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1998 Ford Mustang ALT 20 amp fuse blows intermittently

Tiny
FATMAN
  • MEMBER
  • 1998 FORD MUSTANG
  • V8
  • 2WD
  • MANUAL
  • 103,000 MILES
New belt, Autocraft alternator, and battery. The 20 amp alternator fuse will blow after a hot run with a/c on. Charges fine when cold or without heavy load. When a/c, fans, hd light on the amp gauge begins to read lower until fuse blows. After a few month of summer it won't hold a fuse, but a new regulator puts things back to normal. This second summer I wen through same senario again. Wiring looks good, batt is 12.6 with car off and about 14 on.
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Sunday, September 19th, 2010 AT 4:43 PM

10 Replies

Tiny
DOCFIXIT
  • MECHANIC
  • 18,828 POSTS
Hi
Think problem is in battery to engine ground left front engine. OHM from ALT body to batt NEG should read 0 Ohms. If connection or cable poor as engine heat and load increases so does AMP draw from increased resistence in ground circuit so fuse melts.
Let me know
Thanks for donate
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Monday, September 20th, 2010 AT 1:24 PM
Tiny
FATMAN
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Docfixit, please check my OHM readings and let me know if it confirms your thoughts. Engine warm, but not hot. Engine off: case to battery negative terminal.5 to.75 OHMs with the meter set at 200. Engine running: Ohms at 9.0 to 10.0 with meter at 200.
Thanks in advance for your reply.

Fatman
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Wednesday, September 22nd, 2010 AT 11:22 PM
Tiny
DOCFIXIT
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Not good replace cable go with a little larger diameter. Clean block contact area well and batt term.
Let me know what out come is.
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Thursday, September 23rd, 2010 AT 12:17 PM
Tiny
FATMAN
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  • 6 POSTS
Docfixit, OK I know this may be becoming a bit tedius, but The Ford replacement cables are $229 and I haven't found any aftermarket direct fit cables. This setup isn't just a simple cable, it has other grounds and power feeds coming off of it so before I went further I wanted to double check my readings. First of all the first readings were from a digimeter that only goes down to 200 on the scale. I double checked it with a old style volt/ohm meter. After checking for 0 at the meter I found 0 with the engine off and.5 on and 1.0 with a/c, blower fan on medium, and engine cooling fan on. Correct me if I am wrong, but I think the 200 scale on the digimeter is misleading. The 10 I had with engine only running on the digi when divided by 200 equals the.5 with the old style meter.

If this all is correct, where do we go from here?

Fatman
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Friday, September 24th, 2010 AT 1:08 PM
Tiny
DOCFIXIT
  • MECHANIC
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Let's try a simple experment attach jumper cable to NEG then to clean block connection. ALT good spot. Drive around see if fuse stays
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Saturday, September 25th, 2010 AT 12:50 PM
Tiny
FATMAN
  • MEMBER
  • 6 POSTS
Docfixit, sorry been busy at work. So jumper cable to clean connection at the block and then from there to alt body? How large does the "cable" need to be? Like a batt cable or test lead wire?

Fatman
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Wednesday, September 29th, 2010 AT 10:23 PM
Tiny
DOCFIXIT
  • MECHANIC
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Battery cable size like a set of jumper cables
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Thursday, September 30th, 2010 AT 2:35 PM
Tiny
FATMAN
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  • 6 POSTS
Replaced neg and pos cables also ground strap from block to chassis and cleaned all connections Ohm reading the same have not given it a good hot run yet Nothing was real dirty but thought i'd replace it as car is at 100k The whole package was very tightly bundled and a real pain in the butt to r&r
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Monday, October 11th, 2010 AT 9:56 PM
Tiny
DOCFIXIT
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Yea think they need to make Enginers have to repair the stuff they design be alot easier I bet.
Hope that is the fix keep me posted
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Tuesday, October 12th, 2010 AT 11:28 AM
Tiny
FATMAN
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  • 6 POSTS
Well, it was hot here in Tucson today so I got to see how it did completely warm with the a/c and blower on mid. It blew the alt field fuse again!
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Tuesday, October 19th, 2010 AT 6:34 PM

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