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Old 03-07-2016, 01:08 PM   #1
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Electical Woes

Hey everyone. I have a 2011 Mustang GT/CS that has developed an issue. My battery reads a steady 14.5v until the radiator fan comes on then it will briefly drop to 11v which causes my headlights to dim to the point that it looks like I turned them off and back on again. This happens either when idling or slowing down off throttle.

I took the car to Autozone to check the battery and alternator and both "passed" but the voltage drop was observed on their machine as well.

Any thoughts on what could be causing my voltage drop and headlight dimming? Thanks in advance.
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Old 03-07-2016, 01:55 PM   #2
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internet guess: either fan motor failing or bad connections to it I would guess connections more likely
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Old 03-07-2016, 02:23 PM   #3
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Quote:
Originally Posted by Jason127 View Post
Hey everyone. I have a 2011 Mustang GT/CS that has developed an issue. My battery reads a steady 14.5v until the radiator fan comes on then it will briefly drop to 11v which causes my headlights to dim to the point that it looks like I turned them off and back on again. This happens either when idling or slowing down off throttle.

I took the car to Autozone to check the battery and alternator and both "passed" but the voltage drop was observed on their machine as well.

Any thoughts on what could be causing my voltage drop and headlight dimming? Thanks in advance.
When you say "briefly drops to 11 volts" does that mean like less than a second, than immediately back to 14.5 volts?
Also, when you say this happens, "when idling or slowing down off throttle" are you saying it briefly drops to 11 volts and comes back again when the fan kicks in or otherwise?
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Old 03-07-2016, 02:35 PM   #4
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Quote:
Originally Posted by Diehard View Post
When you say "briefly drops to 11 volts" does that mean like less than a second, than immediately back to 14.5 volts?
Also, when you say this happens, "when idling or slowing down off throttle" are you saying it briefly drops to 11 volts and comes back again when the fan kicks in or otherwise?
Correct, less than a second. It'll go 14.5v to 11v spike to around 14.7 then settle back at 14.5. I would imagine that the fan coming on would draw power but I have never had it dim my lights like it has since last night. It only seems to happen when the fan turns on.

I have noticed that the fan is coming on, voltage drops and then the fan turns off. Does this about every 10 or so seconds once my car is warmed up.

Thanks for your replies so far.
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Old 03-07-2016, 04:49 PM   #5
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internet guess: either fan motor failing or bad connections to it I would guess connections more likely
I would take Mud's advice and check the connections. This could cause this issue and it won't cost a dime to check it out.
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Old 03-07-2016, 05:29 PM   #6
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If connections check out, you may want to look into checking the current draw on the fan.
Although I have no first hand experience with these fans, I tried a little research and could only come up with a Youtube video that determined the draw on a Taurus single speed fan was about 30 amps after an inrush(momentary) current of about 60 amps.
The online manual shows that it is a 2 speed fan with a common 40 amp relay and 2 individual 30 amp relays. No indication of actual current draw. So if you have a little electrical knowledge or know someone that does, you could check for amperage draw. If you don't know what you're doing, you may want to leave that to a professional.

Ford Service Manuals - Wiring Diagrams

EDIT: Just realized from your description that the fan doesn't stay on for more than about 10 seconds at a time. Not sure if that's normal or not. Never noticed in my car. If someone can confirm that that is NOT normal than perhaps it kicking in and out due to excess current draw.
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Old 03-07-2016, 06:08 PM   #7
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Diehard and Strange Mud thank you both for your replies so far! I'll check out the connections and go from there.
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Old 03-07-2016, 07:01 PM   #8
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Connections look good.

I'm not very versed in electrical work so I suppose it will have to go to the shop.
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Old 03-07-2016, 08:57 PM   #9
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Connections look good.

I'm not very versed in electrical work so I suppose it will have to go to the shop.
Please let use know the outcome.
And good luck!
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Old 03-08-2016, 06:07 AM   #10
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have you checked/cleaned battery connections? Also look at alt connections. Another experiment would be to turn on fan/ rear defrost/anything that draws power to see if your lights dim with other draws. If nothing else dims your lights (although gut tells me cooling fan is a big draw) I would lean more to the bad fan motor/wiring.

side note: I had an old F250 that would only squeal at night. It ended up being the alt had a bearing failing and under the load of my headlights it would make noise. I felt a lot better when I figured that one out.
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Old 03-08-2016, 12:21 PM   #11
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Looks like it was something with the fan. It was drawing 80 amps when it'd turn on and had a constant draw of 30 amps.

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Old 03-08-2016, 01:17 PM   #12
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Looks like it was something with the fan. It was drawing 80 amps when it'd turn on and had a constant draw of 30 amps.

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Definitely sounds excessive.
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Old 03-08-2016, 04:09 PM   #13
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Quote:
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Looks like it was something with the fan. It was drawing 80 amps when it'd turn on and had a constant draw of 30 amps.

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Wow, that does sound excessive. I don't know what it should draw but I'm pretty sure it isn't that much.
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Old 03-08-2016, 07:51 PM   #14
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My new one hits 40 amps at start up and runs at 18 amps. All is good again.

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Old 03-08-2016, 08:20 PM   #15
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My new one hits 40 amps at start up and runs at 18 amps. All is good again.

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Excellent!
I'll have to make a note of those amp draws for future reference, since I couldn't find it anywhere.
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Old 03-08-2016, 08:28 PM   #16
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Great news!


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