FTE folks, I have been researching and reading as much as I can about this test.

I have pinpointed my charging problem to the alternator, but I need to be sure I'm doing the Full Field test correctly.

I first checked the charge of the battery.
Engine off - 12.6 volts.
Engine running - 14.6
Engine @ operating temp - 14.4
Engine @ operating temp, (lights, blower on High) 13.8
Engine @ operating temp, (lights, blower on High, transmission in drive 13.2 and dips into the 12.9 range.
Engine @ operating temp (NO Load) IAC valve disconnected 12.8 and finally drops below 12.5.

Basically if the rpm drops to 700 or less, the alternator doesn't charge.
The belt is new and plenty tight.

I tried to perform the Full Field test by grounding the "F" terminal to the case, the bracket, the ground on the plenum and finally the battery ground, but never got it to "perform" max output.

Does this mean the alternator or regulator , or both bad?

 

I would suspect the alternator itself is weak if the full field test didn't work out.

 

Thanks Lead Head.

I swapped it out this morning with a another alternator. It's the one that came with the truck when I bought it.

I was able to perform the Full Field test, but it performs the test if the voltage is above 14 Volts. If the voltage is around 13.5-13.3, it won't perform the test.
As soon as voltage reaches 14 volts, it shoots up to 15.10 volts.

Does this point to a bad voltage regulator?

Thanks again!

 

Does the "A" terminal have full battery voltage during these tests?

 

I didn't have a chance to test the "A" terminal today for 12 volts, but will in the morning if it isnt raining.

I'm assuming it does since the volts only drop out once it's below 700rpm.

Both alternator's are Reman's, so I'm betting there are just weak.

I have been looking for a 130amp alternator, but $100 is a lot to me right now.

I'll post my findings in the morning after work.

Thank again for the help!

 

For older trucks it would be wise to perform a voltage drop test on both positive and negative sides of the charging circuit. High resistance is a common issue - even just a few hundredths of an ohm will cripple any alternator by about 30%

 

If I'm understanding what your trying to do is turn everything electrical on and expect the alternator to put out full current at idle? That's like asking an engine to do full hp at idle, you need to get the rpms up to 1500 then do your field test and watch it jump to 18v or so.

If the alternator is supplying full power that it can at idle and you field test it (ask it to do full power) you won't see any difference as it's already doing what your asking it to do.

What you posted for voltages is completely normal to me, if you want better go to 3g or change the pulley to a smaller one so it will spin faster and have better output under 700rpm.

 

Tedster9 & ifrythings,

I completely understand your points for the charging system.

Cables are clean and new cables have been ran.

Both of my other trucks are older and I have converted both to 3G 130 amp alternator's.
The truck I am working on is my '95, 302, Eddie Bauer. It came stock with a 95 amp alternator.

Both of my other trucks have same pulley sizes. Both still put out 13.5 with Load and in drive or reverse and they don't drop in voltage steadily, even if the rpm's drop.

So, it could reason to stand that both the alternator's I have tested are weak. Both are Reman's with unknown run time.
I have also lessened the electrical load on the system by swapping a lot of the incandescent bulbs to LED's.

Thank you for the help and keep the ideas coming.

 

Good point about idle charge voltage, are you measuring with the alternator loaded up with all the lights and accessories on at around 2000 or 2500 RPM? Voltage will sag a little bit at idle. I'd still want to measure the voltage drop even with new cables, if only to see the result of my handiwork and verify. The alternator itself needs a clean connection with the block, modern vehicles use a lot of dissimilar metals (Aluminum/Iron) that tends to corrode over time, especially in northern states where lots of salt is used in the wintertime.

 

Ok, so I tested the "A" terminal and it does have 12.5 volts, so does the "F" terminal.

I also tried another test.

With the truck running, not up to operating temp, if I disconnect the IAC valve and keep the rpm's at 500-600, the alternator still keeps the voltage at 14.5
This leads me to believe the alternator is weak, as it's a cheapie Reman. It's only when its up to temp does it not want to keep the charge when warm.

I also made sure the mounting points of the alternator are clean, and used aluminum anti-Sieze on the the threads of the bolts.

Edit:
I checked the "A" terminal, engine running, operating temp. I get 14.62 volts.
"F" terminal, I get 10.00-10.20 volts engine running, operating temp.

 

If it otherwise charges fine, I'd just leave it until you have enough cash to replace the entire thing. Especially since you seem to have a spare working alternator as well.

 

I had another thought...

I have another 130 amp alternator, but the brushes destroyed the brass slip rings.

I'm thinking of soldering out the rectifier and swapping it out with the 95 amp alternator.

Id like to hear any ideas on that.

 

I just don't really think it's worth the effort. Trying calling around your local alternator/starter rebuild shop. They may be able to significantly undercut the $100 cost of a new alternator.