TR6 Tech Forum

1973 TR6 alternator issues

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dicta dick Taylor
Downey, Callifornia, USA   USA
In reply to # 1508607 by ghutch I AM CONFUSED!!!!!

I don't understand what is happening!

I have replaced the alternator with a new unit. I have it mechanically installed to be sure that the pulley is aligned and everything is back to the same configuration as my old Lucas.

My original Lucas had the standard 3 wire plug plus a single spade lug. My understanding is that the single spade lug is not necessary if the alternator does not have the additional connection. The unit I purchased does not have a single spade lug so I taped off the additional wire.

I connected the alternator and a digital volt meter. Before starting the engine I charged the battery and was reading 12.4 volts. I started the engine making sure that the alternator was spinning properly. The volt meter showed about 14.6 volts. I operated like this for at least 20 minutes and the car came to operating temperature.

I drove the car around the block but noticed about half way the dash volt meter was reading close to 12 volts. confused smiley By the time I got home it was showing less than 12 volts.

I attached my digital volt meter and the charging system was showing 11.8 volts and no "ignition" or "alternator" light showing on the dash. I shut down the engine and disconnected the 3 wire plug from the alternator and restarted the engine. I measured the output on all three alternator connections and read no voltage at all.

I experienced this same thing several years ago when I attempted to replace the Lucas. At the time I felt that I may have had a defective alternator. Having the EXACT same thing happen again makes me angry. angry smiley

Does anybody have ANY idea what is going on? My original Lucas was functioning perfectly until just recently when the diode plates failed. Connecting a new alternator works for a short period of time and then the new alternator seems to fail and stop charging the system.


Gary --- I had something similar happen when changing from the original Lucas alternator to the Delco 65 amp. A better ground between the battery neg. post and the car body was all I needed. It now charges at 14.6V no matter how long I run the car.


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Tommys4 Thomas G
Ojai, Calif., USA   USA
Some years ago I owned a auto repair shop, a fellow came in with an old Volvo camback 6volt had a starter issue. Checked the normal stuff. FYI, this was a reman starter. took it back to the parts store.They did a test
and it was OK. I tested everything 3 ways to Sunday. The starter was the issue! Took it back to the store. They tested it And I said show me, They where testing it on a testing machine. I said wait one second here
that machine isn't starting this car, so try a battery...........Guess what ! starter was bad............replaced the starter with a new one. Worked fine........WE all know about problems we sometimes have with parts we buy
for our L.BCs. But we all persevere

I hope everyone has an awesome year........................TMG

ghutch Avatar
ghutch Gary Hutchison
Apex, NC, USA   USA
1973 Triumph TR6 "Carmine"
Now I have done some diagnostics. It looks like the rotor, exciter coil is open. With the alternator removed from the car, an ohm meter across the brush connections showed no conductivity. I removed the brushes and checked the rotor directly - still not conductivity. My old rotor shows about 15 ohms across the brushes.

I will be calling the company I purchased this alternator from today. I just wondered if someone here knew why a rotor would become damaged within the first few minutes of running the car?


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glcaines Avatar
glcaines Silver Member Gary Caines
Hiawassee, GA, USA   USA
I have a 73 TR6 with a Bosch alternator installed. No issues whatsoever. You might want to try Bosch as a replacement if you can't get your rebuilt.

Current: 1973 TR6 W/Overdrive

1963 TR3B W/Overdrive
1962 TR3A
1961 TR3A
1960 TR3A
1960 TR3A

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
Even 15 is on the high side. The workshop manual gives values around 3 to 4.5 ohms, depending on alternator model.

The rotor spins over twice as fast as engine rpm, so the connections inside the rotor get exposed to lots of centrifugal force in operation plus magnetic attraction and high frequency vibration. Obviously something couldn't take it.

56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

ghutch Avatar
ghutch Gary Hutchison
Apex, NC, USA   USA
1973 Triumph TR6 "Carmine"
Thanks Randall,

I was focusing quite a bit on electrical issues and failure mechanisms. I hadn't really thought much about mechanical issues. Without the alternator making any unusual noises the possibility of mechanical failure didn't really dawn on me.

You are right. I need to consider those issues. I spoke to the company I purchased the alternator from. They also stated that a failure in the rotor is unusual but possible. Anything from an exposed wire rubbing to a bad solder joint from the winding to the commutator. They will be replacing the bad alternator. smileys with beer

Maybe I will add crossing my fingers to the installation procedure next time. drinking smiley


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