Amongst other things, my car is now having trouble keeping a charge. The battery is brand new. The stock volt meter reads about 12v at all times when the battery is charged. I know when the alternator is good and working it should read around 13.5 or so. When I checked the alt at Discount (Advance, whatever) it read at about 13v, which isn't the best, but it means that somehow the alt isn't sending any power to the battery. I have taken resistance readings from the ground post to several ground sources on the chassis including the engine and all read below 2.5 ohms, the closer it was the lower the resistance. From the alt power wire to the battery positive post was 0.5 ohms. That tells me that the problem is probably in whatever activates the alternator. Anyone have any ideas on what I should test next?

 

I would say your alt. is weak ..you might want to start with a new alt. and go from there if its still weak then I would say you have some wiring problems.I would check the junction block which is connected in series or inline with the large red wire at the back of the alt.Mine is near the radiator side tank on the pass. side.Make sure all the fuses are good and fusable links there are some at the junction block and down at the starter and all else fails get a wiring schematic and follow each circuit.[

 

checking the ohms isn't a very good test. the circuit isn't loaded like that. a voltage drop will tell you more.

first to start off the battery needs to be fully charged.

with the motor running & head lights turned on bright, check the voltage output of the alternator from the output terminal to the alternator case. it needs to be 13.2 minimum which is a border line alternator.

next with the positive test lead on the positive battery terminal put the negative test lead on the alternator output terminal & write down the reading.
now put the negative test lead on the negative battery terminal & the positive test lead on the alternator case & write the reading down. next move the positive test lead to the motor & write that reading down.

now move the positive test lead to the fender or other metal body part & write that reading down.
lastly, move the positive test lead to the motor & the negative test lead to the body & write that reading down. if you get a half volt total drop, something is wrong. clean all the connections that showed more than 0.050 of voltage.

 

The alternator wasn't putting out any current on the car, but worked fine in Advance Auto. This made me think that the problem lay in the circuit that drives the alternator. Following the wiring diagram I determined that power to the alternator driving circuit is routed through the fan fuse... why? I have no clue. Anyway the fuse was blown causing my alternator not to be kicked on. Fuse replaced, problem solved. Runs a little over 14v now.

 

Ok, i just did these test to see if they worked before i put my car into the conditions ( also was still in open loop so was idling high and also in park ) where it only has like 9 or 10 volts ( everythign goes dim and checked with volts meter ). Here is the conditions i'm having. I go into town at night and say i have my headlights on and my blower fan on inside the car and i come to a stop. My volts drop almost to red and read like 9 on my volts meter. That is WITHOUT anything else on. ex. Stereo, fog lights, my fans, turnsigles dome lights. I mean without ANYTHING! To drop that low is no correct and i know it. I can litterly make my car die if i have these ( * ) on and i've been cruisin town for a while. * Headlights, foglamps, stereo ( not even bumpin extremely hard PLUS a power cap ) blower motor, fans. It will go almost to 8 and i say turn on my hazard lights and kick on all the lights inside my car just dies cause lack of voltage i guess. Its funny but not ya know. Once i get it the rpm ups my voltage goes up but only to like 12.5 to 13 cause after having to sit at the stop lights its drawnin more juice off my battery than what my alt can put back into it. ( on the side note. Just the headlights on and nothing else it will still drop way down after awhile, just takes longer cause my batter is at full juice ya know ) Once i get on the interestate to go home BAM! around 14 where it should be cause i'm running 2 grand plus for a period of time. I can run around town holding a high rpm but never have a long enough straight away to get up that far. ( big cruise strip. Probley a mile plus on a section before i have to stop again! and that should be plenty of enough time) Every place says i have a good alt and its a BRAND NEW battery. ( Looks like a bran new alt too!!! was there when i bought the car in feb and the guy said he just put it on so i'm assuming he "tried" fixing this problem or some shiznet) The alt says its reman. and its a az duralast.... OH btw it idles at 600 and has brand new belt also.... Also when i did these test i noticed this noise. With the key on and nothing else on my alt makes a very high pitched quiet whine.. Almost like the key is in the ignition but 100X quieter. Had to put my ear about 4 inches from alt to realize that is where its coming from.... Wtf is that about!

Ok now u know my problems and this is why i tried these test....

Ok what I got the steps in bold and down below is what i expeerenced... maybe its just me... i donno never hurd of these test but i tried them and here are my results number to whats in bult in the quote....

1. Mine was reading like 14 something ( Cold start not in gear )
2. I did it even though it don't sound right. Wires got hot on my meter ( lol knew it would ) and it read nothing......
3. got no reading......
4. still no reading......
5. hmmmmmmmmmmmmmm stilllll no reading......
6. ummm nope gettin 0.00 still

most of these test don't make sense to me. I had to re-read them about 10 times cause i was like wtf and he expects to get a reading like this.... But tried it. So ummm wtf did i do wrong or is the test wrong or what......

Thanks Derrick

 

Before we go any farther, I see you have a 1990 model year car. I want you to check your FAN fuse. I know it sounds crazy, but take a look and tell us if it's good. It should be a 20 or 25 amp if memory serves, and I think it's in one of the corners of the fuse panel. The alternator will not energize if that fuse is blown, at least on 1990-92 model year Fbodies.

 

Ya my fan fuse is good. I've whipped out the chiltons book and traced every wire that would have something to do with the alt to make sure they are all good.... It charges. But ****ty at low rpms. For instants when i cruise town with or without accessories.... U have anyidea what that buzzin/hummin noise would be. Its really faint but i've heard an alt do that.... And if i unplug the plug that goes into the top of the alt ( with the 2 wires ) it quits humming.

Thanks
Derrick

 

I'm not sure where to go from this point. If that fuse is good, You have to have a break in the line somewhere, yet the only time I can recall the "humming" you described was when my fan fuse was blown. Strange indeed.

 

moved reply to other post

 

Another

Once had similar charging problems -- battery wouldn't stay charged, and alternator tested okay. Finally traced problem to corrosion. Not only on the main batt terminal, but the junction block where the alt. power connects in,
which is located in close proximity to the battery. It wasn't so bad as to be obvious at a glance, but apparently bad enough to DEGRADE the charging circuit. (With NO alt. charge at all, the battery will deplete pretty quickly.)

Good luck.