Battery charging

[chopthebass]

I started taking my car to inspection but I noticed the volts were dropping until eventually the car died. When I used booster cables on another batter it read 13.5v and the car started. So its clearly not charging, and running off the battery.

I took alternator out and got it tested. All is good. I checked for bad connections and all is good. Put it all back in the car and no change. HOWEVER... 15 minutes later I start again and its reading 13.5v on the gauge, so for that one moment it seemed to be doing what its supposed to. Tried the next day, and back to just under 12v and not charging.

I'm out of ideas now. Could the alternator have an intermittent problem? Seems odd that when the store tested it (twice) it said it was ok. It has 3G connector. Brown FFR wire goes to light green on right of D plug. White middle connector goes to stator plug, left yellow wire goes to alternator post. The post then connects to battery via a breaker.

Any suggestions would be most appreciated.

[Sawbulba]

Seems silly but make sure there is enough tension on the belt and the alternator pulley is not slipping. Quick visual inspection with the hood up should cross this one off the list.

[Gromit]

x2 on Belt slippage it takes a bit of torque to spin an alternator that is charging a battery especially one that is drained. Also I would be looking for a poor ground. alternator to block, block to frame, frame to battery.

Chris AKA Gromit

[phileas_fogg]

I seem to recall that some models of alternators don't charge below a certain RPM. So if your alternator doesn't charge until 1200 RPM and your idle is 900...

Hopefully someone who knows more can narrow down the models with this "feature".


John

[chopthebass]

Fan belt tension is good. I will check grounds though.
RPM doesn't change anything. I was trying to drive to my inspection, so revs were up. Its weird how it worked once last night when the gauge read 13.5 volts.

[edwardb]

13.5 volts is a little low for a 3G, especially if the battery is low and it's charging. You should be seeing 14's. Something isn't quite right. RPM shouldn't matter. 3G's work well at low/idle RPM's. Low idle/low charge is mainly a GM 1-wire thing, and even then it depends on the alternator, regulator, etc.

Has it worked differently before? What about your battery? You say the alternator tested good. What about the battery? Maybe also wouldn't hurt to check the voltage with a VOM, just to make sure the gauge is indicating correctly.

[chopthebass]

Ed,

The battery is one of those fancy yellow ones (can't remember the make). I tested across the terminals and it matches the gauge. I know that with the engine running if I measure across the battery it will show what the alternator is pumping out. And I get no change.

[chopthebass]

Ed I looked at the photo I snapped and it was real close to 14v, not 13.5v

[Railroad]

I had an alternator in a 85 Nissan pickup, that would charge sometimes. It made sense when I looked at the brushes. They were so used up the spring could not keep them against the armature, if that is the correct term. New brushes, OK.

[awd-turbo]

Sorry this might not pertain to you, since I am really new to wiring these things. In the Rf harness aren't you supposed to have a resister for charging?

[CraigS]

Is there an inline fusible link in the wire coming off the alternator? I had one of those go intermittent on me 5-6 yrs ago.

[Just puttering]

A few thoughts,
Link below is what I assume your wiring is.
1 check voltages at the alternator and battery when its charging and not charging make sure the voltage is the same at both. If there is a difference in voltage you have a wire problem. When not charging and both the same see 3 below.

2 you have a circuit breaker on the wire to the battery, it could be bad but not tripping the circuit from an overload, when its not charging check the voltage across the breaker it should be 0 volts.

3 when not charging check the brown/green wire for 12 volts - this one turns on the alternator. You could have an intermittent "open" on this wire that is not turning on the alternator.



http://www.allfordmustangs.com/forums/attachments/classic-tech/21965d1166149617-late-model-alternator-early-harness-3ginstall.jpg

[chopthebass]

Thanks JP. I will do those checks

[R Thomas]

As Craig mentioned above the R F wiring instructions call for pulling a resistor from the gauge panel and jumping some leads. Says it won't charge with out it.

[chopthebass]

Hey JP, If I have an intermittent "open" I could try running a new wire. Where does the other end of the brown wire terminate?

[Big Blocker]

Circuit breaker on an alternator ? ? ? any chance it's tripping and going open until it resets or needs to be reset? What is it rated at compared to what the alternator is capable of? Most alternators need some kind of "exciter" wire or circuit to function - older models used a "stator" wire to accomplish this function. Not to sure what the newer ones use to get the field windings HOT so they will produce power. That resistor that everyone is mentioning is a possible fix, no resistor (exciter circuit), no charging.

Doc

[Frank Messina]

It sounds like you have missed something in the wiring. Do you have a volt meter or a dash lamp? Did you add a 500 ohm resistor in parallel to either one of those? If you look on the other forum there are any number of threads with diagrams showing how to properly wire a 3G. A 3G should show 14.5 volts on startup and idle afterwards. With all the lights and flashers on you should see 13.8 volts. Is your alternator new or used? If it's used replace the voltage regulator as well as the brushes if they are worn, its a 10 minute job.
Frank

[chopthebass]

Frank its a new alternator. I have the volt gauge. No resistors as my engine guy said its only needed with the donor gauges. One time only the gauge read nearly 14 v on idle. So I assume that it was charging then. All other times the voltage is identical to taking a reading of the battery. I'm pretty confident its wired correctly. I am thinking its an intermittent alternator problem, bad ground, or bad connection. I am going to pay my engine guy to diagnose it!