Alt fuse blowing at WOT, what is going on there?

99FiveOh · Feb 24, 2009

I just rebuilt my alternator to fix some charging issues and thought that maybe the bad alternator was the reason for my blown alt fuse.

Well, today on the way to work I got on it really hard and my battery light came on, damn alt fuse blew again! I've never had and alt fuse blow in any of the last 8 Mustang's I've owned so now I'm concerned. I'm guessing there was alot of power being supplied to the charging system under WOT and the alternator probably overloaded the system and took out the fuse. But what would cause this?

My first guess is poor ground, I'll check that tomarrow. Any other ideas?

HISSIN50 · Feb 24, 2009

Do you overdrive the alt? Critical speed on a 3G is about 15K RPM as I recall.

Why wouldn't the regulator taper the output off? I'm actually asking you since you rebuild these things and I dont know why it wouldn't.

Check your charge-cable condition in case you have a huge voltage drop (sensed by the remote sense circuit, ironically being what the alt fuse is in series with). The alt would really crank out juice in such a case.

Bump for ya.

99FiveOh · Feb 24, 2009

This is puzzling actually. The voltage regulator is regulating my voltage just fine. As the RPMs climb the voltmeter shows rock solid in the same place. One thing the voltage regulator has no control over however, is amperage. So, for some reason, there is more amperage running through that circuit than should be.

I know you have access to various diagrams, could you maybe check and see what that circuit connects to and maybe where it's routed?

This alt is like a completely different one now accept for the regulator, I have a stock ford unit (better IMHO) that I may try since the regulator is the last remaining part from the defective alt.

Thanks for the help!

BlownFiveLiter · Feb 24, 2009

I had an alternator pop itself at WOT with a stock pulley. Everything got bright, then very dim. I'm betting the regulator has an issue. If you're able to, get a multimeter on it, give it a good rev, and watch the volts. Over 16.6 V (IIRC), the light will come on, and it may very well pop the fuse.

HISSIN50 · Feb 25, 2009

The alt fuse is a real simple-retarded circuit. It just has battery power into it (via the PDC) and then the white/yel wire leaves and heads to the reg.

Alternator schematic

I have the same feeling that Matt and you do - the reg might be the issue. I just dont really know how to test one and know it's the issue, so it's the last thing I replace (after wiring checks, etc).

99FiveOh · Feb 25, 2009

since it was on the bad alternator(regulator), it stands to reason that it may have been damaged by that bad alt. I'll change it later today and see what happens. I''ll also check the wiring to see if anything is frayed or exposed.

And hissin, thanks for the diagram, really simplified it for me. I know the alternators pretty well, but car wiring is a little bit of a mystery to me.

99FiveOh · Mar 6, 2009

Well, I swapped in another regulator and revved it up while sitting still up to 5K rpms, and POP! Fuse blew again. I checked out the alt harness pretty well, but didn't see anything wrong with it. It's still wrapped up pretty good.

I don't know where else to check, it's such a simple circuit. Maybe I need to try a completely different alternator in there and see what that does.

madams74 · Mar 8, 2009

Get a new alt. You may have a short happening from the output to the casing, a leak if you will.

moosetang408w · Mar 9, 2009

If it was shorting from the output to the casing it would not pop a fuse just melt your regulator and/or alternator. You are putting a short in parellel with your charging circut which would cause the output voltage to sag to almost nothing as it pumped as much current as it could through your ground until it stopped working. I think it regulates the current output by voltage differntial so I would believe the culpurate is the voltage regulator expecially if your lights got bright. A short would just pop the fuse.

madams74 · Mar 9, 2009

moosetang408w said:
If it was shorting from the output to the casing it would not pop a fuse just melt your regulator and/or alternator. You are putting a short in parellel with your charging circut which would cause the output voltage to sag to almost nothing as it pumped as much current as it could through your ground until it stopped working. I think it regulates the current output by voltage differntial so I would believe the culpurate is the voltage regulator expecially if your lights got bright. A short would just pop the fuse.

Not so sure about the alternator melting part. There are 2 fusible links that are designed to melt from the main wire off the alternator. The links are made of a special material. They are attached in line near the main fuse panel in the engine compartment. They have a black material like a block of plastic. I had an alt short out from the output lead to the case. Boom! Smoke everywhere, and the car died on the road. It did start back up, but had nothing but battery power from point. I made it home barely before loosing injector voltage and found the links were melted in 2. Ran a continuity check and from the positive post to the frame was a dead short. Get a new alternator man. All the springs, brushes and junk in their are not worth the hassle.

99FiveOh · Mar 9, 2009

Well, tonight it started doing something wierd. I started it up after work and watched the voltage gauge, something was putting a HUGE load on it intermittently. And I know it wasn't the fan because the car was still cold. After about a minute it was charging fine and solid. When I pulled into my driveway at home, it started acting funny again and the lights went dim a couple times. I think whatever ate my last alt is trying to eat this alt too.

Here's my theory now: For the past week I've been feeling a slight jerk in the car while cruising and at times I see the volt gauge jerk as well. And just last night the car wouldn't start at all for a while(cranked over, but no fire), no spark from the coil, but it was getting power and pulsing.

Since I've had the car I can hear a noise coming through the stereo, but only after driving for a little bit, and RIGHT AFTER I see the volt gauge jerk. So I'm kinda leaning toward the coil going bad. At high RPMs it could be causing the alt fuse to blow. Not sure how, but I'm going to replace it to be safe and see what happens. I can always return it if it's not the problem.

madams74 · Mar 9, 2009

Fuses blow from dead shorts or amps past the rating of the fuse. What size fuse are you using? When you say fuse are you talking about the fusible wire links or an actual buss fuse in a socket? I can pull up the wiring if you give me the fuse location.

99FiveOh · Mar 9, 2009

JT (Hissin) already linked me to a wiring diagram for the charging system. It is the buss fuse blowing in the underhood fuse block.

If there's a slight short, it has to be in the alt. I need to just throw a completely different one in there and see where I'm at. I didn't change the rotor inside the alternator, so it could have a small short. I might just pull the alt tomarrow and have it tested at a Alt/Starter shop, they have better test equipment than parts houses.

99FiveOh · Mar 10, 2009

I just got off the phone with a good buddy at the local auto electric shop and as soon as I said "alt fuse popping at high RPM" he automatically replied "rotor is bad" or armature for those who don't know what a rotor is.

He said the windings are shorting out at high rpms due to the centrifigul force pushing them together, and there's a break somewhere in the windings. So now I'm on the lookout for a new rotor.

99FiveOh · Mar 13, 2009

Problem solved! Pulled the alternator apart again and it was obvious. For those who know their way around the inside of an alt, there a slip ring (part the brushes ride on) and at the bottom of the slip ring there's two metal terminals that reach toward the outside of the rotor/armature. They get soldered on at the ends to the rotor itself. Well, on one side the solder job was crap and came loose allowing the terminal end to move outward and touch the metal on the rotor. I'm sure it took alot of RPM (5000 to be exact!) to make it short.

so I resoldered it and now it works perfectly!

toyman · Mar 13, 2009

I'm sure it took alot of RPM (5000 to be exact!) to make it short. so I resoldered it and now it works perfectly![/QUOTE said:
Since the ratio of crank to altenator pulley diameters is probably greater than 2:1 I'd guess that the alternator was spinning at 10k+ rpm.

madams74 · Mar 13, 2009

Actually they spin the same speed since they are connected together. If it were a gear with cogs and the teeth were different ratios, you would actually see a slower alternator. Take a CD and another smaller circular item. Spin the CD one revolution and the object sitting on top will also go 1 revolution. Now if it were internally 2:1 then the pulley would spin 1 revolution and the alternator interior would actually be 1/2 of that running slower. Now that ratio is important that you mentioned to the overall speed of the alternator in relationship with the crank pulley.

99FiveOh · Mar 14, 2009

I really don't want to argue this fact, but if you were to mark the crank pulley, and the alternator pulley, you'd see the alt pulley spin about twice per once revolution of the crank pulley. This is just physics at work here. It doesn't make a difference whether it's cogs, gears, or belts. It's all the same principal.

And one big thing that totally blows your theory out of the water? UNDERDRIVE PULLEYS! If your theory was sound, then underdrive pulleys would be useless!

madams74 · Mar 14, 2009

I believe it was originally stated that the alternator was spinning at 5,000 RPM and it would allow the internal part of the alternator to be spinning at 10,000 RPM. If not, sorry. I took it as someone thought that a 2:1 ratio from the pulley being spun, meant that the internal parts went faster. We agree this is not true, so were cool on everything here.

99FiveOh · Mar 14, 2009

That statement wasn't made by me, it was made by Toyman. Further, when I was talking RPM's I was stating that when my tach in the car would reach 5K, it would blow my alt. fuse.

And I believe I understand what you were trying to say. You were talking about the relationship between the rotor in the alt and the alternator pulley and that they spin at the same speed. That is correct. But if the engine is turning 5000 rpms, then Toyman was trying to say the alternator itself is probably turning 10K rpms.

Alt fuse blowing at WOT, what is going on there?

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