As much as I agree that the alternator can be the culprit, there is a very easy way to check them sitting in your driveway with you're everyday volt meter. You simply switch the voltage to AC and touch red to the alternator output and black to ground or the alternator body. The voltage should not exceed .020 at idle and nothing over .1 at 2500 rpm with all electrical components turned on. If the numbers are higher than that then the diodes are shot. So replacing the alternator is OK but always check before throwing money at parts.

Dont check through the batteries either because they can filter some of the sine wave and give a lower reading than directly from the alternator.

Lastly, National Alternators are very high quality and are very reasonably priced too. You have to be careful looking for higher amp alternators because they sometimes come with smaller pulleys which means having to run a shorter belt.

As for batteries, yes, they play an important role in alternator AC as well. Because if the batteries are weak or have dead cells then the alternator is running overtime trying to charge something that may or may not ever charge which generates excessive AC voltage. The same thing applies to corrosion, loose connections, and bad cables.

I feel the best batteries are not so much the higher CCA but more the higher "reserve amperage" numbers. The higher the reserve the more capable the battery is at doing the job intended while maintaining its optimal charge and the less work the alternator has to do to keep that battery in tip top condition. But that said, any higher reserve amperage "starting" battery will also have higher cranking amps. When I was looking around at batteries I found the group 65 models were the best for what I wanted. Over an beyond that, an AMG battery will be outstanding but they're expensive.

 

About the cables- yes there are different qualities, and they do have a lot of influence on how your truck performs.
Something that is often overlooked- battery connections. They need to be clean and tight, because the batteries are the main "noise filter" in the system.
I did not find too many bad cables in my work, but lots of bad and dirty connections.

 

It's fairly simple to eliminate the alternator as a suspect without replacing it. I just disconnect the field connector from the alternator and take the vehicle for a short drive, just long enough to determine whether the condition still exists. You'll be driving on battery power alone, so don't drive like that all day, but you'll have ample time to see if it's made a difference.

If you replace your alternator with the Ebay one, don't throw away the original. It's quite possible the new one is a reproduction that doesn't have near the quality of the one that your truck was built with. If you get tired of replacing the new one, you may be better off to throw a set of brushes and a rectifier into the old one so it will go for another 120K miles.

There was another repair procedure for your condition that I used a few times, and it corrected the problem every time. It involved overlaying the signal wire from the APPS to the PCM. I'll try to find you the link, if it's still around. Like I said, I had good success with it.

 

The concept of disconnecting the alternator works but just be careful as if the battery conditions are not good or have bad cells then you could experience other phantom issues since sensitive electronics run on very specific voltage parameters.

 

wow--just came back to check posts... didn't expect to receive this much education... mucho thank you's!

KATOOM: Will attempt the alternator check you described ((I hate electricity...I tend to burns my eyebrows off...and that burnt hair smell stays in nostrils for days...)) grats!

d

 

ok, Assuming I used the volt-meter properly, and assuming I touched the probes to the correct places (RED to bolt on back of alternator with a few cables attached and black to the alternator body/or another ground); while running @ idle, the reading bounces anywhere from 39 up to 50-some. (my cheap yellow voltmeter from HarborFreight doesn't have a setting with "." (point).

I didn't attempt the method of disconnecting alternator and going for a drive....getting dark and I don't want to chance stranded somewhere.

CONCLUSION: I want to believe the new alternator will fix the issue... d

 

You'd have to post a picture of what you're HF volt meter looks like to know what numbers you were seeing.

 

Also worth mentioning is to clean your ground connections. My torque convertor did this too about 8 years ago. Cleaning the grounds involved a wire brush and some sandpaper, and it fixed my issues.

There are 6 key points to clean. Both negative battery terminals. The grounding connections going to the frame/body from the battery terminals. And the last two are the PCM ground on the passenger side firewall. Make the terminal ends and contact points shiny bare metal.

That said, age and environmental conditions don't do our electronic stuff any favors on these trucks.

 

Thanks Coastie! Was wondering about all the grounding points. Gratz!
New alternator arrives tomorrow.

 

My 98.5 24v was doing this for about 1 month. as soon as i'd hit 45mph it would fluctuate roughyl 1-200 RPMs. i cleaned all my terminals and checked the wires. My issue has gone away. I did plug in a snap on scanner when it was occurring and it said it was the TCC.

 

Ground loops and corrosion can cause some funky things with electronics.