bad alt or bad VR?

[discharger1332]

Yesterday while driving my 73 dart(/6car) i noticed the alt guage started discharging. when i got home i ran a test with multi meter, same results with car running and not 12v so i threw a spare alt in. when i was running it after the guage was running high, and would peak when i revved it.
my question is...is this normal because the new alt is charging the low battery after driving with a dead alt?
also what would i see on the guage if the VR were bad?

 

[circlepilot]

Prior to installing the "different" alternator, it would of been a good idea to of checked the "static voltage" (nothing turned on) of the battery. If it was below 12V, a little charging would of also been a good idea, however it must of had enough power to start the car. Yes! The gage would show a high rate of charge (due to the low voltage in the battery) and diminish as the battery comes up to a full charge, if you didn't have any lights or accessories turned on. Check the voltage at the battery with the engine running and nothing on. I believe you should read between 12.9V to 13.9V. (someone will correct me if wrong) If the high charge rate doesn't come down to normal, there maybe a problem with the system's regulator. Over charging can cause a battery to explode.

 

[discharger1332]

it was 12v with no key in ignition. it was 12v running. now it is 14v running and stays there when i rev it. but guage is still reading high and peaks when i rev it. when idleing guage reads half way on the high side and gets close to the 40 mark when i rev it. also alt is getting hot, but wires are not hot.

 

[67Dart273]

Since you disconnected the VR and it stopped charging, this indicates the VR circuit is not faulty. Next is to determine "is it a bad VR?" or "is it voltage drop?"

To find out.......First go to MyMopar and look around, download a service manual and the aftermarket wiring diagram, both free

MyMopar - Mopar Forums & Information - MyMopar Tools/Reference

Next Understand that the "ignition run" voltage coming from the switch (as well as the ground circuit) are what might be the culprits here

The circuit "in general" is battery..........starter relay.......fuse link.........through the bulkhead connector.........to ammeter circuit.........to welded splice in the under-dash harness..........to ignition switch connector.........through the connector........through the switch.........back OUT the switch connector on the IGN1 "run" wire (usually blue).........back out through the bulkhead connector and into the engine bay

Out in the bay, that wire branches off and feeds several circuits, by the way NONE of which are fused. "Generally" feeds

ignition system
Voltage regulator
Alternator field (light blue)
electric choke if equipped
some smog du dads on some cars

NOW find one of these the best is the "high" (key) side of the ignition ballast. Turn the key to "run" engine off. Probe your meter on the ballast and find the HIGHEST reading

Leave that probe on that terminal, and move your GROUND test probe to the battery POSITIVE.

NOW you will read a very LOW reading, the lower the better. Zero would be perfect. More than .3 tenths (three tenths of one volt) needs investigation. This higher voltage (mine was 1.5) ADDS VOLTAGE to the VR setting. That is, in my case, the harness drop was 1.5, and assuming the VR was properly set at about 14, this means the battery WAS CHARGING at 15.5 volts!!!

If you do NOT read more than .3 then the harness/ switch/ connections is OK

=====================

Remove the VR, scrape around the bolt holes on the rear of the VR, scrape around the firewall, and remount tight, using star lock washers, and retest for charging voltage.

If the above does not change anything, try a new VR

 

[discharger1332]

update, this morning a threw a spare battery on the charger... once it was charged up I put it in the car and bingo everything is normal. I put the cars battery on the charger and I am surprised it started the car, it was dead.

Thank you for your reply's and help. I will download the manuals and start getting myself familliar with this stuff.

 

[67Dart273]

^^I almost mentioned this, or maybe I did LOL^^ Sounds like you are making progress. Once you get the thing "normalized" again, do yourself a big favor and with the battery charged and warm and running, check charging voltage. If it's much above 14, do the test I outlined

 

[discharger1332]

oh sorry i forgot to say i did that while revving and it stayed at 14. but it stayed at 14 with the dead battery so i thought i was safe but just threw the spare battery in to see and there it was. I am going to go through and clean all the grounds today and do some basic maint.