TR2/3/3A Loss of Power - ride of shame on flatbed to the house

[jfarris]

1956 TR3, stock engine rebuilt 1,200 miles ago, distributor by advanced, carbs recently rebuilt.
Yesterday, the wife and I took a drive downtown to get some Moss Motors Challenge pictures. Heat index was 102-105, we drove 50-60 miles over a 3 hour period. Car performed great with just a touch of added heat at long traffic lights. We decided to take the interstate home. After 14 mile of cruising at 70 in OD, the car started a gentle sputter. I thought fuel clogging as it was intermittent. It got progressively worse with some surging, if I increased the throttle it would clear up and run as if nothing was wrong. We made it to our exit, another 4 miles, with the car still running, it died at the stop sign. Pushed it off the road.
Checked the fuel pump bowl - full.
Checked the spark at the coil wire - no spark - called the tow truck.
Did I mention it was really hot - the wife did a couple of times!
Today: Power is good to the plus side of the coil.
Test light flickers at the negative side as the points open and close.
Points gap is at .013, should be .015 but I don't think that would keep it from running.
Is there a way to test the condenser - I may or may not have a spare?
The rotor and cap look good.
I checked the plugs, #1 was pretty dark, can't tell if it was oil or too rich, but the other three were fine, #3 and #4 actually looked a little lean. I'll do a compression check when I get it running again.
Any ideas other than the condenser?
Thanks in advance!

 

[charleyf]

The coil is also a possibility.

 

[mt10flyer]

Did you check the low tension lead (from the coil to the distributor). The spade connector, especially at the dizzy, gets vibration and the wire tends to separate at the connector stem.

 

[TFB]

Did you check point gap?I have had significnt change in initial point gap after first 1kmile.
If you condenser went bad your points would look burnt if it conttinued o run,not shorted to ground.
Tom

 

[CJD]

My guesses:

Top guess...coil has died. It may even work again when all is cool, but would still be bad.
Next guess (much lower chance)...the high tension lead to the cap/the cap/or rotor have failed.

If it were condenser, it would either spark erratically, or it would short and you would not get the cycling to the coil. So possible, but lower down list. They can be tested, but are so cheap it's normally easiest to swap it out.

 

[tinman58]

I had the same thing. Coil was bad.

 

[TR3driver]

IMO the only reliable way to test the condenser is try another one. I actually have one on the bench, that is clearly bad (car runs terrible with it, fine with another condenser), but two different capacitor testers say there is nothing wrong with it! It appears that only the rather extreme electrical violence of actually running the engine makes it act up.

Coil is another one that is best tested by substitution IMO. Before that, check that the center coil wire is making good contact inside. If you have an original type screw-in coil, then you must have a screw-in lead IMO; and vice versa. I've seen a lot of people try to stuff a more modern style push-in connector into a screw-in coil; it works at first but then gets flaky later on. If you do have a screw-in lead and coil, check that the center conductor is still attached to the washer and hasn't broken right there.

Also check that the screw that secures the condenser is tight, and the clamp (if present) isn't broken away from the capacitor housing. That is the ground path for the condenser, engine wont run (or run poorly) without it.

Other possibilities (way down the list from the ones mentioned above) include intermittent breaks in the wire from the distributor terminal to the points, and the ground wire for the point plate.

While I would change the condenser first (I always keep a "known good" spare, aka the one I took off at the last tune-up, in the spare tire well); the coil is my bet.

 

[jfarris]

Thanks to all who have responded. I have a coil coming to the local O"Reilly"s tomorrow morning. I'll try that first, but I do have a spare condenser in my old distributor, that will be the next step.
Can one of our EE's tell me why the coil will still respond to the points opening and closing (test light comes on connected to the negative side) when points open?
Best outcome so far is my wife's comment that if I know these things are going to break, we should take some extras with us - yes!
You guys are great - update tomorrow!

 

[sp53]

Coil, Moss sent me another new one, and I did not even need the paperwork; they just sent it after I called. I did not put the new coil they sent back on the car. I had some old extra ones, and now I carry one of those with me. I do not want to go through that again.

 

[CJD]

Remember the coil is simply 2 windings. The primary side, which you are asking why it cycles with the points, is a rather low number of windings. Power comes into the neg side of the coil (positive ground), and runs through the primary winding. If you measure the voltage at the neg, it will always be 12.6v. If you measure the positive side of the winding, it will measure 12v if the points are open, as the voltage has no flow, so it sees no resistance in the coil. Once the points close, then the electrons flow through the coil to ground (points). So, with points closed the voltage at the pos lead of the coil is 0v, give or take a bit. The primary winding acts as a large resistor with the points closed.

All that said...your failure is in the secondary, or "high tension" winding of the coil. With age and heat the high voltage in the secondary starts to jump between the windings, burning the internal insulation as it does so. What you are measuring at the primary leads of coil has no relation to the secondary windings.

Randall had a very good point...Most TR coils from the factory were "screw" in for the high tension. My TR3A came to me with a standard "press in" lead, and I had no knowledge of the screw on type. The car ran great for a year, but finally the press in wire started to arc and make the car run poorly. So, you may be in the same boat, where a PO has used a push in wire in your screw in coil.

 

[sp53]

The basic stuff I learned was it takes 20,000 volts to go through/arch 1 inch of air, so the demand on the windings is high. The coil will work with the + and - leads flipped around, but when hooked up with the incorrect polarity the rule I remember was a 10 to 20 % of voltage dropped, so it is something to watch.
steve

 

[TR3driver]

The basic stuff I learned was it takes 20,000 volts to go through/arch 1 inch of air, so the demand on the windings is high. The coil will work with the + and - leads flipped around, but when hooked up with the incorrect polarity the rule I remember was a 10 to 20 % of voltage dropped, so it is something to watch.
steve

It's not less voltage from the coil, but rather it takes more voltage to fire the plugs the wrong way. It's easier for the spark to jump the gap when the center electrode is negative.

Barney did a nice write-up with more details at https://mgaguru.com/mgtech/ignition/ig104.htm

 

[jfarris]

EUREKA! I installed the new coil and the car started on the first crank
I'll look at why #1 looked sooty in my spare time.
Thanks for all the help and explanation of how those electrons move around.
The depth of knowledge and experience here is amazing!

 

[mrv8q]

Just for curiosity’s sake, is there a manufacturer date stamped onto the bottom of your defective coil?

I’m still driving on an 08/59 coil, and always wondering... is today the day?

 

[CJD]

My coil is 07/55. It’ll likely outlast me!

 

[Geo Hahn]

I have given up on new coils, the ones I am using now came off of eBay and are from the 60s - quite reliable. Plus they are the Buck Rogers inspired design Lucas used for the classic Sports coils.