alternator question

Hey guys,
I've been doing a lot of touble shooting on my truck recently to figure out why my batt wasn't charging properly. I finally hooked it up to a scope at work and found that with the batt connected i had ~.05VAC present. I then removed the NEG batt cable from the batt (thus running the truck off the alt only). I then found that the AC voltage had gone up to ~1VAC. The DC was always around 14.5VDC. This doesn't seem right to me, but wanted to check what you guys thought before I go to the trouble of changing out the alt.

If any one could give this a try and see what they get that would be awsome, the test can be done with a DMM.

Thanks Fletch

That's not enough AC to be a problem. If you have 14.5vdc and your batterys not holding a charge then you have a bad battery or something is drawing power with the key off. Your ECM and Radio memory should draw about 700ma with the key off.
With the Engine off turn your headlights and heater blower on for 5 min then turn everything off and read your battery voltage.
If you have less that 12vdc your battery is bad.

key off batt draw is 7mA [thats ECM, clock and radio]. I took the batt in for load testing and it put out 980A so they said it was fine. The guy at the shop told me that the dirt on top of my batt was drawing it down. Gave him the benifit of the doubt and cleaned it, and to my surprise [not] it made no difference.

Just turning the problem around in my head some. If the battery is a serviceable type, do a specific gravity test of the electrolyte every few hours. If you can look in the water fill hole, see if the plates are covered with a white buildup, lead oxide or calcium (which really slows down loading, but doesn't affect the high amp discharge properties as much). If you have a sealed battery, they don't like being completely drained and refilled much, they don't seem to want to hold a charge as well as they used to.
It's possible one of the diodes is leaking a little, but it doesn't sound serious. If the alternator has had a coolant bath lately, wash it out with a mild soap and hot water under moderate pressure. Coolant is a pretty darned good conductor. much better than water or water/dirt. Batteries will leak down if they are covered in dirt, the open type battery vents acid fumes and these crystallize on the battery, the dirt holds moisture. If you listen close you can sometimes hear the electrolyte/acid, water and dirt mix on a battery crackle as it discharges your battery.
If the alternator is putting out close to 14 volts, chances are the diode pack is good and the regulator is working. After it has been running for awhile, feel some of the connections, if there is resistance at some connection or even inside the insulation on the wire it will get warm.
The small wire at the rear of alternator is what tells the regulator (and activates it) when the key is on (if it has power with the key off, something is wrong, if a connector is corroded and it has low voltage, it can also cause problems). On the older Delco alternators it was possible to have the battery discharge back words through the alternator, I'm not sure about the CS model. Maybe unhook the positive lead and do a volt test from the positive pole of the battery to the alternator/battery feed cable with the key off. You might also try a resistance/continuity test here, ohms should be infinite/no continuity.

well I should have said in the begining that its an Optima Red top. It runs through a diode isolator, I have a Yellow top aswell. There is a 1 volt lose fron the isolator. The yellow was totaly done. Needed replacing. The red is 28 months old. It also never gets a full charge and if I put it on a charger it doesn't seem to hold it for long.

on a side note. I notice on the dah volt gage that it reads lower there then at the batt. On start up it reads a hair below 14, then as I drive it drops and goes to the line above the red section [11.5]

This whole thing is really anouying hasta

Fletchs90xj said:

well I should have said in the begining that its an Optima Red top. It runs through a diode isolator, I have a Yellow top aswell. There is a 1 volt lose fron the isolator. The yellow was totaly done. Needed replacing. The red is 28 months old. It also never gets a full charge and if I put it on a charger it doesn't seem to hold it for long.

on a side note. I notice on the dah volt gage that it reads lower there then at the batt. On start up it reads a hair below 14, then as I drive it drops and goes to the line above the red section [11.5]

This whole thing is really anouying hasta

Click to expand...

I have also had the same troubles with Optima, there just not worth the money unless you need to lay them down or something.

Optimas routinely pass the Bear ARBST tester at my local - even when they should fail. It's a quirk of either the tester or the battery - I haven't really decided which. Here's a better test:

Put the battery on charge - overnight "trickle" charging is best, but any low-rate (under 50A) will serve. Note the charge voltage once you connect the charger.

Before you disconnect the battery, note the charge voltage again. It should be higher (meaning the battery is charging, and is placing less of a load on the circuit.)

Once you've disconnected the battery, put it in the freezer - or pack it in ice (don't cover the top...) for about an hour.

Once you take it out - have it connected w/in five minutes of removing it from the cold. Have a helper crank the starter, with the ignition disconnected, while you note the batteyr voltage. It should not drop below 9.6VDC - if it does, you're in line for a replacement.

I find this a more "real-world" test, and more accurate than any electronic gizzie. Back home, there was twice every year when you KNEW your battery was wanting replacement or not - at first frost, and at first "dog day" of summer. This simulates first frost, and that's a harder test for a battery to pass, anyhow...

5-90

We just had an alt go out on one of our TJ's, same symptoms, startup it would be fine, hair under 14v, then after about 30 min it would drop down slowly and the 'check instruments' light would come on. Replaced the alternator and it's good so far. Oh, all our jeeps have red tops.

so all in all is 1VAC from my alt acceptable?

how do you take the alt out? im in the proccess right now and i already took the two bolts off then the other two that connect the electrical but im not sure if there is only two are there more?

only two, sometimes it gets pinched in the mounting bracket on the side of the timing cover

On my way home tonight I tried meetering the alt again and set the DVM on the min hold setting it showed 8.5VDC. Then I set it for Max hold and it showed 5.6VAC with the NEG cable removed from the batt.
I turned on the headlights [batt still disconected] and the truck died. reconected the batt and all was fine.

Also I've noticed a pulse in the voltage everynow and then when at idle.

ok how do you pull the alternator out of a 4.0? ive unbolted all the bolts and battery and everything. it is such a tight fit i dont even know how to get it out it wont fit anywhere in any direction. yes i have an a/c. how do i get it out?

Fletchs90xj said:

only two, sometimes it gets pinched in the mounting bracket on the side of the timing cover

On my way home tonight I tried meetering the alt again and set the DVM on the min hold setting it showed 8.5VDC. Then I set it for Max hold and it showed 5.6VAC with the NEG cable removed from the batt.
I turned on the headlights [batt still disconected] and the truck died. reconected the batt and all was fine.

Also I've noticed a pulse in the voltage everynow and then when at idle.

Click to expand...

Here is a site that explains the CS series alternator pretty good. http://www.alternatorparts.com/cs130_sbpage1.htm

It's possible to get a little AC to cross over onto the DC side of the diodes through dirt, moisture, coolant or what not. It's also possible a diode is failing.
On the older style Delco and Mopar alternators it was pretty easy to troubleshoot. Each diode accounted for 33% of the output voltage. Any 3-5 volt drop, was usually a diode.
All of the CS type alternators I've had fail, where the regulator. I've never had the diode pack fail on a CS, but have heard they don't follow the 33% rule, newer technology.
I've also had the slip rings covered in mud, oil or coolant and the alternator to charge low or not at all. After cleaning off the slip rings, everything worked just fine again.