Coil to points wire burned up--why?

OK--I'm working on my dad's Ford 3000 with a backhoe/loader on it.

It was running well this spring but when I went to start it this weekend it was clear I didn't have any fire--checked the distributor and saw the wire between the coil and the points had deteriorated, which did not surprise me. Dad's had it probably 10 years and it as never seen the inside of a building.

So, I put on a new coil wire, and points and condenser for good measure since they were the worst I ever saw--condenser case completely covered with rust. Reused the old coil since it was fine in the spring. I gapped the points and ohmed them to make sure I wasn't shorted to the distributor case on the coil wire side, and it all looked good. Put the caps and wires on (no picnic on this tractor), and attempted to start it up. Within seconds I had smoke as the coil wire insulation burnt up!

So--I usually work on one of the two 8Ns he also owns and have changed points and coil wires before. I know it is not uncommon for a points to slip closed after being gapped but I have not see this to be a cause of a coil wire burning up. Maybe that is what happened to the old one.

What do you all think? I know there is a dead short somewhere. Maybe I caused it when I was fumbling with the cap but I don't know, the screws were tight. Feel pretty dumb about this.
 
something was definitely touching somewhere it shouldn't.

What gauge wire did you use? I believe it should be #14.

How many wires were at the coil? Should've been 2 to the + side (key on/run resistor wire and a 12v jumper from the I terminal of the solenoid) and 1 to the - side (from the points).

Sounds to me like maybe the 12v solenoid wire was on the - side with the points wire?
 
Well, Gears, I think it is wired correctly. There are two wires going to the + terminal and it impossible to get that wrong because they are crimped together in one ring terminal.

I don't believe that the wire was as heavy as 14 gauge though. Maybe it was 18?

I should add though that the coil was hot when I was doing my autopsy. I could not really see the points and condenser at that point though. It was getting dark (so was my mood!)
 
New condenser could be bad right out of the box and have a short in it which might cause that. Or the insulator in the side of the distributor may be shorted out and that too would cause that. But still a bit odd since that wire has voltage on it all the time unless some how the coil it self is also shorted out putting full battery volts on that wire and full battery amps
 
Charlie,

Most (if not all) of the guys on this forum know more than I do, so take their advice first.
So, my opinion would be that you need a new coil.


I believe that the primary winding of the coil has a dead short in it. If the points slipped and stayed closed all of the time, if the condenser were shorted, or if the wire from the coil to the points were shorted to the distributor, the wire from the coil to the points would not get hot enough to melt the insulation.

All of those conditions are essentially the same as turning off the engine, and then turning the key back on while the points were closed. Sure enough, the battery would run down, but the wire from the coil to the distributor would not get hot enough to melt the insulation.

If, on the other hand, the primary winding in the coil were shorted out, you would have a dead short from battery plus voltage to ground through the wire from the coil to points. That would be a HOT situation.

Please let us know what you finally do to fix the problem. That would be of educational value.

Tom in TN
 
Does the coil have internal resistor? When most people changed 6 volt systems they sometime got a 12 volt
coil that had to use a external resistor. This would burn points really fast if they didn't use a
resistor. Have you replaced the coil anytime?
 
Thanks Tom, Old, and Miner09 for all your replies.

I'm leaning toward the old coil being the problem, fellows.

Miner, I've never changed the coil, and it has no markings on it. I don't see an external resistor anywhere--the wires come out of what would appear to be the original wiring harness so would it be hidden somewhere? It definitely is 12V and the voltage regulator box is Lucas, but it looks like the one that was on my 1967 MG Midget, not the one on my 49 8N.

So do you all suppose I toasted my new points and condenser? it was dark but the insulation on the condenser didn't seem to be melted--I couldn't see the points. I was kind of in shock to tell you the truth.
 
Replace points and condenser and check where the wire goes through the distributor. It has a insulator to
keep it from grounding out. When you buy coil make sure it has internal resistor. Not sure if you can
even buy coil using external resistor. I see a lot of tractor with it. Good luck.
 
Charlie,

I have a 1973 Ford 2000 3 cylinder gas. The "resistor" that is between the ignition switch and the coil is actually a long piece of resistive wire. It looks just like regular old wire, but it isn't. Mine is just plain old coiled up wire in a coil that is about 3 inches in diameter and has about 5 or 6 loops in it. I think it is brown in color, but I'm not positive about that.

You should have two wires going to the plus terminal on the primary winding of the coil. One is a short jumper from one of the small posts on the starter solenoid (Ford actually calls it a starter relay). The other is from the ignition switch. In the case of my 2000, that second wire is the resistive wire that is coiled up and taped to the jumper wire from the starter solenoid to the coil.

I don't know about your tractor. It might have an honest-to-goodness resistor somewhere, or if it's like my tractor, it's that coil of resistive wire.

Tom in TN
 
NO COIL has a built in resister they are just made different so as to work on 12 volts with out a resister in the system any place
 
Well I'm a bit baffled as to what coil to buy.

There are a pair of brown wires, which went to a single ring terminal, which went to the +side of the coil. One wire goes to the solenoid post, the other into the bundle and on to the ignition switch. But there is no coil of looped wire as you mention. The two wires are about the same length.

If this is a resistive wire, wouldn't that drop the voltage? I have 12 volts there. So, I would assume I would get a coil that says "no external resistor required"?
 
Charlie,

When you say that you have 12 volts there - - 1. is that with the wire connected to the positive side of the coil? AND, 2. is that with the wire to the points connected to the negative side of the coil? AND, 3. are the points closed right now? If so, the wire to the points should be on fire right now.

This is a really important point because if all three of the above questions are answered "yes", your problem is probably NOT the coil. I doubt that all three of the answers are "yes", but it's really important to know.

Tom in TN
 
Hi Tom
Well, everything is disconnected now, of course. But--yes, the 12V wire (presumably from the ignition switch) was connected to the + side of the coil. This seemed normal to me, since I have worked on converted 8Ns that have no resistor in the line.

I'm sorry, I can't tell you what the voltage was on the — side of the coil at this point. (the actual tractor is at my dad's 20 miles away) I never measured it, but wouldn't it

Remember, this was a running tractor last spring, so the absence of this coil of resistive wire was not an issue then. I started working on it because the coil to dizzy wire had disintegrated and that is when the new points and condenser were installed.
 
Measure the wire from the coil to the ignition switch with an ohmmeter. If it is not a resistance wire it should measure zero resistance, or maybe a couple of hundredths of an ohm. If it is a resistance wire then it should measure (if my memory serves me correctly) somewhere around 1.5 to 2.0 ohms.
 
Charlie, did you take a resistance reading on the coil? That should tell you if you have a burnt/shorted winding on the primary side (I believe it should show 3 ohms or so of resistance if it's 0 then it's shorted).

I was thinking it was wiring at the coil because it didn't happen until you changed out the points/condenser. But it could be a bad condenser or a wire got pinched under the cap.

If you have a volt meter you can check for continuity between the points wire and ground without taking the cap off.
 
Sounds like the coil is fried and need to be replaced. You can hook up a wire right form the battery to the ignition side of a coil and run most any engine wired that way with out any wires melting when you do so. So the coil has to be shorted up and that would let things smoke.
 
if the electrical system is 12V make sure the coil is around 3.2 ohms resistance. If it"s a 6 volt coil on a 12 volt system, the resistance will be around 1.6 ohms and allow the ignition current to be doubled from ~4 amps to ~ 8 amps. That could be the problem and that high of an ignition current can burn you points and eventually your coil.
 
Well, if he does go purchase/install a 3 Ohm coil, he needs to be sure that the factory wire-type ballast resistive wire is no longer there.
 
Thanks for your replies, fellows. So, let's just assume that the existing coil is bad. So the question comes down to what coil to buy.

To review--there are two wires going to the plus side of the coil, one terminates at the solenoid The other goes back into the harness, to the ignition switch, presumably. They are about the same length.

So it seems possible that at some point in the long life of this old tractor that someone might have just snipped off the extra resistive wire.

Sean suggested ohming the wire between the ignition switch and the coil--however I don't see how to get easily get access to the ignition switch though. Any other way to tell what we are looking at here?

I am not sure I understand why Ford would use the resistive wire here--it seems to make this unnecessarily complicated.
 
Thanks for your replies, fellows. So, let's just assume that the existing coil is bad. So the question comes down to what coil to buy.

To review--there are two wires going to the plus side of the coil, one terminates at the solenoid The other goes back into the harness, to the ignition switch, presumably. They are about the same length.

So it seems possible that at some point in the long life of this old tractor that someone might have just snipped off the extra resistive wire.

Sean suggested ohming the wire between the ignition switch and the coil--however I don't see how to get easily get access to the ignition switch though. Any other way to tell what we are looking at here?

I am not sure I understand why Ford would use the resistive wire here--it seems to make this unnecessarily complicated.
 
(quoted from post at 23:19:25 10/19/16) Thanks for your replies, fellows. So, let's just assume that the existing coil is bad. So the question comes down to what coil to buy.

To review--there are two wires going to the plus side of the coil, one terminates at the solenoid The other goes back into the harness, to the ignition switch, presumably. They are about the same length.

So it seems possible that at some point in the long life of this old tractor that someone might have just snipped off the extra resistive wire.

Sean suggested ohming the wire between the ignition switch and the coil--however I don't see how to get easily get access to the ignition switch though. Any other way to tell what we are looking at here?

I am not sure I understand why Ford would use the resistive wire here--it seems to make this unnecessarily complicated.
This isn't an 8N. The wiring is quite a bit different.
It's not all that complicated though. Similar to what they
used on the cars and pickups with the four post solenoid.

Why not just disconnect the coil and measure its resistance?
You can't always tell if a coil is good or bad that way, but if
you have a decent meter and the coil is a dead short your
meter should be able to tell that while it's disconnected.

Granted, it would be more capable of measuring the wire.
You may have to remove the nut holding the switch to the
cowl and drop the switch out underneath the tin to get to it.

I'd hate to see you go spend money on a coil when it may not
be needed. If you get the original one and the resistor wire is
not there it may still burn wires and/or the new coil.
If you get a true 12V coil and the resistor wire is there and
intact, the tractor may not start even if everything else works.

As far as I know, all Ford 3000 tractors were 12V so you
shouldn't have to worry about a conversion issue.
Someone please correct me if that's not correct.
 
(quoted from post at 23:57:39 10/19/16)
(quoted from post at 23:19:25 10/19/16) Thanks for your replies, fellows. So, let's just assume that the existing coil is bad. So the question comes down to what coil to buy.

To review--there are two wires going to the plus side of the coil, one terminates at the solenoid The other goes back into the harness, to the ignition switch, presumably. They are about the same length.

So it seems possible that at some point in the long life of this old tractor that someone might have just snipped off the extra resistive wire.

Sean suggested ohming the wire between the ignition switch and the coil--however I don't see how to get easily get access to the ignition switch though. Any other way to tell what we are looking at here?

I am not sure I understand why Ford would use the resistive wire here--it seems to make this unnecessarily complicated.
This isn't an 8N. The wiring is quite a bit different.
It's not all that complicated though. Similar to what they
used on the cars and pickups with the four post solenoid.

Why not just disconnect the coil and measure its resistance?
You can't always tell if a coil is good or bad that way, but if
you have a decent meter and the coil is a dead short your
meter should be able to tell that while it's disconnected.

Granted, it would be more capable of measuring the wire.
You may have to remove the nut holding the switch to the
cowl and drop the switch out underneath the tin to get to it.

I'd hate to see you go spend money on a coil when it may not
be needed. If you get the original one and the resistor wire is
not there it may still burn wires and/or the new coil.
If you get a true 12V coil and the resistor wire is there and
intact, the tractor may not start even if everything else works.

As far as I know, all Ford 3000 tractors were 12V so you
shouldn't have to worry about a conversion issue.
Someone please correct me if that's not correct.
ounds right to me.
 
Disconnect the wire from the coil to the ignition switch at the coil end and measure the resistance from that end of the wire to the hot post on the battery when the key is in the run position. No need to try to get to the back of the key switch.
 
Ok--
So I took off the old coil this morning and measured the primary circuit--zero ohms--so we can conclude that it is bad.

Per Sean's instructions, I turned the ignition to the on position and measured the resistance between the + post on the battery and the end of the wire that goes to the + post on the coil. That measures 39 ohms--so I do have resistive wire in there. I'm sort of surprised it is that much. Does that sound right?

So I guess I am looking for a coil that says, 'use with resistor,'? Yes? Anyone have a part number?

Thanks. Charlie
 
(quoted from post at 19:59:00 10/23/16) Ok--
So I took off the old coil this morning and measured the primary circuit--zero ohms--so we can conclude that it is bad.

Per Sean's instructions, I turned the ignition to the on position and measured the resistance between the + post on the battery and the end of the wire that goes to the + post on the coil. That measures 39 ohms--so I do have resistive wire in there. I'm sort of surprised it is that much. Does that sound right?

So I guess I am looking for a coil that says, 'use with resistor,'? Yes? Anyone have a part number?

Thanks. Charlie
y assessment is that you measured the 39 incorrectly, thus meaning that you missed the zero Ohms, too.
Buy a few resistors around one ohm and around 10 Ohms and practice measuring those.
 
Charlie,

39 ohms is too much. Whether there is a discrete resistor hiding up in there someplace, or if it is only resistive wiring, the resistance should be about 4 ohms. Is there any chance the 39 ohms is actually 3.9 ohms?

Maybe someone else will correct me, but that sounds to me like way too much resistance in that primary circuit.

Tom in TN
 
Yes, it was 10x what I was expecting, is possible I misread, although I am using a digital Wavtek WDMx25 meter--so whether the decimal point is in place or not is pretty obvious.

Electronics isn't my day job, JMOR, but my hobby for many years has been building nixie clocks on boards I designed myself with 1960s-era transistors and CMOS chips. I've made hundreds of thousands of measurements with that meter. I'm no stranger to points and condensers either.

Still, one can err.
 
IF you need one, CNH lists the coil part number as 87726663.
I would assume there are local cross references cheaper.
Although it only lists at about $26 online.

Doesn't taking the measurement of the resistor wire the way
you did leave several connections in the path? Including the
switch itself? I'd want to narrow that down if it were me.
Rule out a bad connection or dirty switch contacts causing
your reading to be too high.

Still, isn't that reading contrary to your symptoms?
It seems if the resistance was that high you'd get low current
and no spark at all. Not burnt wires. Just thinking out loud.
 
(quoted from post at 22:44:53 10/23/16) IF you need one, CNH lists the coil part number as 87726663.
I would assume there are local cross references cheaper.
Although it only lists at about $26 online.

Doesn't taking the measurement of the resistor wire the way
you did leave several connections in the path? Including the
switch itself? I'd want to narrow that down if it were me.
Rule out a bad connection or dirty switch contacts causing
your reading to be too high.

Still, isn't that reading contrary to your symptoms?
It seems if the resistance was that high you'd get low current
and no spark at all. Not burnt wires. Just thinking out loud.
agree, Royce. It will be interesting to hear the "rest of the story", even though we often never do. Res that high does not lend itself well to burned ~14ga wires and the shorted coil doesn't either, because of the tiny 24-26ga wires in the coil windings will fuse well before the tractor wiring.
 

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