[Mg-t] Dynamo output

[Dave and Liz DuBois]

John,

Look at the number on your dynamo.  If it is C39PV, then the spec is for 
17 amps (204 watts).  If the number is C39PV2, the spec is 19 amps (228 
watts).  The original head lights for the car were rated at 35 watts per 
bulb on low beam and 45 watts on high beam.  The present day Sylvania 
Silverstar (high output sealed beam halagen bubs) pull about the same 
power.  I tested one that I have in the garage and it tested at 3 amps 
(36 watts) for low beam and 3.9 amps (47 watts) on high beam - 
essentially the same as the OEM bulbs.  LED light bars pull a very small 
amount of current, so can be discounted unless you are very near to 
total capacity.

A couple of considerations.  If you have th3 C39PV dynamo rated at 17 
amps, you can get a repair kit for it from Holcombe Armature Co. 
800-241-3027 that includes a new armature that is rated at 19 amps to 
upgrade your present dynamo.  I would also get the solid state regulator 
modification that Bob Jeffers (bobj50 at comcast.net) makes that contains a 
protection circuit so that the dynamo will never produce more current 
than its rated value.  This will protect the dynamo in case you get more 
stuff drawing current than the dynamo can handle.  Finally, most of the 
current being drawn by the electrical system comes from the battery, 
with the dynamo primarily keeping the battery charged.  Present day 
batteries will supply a lot of current for a relatively long period of 
time, so if you are pulling 25 amps while driving on a cold rainy night 
with the high beams, windshield wiper, and heater blower on, and the 
cell phone charger plugged in, the lion's share of that is coming from 
the battery, with the dynamo trying to keep up with the discharge of the 
battery.  The result is that the battery will be slowly discharged.  If 
you are not on a marathon run, you will get to your destination ok, but 
the battery will be in a somewhat depleted state, but if it is a 
relatively new battery, everything will be fine.  I would suggest that 
you also get one of the battery tenders that can be left plugged in when 
your car is not being used.  This will restore the charge on the battery 
without overcharging it.  Bottom line, if you don't go wild with the 
headlights (putting in the 2 million candle power bulbs that are so 
loved by the big 4 X 4 crowd) you will be fine with the OEM dynamo.

Cheers,
Dave