It would appear that my alternator is not charging the battery while the car is running. However, I have taken the alternator out and taken it down to a shop to have it tested and the shop tells me the alternator is good. The alternator I'm running is one I got from summit a few years ago, its a 120 Amp alternator.

At any rate, I checked the connection from the battery to the switch (in run position) and back to the brown wire connection at the alternator. The resistance varies constantly. I don't know what it should be as the diagram in my Helm service manual only shows a resistive symbol in the circuit. Does anyone know what the resistance should be? Or can anyone here with a meter try doing this test in their car to see what they get?


Also, I then measured it with the battery connected and the key in the run position--got 4.5 VDC. The service manual does not tell me what the voltage should be, does anyone know? Or can anyone here try this test as well with a meter to see what kind of output they get?

Any help/info/suggestions are appreciated.

 

When is the last time you charged your battery? Did you have your battery tested? I'd suggest having the battery tested. The alternator isn't a battery charger, its a battery maintainer. If the battery has problems (dead cell, etc) or has a really weak charge, the alternator won't be too happy, and it could actually be damaged.

What do you have running when it doesn't seem to be charging like it should? Aging blower motors, electric rad fans, etc. suck alot more amps than now than they did when they were new.

 

You need to do voltage checks with it running on the car.
(good point, Adam. I almost blew up a battery with a shorted cell by forgetting about that)
Here is a troubleshooting procedure for alternators with a 3 wire plug.
Engine off, key on. The red and brown wire should light a test light.
Red wire: hot always, brown hot in run. If these check out, check the voltage on the output terminal of the alternator at idle. If it is above 13.5 check voltage drop between alternator terminal and battery positive terminal. Do this by positive meter lead on alt terminal and negative meter lead on battery positive post. If voltage drop above 1 volt check wiring circuit from alt to bat and that battery thing Adam mentioned.
If the voltage is below 13.5 the alternator may not be being "turned on" by the excitation wire (brown with white). The circuit is from ignition circuit thru a resistor in the dash cluster. The resistor value is different with different year models. It is easy to test with a test light. With the key on, engine not running and the plug unplugged from the alternator, the wire should have ignition voltage on it (about 11.5v). Use the test light and check the resistor circuit. The wire should make the test light glow faintly. If the excitation wire does not check out don't panic and order a new dash. You can add an external resistor to replace it's function.
If the 3 wires check out and you don't have 13.5v on the alternator output, then the alternator is bad.
If one of the 3 wires don't check out here is the schematic for an '88 to trace the problem down.

 

Thanks guys, I appreciate the information. I replaced the battery not too long ago and when I put a battery charger on it, the charger gauge goes back down to about 0 (between the 0 and 1 on the gauge anyway). Also, I keep a trickle charger on the battery when I leave the car in storage.

I forgot to mention that my car is a 1983 Trans Am with the 305 Cross-Fire Injection engine. Thanks again for the information, it gives me a good place to start checking for problems. I'll check it out and replay here when I find out more!

 

I'd suggest letting it trickle charge for a couple days, then bring it to a GOOD repair shop (not Canadian Tire, Auto Zone, etc) and have it load tested a few times. One load test won't tell you much unless the battery is REALLY bad. You need to have it load tested at least 2 or 3 times to see what state the battery is at. It shouldn't cost more than about $10-15 I would think. My shop doesn't charge any more than that.

 

The alternator has been checked to be okay, and the battery checks out good.

Can anyone here try putting a digital meter on the brown/white wire and the red wire going the alternator (being connected and disconnected) with their key in the run position (the car doesn't need to be running)?? Also, it would be helpful if you could unplug the alternator wire and measure the resistance between the brown/white wire and the positive terminal of the battery! The manual shows a 10 Ohm resistance in the brown/white wire, is this a physical resistance or is it in the wire itself, or some other item (if known, where would the resistor be found)?? This would sure be helpful!!

 

The brown wire should have close to zero volts on it with the key on and engine not running. Should have about a volt higher than the big red terminal with the car running and everything working right.

The resistance is part a wire, and part the warning light.

The alt WILL NOT WORK if the bulb is bad.

Does the "Alt" light (might be labelled "Choke") come on when you turn the key on with the engine not running? If not, replace the bulb. That'll probably fix it.

Don't argue, replace the bulb.

 

In my gauge, the light is labelled "Choke" and it did not come when I put the key in the "Run" position.

I have verified all my fuses are good and I replaced the light bulb that lights up "Choke" on the gauge. However, the light still does NOT light up when I put the key in the "Run" position.

The wiring diagram in my Helm manual refers to something called the "Choke Heater" (see pictures attached). However, I do not know where this is located. My car has the "S" in the VIN number and I am working with a 305 V8 Cross-Fire Injection engine.

Any help is appreciated!!

 

Did you do this simple test shown above?
Does this car have a "volts" gauge in the dash cluster?

 

My car does have a "volts" gauge in the dash cluster. Does anyone know where this "Choke Heater" may be?

 

the choke heater is on the side of the carburetor. with cross fire you don't have one.

 

If it does then there is NO bulb in the circuit to the alternator excitation circuit, only the 10 ohm resistance wire as shown in the wiring schematic.

 

Okay, I was just going by what I read earlier that stated: My gauge's light is labeled "Choke" rather than "Alt" and this is the bulb that is not working.

 

So, did it start working right when you replaced the bulb?

 

No, I have verified I have a good bulb put in and it still does not light up.

 

Any other ideas?

 

Has anyone had time to measure what the resistance is on the brown wire going to their alternator?