Charging System Troubleshooting--Blown Ignition Light Fuse?

[patrickt]

Here, this test will take less than five minutes, requires no wire cutting and, if successful, will make your car drivable (excluding wipers, gauges, and heater).

[ACHiPo]

Originally Posted by patrickt

Here, this test will take less than five minutes, requires no wire cutting and, if successful, will make your car drivable (excluding wipers, gauges, and heater).

I got a chance to try Patrick's diagnostic, and I got something, but not what I hoped for.

Removed the ignition light from the dash for access. Pulled the double green wire off the ground spade of the light socket. Ran a fused jumper between the accessory terminal on the ignition to the grounded lug of the ignition light.

Turned key to ON position. No light.
Started engine and light came on while cranking and extinguished when engine started.
Ammeter was slightly negative (~1A) with engine running. Turned on headlights and ammeter dropped to ~-12A. Revved engine, but ammeter didn't budge.

I'm sure I learned something, just not sure what. Maybe that I can definitely eliminate the gauges as a short path?

[patrickt]

That's progress. For the next set of tests, you will leave the double green wire disconnectied from the light for everything below.

1) Remove the G/R wire from the ignition light but leave your fused jumper wire running to the light. Turn the ignition switch ON but do not start the car. Touch a known good ground lead to the exposed pin of the light and observe that it illuminates. Then, using a VOM, test for either a) 12v+, b) 12v-, or c) neither one at all at the disconnected G/R wire. Report the findings.

2) Plug the G/R wire back on to the ignition light. With the ignition ON, and the car not running, the light should be "ON" but you report that it is not. Try running a known good ground wire to the alternator and voltage regulator and touch different spots on both those components just to see if you can get the ignition light to illuminate. For instance, touching a known good ground wire to the VR case and making the light come on each time you did that would be a really nice sign. Report back.

3) Insert a fuse in to Fuse #3 and turn the key ON, but do not start the car. See if your gauges, heater, and wipers work or if you blow the fuse doing that. Report back.

We are making progress.

[ACHiPo]

Originally Posted by patrickt

That's progress. For the next set of tests, you will leave the double green wire disconnectied from the light for everything below.

1) Remove the G/R wire from the ignition light but leave your fused jumper wire running to the light. Turn the ignition switch ON but do not start the car. Touch a known good ground lead to the exposed pin of the light and observe that it illuminates. Then, using a VOM, test for either a) 12v+, b) 12v-, or c) neither one at all at the disconnected G/R wire. Report the findings.

2) Plug the G/R wire back on to the ignition light. With the ignition ON, and the car not running, the light should be "ON" but you report that it is not. Try running a known good ground wire to the alternator and voltage regulator and touch different spots on both those components just to see if you can get the ignition light to illuminate. For instance, touching a known good ground wire to the VR case and making the light come on each time you did that would be a really nice sign. Report back.

3) Insert a fuse in to Fuse #3 and turn the key ON, but do not start the car. See if your gauges, heater, and wipers work or if you blow the fuse doing that. Report back.

We are making progress.

First test did not show any voltage or illumination. I ran a ground wire from the engine block to the center pin of the ignition bulb with the ignition "ON" and no illumination. Checked with a DVM and sure enough no voltage across the connections.

Running second test now...

[patrickt]

Can you get that ignition light to illuminate by just running 12v+ to one pin and 12v- to the other pin? All we're doing here is just testing the light bulb to see if it will come on.

[ACHiPo]

The resistance across the ignition light terminals is 7.5 Ohms. It lights when battery power is applied.

Need to head to work, but will check back later today.

[patrickt]

OK, fuse out, run a 12v+ line to the pin on the light that originally had the two green wires that you removed, and the G/R wire is now attached back to its pin. When you do that does the light illuminate? If not, and you're absolutely 100% positive that the light works and that you are giving it 12v+ on one side, then that means it is not getting a return path on the other side through the VR. Double check this test as it is important.

[patrickt]

All we're doing on this test is just making sure the ignition light is not broken somehow and that the G/R wire coming from the voltage regulator does indeed provide a path to ground when the car's alternator is not charging. When behaving normally that G/R wire will provide a path to ground until the car begins charging, and then the path is gone. That's what makes the ignition light come on and off.

[ACHiPo]

Originally Posted by patrickt

OK, fuse out, run a 12v+ line to the pin on the light that originally had the two green wires that you removed, and the G/R wire is now attached back to its pin. When you do that does the light illuminate? If not, and you're absolutely 100% positive that the light works and that you are giving it 12v+ on one side, then that means it is not getting a return path on the other side through the VR. Double check this test as it is important.

+12V to the two green wire pin with the G/R connected--bulb lights up. Makes sense given that the bulb lit up when I cranked in the previous test. Not sure why the bulb isn't lit when key is in the "ON" position?

I'm back to suspecting the voltage regulator, although maybe I should disconnect the fan and wipers to eliminate them as the source of the short (assuming I can get to the connections)?

Originally Posted by patrickt

When behaving normally that G/R wire will provide a path to ground until the car begins charging, and then the path is gone. That's what makes the ignition light come on and off.

The fact that the bulb is not lit when the key is "ON" suggests that when the key is in the "ON" position, there is no power to the ignition light. Could I have a short in the ignition switch? Seems unlikely when the car starts and runs ok?