Resistor voltage drop.

Phil Gillespie

Phil Gillespie

Well-known member
Been checking some resistors as i have my resistor mounted just before the coil. 39 stanadard coupe 6v running a Skips coil.
These resistors are only showing 2.5 plus volts. About 3 i have checked are in this area.
This is too low and understand it should be in the region of 4.5 volts is this correct.
Where to get a decent resistor.
Phil NZ
 
If the resistor's voltage drop is showing 2.5+ volts out of 7+ running volts, then the voltage drop across the coil is about 4.5 volts. This seems about right to me. Jack E/NJ
 
A thousand distributors and a million retailers in the US have resistors and can get the value you need. Automotive use is probably only 10% of those applications that power resistors are used in. Oh yes, north or south island?

Phil - Firstly determine what you have and what you want and we'll get one for you. Regardless of which island including Tasmania.

Are you able to read ohmic value on your resistor? If so, what is it? If not, are you able to put an ammeter in the coil circuit and read the amount of current through the coil with resistor? Then and only then, read the voltage across the resistor.
 
The car was running good at idle showing a steady 19 to 20 inch of vaccum,
Changed out the distributor for a reconditioned unit with a vertex magneto condenser.
Distributor is fitted with a Skips coil. I have the resistor mounted on the coil.
Now the car is not idling so good. And only showing 13" of vaccum.
Hence my thinking that resistor could be at fault?
Any comments nd help would be appreciated. Thought this would be just the final bit of fine tuning. Now running like ****. To find the reason.
Phil NZ
 
Make sure not to make any voltage or current readings with the engine running as you will get false readings. This is because of the points opening and closing casing the volt/ammeter to read the "average" voltage/current rather than the "steady state"
 
Engine working good, changed distributor and engine doesn't work any more?

Was that super duper distributor with Skips coil on a 12v car prior to your scoring the great deal? Just askin'
 
With engine running jumper the resistor and see the effect it has on running. 2.5 is a little low on drop . Is this a six or 12 volt??
 
Its a 6v system. Have swapped a couple of distributors and same results.
The only item not changed is the coil. i am awaiting a distributor and coil from Bubba so will wait til this arrives and this will narrow it down to perhaps just the resistor?
Appreciate all assistance.
Phil NZ
 
If it still runs badly bypassing the resistor with a wire like Bubba suggested, you'd narrow it down as well. Jack E/NJ
 
Resisters are good or bad unless loose connection. I checked my 39 resister think it was .5 ohms. Macs auto has one .4 ohms. 6 volts cars run about 3 amps, 3x.5= 1.5 volts.
By measuring across resister, that means what ever is left over is the coil voltage .
So if running at 6.8 minus 1.5 volts plus or minus is about 5 volts at coil.
 
Thank you George and others for this supplied information.
I was not checking also to see if points were closed when taking voltage to coil so will remember this and look for a resistor in the 0.5ohms range.
Phil NZ
 
Phil, Can I suggest that you stick with the original Genuine Ford V8 resistor and not mess around with various aftermarket resistors which you don't know the operating specifications of. The Ford resistor has the correct type of resistor wire and the correct resistance in ohms both hot and cold. Others could vary in this regard. Check the voltage at the coil with the engine running, that's between ground and the coil terminal and not across the resistor terminals. Also check the coils current draw in amps with the engine running, using an ACCURATE ammeter. Some like to measure these with the engine stopped but all these components are working with the engine running arnt they ? Report back your results. In my opinion you don't have a resistor problem. Have you replaced the condenser or had the condenser tested on a proper tester which tests for leakage, series resistance and capacity in micro farads. Should be around .3 mfd depending on year of car or truck ? Regards, Kevin.
 
Doing some reading and info gathering on this topic.
I see in the Bulletins for 38 to 40 the total resistance for the primary circuit should be 1.00 to 1.35 ohms. So if the primary for the coil is 0.6 ohms would you agree that the resistor will be I the range 0.4 to 0.76 ohms according to the Ford specs?
Phil NZ
 
Resistors are listed at .36 ohms cold and .56 ohms hot. Most I have tested are .5 to .6 ohms. You can run around in circles worrying about this. Just do the tests I outlined in post # 13 with the engine running and post the results. If you do the tests for voltage at the coil and coil current draw with the engine stopped you need to have the distributer timing points closed to load the resistor up. Depending how long you have the ignition turned on for then the ohms will increase as the resistor gets hotter. Voltage at the coil will change and so will current draw so that's why you do the tests with the engine running or under actual ignition operating conditions. Regards, Kevin.
 
Ok Kevin will follow this as you have outlined. Today got the ride on mower operational and grass cut this was another one down. Car is next.
Thanks,
Phil NZ
 
koates said:
Phil, Can I suggest that you stick with the original Genuine Ford V8 resistor and not mess around with various aftermarket resistors which you don't know the operating specifications of. The Ford resistor has the correct type of resistor wire and the correct resistance in ohms both hot and cold. Others could vary in this regard. Check the voltage at the coil with the engine running, that's between ground and the coil terminal and not across the resistor terminals. Also check the coils current draw in amps with the engine running, using an ACCURATE ammeter. Some like to measure these with the engine stopped but all these components are working with the engine running arnt they ? Report back your results. In my opinion you don't have a resistor problem. Have you replaced the condenser or had the condenser tested on a proper tester which tests for leakage, series resistance and capacity in micro farads. Should be around .3 mfd depending on year of car or truck ? Regards, Kevin.
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Kevin,
Had another look today. Put the multimeter ( a cheaper digital type)
in series to inlet side of resistor. No start at all. Cranked over.
Then with engine running put the multimeter on inlet pole to coil.
The voltage was bouncing around in the 18V up and down. Not able to get a reading as all over the place. This was the same when measuring on input to resistor.
I have the old type original coil mounted on top of the distributor.
I have my resistor mounted directly at the coil input.
Have it set up like this as previously the car was fitted with the remote mounted, newer style replacement coil.
Engine is still nor idling very smooth. Original condenser is fitted as was in use when all was good and smooth.
Phil NZ.
 
Phil, Not a good idea to use a digital multimeter, especially a cheap one on an old car. They tend to jump around all over the place. I always use analogue test set voltmeter and ammeter, the ones with a dial and needle pointer. Just for an easy test swap the condenser, take it out of the coil and substitute a new modern type using some jumper wires for a test. Condensers give that sort of trouble you are experiencing. When checking coil current amps draw you have to set your meter on amps (not volts) and might have to move one of the meter test leads into another plug marked amps. It should be a meter able to handle up to 10 amps. See if you can borrow a good quality digital multimeter from a mate. That's what mates are for. Regards, Kevin.
 
Did you find a .5 resister? If not orginal get one with .5 ohms 10 watts.
If you have a newer 6 volt coil round like a 50 ford or volkwagon 6 volt and use a vw condenser may be easer to find.
Only a 32 to 48 ford uses a resister for 6 volt cars.
They put the resister under dash out of sight out of mind.
Are wired to have any other things runing off ign. The most that ever run was a heater and radio.
I like to measure across the resister,now if you had a know .5 ohm resister and you had 2.5 volts.
2.5 volts divided by .5 is 5 amps way to high. Only need a good volt meter didatal no good.
 
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