Mega Fuse Question - CEG Archives

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I have been having charging problems with the alternator to battery wiring. I pulled the coil pack out and the mega fuse is hanging there, now I took my multimeter and grounded it, then stuck the red portion on the battery side of the fuse, getting a steady voltage there, on the other side, I get nothing. Would this indicate a bad fuse? My logic states that I should be getting voltage all along the line, but I could be wrong. Thanks in advance!

To test a fuse you would "ohm it out", ala testing the resistance across the fuse itself. Should nil, none, 0, or at least CLOSE. if it is reading "XX MEGAohms", an infinity symbol (an 8 on its edge), or anything higher than about 2-3 ohms, then you have a bad mega fuse, which will cause:
ZERO charge to the battery
Constant charge indicator light on the dash

To also narrow down what it happening here, measure the DC voltage on the battery with the car running, across the terminals it SHOULD be 13.9ish-14.5-ish

Then shut the car off, turn on the headlights and leave them on for 60 seconds to remove the "surface charge" off the battery. Test across the terminals with the car off and you should have 12.0 volts or higher.

turn off you headlights and let us know what you found regarding the voltages and the resistance of that fuse.

Your logic is correct.

Under no-current conditions, the voltage in all parts of the circuit should measure the same voltage potential.

Sounds like your megafuse is open.

Steve

Thanks for your support, oh by the way, the ford dealer here messed up. They told me over the phone they would cover my car under the wiring harness recall, I had it towed in this morning for the service and they refused to cover it in person

. I made them pay for the tow in and out at least, in any case I'm happy to be making progress on this.

One more quick question, can I buy the megafuse only at a ford dealer?

Originally posted by Shtoms:
One more quick question, can I buy the megafuse only at a ford dealer?

Nope. It should be available at any decent autoparts store. Ask for a Littelfuse MEGAfuse 175 Amps. If the drone behind the counter gives you a blank look, you're at AutoZone.

Steve

Here is a good picture of a megafuse from Littlefuse

OK then. A few burns and some good scrapes later I now have 12+ Volts all the way to the alternator post. The mega fuse fell apart in my hands when i took it out, it was corroded and shot, wouldn't hold a clean signal through it. I went to Ford too get my new one because Autozone couldnt understand what I wanted

. Now, I am still not getting a charge. I used my multimeter to test every point on the line, 12 V at the alternator post when it's running. 12 V when its off. Correct me if I am wrong, I should see 14.5 on the alternator post when its running right?

The battery is an Interstate 60 month battery, that has been drained completely 4 times since January, 3 times in the past 2 weeks. The battery passed Sears die hard testing last week with flying colors.

Do you think it is a bad alternator? If so its going back to the shop for a warranty replacement.

If you have 12V at the alternator itself while running, my first thought would be to make sure you have the alternator ground hooked up to the chassis, and the alternator.

Secondly, a faulty alternator.. yes.

Quote:

Do you think it is a bad alternator? If so its going back to the shop for a warranty replacement.

Yes, it is bad.

Didn't I warn you about AutoZone?

Steve

This alternator is from CarQuest actually, th 1st one in was an autozone one

Ok the saga continues, My mechanic says the harness that snaps into the alternator is bad. He tells me that it controls the alternator and tells it when to charge etc. He said he jumped the wiring and it started to charge. So he wants to rewire the section and see if it will work. help me out here, I thought that alternators were always pumping energy when spinning, I wasnt aware that a computer told it to start sending 14.5V out. As before, there is 12V from the battery to the alternator post, running or not running. All the grounds are in place and everything is tight. Thanks for your input.

He is correct.
http://www.contour.org/ubbthreads/showflat.php?Cat=&Board=trouble&Number=1043318&page=&view=&sb=5&o=&fpart=all&vc=1

Originally posted by Tony2005:
He is correct.
http://www.contour.org/ubbthreads/showflat.php?Cat=&Board=trouble&Number=1043318&page=&view=&sb=5&o=&fpart=all&vc=1

So I'm guessing I should try this fix? I have nothing to lose

Originally posted by Shtoms:
..So I'm guessing I should try this fix? I have nothing to lose

Correct.

OK, so what gauge wire should I be using to splice with etc? Thanks again.

Alrighty, I finally got sometime today to pull the harness of the alternator and test it. I am getting the same voltage ( approx 12V ) end to end, from the battery to the harness ends I get 12V. there are 3 wires, one is the red one, a blue one I think, and the grayish one that clips into the alternator at the bottom. What do all these wires do? I know I need to make sure the alternator is grounded, how do i do this? And finally, if all these things pass, the voltage passing all the way, the grounding is good, etc. Then would it be safe to assume the alternator is no good? Any opinions are greatly appreciated. Off to patch up my scrapes and bruises....

What year and what engine?

Autozone has some good schematics. Try Figure 10 - Diagram 9

http://www.autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/1e/b3/2d/0900823d801eb32d.jsp

Grey to ground, big Red to Megafuse, Green/Black to instrument cluster (battery charge light), and the small Red wire is supposed to be cut and spliced on the big red connector at the alternator (look at the diagram posted by Goldentour).

it is a 95 mystique, duratec.

Yay, the wiring fix made the difference! Thx again. I am charging away once again.