I have an 88 F150 4X4 with a 302. We have changed the alternator now three times, and it is not charging the battery at all. We do not use this truck very often, so we don't know if any of the alternators actually worked or for how long. We bought the alternators new from Autozone (actually bought one new, exchanged the other two.) It was like pulling teeth to get them exchanged because according to autozone's tester, they all passed and worked fine. We have replaced the starter silenoid and rewired the harness, checked the connectivity on the harness and everything. It all works fine, but the voltage only reads what the battery is putting out. We have no idea what the heck could be wrong with it since everything except the alternator seems to work. Can we have gotten three crappy new alternators from autozone?



Anna

 

I'd be really surprised if the alternators are all three bad. It could be that the original one isn't bad either. Have you had the battery checked? How do you know the battery isn't charging? Is it simply that the truck doesn't start? My guess is a bad ground connection somewhere, but need more info.--DIY

 

bad ground is what I would say too.
I have a 92 with an inline 6 4.9l and that thing has had 5 alternators since we have owned it, and we didn't buy it new. The last alternator only lasted a year. We (my dad and I) did alittle digging and took some good advice and replaced the wire that grounds the terminal to the fender. It's a bit early to tell for sure but this alternator is already acting stronger than the last one did after this much time (only a week!). Sad to say but last time the old one did show lack in performace in such a short ammount of time.
lemme know how it turns out and get the warranty with your next alternator.

 

I trust that you are using a volt-meter to check that the voltage is not rising above battery voltage?

Is the voltage regulator separate from the alternator in your truck? You might check that wiring if it is. A bad regulator will never sense low voltage and never signal the alt. to supply more power.

Remove, clean and re-tighten the battery cable to ground ( usually on the block ) no matter what it looks like if you haven't done it in the last year.


I once replaced a battery, then a voltage regulator, and finally an alternator on a car I had that was not charging. Turns out it was just a loose belt on the alternator. The belt was just tight enough that the alternator charged the battery while the car was at idle so everything appeared to be ok. When the revs picked up, the belt slipped enough that the alt. quit charging. I didn't check the belt until I went to replace the alternator with a new one. Of course, that car did not have a sepentine belt with a spring loaded tensioner.

Good Luck

 

Sorry about that, the battery is brand new and we are using voltmeter to test it. The serpentine belt is also pretty new and seems tight enough. I will have my husband check the grounds and see if that is a possibility. I think the regulator is built into the alternator, if not I have NO CLUE where it is

Anna

 

Yea, check all the wiring, especially the ground cable at the block. Also look at all wires between the alternator/generater and the battery. Make sure the wires themselves are non-corroded and non-brittle. The worst corrosion usually happens right next to the wire lugs but can sometimes be under the insulation.

If you haven't figured out yet if your voltage regulator is seperate - call the local parts store and see if the sell it. They used to run about 15 bucks.

 

Here's a quick trick I've used in the past, it saved me changing alternators at least twice. Get a short chunk of 2x4 wood. Next, get a pretty big hammer (or axe), no this is not a joke. Rest the block against the alternator housing NEAR THE MOUNTING BOLTS where the metal is stronger. With the engine running idle and the voltmeter on the battery terminals, give 'er a good crack of the hammer. Check the voltmeter for an increase in voltage. If that fixed it, youpie !
Try this for fun. Start & let idle, unhook the negative battery cable. If it stalls, it is officially not charging. Using an ohmmeter, check each wire that connects the alternator to the battery terminals/fusible links, as much as you are able to trace them, looking for a bad wire.
Also, I've seen terminals get weak and not hold onto their matching male end on a plug or whatever it goes on. They get spread open and don't hold a good contact, so you think it's connected but the terminals don't actually touch. Replace any suspect terminals or squeeze them back together when you can.

 

WHOA.
Don"t EVER take the battery off a running vehicle that is equipped with a computer unless you absolutly KNOW that it will not damage that computer.

Lots of systems use the battery to smooth voltage spikes off of the alternator/generator. Removing a battery off of a running engine can and does fry some system computers.

A new alternator probably doesn't need a crack with hammer either. Trust me, if its the third one you have tried - that ain't the problem

 

There are two alternators for this truck one has an external fan the other is internal. They both have the regulator built int them. You gotten some good advice here on checking this-Accept for disconnecting while running]. Another check is to connect a test light to the disconnected neg post and the cable clamp[key off position]. If the light does not come on its no drain from a short if it does the short must be repaired before moving on. Haynes covers all this well and I always suggest if you don't have one its a good investment of around 20 bucks. Good luck.