rv969wf

rv969wf

Well Known Member
Heres the problem. I've got a ND alternator on my RV-6 that has worked great for 500hours or more. It charges upon start up of the engine and works for about 30 minutes into a flight then quits until I slow the engine to around 1,500 rpm, speed the engine back up to cruise speed and it quits, slow the engine again back to 1,500 rpm and it starts charging again. Can anyone tell me what the heck is going on? I've replaced the voltage regulator, checked all the connections, grounds, wiring is solid, no corrosion etc. I've taken the Alt off and put it on a bench test and can't find any problems. I thought it might be heat related, but why will it charge when slowed down and still heat soaked??? Any help would be great, Next step is to replace the Alt. but I'd like to have some input a to why it is acting up like this. Thanks.............
 
I would be suspicious of the regulator - it's possible (I've seen it happen twice on other regulators) that the regulator is keeping the field current wide open. At low RPM, the output is lower and the battery can absorb the current without the voltage going too high. At high RPM with a full field current, the output is too much for the battery to absorb and the voltage rises to the point that the overvoltage circuit trips the alternator offline.

Plug in a digital voltmeter into your system and watch it as you raise the RPM from idle to full power. The voltage shouldn't change - if it rises with RPM that is your culprit.
 
Another thing you can check is belt tension. Put a wrench on the alternator belt pulley and see if you can turn the alternator without turning the engine. If you can, belt is too loose. Sometimes this can produce the same type symptoms.
 
Its the brushes

Take them out and replace...There are two brushes, whats happening I bet is they are worn down (they are usually on a spring) and because of the reduced length the sping does not have enough force to push them onto the (now dirty) slip rings.

As speed increases the brushes lift and it stops charging..Voila!

Frank
 
THANKS,,,,,To everyone!!!

Thanks for all the great input and help, everyones comments gives me something to check into and look at! :) I'll let everyone know how things check out. I need to fix this Alt. issue before heading too OHSKOSH!! Anyone else going to OSH????
 
OSH

Thinking about it...My buddy is supposed to be making arrangements, he is going to decide for sure today..Its a 2000 mile trip each way for me...and gas prices being what they are means I need a paying partner or its not really worth it to me.

Oh on the brushes thing..I have seen this many times on alternators that have acted like yours..If you can't get new ones in time you can stretch the spring a bit in a pinch.

Frank
Western Oregon
 
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