10 Amp Engine fuse on an 89 FI

[big_kid2]

Advance Auto Parts p/n R4169 - $112.99 use coupon TRT30 takes it to 79.00 plus tax

[tooldude]

Yes I still have it

Sent you a PM.

[tooldude]

Oops never mind

[tooldude]

Nitro, I don't know what that is, so much I don't know about this site and computers etc.

[nitrofarm]

Amazing what I just found online googling resistors. I would not have made another post if I had not seen this. You may have heard, as I have, of resistors gaining in value as they age, but not losing resistance. I found some ham radio operators discussing how they go up or down in value as they age. This is the only way this problem makes any electrical sense to me. The main relay coil is good so the engine will crank & run, but the resistor in parallel has lost resistance and is allowing too much current once the motor is running. If the value gets too low it may leave V Reg vulnerable too if it loses its clamping ability. I never dreamed they could do that. This post is for all who like to fully understand these circuits. gunnerw do you still have the old relay? I sure would like to check it out.

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[gunnerw]

Yes I still have it

[tooldude]

Amazing what I just found online googling resistors. I would not have made another post if I had not seen this. You may have heard, as I have, of resistors gaining in value as they age, but not losing resistance. I found some ham radio operators discussing how they go up or down in value as they age. This is the only way this problem makes any electrical sense to me. The main relay coil is good so the engine will crank & run, but the resistor in parallel has lost resistance and is allowing too much current once the motor is running. If the value gets too low it may leave V Reg vulnerable too if it loses its clamping ability. I never dreamed they could do that. This post is for all who like to fully understand these circuits. gunnerw do you still have the old relay? I sure would like to check it out.

[tooldude]

That's really great gunnerw, getting it fixed is what counts! It never quite added up to me. I have to stick with measurements and specs.

[dalebwilson]

Hip Hip Hurray!


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[nitrofarm]

AWESOME!

[gunnerw]

Ok everyone. I found another main relay on Ebay. Put it in and that seems to have been the problem. Bad main relay. Thanks for everyones help. Now to continue... Next, new value cover gasket, exhaust and windshield.

Thanks again everyone

[tooldude]

Good question Nitro - what can we think? I've been trying to figure how to analyze this with little to go on. It began as looking like a bad alternator, as removing it from the circuit appeared to stop the fuse blowing. That can be misleading as unloading a circuit with a good part could remove enough amps to hide the true problem. That was looking at things superficially, but it seemed probable as the Alt seemed to have improper specs. What's needed is actual measurements and specifications. We can test current flow through those 2 relay coils & if excessive enough we would know the relay is bad. If we know its resistance we can accomplish this easier with a voltage drop and be sure about what any measurement means. A relay coil can't have 0 resistance and operate as no magnetic field would be generated. So what percentage of the coils can be shorted and still have function? As tolerances go, could one drop 20%-30% of its resistance and function? IDK the specs but most of these little coils are 40-60 ohms at 225ma-350ma. Losing over 50% of resistance to 20 ohms makes 680ma and that will not blow that fuse. I was looking at them superficially. If we get a 3 ohm resistance in circuit there will be about 4.5A current. So how much amperage is in that circuit?- good question. Looking at a time-current chart from Littlefuse gives some idea. These are approximate, but reasonably close, over-current specs for a 10A fuse: 40% blows around 300 sec/ 44.5% in 100 sec/ 46.2% in 60 sec/ 51% in 20 sec. Sounds like what gunnerw was saying.10A fuse makes an easy calculation of about 4 extra amps, 14A blowing the fuse - if correct, not a relay. That leaves the other possibilities of something connecting into the B/W wire or a "new" alternator bad right out of the box. I've had that happen. Funny, the spec for Alt field coils is 3- 6 ohms. What do you think?

[nitrofarm]

^ LOL Thanks,appears the OP is MIA.......

[dalebwilson]

1st drawing rotated right side up

[tooldude]

Just shows we Festiva types are a determined bunch, dedicated to finding out what's causing all that aggravation and will do whatever it takes, even if we have to get a picture in edgewise, right Nitro? Sorry you had to read that sideways, but at least you know you can still do it. Heck, that might come in handy under the car.