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Dead alternator but no idiot light - 2004 Ford Escape V6

  • JeffBirt
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05 Sep 2019 08:23 #33427 by JeffBirt
Hi all,

I just replaced the alternator in my daughter's 2004 Escape which has the V6. I did the basic tests, i.e. checked voltage from positive post to negative when running, then form positive to engine block to check for a bad ground. I found no output form the alternator but I could charge the battery and it would hold a charge.

The idiot light on the dash came on when the key was moved to the run position but when running, and not charging, it was not illuminated. I could not find what I was 100% sure was the correct wiring diagram but after inspecting the control wiring at the alternator and seeing there were only two wires I was certain it was not a PCM controlled alt. The wiring diagram that I found that looked like what I saw on the car showed a standard circuit from +12V through the idiot light to alt so if the alternator was not putting out 12V the idiot light should light. (I'm assuming this circuit still provides the excitation voltage to get the alternator going as well.)

So I put the new alternator in, with a bit of cursing at Ford for having to pull an axle to change the alt, and it works. It bothers me though why the idiot light did not come on. I can only guess that a failure in the voltage regulator cause that circuit to be open internally (so it neither grounded the lamp or provided excitation voltage). If I had not already disposed of the old unit I would ohm that out to be sure. I'm guessing one other diagnostic check I could have done initially would be to ohm out that circuit with the alternator still in place, to see if it were open or not.

While the new alt solved the problem, it bugs me to not know for sure what failed and how I could have been more certain before ordering the part. (I was 90% sure based on previous experience but...). Have any of you pros seen this symptom before?

Thanks!

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  • Tyler
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06 Sep 2019 05:33 #33438 by Tyler
I've seen this a few times, on internally regulated and PCM controlled versions. I never took the alternators apart to find out? :silly: But I think your theory is correct - a failure of the voltage regulator that prevented it from switching the light circuit to ground.

You COULD ohm out that circuit. But, the battery light comes on with the bulb check, so I'd say you're in the clear. :) If you still want that 110% confidence, you could backprobe the light circuit pin at the alternator connector and ground it with the engine running. Bulb should light.

The same test could also be done with the connector unplugged, but just getting to this connector in the first place is some work...

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  • JeffBirt
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06 Sep 2019 12:23 #33447 by JeffBirt
Thanks Tyler. Back probing the circuit to supply ground is a good idea and I'll keep that in my mental toolbox for the future. As you say it is a real PITA to get to the connector; I was just able to get an inspection mirror in from the wheel well to take a peek at it but don't think I could have unplugged it without being 90% committed to removing it anyhow.

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