burning up coils / hot coil

[greentruck]

I have a 1970 with a 302 and am still running points style iginition. I have read all the previous post on this but still can fix my problem. I am on my third coil in 3 months (all stock coils). I have only owned it 4 months and the previous owner did not tell me of any ingintion problems.

-The resistor wire reads 2.0-2.2 ohms at the firewall junction.
-Motor off - The resitor wire reads 7.8v with the key in the run position BUT only with the motor not running.
-Motor running - If you start the motor the resistor wire at the firewall juntion reads 12v with the key in both the start and run position.
-The hot wire going to the coil constantly reads 12v in the start and run position.

Thanks for your help,
paul

 

[BUCKNBRONK]

The resister should cut the voltage by 15-25%, so with it on u should be getting like 7-8 volts. Have u tried a new one? What is your alt putting out? , maybe the regulator is bad and while it is running you are putting out more than 14.5 volts. Sounds like it is dropping the voltage strait from the battery but when the alt kicks in it cant handle the xtra voltage. Check out your alternator output 1st, if that is within spec's try a new resistor. hope this helps

 

[RIbronco]

Somethings odd, it should revert back to ~6V after starting.

 

[Old School]

Too many volts.
On my rigged up 69 F-250 firewood truck which was messed up mercilessly by the PO, I finally ran one of the ceramic Chrysler style resistors to get the voltage down.
Ugly but works.

 

[motoman]

Make sure your coil is mounted upright and not on its side... the oil inside stock coils does not cover the coils when on it's side. Will cause overheating and intermittent failure or cause the coils to die......

 

[greentruck]

The coil is mounted upright. The alt is only putting out 14v. Do the resistor wires ever go bad? Is it possiable that the starter bypass is somehow staying engaged and consistanly pussing 12v?

- old school were did you splice in the ceramic resistor? I am assuming you us a 1.4 ohm resistor?

Thanks for your help

 

[dbevans2249]

The coil is mounted upright. The alt is only putting out 14v. Do the resistor wires ever go bad? Is it possiable that the starter bypass is somehow staying engaged and consistanly pussing 12v?

- old school were did you splice in the ceramic resistor? I am assuming you us a 1.4 ohm resistor?

Thanks for your help

The starter bypass wire would be my guess. IIRC it should go from the starter relay I terminal to the coil + terminal. You may want to start it, disconnect the I terminal amd then measure the voltage at the coil if you can. You could trace the wiring some to see if some creative PO spliced anything in some place.

 

[Explorer]

"You are checking at the right spot, but you will read battery voltage whenever the points are open. This is due to Kirchoff's Law of Voltage - an incomplete circuit will read the full applied voltage at any and all points. Both the resister wire and the OE coil are ~1.4 ohms, so you should read half battery voltage when the points are closed. To verify the presence of a resister wire, connect a jumper wire between the coil negative terminal and a verified good ground. With the ignition on, you should read ~1/2 battery voltage at the positive terminal. Do not leave the ignition on for very long to do this check, though, as the constant current through the coil can overheat it.

The purpose of the resister wire is to limit current flow through the coil and points, and you definitely need it if you are using points. Without it, you wold have ~10 amps running through your primary ignition circuit, and it would be a race to see whether your coil overheated or your points failed first. With the resister wire, current is limited to ~5 amps."
Copied this from another forum. Simpliest written explanation I have seen for awhile.

 

[greentruck]

The starter bypass wire would be my guess. IIRC it should go from the starter relay I terminal to the coil + terminal. You may want to start it, disconnect the I terminal amd then measure the voltage at the coil if you can. You could trace the wiring some to see if some creative PO spliced anything in some place.

I tried this and still read full voltage at teh positive coil terminal. So I guess that eliminates my theory about the starter bypass. Off to chase wires again to see if I can find a splice or fray in the resistor wire that may be causing this.

 

[greentruck]

"You are checking at the right spot, but you will read battery voltage whenever the points are open. This is due to Kirchoff's Law of Voltage - an incomplete circuit will read the full applied voltage at any and all points. Both the resister wire and the OE coil are ~1.4 ohms, so you should read half battery voltage when the points are closed. To verify the presence of a resister wire, connect a jumper wire between the coil negative terminal and a verified good ground. With the ignition on, you should read ~1/2 battery voltage at the positive terminal. Do not leave the ignition on for very long to do this check, though, as the constant current through the coil can overheat it.

The purpose of the resister wire is to limit current flow through the coil and points, and you definitely need it if you are using points. Without it, you wold have ~10 amps running through your primary ignition circuit, and it would be a race to see whether your coil overheated or your points failed first. With the resister wire, current is limited to ~5 amps."
Copied this from another forum. Simpliest written explanation I have seen for awhile.

After grounding the coil's negitive terminal, the voltage at the positive coil terminal with the ignition on reads 5.5-6.2 volts. Does that mean my resistor wire is in good working condition? Also, by ignition on you mean the key in the run position, but not the motor running correct? I would not think it start with coil grouneded out.

If so, why else might I have gone through 2 coils in 2 months with light driving?

Thanks for your help, paul

 

[Explorer]

Yes, it would seem resistor wire is intact. The coil itself should read .08-1.6 ohms across pos and neg, either one to center post, 7700-10500 ohms(no wires attached). I find it hard to believe you would have gotten that many bad coils. Make sure a PO hasn't wired anything else into that circuit.

 

[greentruck]

2.1 ohms from peg to peg and 10680 from peg to center post. It is a BWD stock replacement coil. I am out of time for today, I will start checking for splices and frays tomorrow. Thanks for your help and advice Explorer

 

[greentruck]

I traced all the wires and did not find anything of note. I have been driving it almost daily for the last week and a half with no issues. The first coil looked like it could have been the orginal and the second coil started to leak a little of oil before it blew. So I guess I just had the original die and got a bad replacement.

 

[Explorer]

Hopefully it holds up. It's a shame, but hard to get good parts anymore and not unusual. Makes it rough when you can't trust a new part and have to question yourself if it's really fixed.