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Old 05-04-2019, 08:38 PM   #1
AwesomeTreehouse
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Default Possible bad wiring to alternator exciter wire?

Right now the current state of my 92' 240 is as follows:
- No warning lights (other than the overdrive arrow) when ignition is in position 2.
- None of the gauges in the instrument cluster are giving any reading.
- 12.4V coming from resting battery.
- About 10.5V coming from battery when under load (1500rpm, radio on, lights on.)
- 12v at alternator exciter wire when ignition is OFF.
- About 6V at the alternator exciter wire when ignition is in position 2.
- Same voltage (12V at OFF, ~6V at position 2) at the exciter wire when it's not connected to the alternator.
Some other details:
- The battery is brand new. Bought from the auto parts store this morning.
- The alternator is less than two years old.
- I have had trouble somewhat recently with power not going from the ignition to the ECU.
- The car starts up with no issue or hesitation.
- I have checked, cleaned, and re-secured the connections on the alternator and battery.
So now I'm just wondering if the issue is in the wiring from the ignition to the exciter wire, the ignition switch its self, or something else that I'm not even considering. Reading around it looks like this isn't a super uncommon issue except for the fact that in my case the exciter isn't getting the full 12v when the ignition is in position 2 (both connected and unconnected).

I'm also wondering if the issue I've had recently with power not making it from the ignition to the ECU is actually related to the exciter not getting full power. My gut says it is but I don't know enough to know if that's possible.

Any insight or help with this issue would be greatly appreciated!
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Old 05-04-2019, 08:57 PM   #2
dl242gt
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From what I can gather your charging system isn't working and you probably have dirty or bad fuses in the fuse box by the door.

The easy quick check for the charging light is to remove the small red wire from the alternator. Then connect it to the case of the altnernator. You can hold it there while you have the ignition on, not running or starting. Just on position where the lights come on in the cluster.

If the light goes on then you have a good excitor wire circuit. If the light doesn't go on. Then connect it to another ground. If it still doesn't come on. Then you need to check the wiring and the light bulb. The wiring is the usual fail right around the alternator or under the crank pulley on the engine block where it gets soaked with oil from the crankseal.

If the light comes on when you connect it to another ground away from the alternator. Then the ground wire on the alternator case to the engine bracket is suspect.
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Old 05-11-2019, 04:12 PM   #3
AwesomeTreehouse
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Thanks for taking the time to reply! So I'm finally able to get back to digging into this.

Today the bolt that the alternator ground wire mounts to sheered right off when I was removing the nut. Fortunately I was able to replace the whole alternator with a new, unused one.

While I had the alternator out I removed the ground wire to check for continuity and everything checked out. Just to be safe I cleaned the area and remounted it.

Now with the new alternator in place I carried out some of the tests you suggested. With the exciter wire mounted to the case of the alternator I still don't get the light. Same with grounding it to a ground that's far away from the alternator.

I'm still getting 12.4V from the battery, 11.8V when the car is running, and around 9V going to the exciter when the key is in position 2. Also still no signs of life from the instrument cluster aside from the overdrive light.

When you say "if it still doesn't come on then you need to check the wiring and the light bulb" do you mean the exciter wiring or the ground wiring? Also is there any chance that the ignition switch could be the problem?

Thanks again!

Last edited by AwesomeTreehouse; 05-11-2019 at 05:58 PM.. Reason: Spelling errors.
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Old 05-12-2019, 10:41 AM   #4
dbarton
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Here's the explanation of the charge light that I always refer to when questions come up:

Quote:
Another good explaination of how a Dash Warning Light works.
A Bosch alternator only needs a short, initial shot of 12 volts (at the D+ terminal) to initialize or "boot" it into operation. After the initial shot of voltage the alternator (generating electricity process) should be self sustaining from then until the car is shut off.

Why is the dash charge lamp lit when you first turn the key to "ON"?
The dash lamp is wired to the D+ terminal. It lights up when you turn key to "ON" because there is a GROUND potential coming through the D+ terminal (at the alternator) and it grounds the idiot lamp circuit. Turning the key to "ON" provides power to the other side of the bulb and the bulb then lights.
As a Bosch alternator begins to spin up, it needs to have that initial excitation charge coming through the dash lamp, which "boots" the alternator into charging. Then as the alternator begins functioning, the ground output from the D+ terminal transitions from ground to +12 volts DC. This transition causes the dash lamp to stop functioning because you now have 12 volts DC standing on both sides of the lamp, thereby the lamp receives no current.

We might think that most any 12 volt lamp hooked to 12 volts on one side and to the D+ on the other should be sufficient to initiate the alternator operation. It doesn't seem to work that way. The danger of too big a lamp (or resistor in line) is that you might cause a condition called "Run-On," where the output from the D+ terminal (through the lamp) can become strong enough to "backfeed" power to the ignition system. If this happens, when you turn off the key the engine continues to run and will only to stop after the wire to the D+ terminal is disconnected.
The below video is quite long, but excellent.
At 10:20 it gets into explaining and testing a dash charge lamp circuit. It shows how you can attach a simple 12v test lamp (one end to battery+ and the other end to the D+ terminal on the alternator, not the brush holder). If the alternator then comes to life, you’ve discovered the problem is RELATED TO YOUR WARNING LAMP WIRE CIRCUIT.
https://www.youtube.com/watch?time_c...&v=--AdY8z1QfU

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Old 05-12-2019, 05:48 PM   #5
AwesomeTreehouse
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Okay so some positive progress.

I connected a 12V test light from battery positive to D+ while the car was running and did see that the alternator began to charge the battery. Awesome! While that connection was in place I checked the instrument cluster to see if everything else woke up but the only change was the battery warning light was illuminated along with the overdrive light. Still no life from the other gauges.

I should also note that I removed the dash cluster yesterday to test for continuity in the PCB and everything was good. The bulbs are also working fine too.

So now I feel reasonably sure that the issue is in the lamp wire circuit like you say. I'm just a little unsure what the next step needs to be to resolve this.
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