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Old 07-19-2011, 11:22 AM   #1
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Default NA fuse/relay box resistance

Does anybody know what the resistance of the ground going into the fuse/relay box (the one in the engine compartment) should be? I am referring to the black wire that looks like it should be a ground and have very low resistance when checked against the chassis/negative terminal on the battery.

My chassis/engine to battery is .2Ω and the black wire in the fuse/relay box was something like 250KΩ. Is that rite?
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Old 07-19-2011, 11:26 AM   #2
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why would a ground be in a fuse box (other than the two relays)?

the Black wire that that connects to the MAIN and Retractor fuses comes directly from the battery.
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Old 07-19-2011, 01:51 PM   #3
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That's what I thought, and the resistance on it is lofty. Gotta figure more **** out now :(
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Old 07-19-2011, 02:08 PM   #4
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What year is the car, and what is the specific problem that you are trying to solve?
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Old 07-19-2011, 03:02 PM   #5
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It is a 90 and I am trying to make sure all of my grounds in the car are adequate.

To answer my own question the black wire that goes into the fuse/relay box is directly from the positive terminal of the battery and the resistance of it was actually 455KΩ so all is well more or less. Thanks again mt!

Last edited by astroboy; 07-19-2011 at 07:41 PM.
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Old 07-22-2012, 08:42 PM   #6
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I keep blowing out my main fuse, the one with the black wire going to it. Previously the negative terminal was touching metal in the trunk and melted to it. I got a new terminal and that is when the fuse blew. Any ideas?
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Old 07-22-2012, 10:30 PM   #7
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Is any fuse other than the main fuse also blowing?
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Old 07-23-2012, 09:07 AM   #8
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I didn't see any other goners but ill look again. I should have been more specific, the battery is in the trunk and it moved so that the negative side was touching metal and melted to it. I banged it off and got a new terminal. At that time I was able to start the car no problem. A few days later only the gauges would illuminate and only recently no gauges and no power, it keeps getting worse which is strange. This is when I noticed the fuse. Changed out the fuse and it immediately blew again when I tried to crank the car, wasn't getting gauges to illuminate at this point either. So is there a possible short and the battery is sending too much power to the main fuse? I have no idea how to fix this and the main fuse is $3 something so I don't want to keep blowing them out
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Old 07-23-2012, 10:31 AM   #9
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Here are a couple other threads regarding the main fuse. They didn't help much though

Main Fuse Keeps Blowing - MX-5 Miata Forum

Blown main fuse
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Old 07-23-2012, 11:53 AM   #10
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Quote:
Originally Posted by TheDriver View Post
the battery is in the trunk and it moved so that the negative side was touching metal and melted to it.
Ok.

The battery's negative terminal is supposed to be securely bonded to chassis- that's where the negative wire goes.

Under normal circumstances, the fact of the battery's negative terminal coming into contact with chassis would cause absolutely nothing- no sparks, no melting, no current flow.

The fact that yours did means that the wire which goes to the battery negative terminal ISN'T making good contact with chassis ground. So fix that first- repair or replace the battery negative cable.
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Old 07-23-2012, 12:52 PM   #11
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I was going to say the same thing. Also if you actually banged the battery to get it off the inner fender you may have damaged cells/plates (something to look into once you've fixed your neg terminal problem).
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Old 07-23-2012, 02:07 PM   #12
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Quote:
Originally Posted by Joe Perez View Post
Ok.

The battery's negative terminal is supposed to be securely bonded to chassis- that's where the negative wire goes.

Under normal circumstances, the fact of the battery's negative terminal coming into contact with chassis would cause absolutely nothing- no sparks, no melting, no current flow.

The fact that yours did means that the wire which goes to the battery negative terminal ISN'T making good contact with chassis ground. So fix that first- repair or replace the battery negative cable.
The negative terminal didn't melt to the metal...my bad. It was the connector on the wire that did, the little metal piece that is clamped onto the wire. I have since replaced it. I see what you're saying though and you might be right. The battery was sliding around the trunk and was probably yanking on the grounding location.

Also, I'm an idiot and didn't know about removing the negative side first and putting the negative side on last ya I'm a noob

Last night I also attempted to use the battery out of my GF's car which is a 2012 sonata so the battery is mint and I still got nothing. It's strange that at first I was getting gauges to illuminate but now nothing.

I think because I didn't remove the neg side first etc I may have a short somewhere in the cable? Possibly the alternator shorted?

I almost want to replace the fuse and try to do the correct negative terminal removal/install and see if it works but I don't want to risk popping another fuse ffuuuuu
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Old 07-23-2012, 02:40 PM   #13
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I am really confused by your description.

First, this business of "negative first" has no real impact on the car itself. It's merely a safety precaution to prevent you from shorting out the battery itself by accidentally touching the chassis with the wrench you're using to remove the positiver terminal. But since the negative terminal is so damned hard to reach on a Miata (at least, on an NA) I always remove positive first (or positive only, if I merely need to disconnect the battery for a reason other than removing it) and I simply make sure not to short it out with my wrench.

To recap: It does not matter in which order you remove or install the battery wires.

Also, wires don't "short" internally. By definition, a wire *is* a short.


Now, I'm assuming that it's only the 80A main fuse which is blowing, and none others. Right? That narrows things a bit, as very few things are fed directly by the main fuse.

(And I'm assuming that the reason you get nothing with the Sonata's battery installed is because the fuse blew again, and not because you hooked up that battery backwards or something like that.)

Obviously you can try removing the heavy white wire from the alternator and re-testing. Also, inspect the heavy white wire to make sure it's not shorting to chassis or engine somewhere. (eg: abraded insulation.)

Does the fuse blow immediately, or does it not blow until after you turn on the key? If the latter, the starter solenoid could also be at fault (Black/Red.)
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Old 07-23-2012, 02:45 PM   #14
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Quote:
Originally Posted by Joe Perez View Post
Does the fuse blow immediately, or does it not blow until after you turn on the key? If the latter, the starter solenoid could also be at fault (Black/Red.)

interesting advice.
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Old 07-23-2012, 03:07 PM   #15
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interesting advice.
Why, because it's correct?

The main power for the starter is unfused, however the power for the solenoid, which comes through the keyswitch and the clutch-interlock switch, is fused only by the main fuse.

I have no idea what the normal operating current of the starter solenoid is, and pulling enough power through the keyswitch to blow the main fuse would certainly generate some heat, but it's a possibility that can't be ignored.
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Old 07-23-2012, 03:25 PM   #16
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Quote:
Originally Posted by Joe Perez View Post
I am really confused by your description.

First, this business of "negative first" has no real impact on the car itself. It's merely a safety precaution to prevent you from shorting out the battery itself by accidentally touching the chassis with the wrench you're using to remove the positiver terminal. But since the negative terminal is so damned hard to reach on a Miata (at least, on an NA) I always remove positive first (or positive only, if I merely need to disconnect the battery for a reason other than removing it) and I simply make sure not to short it out with my wrench.

To recap: It does not matter in which order you remove or install the battery wires.

Also, wires don't "short" internally. By definition, a wire *is* a short.


Now, I'm assuming that it's only the 80A main fuse which is blowing, and none others. Right? That narrows things a bit, as very few things are fed directly by the main fuse.

(And I'm assuming that the reason you get nothing with the Sonata's battery installed is because the fuse blew again, and not because you hooked up that battery backwards or something like that.)

Obviously you can try removing the heavy white wire from the alternator and re-testing. Also, inspect the heavy white wire to make sure it's not shorting to chassis or engine somewhere. (eg: abraded insulation.)

Does the fuse blow immediately, or does it not blow until after you turn on the key? If the latter, the starter solenoid could also be at fault (Black/Red.)
I was thinking that maybe removing them in the wrong order and touching something that they weren't supposed to it may have caused a short. I hardly know anything about wiring as you probably can tell.

I think I am going to try what you mentioned above and make sure the negative cable is grounded appropriately.

I'm unsure as to when the fuse blows exactly. I got a new one and turned the key and nothing happened, I looked under the hood and of course it was toast.

Indeed only the main black fuse is blowing. I wish there was a knowledgeable North Carolina guy that could come get this thing firing again!

I want to thank you for being patient and helpful. I'd buy ya a beer if I knew you...and if you fixed the problem I'd buy ya a keg

Last edited by TheDriver; 07-23-2012 at 03:43 PM.
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Old 07-23-2012, 03:48 PM   #17
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Quote:
Originally Posted by TheDriver View Post
I was thinking that maybe removing them in the wrong order and touching something that they weren't supposed to it may have caused a short.
That's a negatory, good buddy.

With the engine off, the battery is the only source of voltage (which is just potentially current, really) in the car.

With either terminal removed from the battery, no current can flow anywhere, no matter what is touching what. You can remove the positive terminal from the battery and connect it directly to chassis, and no harm will come of it.

The only things you can't do without potentially causing some damage are:

1: Connect the battery backwards,
2: Create a short-circuit between the positive and negative terminals of the battery, such as by creating a connection between the positive wire and chassis while both are still connected (as with the handle of a wrench), and
3: Attempt to operate the electrical system (especially the starter) while any of the ground straps (engine to body, PPF to body, body to battery negative) are loose, frayed, or otherwise sub-optimal.



Quote:
I think I am going to try what you mentioned above and make sure the negative cable is grounded appropriately.
This is a good idea in general, however a bad ground isn't causing your main fuse to blow.



Quote:
I'm unsure as to when the fuse blows exactly. I got a new one and turned the key and nothing happened, I looked under the hood and of course it was toast.
Well, we need to find that out.

Disconnect the white wire from the alternator, insulate it, and leave it hanging. Try powering the car up (key to ACC, then RUN, but not start) and see if the fuse blows. Inspect that wire carefully.

If everything is working, try starting the car. First with just key to START but without depressing the clutch. Did that blow the fuse? If not, proceed to actually start the engine. Did that blow the fuse? If so, you found the bad circuit. If not, you still found the bad circuit. Either way, you'll have your answer at that point.
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Old 07-23-2012, 04:30 PM   #18
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Originally Posted by Joe Perez View Post
Why, because it's correct?

The main power for the starter is unfused, however the power for the solenoid, which comes through the keyswitch and the clutch-interlock switch, is fused only by the main fuse.

I have no idea what the normal operating current of the starter solenoid is, and pulling enough power through the keyswitch to blow the main fuse would certainly generate some heat, but it's a possibility that can't be ignored.

see my DD thread.
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Old 07-23-2012, 04:34 PM   #19
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see my DD thread.
I stopped following that thread when the first idea I had in it proved to be wrong.
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Old 07-23-2012, 06:08 PM   #20
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Originally Posted by Joe Perez View Post
That's a negatory, good buddy.

With the engine off, the battery is the only source of voltage (which is just potentially current, really) in the car.

With either terminal removed from the battery, no current can flow anywhere, no matter what is touching what. You can remove the positive terminal from the battery and connect it directly to chassis, and no harm will come of it.

The only things you can't do without potentially causing some damage are:

1: Connect the battery backwards,
2: Create a short-circuit between the positive and negative terminals of the battery, such as by creating a connection between the positive wire and chassis while both are still connected (as with the handle of a wrench), and
3: Attempt to operate the electrical system (especially the starter) while any of the ground straps (engine to body, PPF to body, body to battery negative) are loose, frayed, or otherwise sub-optimal.



This is a good idea in general, however a bad ground isn't causing your main fuse to blow.



Well, we need to find that out.

Disconnect the white wire from the alternator, insulate it, and leave it hanging. Try powering the car up (key to ACC, then RUN, but not start) and see if the fuse blows. Inspect that wire carefully.

If everything is working, try starting the car. First with just key to START but without depressing the clutch. Did that blow the fuse? If not, proceed to actually start the engine. Did that blow the fuse? If so, you found the bad circuit. If not, you still found the bad circuit. Either way, you'll have your answer at that point.
Just so I'm more aware of what is going on...what would be the possible cause and solution of each scenario?
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