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Coil resistance values for 12v To-35

2

257 Roberts

Member
Hey gents. My 1955 TO-35 occasionally develops a sputter and miss under load, the classic weak ignition symptoms. I'm thinking my coil is marginal and needs to be replaced. The tractor is 12v and was converted to use an alternator before I got it. I put a Pertronix kit in the distributor. Cap, rotor, and plugs are new, wires were replaced a couple years ago and are clean and secure. I checked the coil (internal resistor), and got 5 ohms on the primary, and 5.8k on the secondary. Seems to me like it should be around 3 and 10k instead. What say you? I'm thinking this coil is marginal and ought to be replaced before I do anything else. Thanks!
 
5 ohms sound off to me but unless you have a calibrated VOM who knows for sure. You can ball park calibrate it wit ha known resister that has a valve you can test and then see where it read and adjust for that when you measure ohms but even then it may only be a ball park reading. Common man does not have a calibrated VOM and to have it down would cost more then most of them cost
 
It sounds off to me too, given that nearly every coil for this application is listed as having 3 ohms. I am using a digital volt/ohm meter that has always been accurate. By calibrated, what are you referring to?
 
Set your ohm meter to lowest ohm scale. Short the probes together and note the resistance.

Then check across the small posts on the coil and subtract the shorted probe reading. I have seen a number of meters show several ohms resistance through the probes.

NAPA IC14SB coil or equivalent is a direct 12 volt replacement
 
Calibrated is a way that a VOM has been tested and set to work correctly and in having that done it is accurate. Most VOMs a person buy are at best ball park readings and the valves they read as close and that is about it and that is if your lucky. Back when I still worked in electronics we would send the VOMs in to have them calibrated but it was not cheap but when your working on Computers on U.S.Navy ships thing need to be correct not ball park because one dos not want a nuke missile to go where it should not go
 
Thanks gents. Yes, I always touch the probes before I use my meter. Yes, working around nukes one should spare no expense for accuracy!
 
Funny thing is we where told we could hit a door knob with those missiles. But one has to remember that missing said door knob by a mile was pretty much still a direct hit LOL. I could tell you some funny stories about those missiles and how well lets just say they did not do as they should have done. Like one film we watched talking about hose missiles where you watch the missile come out of the tube on the sub and go up then lay over and head right for the camera and the last thing yo use is the camera falling over as the missile almost hits it and the guy behind the camera. But that was back in the 70s on a SSBN sub built In the late 50s or early 60s
 
Have you checked to see the condition of your spark. Will it jump 1/4 inch to 1/2 inch?
Not reddish or yellow but blue? Engines can run good no load and then miss out under load.
Even new sparkplugs can do this after a short time. Some tractors are worse about this
than others. If you have and old set try them and see if it helps. I have a neighbor who
had brand new plugs from John Deere in a D. It went to missing under load. He went
through everything, even fuel tank with same results. Finally changed plugs and corrected
the problem. By the way I think 4 ohms is about right on the primary. I assume your engine is in good shape. Oil fouling can also cause a miss.
 
Thanks. The deal is, 90% of the time she runs fine. After mowing for about an hour it starts missing with the classic weak spark symptom. No oil fouling, it doesn't use a bit of oil. My coil actually reads 5 ohms on the primary, Pertronix says their module requires three ohms so it makes sense to me to correct that before anything else.
 
Is that 5 ohms before you fire it up or 5 ohms after you have run it an hour or so. ALL wires disconnected from the coil or you can get a false reading????
 
The 5 ohms stays constant whether it is cold or warmed up. I didn't realize you had to have all the wires disconnected, I'll check again and see.
 
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