still chasing intermittent misfire - ballast wire question

Mick Rae

Active Member
Still here, still chasing intermittent misfire. Between misfires, car (2200sc, that someone in the mists of time added TC instruments to so has rev counter) is running perfectly. I've replaced everything (coil, leads, condenser, points, rotor, cap, leads) one thing at a time, twice over. Tried a different LT wire, no change. Sometimes can go for 10 miles fine, other times, 100 yards before misfiring. Feels like the car just cuts power for a half second. Rev counter not affected. Tried running without rev counter, not that. So tested everything again. Everything checks out - but I am suspicious that the ballast wire feed is a bit on the low side - its feeding 5.5 volts to coil (with ignition on). I thought maybe this should be closer to 9Volts? Could it be possible that it has magically gained additional resistance, and that this is causing an intermittent hiccup?
As you can likely tell, I am clutching at straws here, but really cannot figure out whats going on. Driving me daft as I'd like to get some driving time in, and kind of feeling that I'm never getting to bottom of this (but equally not prepared to give up!).
If anyone can say that this feed to coil from ballast wire is too low, then i figure (with assistance of a friend who actually knows what they are doing) I could try a 12v coil, bypass the ballast wire with a 12v feed and see if that makes a difference?
Apologies in advance if I am simply talking guff here.
thanks
mick
 
sounds like wrong/failing coil. It will only read 9V as in a potential divider if the coil is of the right resistance. A ballast coil should be 1.5ohms which implies the ballast is 0.5ohms.

To get 5.5V, that implies your coil is 0.4Ohms and would, without electronic control overheat in time.

Even with a simple electronic ignition system you can't run a coil this low, you need one that runs a feedback loop rather than a simple points replacement.
 
Another thing is your tacho RVI or RVC? It'll say on the face. RVI means the ignition is wired through the tacho and can cause issues. TVC is in parallel.
 
Hi Peter,
coil is 1.5 ohms, labelled ballast coil. Have replaced just in case, and this new coil is also 1.5 ohms. Doesn't appear to get overly hot. In case I have said the wrong thing, I meant that the voltage from the ballast wire itself read 5.5V (not connected to coil that is, with ignition switched on). Apologies if this is a daft thing to say - I'm not at all high on the electrics understanding scale :). Car runs spot on other than this intermittent misfiring, and even after a drive, coil is warm, but not hot ie wouldnt cause you to draw hand away.
many thanks
Mick
 
we crossed over Peter. The tach is RVC. I disconnected it to rule out, but sadly made no difference.
 
If the coil is correct then it can only be the ballast. Most likely cause is high resistance joint in the circuit. There is no ballast resistor, it's a length of resistance wire it can't really "burn out as such very easily. 5.5V from the coil to ground is as you say too low.
 
Thanks Peter, makes sense. Do you think that could result in an intermittent misfire? I'm also wondering what you think of my plan to bypass the ballast wire with a 12v feed and use a non-ballast coil - a sensible workaround? I'd like to claim I came up with that idea, but it was my learned friend - I just make the coffee!
really appreciate your time and replies
Mick
 
Hi, Bypass the ballast wire to run a 12V feed down to the coil area and have a stand alone ballast resistor.

Colin
 
Thanks Colin. Is there a good, easy access 12v you would recommend to feed a ballast resistor, or should I take off from fuse box (if fuse box, would there be a preferred fuse.....I'm thinking one of them will already be the ballast wire feed, but am on a bus pondering, not sitting with manual. And being on a bus reminds me why I want to get the car running right :-(
 
Hi, Disconnect the existing ballast wire from the switch or the tachometer and run a replacement new wire to the coil area. If it's successful then cut or tape up the old and secure the new to the loom run. Traditionally ignition coil supply is unfused otherwise you lose it if something else blows the fuse.

Colin
 
For a short test you can take 12v off your windscreen washer pump feed. Just put a wire from the pump + across to your coil and see if it runs clean.
I bet your ignition problem is fuel.
 
With the engine running on my TC , it was reading 9.8 Volt at the coil input , which is right . As mentioned I would run it for a short while on 12v to just check, but I wouldn't want to do it for too long as it would overheat the coil.
 
With the engine running on my TC , it was reading 9.8 Volt at the coil input , which is right . As mentioned I would run it for a short while on 12v to just check, but I wouldn't want to do it for too long as it would overheat the coil.
You can’t get an accurate reading at the coil with engine running. You need to have the ignition on with the points closed to measure voltage at coil. If you have electronic ignition by pass the module to read voltage.
 
update. I didn't have a resistor, but did a 12v coil. Hooked up to 12v feed. Thought it was cured, but a quick miss later, nope. Switched back to ballast coil and ballast feed, yep, still missing (why wouldn't it be of course). So, not convinced it's the coil feed. But all these 'try something, go for test drive, curse and drive home' sessions I have been doing, I am noting the problem is becoming less frequent. Of course that's the issue with intermittent stuff! However, hard to see electrical issues doing anything but get worse. Perhaps three misses on a 20 mile drive now, originally is have that in 2 miles. I also noted Cobraboy's comment above about fuel. You know, I got my head fixed on ignition, but have a new half baked possibility. The car had run fine for plenty miles end of last year then stored for winter Ran great again spring for first week of driving. At end of week, filled up at a country station, where the amount of low ethanol fuel sold will be not much (never seen anyone use it, and I get std petrol for my modern car there every week, so am in frequently). And twenty or thirty miles later the missing began. Could it be possible that E5 fuel that's been around a while in petrol station underground tank could dissolve some water in the ethanol in the fuel? Or just water from condensation even? The fuel lines all look fine, well, those I can see. From the outside of course, so not informative......Yep, more straw clutching. Think I'll Syphon sone fuel from bottom of tank just in case, or daft thinking? Also think I'll just drive the car and get a fresh tank of fuel. Worst can happen I break down completely, then it should be easy to find out the electrical fault, if there is one
 
Hi - its the gap specified in the workshop manual (my brain has failed me for a moment) - bought fresh plugs and gapped to book is all I can recall. I think I have checked, and rechecked everything i can adjust about three times now. But hey, thanks for the thought - its the thoughts on here that are keep me keen currently!
 
I have during my motoring life about four times when the fuel itself has been the issue with a misfire . Popping, misfiring , not been able to accelerate etc etc. . The first time I spent ages checking all the ignition & then the carb. Eventually I needed more fuel. Filled up with new good brand fuel and problem resolved. On one Land Rover I had two tanks, good fuel in one, duff in the other. I could switch tanks and the systems went away, swapped back and they would re occur , so certainly the petrol was the issue. Each time fresh fuel resolved the problem. If I leave E5 in my mower over the winter it wont run properly until I put in fresh fuel , so I wouldn't discount the fuel.
 
That is very interesting Roverp480. Wouldn't I feel daft if a new tank of fuel solved my 'ignition' problem! Well, yes, I would, but I'd assuage this by thinking all's good, and better, there is a lovely well checked out ignition system also (I know it all much better than I ever would have - always looking for a positive!). Sad man that I am, a little bit of me quite enjoys a conundrum - takes me away from work, and unlike work, when eventually a problem is solved, its solved for good!
 
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