what voltage needed for coil

A

allford

Member
I'm almost ready to test fire my 39 merc. engine. My question is what voltage is needed on the coil. Seems like I rememmber reading that less than 6 volts is nneded on the coil. I plan having 12 volts system on the 37 truck that this motor is going into, I plan on using a 6 volt resistor, but do I need another resistor behind the 6 volt resistor? Everything is orginal on the electrical system. ( set up to run on 6 volts)
I have been bulding this motor all year and I got it back together except rebuilding the carb. which is a model 91-99, and new brushs and bearings on the starter. Any help will be greatly apprecated, and thanks in advance.
 
The original coil uses about 4 volts, meaning 2 volts lost across the dropping resistor. So at 12 volts, you'd need the equivalent of 4 of these dropping resistors connected in series to lose 8 volts. In theory, a 6 volt dropping resistor connected in series with the original 2 volt dropping resistor should work if it can handle the current.

Jack E/NJ
 
A ballast resistor some where in the neiborhood of 1.5 Ohms should work. Otherwise use a Dropping resistor like Randy Rundle sells to drop the voltage to 6 volt just prior to the original ballast resistor. Check out Randy's web-site. www.fifthaveinternetgarage.com

Kerby
 
If you want a 12 volts system,I would get a 12 volt coil and resister,and remove the .5 one under dash.Those old coils are nothing but trouble.If you want to keep the 6 volt coil, they draw about 3 amps,so you would need about 2 ohms with wattage of 18, 3amps x 2 ohms =6 volts.or as said total of 5 x.5 resisters.
 
Agree, buy a 12V coil...get a good brand, like Echlin, and order it for a specific application (like 1965 Mustang, say) and order the correct resistor for that application from the same source. There are lots of different resistance values for different 12V vehicles and you want a match. Remove original Ford 6V resistor from the circuit.
 
Whats wrong with old six volt system,i have two 35,s on six it works just fine. Just put 2700 mi on the car Oct. 13-23 ran just fine.You set them up the why ford built them and they work.Dont forget they sold millions of them.your working on one now!
 
There is a twelve volt coil with a built in resistor that is used on industrial engines just run a twelve volt wire to it.I think the delco # is U501 it was used on hyster fork lifts before going to HEI ign. John
 
Whether you bug coil plus resistor or integral resistor coil in 12, get ones meant for same application! Lots of people on here and the HAMB seem to bu A coil and then a resistor with no relationship at all...there are many different values even within corporations.
 
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