i have an 87 T/A with a 305 TPI, with auto.
tranny.
i've been having a charging system problem for quite some time now, and i've done all i could to fix the problem, and still i'm having the problem.
here's the problem:
when the car is warmed up, i'm in drive while holding the brake at around 500rpms., having the headlights and the fog lights on, and few other accessories, the voltmeter drops below 13 volts. Now there have been a few modifications that has been done to the car. Like since i've taken the A/C system out, i've spliced the wire from the a/c fan to the primary fan, so that way i got true duel fans running simutaniously, and also i've taken out the rear spoiler and the 3rd brake light has been disconnected.
Now when at a stop, the lights will turn dim.
Before i did any testing, i checked and cleaned any and all grounds and terminals, from the battery, to the starter, to the fog light connections, the rear window defogger grounds, the body ground, engine ground, blower motor connections, etc. and all is good. after that was done, and done a charging system check, i figure that i would use a underdrive pulley for my alternator, that way the charging system rate would keep up while at a stop.
<note: i've installed a new alternator about 5 months ago, and also replaced battery>
When i was using the ol VAT40 to check the charging system, everything looked good when the car is at park, idling around 800rpms,
and all accessories on, which displayed about 12.6 volts to about 13., can't remember how many amps. though.
after putting it in drive, and while holding the brake, the volts started to drop rapidly to under 11.5!!!,so i'm thinking maybe the battery cables are corroded. Well took my dvom, and tested the battery cables, shows that i've only have about a .8 volt drop as the total of the two cables,which isn't that bad at all... stumpped as i was again.
i started testing each accessory indivually... and i'll keep ya posted if i find anything on that.
in the mean time, anyone with any suggestions or help on what i should do?

 

With an engine speed of 800rpm, I would want to see 13.8 - 14.9 volts (dependent upon outside temperature since the alternators' output is temperature compensated). For voltages less than that, I would consider that the alternator sees a large load, and therefore cannot provide enough current to maintain 13.8 - 14.5 volts. That could be due to a large load (battery has a very low charge, or any other load on the system, etc.) or a defective charging system.

When first started, the alternator must also recharge the battery. Once charged, it can provide more current to other accessories. Therefore, what voltage do you see after the car has had a chance to charge the battery? (Depending upon the condition and state of charge of your battery, that could be from never [battery is bad because it cannot receive a charge and appears as a constant load to the alternator] to a few minutes, depending upon many factors - temperature, alternator speed, state of charge, accessories in use, etc.)

My first suspicion would be a slipping alternator belt. When the alternator is under load, the belt may be slipping, causing insufficient current (voltage).

Check the easiest things first...


You stated that you put an underdrive pully on the alternator. To clarify, is the pully bigger or smaller than what you had? If bigger, then the alternator will be turning slower, which would lower the amount of current it can produce at any one rpm.

Be advised that when you put the vehicle into gear on an automatic, your rpm's will drop. Since you stated that you only saw about 12.6 - 13.0 volts, which I consider to be very low for a running alternator, putting it into drive will reduce that even more. Once you see less than about 12.6 volts, you can safely assume that the battery is providing some or all current. Above 12.6, you can assume that the alternator is providing all the current needs.

 

Try disconnecting the secondary fan, so that it does not run all of the time - it is not necessary to have the secondary fan on all of the time - in fact, if you have removed the A/C system, it is probably not necessary. A fan motor draws around 8 to 9 amperes, so that's quite an additional load on the alternator. If you did install an underdrive (larger diameter than stock) alternator pulley, you need to remove it, as Stuart has indicated. If anything, you should install an overdrive (smaller diameter than stock) pulley on the alternator. March makes overdrive alternator pulleys.

Tim

 

okay let me make myself cleared on the underdrive pulley it's a smaller pulley. So it should help with the charging status.

now when i did these tests i did in room temperature, like around 65 degrees. <our shop temp.>
and also when the engine was warmed up and the car was in park @ 800 rpms i had all accessories on and vat40 displayed 14 volts @ 20 amps,
also the cooling fans weren't running at this time. If it was then the amps would increase to 40 amps.
Now the tension on the alternator belt is good, no problems there.
another thing i wanted to make myself clear was that the a/c fan only comes on when the primary fan comes on as well. so when the primary fan goes off, the second fan goes off as well.
Now my suspicion,is that it sounds like the alternator isn't doing it's job,
i've already tested the battery seperatly and i've got about 12 volts @ 15 sec. when doing the load test. So the bat. is good.
But when i did the alternator load test,
i've got around 80 amps at 12 volts?!
so it sounds like the alternator is doing it's job as well?!
once again i did another test on the car,
what i did was:
i let the engine warm up, the car was put in drive, rpms was at 500, while i'm holding the brake, having all accesories on...
readings: 12 volts, at -10 amps??? is that possible because that what i got from the vat40, and it's working properly.
so i really don't know what's going on here.