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TR6 Losing voltage at + side of coil

cmacmillan

Senior Member
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Been dealing with a no-start situation. I think I've traced it to a bad electronic ignition unit so the points/condenser are back in circulation (glad I keep everything I take off!). Anyhow, my ignition wire seems fine...getting 12V at battery and + side of the coil. But when I connect the dist wire to the - side, my + side reading drops to 5V (no voltage on - side, so that's a better place than where I started). Anyone encounter this?
 

TR3driver

Great Pumpkin - R.I.P
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There is a ballast resistance built into the wiring harness on later TR6 (roughly 73 onwards), so it's normal for the voltage on the coil hot terminal to drop when the points (or virtual points in an electronic ignition) are closed. But normally it shouldn't drop that far. Sounds like your coil might be lower resistance than usual (perhaps an internal short), or the resistance wire is high.
 

dklawson

Yoda
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+1 to Randall's comments.
Normally an inline ballast resistor would result in the coil (+) terminal having somewhere between 6V and 9V on it when the points/module are providing a path to ground.
 

poolboy

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It wasn't clear to me how you performed the test, but do not rely on the points being closed to create a ground for the coil when you check for voltage that is feeding the coil.
Use a jumper wire connected from the negative terminal of the coil to a known ground.
Then use your VOM meter in the normal fashion.
 
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cmacmillan

cmacmillan

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Update - tried a second coil, same thing. I'm getting 5.7V at the - coil terminal with the ignition on, points closed. Grounded to the block with a secondary wire, same reading. I'm finally getting spark at all 6 plugs but no-start....drained the battery trying, so charging it now. How worried should I be about the 5.7 V? Should I replace the dist to coil wire?
 

TR3driver

Great Pumpkin - R.I.P
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Update - tried a second coil, same thing. I'm getting 5.7V at the - coil terminal with the ignition on, points closed. Grounded to the block with a secondary wire, same reading.
Don't you mean the positive terminal?

And you've checked that it goes back up with the engine turned so the points are open, right?

I doubt that the 5.7v is the problem, but for a test you could try running a temporary jumper from the coil positive terminal to the battery positive. That will deliver full battery voltage to the coil (which is supposed to happen while cranking anyway).

Don't leave it for a long time though. 10 minutes shouldn't hurt anything. And obviously, once started, the engine isn't going to shut off until you disconnect the jumper.
 
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cmacmillan

cmacmillan

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Oop - yes, you're absolutely right...5.7V at the + coil terminal. Didn't check with the points open...will give that a shot as well as the direct cable.
 
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cmacmillan

cmacmillan

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Ok - 12.2V with points open, back to 5.7 closed. Ran a jump direct to the coil and no difference. Sparks are all there but I'd say are white-yellow (is anyone getting blue spark?). Made sure fuel pump was sending fuel. Finally hit the carbs with quick start...annnnnd it worked. Worried it's the newly rebuilt cards, but it was running under it's own steam after the initial quick start.
 

poolboy

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Yes, I get a blue spark.
If you are not feeding the coil enough voltage going in, you are going to get a weaker spark coming out.
When I had a ballasted ignition the coil would get fed battery voltage when the starter was engaged.
Once the engine started and the starter relieved of it's duty, the coil was fed 9.7 volts thru the ballast resistor wire.

Does your engine block have a good ground connection to the battery ?
 
Last edited:
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cmacmillan

cmacmillan

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Hey all - sorry...hasn't really been "car working" weather in this part of Canada! Some follow-up - I did check engine ground which seemed fine. It's putting out decent spark - but still hard starting unless I hit it with some quick start, in which case it fires right up....I think it's a fuel/air mix issue...more on that in my next follow-up post. Thanks again for weighing in!
 

TR3driver

Great Pumpkin - R.I.P
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It's putting out decent spark - but still hard starting unless I hit it with some quick start, in which case it fires right up....I think it's a fuel/air mix issue...more on that in my next follow-up post.
Just a thought, is the fuel fresh? If it sits for a long time, the more volatile components can evaporate, leaving something that looks and smells like gasoline, but acts more like kerosene. (Meaning really hard to start.)
 
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cmacmillan

cmacmillan

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It *should* be ok - when I started this process I drained the old stuff and added new gas...that said, it's now been 4-5 months. I did add some fuel stabilizer to the tank just before Christmas.
 

AltaKnight

Jedi Knight
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Sounds to me like you have a problem with the choke system, make sure the cables are fully actuating the chokes properly
 
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