Charging system problem- dash indicator light

[greene3]

I just replaced my alternator for nothing... I believe the reason the alternator is not charging is because the circuit is bad between the D+ terminal and the instrument panel battery indicator. The bulb is good, I tested it, but it doesn't light with the engine stopped and the ignition on, thus it's not completing the circuit to energize the alternator. I've verified that the alternator is working fine by placing a test light between the + terminal on the battery and the D+ terminal on the alternator while my wife started the car. As expected, the test lamp goes out as soon as the car starts, and the voltage between the battery terminals is now a healthy 14V.

Any ideas on how to troubleshoot the circuit? Maybe there's a fault in the instrument cluster? If yes, is this a common problem and is there a known fix? I'm thinking of a possibly just doing a workaround by wiring a switched hot lead to the D+ terminal. If i do that, should I wire in a resistor?

Thanks!

 

[jmackro]

I believe the reason the alternator is not charging is because the circuit is bad between the D+ terminal and the instrument panel battery indicator. The bulb is good, I tested it, but it doesn't light with the engine stopped and the ignition on, thus it's not completing the circuit to energize the alternator.

Have you replaced your stock alternator with a modern, internally-regulated model? I don't think the original set-up needed current from the indicator light to tell the alternator to begin charging. But yes, the later models do need that power.

I'm thinking of a possibly just doing a workaround by wiring a switched hot lead to the D+ terminal. If i do that, should I wire in a resistor?

I don't think a resistor is needed, though in the original set-up, the bulb serves as a resistor. Automotive dash bulbs are typically 5 watts. A bulb would need to have about 30 ohms of resistance to dissipate 5 watts of power at 12v. So you could put a 30 ohm resistor in the lead (or better yet, an indicator bulb) to match the stock set-up. Just don't use an LED!

 

[greene3]

Have you replaced your stock alternator with a modern, internally-regulated model? The original set-up didn't need current from the indicator light to tell it to begin charging. But yes, the later models do need that.

Forgot to mention that the engine (and alternator) is from an 86 635CSI. I've verified that it does need the excitation for it to put out proper charging voltage.

 

[Mike Goble]

If you have the original tester socket, there are two of the contacts on it associated with this circuit. Open another tab and open this electrical diagram:
http://www.e9-driven.com/Wiring_Diagram.pdf

I realize that it's an e9 wiring diagram, but the circuits are basically the same, a 12v source, a bulb, a test socket, etc.

Near the top the first page is the alternator, and attached to the alternator is a blue wire. If you follow this blue wire down the page, you will see it connected to a small triangle with the number 8 in it. This represents the connection to pin 8 of the tester socket. ( You can see the tester socket about halfway down the page, a box with 9 numbered dots. ) Follow the blue wire across the page to the right, it exits the page between a brown-green and a black wire.

Scroll down to the next page and pick up your blue wire on the left side of the page. It goes thru a connector ( represented by the box ) and into your instrument panel. Follow it to the right and up to the circle of bulbs labeled instrument light, and your alternator light is the top right bulb.

The hot side of the bulb is a green wire. Follow this wire down and to the left, back thru the connector, then down to a junction near the dimmer switch. Follow the green wire from the junction back up the page and to the left, where it exits between a green-white and another green wire.

Scroll up to the first page and pick up the green wire on the right side and follow it to a fuse with a plethora of other green wires on it. Also attached to this fuse is a triangle with the number 2 in it, representing pin 2 of the tester socket.

If I was troubleshooting this circuit with your circumstances, i.e. the alternator light doesn't light with the key on and the engine stopped, I would first disconnect the blue wire from the alternator to get the alternator out of the circuit. I would then turn the key on and put my voltmeter on pin 2 of the tester socket looking for 12v. If I have it there, indicating I have power to the circuit, I would move to pin 8, again looking for 12v. With the blue wire open, I should read 12v on pin 8, indicating a complete circuit through the instrument panel.

If 12v is present on pin 8, grounding pin 8 should light the alternator light. If 12v isn't present on pin 8, the two most likely suspects are the connector on the instrument panel or the bulb socket, so I would move to that area and use a sharp probe to pierce the insulation on the associated wires on either side of the connector to measure the voltage.

Keep in mind that your symptoms can disappear at any time by slightly moving a connector or wire and completing the previously faulty circuit.

 

[jmackro]

Keep in mind that your symptoms can disappear at any time by slightly moving a connector or wire and completing the previously faulty circuit.

Evidently there is a break in the connection between the back of the alternator and the bulb socket on the gauge. It is unlikely that the wire itself is broken, but the two least reliable points in that path are:

- The connectors behind the dash that allow the dash harness to be disconnected. Unfortunately, those connectors are pretty inaccessable - you can see them by removing the speedo and tech. On my coupe, the little "fingers" that hold the connectors together had broken off as the plastic became old and brittle. That resulted in intermittant connections. Wrapping a tie wrap around the two halves of the connector restored reliable operation.

- The indicator bulb socket itself. This is sort of a complicated socket in that it has two leads: one for +12V switched, the other going to the alternator.

This would be sloppy, but running a dedicated wire between the alternator and the bulb socket on the gauge will tell you something, and will probably be a more elegant interim fix than just running the wire you proposed in your first post.

 

[greene3]

I pulled the connector to the instrument cluster. I've verified switched power on the green wire and determined that there's no continuity dorm the connector to the alternator on the blue wire. I spliced into the blue wire at the connector and ran a temporary wire to the alternator. Everything works now. The indicator light works when you move the key to the on position, and after starting, the alternator is putting out 14V.

So now I'm just going to bypass the original blue wire and run a new wire to the alternator and call it a day.