odd dynamo diagnosis

[JonE]

Odd one - posting to see if someone immediately suggests the weak point....

Charging system with converted CFR2 ( Conrad type rectifier diode for cutout and a resistor to replace the old coiled resistance wire), PLC2 switch and 3 brush late dynamo was fine and running well. Some tweaking of third brush seemed to draw an end to its function, with the red ign light remaining on at all times.  

Stripped dynamo off and re-soldered the 3rd brush lead; checked everything, re-assembled.

No difference.

Stripped dynamo off with distributor still attached. Connected joined top terminals to battery positive and case to negative. It motored.
Should I have noted how fast this should be?

Put it back on - no difference.

There are no recent changes in the wiring locations and its all new. I've checked for wire fall-outs and checked continuity between D and F locations on the CFR2 and the end of the loom to the dynamo - all fine.

I'm presuming the resistance is not at fault - could this be a diode failure and how likely is that? Is it possible that the dynamo motoring was too slow and there is some partial fault in the innards? I'm thinking that the 3rd brush wire break COULD have been as a result of pulling the end of the dynamo off, so the "repair" there may be a red herring.

Hope someone recognises something they have been through before...

[Bob Culver]

Was the motoring polarity same as car polarity? My car very occasionally failed to generate but responded to revival of the residual magnestism. Tested using a lamp or whatever there should be low resistance continuity between the isolated F terminal and earth, with no interruption as the engine is slowly hand cranked.

[Dave Mann]

    When I've done the motoring test you describe I then use an electric drill to drive the dynamo and check the dynamo output voltage which with a good dynamo off load will be at least 18 volts. It is not unusual for a dynamo to motor and not generate and if all the wiring is sound my attention falls on the armature  paying particular attention to the commutator soldered connections. If there is any sign of the solder having melted it's a rewind because if it's got hot enough to melt the solder the winding insulation in the armature slots will be cooked.

[JonE]

thanks both. Very interesting.
I did put negative earth on the body which is what the car is.
Can I profitably do any tests between commutator segments of armature to check out the above?
and the dynamo output voltage as per last photo... measure between D and earth?

[Bob Culver]

What Dave states is basically true. Neverthelss thrown solder at some stage was/is more or less the norm for Seven dynamos and certainly worth trying. (I have never run a dynamo which has not suffered! Unlike modernd Sevens are not quickly immobilised if generation fails). Usually enough solder remains for electrical connection. The 2 pole armatures are wired 1 to 2 to 3 etc around to 1. There are therfore two parallel paths between adjacent comm segments, one longer than the other. Prior to digital meters it was a bit of a fiddle to check. But digital meters read very low ohms and the resistance between adjacent bars (without brushes) should be very comparable. Poor connections show and can often be restored by resoldering those bars (taking care not to get flux in the winding), or all.

[Dave Mann]

Both the motoring test and the test with an electric drill are done with a link between the D & F terminals and the voltage measured between the case and D terminal. Once I am sure that all the connections and the field coils are sound with no insulation faults it only leaves the armature as a possible cause of failure. I don't have a growler, for testing armatures, and I have in the past put a known good armature in a dud dynamo to prove that the armature was the cause of failure.

[JonE]

I'll do this next! I'll find another dynamo to substitute if the voltage is too low. All this toing and froing is at least making me more adept at it all.

My only other thought is what is the outward diagnosis for the failure of a Bridge rectifier [in place of the cutout]?
I've blithely used this miraculous two quid part, but others have advised 'carry a spare' - and I'm now very conscious of Bob's pronouncements on such things!

[Robert Foreman]

I agree with Dave, 
the best way to test an armature is to use a Growler.

[Dave Mann]

One of the reasons for the spin test with an electric drill, it will identify whether the fault is in the dynamo or the car electrics.