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Hot wire under dash resistance wire getting to hot Rate Topic: -----

#1 User is offline   Classic Cars 

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Posted 19 October 2011 - 07:12 PM

I am new to FBN. I am working on my daughters 69 bird. Engine is a fresh rebuild 350 ci. no miles on the engine. I have a resistance wire under the dash that gets hot when the ignition is in the run position but engine not running. When the engine is running this wire does not get as hot. This wire is the blk pink print according to the factory schematic. In the car the wire is blk with writing that says (resistance do not cut). The wire ties in from the fuse block, with the backup lamp switch, the directional signal flasher then to position E on the ignition switch 6 pin connector. I have disconnected the 2 lamp switches and turned on the ign. to see if the problem goes away. This did not solve the problem. Eventually I had an idea and disconnected the coil from the load wire and the resistance wire does not get hot when ign was turned on. I have solved a lot of electrical issues in my days but this one has me stumped.
We have purchased a new engine harness, the old one was butchered. There is one other thing to point out, the coil load wire does not get hot as well as the other wires that are in this part of the circuit, only the resistance wire is getting hot to the point where you can smell the insulation. Is this a problem with the msd coil pulling to much load or something else i am not seeing in the schematic? Please, Any help will be appreciated.


Thanx
Robert AKA Classic Cars
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#2 User is offline   Formula71 

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Posted 20 October 2011 - 01:39 PM

http://i582.photobucket.com/albums/ss266/melissajean88/Sweetpea%20Smilies/bump.gif


Great to have you and your daughter here Robert. Post some pictures of the car when you get a chance.


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#3 User is offline   GaryDoug 

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Posted 20 October 2011 - 02:16 PM

I think that is normal "resistance" wire that took the place of resistors in series with a coil, that was used in other/older cars. It is normal to get warm when the ignition is on and the engine is not running. It keeps the points from burning up while the engine is stopped. While it is running, the circuits get less average power. I wasn't completely sure about this until I searched and found this in another forum: "The resistance wire was indeed used to reduce voltage to the coil primary circuit for normal running. Other makers used an external ballast resistor for the same purpose." FYI, it is made of nickle-chromium aka "Nichrome", not copper. Don't cut it unless you want to make one with less resistance.
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#4 User is offline   Classic Cars 

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Posted 20 October 2011 - 03:32 PM

Already done. check the garage.

View PostFormula71, on 20 October 2011 - 01:39 PM, said:




Great to have you and your daughter here Robert. Post some pictures of the car when you get a chance.




Thanx GaryDoug This makes sense just never seen it done this way before.

View PostGaryDoug, on 20 October 2011 - 02:16 PM, said:

I think that is normal "resistance" wire that took the place of resistors in series with a coil, that was used in other/older cars. It is normal to get warm when the ignition is on and the engine is not running. It keeps the points from burning up while the engine is stopped. While it is running, the circuits get less average power. I wasn't completely sure about this until I searched and found this in another forum: "The resistance wire was indeed used to reduce voltage to the coil primary circuit for normal running. Other makers used an external ballast resistor for the same purpose." FYI, it is made of nickle-chromium aka "Nichrome", not copper. Don't cut it unless you want to make one with less resistance.

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#5 User is offline   Classic Cars 

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Posted 20 October 2011 - 03:37 PM

One more question Gary. Do yoiu think I should isolate this wire from the rest of the harness? I am thinking i should because the MSD coil might be using more power/load. What i mean is to put this wire by itself outside the rest of the wires in the harness.

This post has been edited by Classic Cars: 20 October 2011 - 03:41 PM

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#6 User is offline   GaryDoug 

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Posted 20 October 2011 - 06:59 PM

View PostClassic Cars, on 20 October 2011 - 03:37 PM, said:

One more question Gary. Do yoiu think I should isolate this wire from the rest of the harness? I am thinking i should because the MSD coil might be using more power/load. What i mean is to put this wire by itself outside the rest of the wires in the harness.


That is a good idea and certainly couldn't hurt. I assume you have only the coil from MSD, not their ignition system also. And that it connects directly to the distributor, right? Did the coil come with a ballast resistor also? If so, was it installed? If the ballast resistor was installed, you won't need (and probably shouldn't use) the resistance wire and can replace it with a regular wire. If there is no ballast resistor and it's a conventional ignition system, the wire will dissipate about 8-15 watts of heat, depending on it's resistance. That is quite a bit of heat to keep bundled up. Keep in mind this resistor/resistance wire is just a fail-safe in case you leave the ignition on and don't have the engine running. Extends the life of the points and coil also.
If it were me, I would try to measure the resistance of the wire. The ballast resistor from MSD is 0.8 ohms. You want your wire to be about the same, give or take maybe 20%. Measuring a resistance that low is a bit tricky. You will need to use some sort of offset/null method (Measure the resistance by shorting the two meter leads together. Then measure the wire, using the same two leads. Subtract the first measurement from the wire measurement). If it isn't fairly close to 0.8 ohms, I would look for a reason. But then, I'm sort of an electrical pedant ;-)

Follow-up: Ok now I have some advice. Get the MSD ballast resistor and install it, then replace the stock resistance wire with a regular one.

I just did some calculations using the specs from the 1972 manual and it look like the stock resistance wire with the 0.7 ohm MSD coil will generate about 48 watts of heat, versus about 8 watts originally. That is way too much heat for that wire's insulation. FYI, the specs were: 7-8 volts across the coil normally, 1.7-2.0 ohms for the coil. Using those values, the original resistance wire should measure about 1.1 ohms.

This post has been edited by GaryDoug: 20 October 2011 - 07:30 PM

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#7 User is offline   Formula71 

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Posted 20 October 2011 - 08:02 PM

Thanks Gary. I knew if you saw this topic you'd have some input.
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#8 User is offline   Classic Cars 

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Posted 20 October 2011 - 08:32 PM

Thanx Gary. You have been a big help. That is why we could smell hot insulation. The MSD coil did not come with a ballast resistor. I will add a resistor in the load wire and replace the resistance wire with a regular wire. This is the original point ignition system.

This post has been edited by Classic Cars: 20 October 2011 - 08:35 PM

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