My car is an 86 GT auto with the extra gauge pod over the center console. This pod contains the voltmeter gauge and the "VOLTS" warning lamp.
I've attached a picture of where this is shown on a wiring diagram in the 86 service manual under the heading for V6 cars (page 8A-30-0). However it's not clear to me if this diagram is correct for the extra gauge pod or if it's only for standard gauges.

The "VOLTS" bulb is very dim during a bulb test because it's not getting enough voltage. The bulbs in this cluster have been replaced with new and I've tried swapping them around. The bulbs definitely aren't the problem, it's voltage.

The Oil warning bulb (which works fine) gets 10.x Volts. I think I measured that with the bulb installed.
At the VOLTS bulb, without a bulb installed, I only get ~8.2 Volts at the solder pads.
With a good incandescent bulb installed at the Volts location, the voltage is only 6.x Volts. This is only enough to make it very dimly lit. This is during Bulb Test, when the light is supposed to be fully on, and when the Oil bulb is getting 10.xx.
I tried wiggling the connector to the gauge cluster but didn't find any issue there. The connection seems completely stable.




I am using a CS130 alternator with a simple 2-wire adapter which I do not believe has any resistor in it. I've been using that setup for several years.

Test
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With an incandescent bulb installed at the "Volts" light
Key on, engine off (bulb test)
The CS130's 2-wire adapter is unplugged from the car's harness. The alternator is still connected to the charge wire and bolted into the car.
On that adapter (which is attached to the alternator), I measure the voltage of the small brown wire. The wiring diagram shows this to be the Ground for the circuit that should light the "VOLTS" bulb during bulb test.
That wire is measuring +4.4V vs chassis ground measured at the thermostat cap. This seems to explain the low voltage across the bulb - unless it would change as soon as I plug it back in to the car.

With engine running, still using thermostat cap as Ground reference:
That same wire (which should now be at battery voltage) measures +13.5V. The + terminal of battery measures +14.4V. That difference seems reasonable enough.

Why is the brown wire from the alternator at +4.4V vs chassis ground at KOEO?
Does this point to a problem in the alternator? Far fetched or highly likely? Again it's a CS130, not a 12SI.

Could it be a problem with the CS130 conversion? Is there any reason not to expect a CS130 to properly ground this wire at KOEO?
This bulb has been dimmer than the Oil bulb for years, but I don't know if it started when I did the CS130 conversion. Even so, I don't know if it was ever *this* dim before. So I'm unsure if this is a new problem or if it should be blamed on the alternator.


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Backstory: I did have a charging system failure recently which was fixed by replacing the ring terminal on the charge wire (it broke off). Having done that the alternator seems to be working fine, but I'm not sure if it's to blame for this issue during bulb test. That bulb has been dim for years but it might be dimmer now than it used to be (not sure).

Alt light Does Not use "bulb test" circuit from I-switch.

Alt Light only key on engine Not running because Alt not running.

check Light Power to Ground, Not to alt. Should be "12v." If not then power side has problems.

"low volt" power to alt means little because the Alt is "watching" the bulb resistance to turn on when engine runs.
Bulb never lights before starting engine then have alt checked on a machine in store to see if regulator etc works.

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quote Originally posted by theogre: Alt light Does Not use "bulb test" circuit from I-switch. Alt Light only key on engine Not running because Alt not running. check Light Power to Ground, Not to alt. Should be "12v." If not then power side has problems.

The voltage across the bulb is very low, but the issue I see is on the negative side of the bulb, not the positive (see measurements in next post).

quote "low volt" power to alt means little because the Alt is "watching" the bulb resistance to turn on when engine runs.

Does this mean it's normal for the alternator to let the negative side Float up to +4V or even higher? If so, then I don't know how the bulb is ever supposed to work reliably.

quote Bulb never lights before starting engine then have alt checked on a machine in store to see if regulator etc works.

It lights but it's very dim. On a sunny day, during an actual charging system failure (broken charging wire), the light was so dim that I couldn't tell if it was on.