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Alternator issues!


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We are getting somewhere now.

Assuming the 3-3.5 volt measurement on my working alternator is OK.that means the resistance of the bulb is too

high(try wiggling it around) or the resistance to ground thru the alternator is too low.

 

You can detemine which by comparing to mine.

Measure the voltage at the bulb w/the alt.plug removed and white/red grounded thru an 8 ohm resistor.

An 8 ohm resistor simulates the alternator well and gives around 3 volts at the plug.(I used a large rheostat)

 

If the 1.6 volts increases much,your alternators resistance to ground is lower than mine.

If the 1.6 volts  stays nearly the same your charge lamp has higher resistance than mine.

 

Seems sorta unlikely that anything w/the lamp changed since you had a working alternator.but,with

the overcharging,who knows.

 

More likely the relacement alternator is different somehow.

Test rig at the shop likely had a different amount of resistance on this wire.

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For furthur diagnosis we need a different measurement.

 

Either measure the voltages at the charge lamp again with the plug disconnected from the alternator and white/red

grounded thru an 8 ohm resistance or measure the current on white/red w/everything connected(suggested).

Haven`t measured mine yet,but,I am expecting something in the 0.5 amp neighbourhood.YMMV

 

I plan to pull the fuse and put the ammeter in its place.

Edited by naru
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That shows the NAPA one had too low resistance since post #11 or earlier.

Probably partially shorted field windings.

It would only start charging w/an abnormally high voltage/current at/thru L.

Surprising that 3 were the same!

 

Cheers

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  • 8 months later...

I’m new to this Subaru forum. But touching on this older post. I have an 84 1800 gl, and it’s not charging. Put new alternator in, new battery, all new battery cables and grounds, new fusible links. Test the fuse on the fuse block and I have battery voltage on power side of block with fuse in, test the other side of block and there is nothing. But when I remove the L plug I get voltage on the other side of the fuse. This is a real humdinger to me

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