voltage 15 volts and higher is to high. Shutdown now to prevent damage to the battery.

Other failures that happen less often are. #1 the wire that grounds the alt light is internaly shorted to ground. Disconnect the battery, and remove the VRM connector. Then reconnect the battery. Turn the ignition switch to run, but don't start. If the alt light remains on the fault is a wire between the VRM and alt light shorted to ground. If it is off the fault is in the VRM, the wiring between the alt and VRM, or in one of the diodes of the alt. #2 rusty ground, or rusty VRM connectors. Bad grounds cause tons of odd problems. Rusty connectors create high resistence connections, and the VRM may not sense the state of charge, or output of the alt correctly.

A bad alt is pretty easy to spot. It either doesn't charge, or what happened to me in Rawlings, WY. Strong winds blew tons of road grit up into everything. Getting into the lose bearing, and siezing it up. The chunks and grit literal came pouring out, It failed while driving. I found out when I tried pass some, and the engine bogged down. The engine can crusie on the battery, but strugles to develop enough current to accelerate.

[390FE1972]

Thanks for your detailed explanation! Glad you got it fixed