RPMs go up, Voltage goes down | Ford Explorer - Ford Ranger Forums - Serious Explorations

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RPMs go up, Voltage goes down

stubborn1

Well-Known Member
Joined
August 15, 2003
Messages
189
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City, State
WISCONSIN
Year, Model & Trim Level
'98 4wd XL
I'm trying to diagnose an electrical problem on my 98. When I go to accelerate, the voltage gauge will start dropping once I am above 2500 rpms and the lights go dim. The higher the rpms, the lower the voltage. The battery light on the dash comes on at about 3500 rpms. When the tranny shifts or I reach cruising speed, everything is fine. My first thought is that the alternator may be bad, so I'm pulling it and taking it in tomorrow.

Is there anything else I might be overlooking?
 



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Xzibit A

Well-Known Member
Joined
November 22, 2004
Messages
710
Reaction score
1
City, State
Ashburn,VA(NOVA)
Year, Model & Trim Level
02 xlt 4.6
no its pretty much the alternator, it goes dwn cause the engine is using the voltage to run, did you check the tension on the belt too and the condition of the belt
 






nascar_intimid

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Joined
January 2, 2005
Messages
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City, State
st.catharines,ontario
Year, Model & Trim Level
'94 eddie ..'02 limited
could be the voltage regulator inside the alt. Just get a new one then you know what tou got without waisting your time repairing it only to have the brushes go on you next and then.... it never ends, just buy a reconditioned one...new if you have $$$
 






lwn

Member
Joined
October 6, 2003
Messages
44
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City, State
Illinois
Year, Model & Trim Level
Explorer 94 Eddie Bauer
Ring on the stator has a short a higher RPM'S very common on Ford altenators. If it charges at an idle then the ring is expanding at higher rpm's causing a bad connection on the wire under it.
 






WDP

Active Member
Joined
June 17, 2003
Messages
63
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City, State
Eastern Pa.
Year, Model & Trim Level
04 AWD XLT
The stator rings are worn/egg shaped. When the revs rise the brushes or whats left of them bounce and make intermittant contact. Don't try to true the stator, it is only thin copper over a ceramic base and there isn't much left. Time for an overhauled one. If you go the junk yard route, get a fresh one, these die at about 90-100 K real regular.

WDP
 






stubborn1

Well-Known Member
Joined
August 15, 2003
Messages
189
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City, State
WISCONSIN
Year, Model & Trim Level
'98 4wd XL
Just an update -

I bought a cheap rebuilt alternator from Autozone and everything is back to normal. I guess 142k miles on the original is pretty decent. I went with the cheap rebuilt alternator with the lifetime warranty because it's an easy pull if something goes bad.
 






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