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Old 08-02-2020, 01:18 PM   #1
dandeluca
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Default Mystery Wire, Possible Purpose?

So I took my little project car (91 240 Wagon) out last night for a test drive and it stranded me as the battery died, the alternator was not charging. No warning lights. I got it home this morning and pretty quickly discovered the "exciter" wire disconnected from the alternator. It appears to be charging now. Great! However I noticed there is a very poorly done splice cable connected to the "exciter" cable. It runs from a yellow wire on what appears to be the wiper motor down to the exciter wire.



Any theories about what this is about? Should I disconnect it?

Thanks!
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Old 08-02-2020, 01:57 PM   #2
iamrolling
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The diagrams for the 89 240 show the yellow wire on the wiper motor to be connected to the fuse block inside the cab. It looks like a cheap hack to bypass a broken wire somewhere. Try pulling it and check whether the alternator and wipers still work. If one of them fails, hunt for a broken wire or at least put it back on with a proper 3-way splice.
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Old 08-02-2020, 02:30 PM   #3
142 guy
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Check here for details

https://forums.turbobricks.com/showthread.php?t=356118

It seems like somebody has done a 'hack' for a failed alternator exciter wire. Setting aside for a minute the construction quality of the hack, it will work; but, it should have some resistance in the circuit to limit the initial bootstrap current to the D+ terminal. A 2 watt lightbulb will do nicely.

The best solution would be to fix the original D+ connection back to the charge indicator light in the dash. However, if the excitation circuit failure was in the dash cluster circuit somebody may not have been prepared to invest the time or $ into a correct repair.
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Old 08-02-2020, 03:41 PM   #4
dandeluca
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Okay,

If I am reading that correctly the little red spade plug is the "exciter" and is also known as D+. That wire should have 12+V flowing to it when the key is on but engine is not. When the engine starts that wire bootstraps the alternator which then provides its own 12+V that eliminates the potential in the circuit and turns off the dash light.

My question is on a properly functioning car should that spade plug only have one (red) wire attached or should their be two?
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Old 08-02-2020, 04:52 PM   #5
dandeluca
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So I disconnected the wire from the wiper motor. It would not light up a test light with the key on, but seemed to be charging with 13.5 volts at battery with car started and wipers still worked. I’m still not clear what the D+ should read on a multimeter with the key on but the car not started.

Can anyone clarify how to test the execiter wire with the key on position 2 with a multi meter?

Right now it seems that wires purpose was to keep the dash light from coming on so I would get stuck on the side of the road.
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Old 08-02-2020, 05:27 PM   #6
dl242gt
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The exciter wire runs in the wiring harness from the back of the engine near the lifting eye. When it passes under the crank pulley the decay of oil and crud over the years degrades the wiring. You can connect to the small red wire at the back of the engine or even under the dash if you want to run it from the fuse box on fuse 12. When I replace the wiring I run it along the firewall at the back. Come forward to the strut tower and from the front of the strut tower I run the wire to the alternator.
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Old 08-04-2020, 12:13 PM   #7
142 guy
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Quote:
Originally Posted by dandeluca View Post
So I disconnected the wire from the wiper motor. It would not light up a test light with the key on, but seemed to be charging with 13.5 volts at battery with car started and wipers still worked. I’m still not clear what the D+ should read on a multimeter with the key on but the car not started.
If the excitation circuit was set up correctly through the dash charge light, with the key on; but, the engine not running you should be measuring 0.5 - 1.5 volts (depends on the alternator circuit set up) between the D+ terminal and ground. Most of the voltage drops across the dash light which limits the current flow to D+. If there is no dash light or other current limiting resistance in the excitation circuit you will likely measure something closer to 12 volts at the D+ terminal. Applying a full 12 volts to D+ when the alternator is not turning can shorten its life if it is sustained for more than a few seconds.
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