New or repro coil

1930artdeco

1930artdeco

Well-known member
Going through my new stash and came across this coil. Tested great cold but still need to warm it up and retest. Is this original or one of the repops?

Mike
 

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FWIW a lot of vintage coils out there perform better than Pertronix coils, including at least one reproduction script coil I tested. If anyone has a box of coils and you don’t know whether they work, I’ll test them for the price of return shipping.
 
I put this one on my King analyzer where I can preheat and heck spark gap from 0" to about 3/4" and cold at least it had a beautiful blue spark all the way! Now I just need to preheat it and see if it does the same thing.

It is black top to bottom Alexiskai. If it checks out good then it will be a good back up coil.

Mike
 
If it’s black, it can’t be original. Must be a reproduction. Which is OK, in my experience the repros are almost always better performing than the originals.

If you have a digital power supply, you can watch the current throughput drop as the coil warms up. Usually they reach a point where the delta is down to -1mA/s, and that’s usually where I do the “hot” test.
 
alexiskai said:
If you have a digital power supply, you can watch the current throughput drop as the coil warms up. Usually they reach a point where the delta is down to -1mA/s, and that’s usually where I do the “hot” test.
Click to expand...

Could a (good) DMM be used for that?
 
katy said:
Could a (good) DMM be used for that?
Click to expand...

Probably. There are two potential issues with using a battery and multimeter to get this data. First, if you're testing a whole batch of coils, you may run the battery down somewhat while doing the tests, so you should periodically check the output voltage to ensure the results are comparable. Second, the numbers go a little haywire when the tester is switched from pre-heat mode to spark mode. A couple of times it's crashed my power supply. So I don't know whether a given multimeter would be able to handle the switching. If you took it out of the circuit before testing the spark, it should be fine.

In my testing, there has been a relatively strong correlation between resistance and spark strength. The strongest coils have resistance when heated of 1.5Ω or less. The lowest I've personally seen was 1.4. Between 1.5 and 1.8 they're usually OK. Above 1.9 the spark is notably weaker. That's why you gotta heat them up, because even bad coils often show resistance <1.8 when cold.
 
I will do the heat and test this weekend. What is the best way to check the ohms? Meter Pos. to either the +/- terminal and the Neg. to secondary (coil wire)?

Mike
 
1930artdeco said:
I will do the heat and test this weekend. What is the best way to check the ohms? Meter Pos. to either the +/- terminal and the Neg. to secondary (coil wire)?

Mike
Click to expand...

I actually infer the resistance by dividing the voltage by the current, both of which are displayed in real time by my digital power supply.
 
The Circle A coils of American Bosch with straight terminals generally have a date code stamped on. It's a single number followed by a single letter. There were lot of reproduction coils for a time so a person has to scrutinize them pretty close to tell.
 
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Well did the heat test and it did quite well.If I keep the gap at 3/8" or less it did fine with maybe a skip once in awhile. Over 3/8" and it would skip a beat and the spark was more erratic. I till think it is a good coil as a back up.
 
1930artdeco said:
Well did the heat test and it did quite well.If I keep the gap at 3/8" or less it did fine with maybe a skip once in awhile. Over 3/8" and it would skip a beat and the spark was more erratic. I till think it is a good coil as a back up.
Click to expand...


Yep 1cm is my rule of thumb for a functional coil.
 
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