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Dynamo not charging [BT7] - replace brushes or buy new?

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fishyboy

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Hi all
I started my BT7 today and the Ignition Warning Light stayed on. I had some intermittent issues with the Warning Light sometimes staying over the last few moths.
I checked the =dynamo output by disconnecting the D and F leads, bridging the two terminals with a jumper wire and checking the output voltage, which was less that 0.5 volts. According to the Bentley manual this suggests a problem with the "field winding".

I have a pair of new brushes in my spare box. Is it worth trying to see if replacing the brushes makes any difference or is the dynamo shot?

Can the brushes be replaced without having to remove the fan/pulley (looks like a tricky job to me)?

Otherwise my only option might be to buy a new dynamo.

Thanks in advance.

Phil
 
I actually don't remember the old dynamo that well since I have been running an alternator for years. But I do know it is not that hard to replace the brushes. I also don't remember the exact reason for the 5 volts on your test but I do remember the test. As I think about it now the dynamo armature slides out easily once you remove the two long screws holding it together. The pulley and fan stays on the shaft with the end plate so you don't have to mess with them. If its not working you have nothing to loose by taking it apart and cleaning it up and putting in new brushes. Also clean the commutator bars with some solvent just lightly, you can clean it more with fine emery paper or fine sand paper, maybe 400 grit or finer. Then be sure to get all the grit out. It you don't have an air hose use the vacuum cleaner. The field windings which are attached inside the case are delicate so don't bang them around or scrape them. Another alternative is to install an alternator. See Hendrix Wire Wheel. I used an AC Delco Alternator. I think it is the easiest to adapt to the wiring and if It fails any auto parts store can sell u one.
 
Hi Phil,

As I remember, the through-bolts will release the rear end bracket and you can pull the end from the commutator. The front drive end can stay in place with the pulley attached to the commutator shaft. I would install the new brushes after cleaning the commutator contacts and stage the brushes with the springs atop the brush holders for reinsertion of the end bracket back on the shaft and over the commutator. Once the end plate is installed with screws tightened, I maneuver the brush springs to lower the brushes pressed by the springs.

As with Vette, I have had a Delco alternator for at least 10 years but have my original generator on the shelf.

Good luck,
Ray(64BJ8P1)
 
A

Yes it is a very simple task to replace the brushes in the dynamo , However , the comunttor may also require tuning and under cutting so i suggest a goog

rebuild by a pro if this is outside your capabilit.
 
Last edited:
If I were to try to rebuild the dynamo in place, I would drop small parts into the black holes under the bonnet, never to be seen again.
 
fishyboy said:
Hi all
I started my BT7 today and the Ignition Warning Light stayed on. I had some intermittent issues with the Warning Light sometimes staying over the last few moths.
I checked the =dynamo output by disconnecting the D and F leads, bridging the two terminals with a jumper wire and checking the output voltage, which was less that 0.5 volts. According to the Bentley manual this suggests a problem with the "field winding".

I have a pair of new brushes in my spare box. Is it worth trying to see if replacing the brushes makes any difference or is the dynamo shot?

Can the brushes be replaced without having to remove the fan/pulley (looks like a tricky job to me)?

Otherwise my only option might be to buy a new dynamo.

Thanks in advance.

Phil
Click to expand...

Another option would be the Stealth Dynamator:
http://www.accuspark.co.uk/dynamator.htm
 
Any good automotive electrical shop can rebuild both dynamos and starters. Depending on what needs to be done--i.e. turning the commutator, new field coils, etc.--it'll probably run ÂŁ100-200. Most shops will replace the rear bush that need periodic oiling with a (better) sealed bearing.

Bear in mind the problem may be in the voltage regulator.
 
Thanks all
Bob, I don't understand why you say "bear in mind the problem may be in the voltage regulator" as i was under the assumption that the "output" test i performed shows clearly that the dynamo is not working. While I haven't tested the control box I assumed that the dynamo had to be fixed first. I don't really want to replace the dynamo if its not at fault. Is the test conclusive?
Phil
 
Phil,

I haven't performed the test you performed so can't say if it's definitive or not. Dynamos require some current, provided by the voltage regulator to energize the field coils; without said current you will not produce much voltage at the dynamo (the field coils may have a bit of residual magnetism, with which they could produce some voltage, I think, but it wouldn't be much). The VR and dynamo comprise a system to produce and regulate electricity; I'm just suggesting you consider the VR as well. If your dynamo is original and has never been rebuilt and has significant mileage say, 100K or more miles, it may well be in need of new brushes, the commutator turned and possibly the field coils rewrapped or replaced (bearings are usually replaced by a good rebuilder as a matter of course). Have you grabbed the pulley and checked for side play at the front bearing?

Did you check the system voltage with the engine running at a fast idle (1,500 RPM or so)? You should see 14V give-or take (13.8 at a minimum).

Bob
 
Hi Bob,
Ah I see what you mean. I had forgotten about the about the current needed to energize the dynamo so I can see how a fault control box could cause a problem. I'll do some more checks tomorrow. I have now found a shop nearby that rebuilds these C42 dynamo. I can take over both the control box and the dynamo and he will bench test both together, which might just be the best solution for me.

I'll double check the system voltage test at 1500rpm tomorrow. I did run at around 1500rpm and the ignition light didn't go out.
Thanks again.
Phil
 
Phil, here is the text from the shop manual about testing your dynamo. Section N.6 in the Bentley manual. As you have correctly done, you removed the cars wiring connections to the dynamo, attached a jumper wire across the two terminals of the dynamo then read voltage between that jumper wire and ground. You will note that during these test the cars wiring harness is removed from the dynamo and the voltage regulator is not involved in the circuit. As per the last sentence in paragraph #6, if you observe a voltage reading of 4 or 5 volts during this particular test, the armature winding maybe at fault. You have been doing it right. I hope my attachments are sized large enough to read. Good Luck.
 

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Thanks vette,
One thing I didn't mention was that when I started the car up I noticed that the rear nearside indicator/brake light had blown. Any coincidence?
Phil
 
Thanks all
Got the dynamo out today after resting and took it and the Voltage Regulator to my local Dynamo shop for bench testing! Hopefully back in a few days and all sorted.
Phil
 
There is a bench test for generators that could show if the generator is functioning. While on the bench apply 12 volts to the field and the generator should spin if all is well.
 
Hi all
Bench test highlighted a problem with the armature (starting to burn out and very worn, but still just serviceable), as was suggested by the initial test results. The Control Box/Voltage Regulator (a proper Lucas RB340) tested fine. Tracked down a reconditioned C45 dynamo (quite expensive) but about the same price as just replacing the armature.
Hopefully, up and running early next week.
Phil
 
Thanks for reporting. Now we know the symptom of a failing armature (though I've never understood how they can fail, being an iron shaft wound with lots of copper wire; the commutators do eventually wear down).

Side note: My '65 Mustang had a flickering 'ALTERNATOR' light; after replacing it I took it apart and saw evidence of the rotor contacting the diode ring and shorting out.
 
fishyboy said:
One thing I didn't mention was that when I started the car up I noticed that the rear nearside indicator/brake light had blown. Any coincidence?
Phil
Click to expand...

Phil, I apologize that I don't pay a lot of attention to Uk traditions and in that I am assuming when you say nearside indicator/brake light I think you mean the cars right rear lights. So if that is the case I must say that there is really no direct correlation between the failed generator and the failed lights. Yet any momentary disturbance to the electrical system could cause an weak component to fail. On the right rear light, the red wire is the running light and it is fed from the headlight switch. This circuit, power coming from the voltage regulator to the Headlight switch and then distributed to the principal light bulbs is UNFUSED! Many are surprised to discover that this circuit is UNFUSED. But that had been the customary procedure for many years, the reasoning we won't go into at the moment. If your other running lights and headlights are working then I would assume that this circuit is ok. The other wire on this right, rear, dual filament bulb would be White/Brown. This wire is fed off the #7 terminal of the turnsignal/brake
relay on the left side inner front fender apron. #7 terminal is the 3rd terminal down on the left side of the relay. This wire gets its power from either the brake light circuit or the turnsignal circuit depending on whether you have a turnsignal on or your foot on the brake. In either case this terminal #7 and the wire to the right rear bulb are fed from the same fuse which is the top fuse in your 2-fuse fuse holder if you have the original 2-fuse set up. If this fuse is not blown
and all the other stuff connected to this fuse are working then I would assume that this circuit is also ok. So the fact that the right, rear bulb has failed is probably a coincidence.
 
Good point dancrim,
As I am staying positive earth do I just disconnect the -ve lead from the battery and run a jumper from the -ve terminal and touch it against the F terminal on the dynamo (which I assume needs to be earthed i.e. mounted on the car). Is this correct?
Phil

Vette,
Many thanks for the description. I'll go back to my wiring diagram and try to work through.
Phil
 
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