Hot wire under dash resistance wire getting to hot
Posted 19 October 2011 - 07:12 PM
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.
Robert AKA Classic Cars
Posted 20 October 2011 - 02:16 PM
Posted 20 October 2011 - 03:32 PM
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.
Posted 20 October 2011 - 03:37 PM
This post has been edited by Classic Cars: 20 October 2011 - 03:41 PM
Posted 20 October 2011 - 06:59 PM
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
Posted 20 October 2011 - 08:32 PM
This post has been edited by Classic Cars: 20 October 2011 - 08:35 PM