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TR2/3/3A tr3 coils

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sp53

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We had a nice day so I drove the tr3. About 5 miles out, my new Lucas sport coil died. Moss is sending a replacement and Haygerty towed me home free, so no real problems. Anyways, I have some old coils around and Randall has mentioned how to check them with an ohm meter, but I never totally understood what he meant. Do I take an ohm meter and put in on the Rx1K (I have an old style) and take a reading from either side and the high tension lead?
 
Looking forward to following this thread!

How'd you know it was the coil that failed? (I've been wrestling with this for several months.) I've also learned that even if the resistance values measure "correct", coils can fail sometimes only when hot.

Thanks.
Tom
 
Well I pulled a plug wire and had no spark. Next I took the cap off and move the points around and they did not spark. Next I took a hot wire from the battery to the coil and the points still did not spark, so I figured the coil must have opened up inside or the condenser was out, so I took a different coil wrapped it in a towel---plugged it in and set it in the engine compartment and it fired right up.
 
Thanks. Sounds like you found it, unless some other element was failing under use when hot. I've had troubles where the coil tested fine, but when it got hot it would fail (no spark). Replaced the coil and all was well - but of course that was after the engine bay had cooled, and the replacement coil was also cool. I hope Randall posts his thoughts on coil (and condenser!) testing.

Tom
 
If you must know, Lucas only puts a certain number of "sparks " into any of its coils.You never
know if its getting empty, it just comes up flat suddenly, often on a perfect day.........carry a spare!!
MD(mad dog)
 
sp53 said:
... Do I take an ohm meter and put in on the Rx1K (I have an old style) and take a reading from either side and the high tension lead?
Click to expand...

Yeah, with the meter on the Rx1k scale, you take the reading between the center terminal connector and one of the small side terminals. And the reading , I don't recall what it should be exactly, will most likely be around 8k-15k ohms.

To test the primary part of the coil, put the Ohm scale on Rx1, or the lowest you have on the meter. Test across the two small terminals. The reading for a TR3 coil should be about 3 ohms.

Edit: There's some more info on the link below for more testing of the coil.

https://mgaguru.com/mgtech/ignition/ig108.htm
 
Last edited:
Thanks Art, much appreciated. Whenever I see a post from you,I want to hear if that rear main started to leak. I wish I had used the correctmandrel on mine.
 
Thanks Art, much appreciated. Whenever I see a post from you, I want to hear if that rear main started to leak. I wish I had used the correct mandrel on mine.
 
The TR3 has been in the garage all winter, but the only oil on the floor is a small drip from the differential cover. The rear main is still hanging in there. Of course, because of some health problems, I only have about 2k miles on the motor since I finished the resto about five years ago. Maybe I should check to see if there's any oil in the sump! :excitement:
 
Sorry I'm late to the party, guys. Art nailed it, except I would use the Rx100 scale for checking the secondary (center post to either side post). And before trying to check the primary resistance, you should short the leads together and either note the reading, or turn the "zero ohms" adjustment to get 0.0 ohms. On those old analog meters, the zero wanders as the battery ages, so you should always re-zero just before taking a measurement.

As usual though, that's just a first level test. The most complete test is the one you've already discovered : hook it up and take a drive.

Ken Gillanders (British Frame & Engine) wrote some years ago of buying a case of new Lucas Sports coils, and finding that some very high percentage (over half) were either totally DOA or came up short on a coil tester. He threw the entire case away and refused to sell them. In spite of that, I bought a Lucas Sports when my old (possibly original) coil failed at TRfest 2009. It died less than a year later, of course while in stop-and-go traffic on a hot summer day. Since then I've been running a Pertronix 'Flamethrower' coil, which has worked well so far.
 
Well I measured them out and 3 of them are the 3 ohm models and two of them are the 1.5--- they all have the 7K when I go from the high tension lead to a small lead though. Anyways, Moss is sending out a new Lucas sport coil for free, but I am going to use it as a spare because the other new one only lasted a year, and also went out in traffic, but I was lucky because I found a business drive way and rolled in.
 
sp53 said:
Well I measured them out and 3 of them are the 3 ohm models and two of them are the 1.5--- they all have the 7K when I go from the high tension lead to a small lead though. Anyways, Moss is sending out a new Lucas sport coil for free, but I am going to use it as a spare because the other new one only lasted a year, and also went out in traffic, but I was lucky because I found a business drive way and rolled in.
Click to expand...

This is good to know. Which coils (mfr, model) were 3 ohm, and which were 1.5 ohm?
Would also be helpful to know the same measurements on the new Lucas sport coil when it arrives.
Thanks.
Tom
 
Some coil makers don't offer a 3 ohm version, but everyone who makes points-type coils makes a 1.5 ohm version. Even the "Lucas Sports" comes in both versions. So does "Bosch Blue" and Pertronix Flamethrower (my Stag has a 1.5 ohm Pertronix, the TR3 has a 3 ohm).
 
Didn't know there are two versions of the Lucas Sport coil. In my Moss TR2,3,4 catalog only one is listed - says it's for non-ballast (no external ballast resistor) systems. Listing doesn't say which coil (1.5 ohm vs 3 ohm) it is.

Is one resistance value for the external ballast-required systems, and the other resistance for non-ballast systems? Or is the internal resistance unrelated to external ballast resistance?

Thanks.
 
NutmegCT said:
...
Is one resistance value for the external ballast-required systems, and the other resistance for non-ballast systems? Or is the internal resistance unrelated to external ballast resistance?

Thanks.
Click to expand...

The difference in the resistance values on the primary circuit are tied together with the use of a ballast resistor. The low ohm (1.5) coil uses the ballast resistor which essentially raises the resistance of the coil as it's in series with it and the ignition switch. The reason for the ballast is to provide a hotter spark while cranking. It does this by shorting (bypassing) the ballast resistor while the engine is cranking over.

Here's some more info on ballast vs. non-ballast coils by Dan Masters from the VTR site...

https://www.vtr.org/maintain/ballast.shtml
 
Thanks Art. Sounds like the systems with external ballast are "hard wired" at the ignition switch to use/not use the ballast (cranking vs running). Edit: and thus, systems with no external ballast use coil's output both during cranking and during running.

Is it pretty safe to assume the 1.5 ohm coils all require external ballast? and 3.0 coils don't? Somehow I'd been thinking that the internal resistance was related to "high output" vs "standard output" coils.

Tom
 
NutmegCT said:
Is it pretty safe to assume the 1.5 ohm coils all require external ballast? and 3.0 coils don't? Somehow I'd been thinking that the internal resistance was related to "high output" vs "standard output" coils.
Click to expand...
In general, yes. Two exceptions that I know of:
1) Some electronic ignitions require a 1.5 ohm coil used without an external ballast. Eg, Crane XR3000, MSD 6.
2) Pertronix specifically recommends using their 1.5 ohm coil without a ballast on V8 engines with points.

You are correct, Moss doesn't sell the 1.5 ohm version of the Lucas Sports coil. Last time I checked, TRF did sell it, but didn't call it "Lucas Sports". However, other vendors do sometimes list it that way.

One other point that may not be clear, you can use a 1.5 ohm coil on your TR3 as long as you add an external ballast resistor. It isn't necessary to short the resistor out during starting (at least not above 0F or so). My FLAPS only carries 1.5 ohm coils, but a ballast was only about $10.
 
TR3driver said:
Last time I checked, TRF did sell it, but didn't call it "Lucas Sports".
Click to expand...
Oops, looks like I was mistaken on that point.

Lucas Sports.JPG

LUDLB110 is a 1.5 ohm coil, described here as "The Classic Lucas Sports Coil".
 
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