my 89 d250 alternator is putting out 14 volts.but,the amps fluctuate.they very quickly change from 5 amps to 20 to 10 to 30 and so on.i checked and cleaned all the grounds i can find. plugged in one of those tester things that plugs in place of the regulator and the amps still fluctuate.tried a new regulator,same thing.can't seem to find any broken wires.fluctuates at any rpms.my amp gauge sometimes works and sometimes doesn't.been that way for years.i never had this happen,and am out of ideas.can anyone help?

 

Could be alternator is fine, but something in the truck is turning on and off, or shorting out.

 

still need help with my problem.can anyone help?

 

i don't really see a problem. a alternator putting out 14volts is perfectly healthy as far as i know. i would not be worried about the amperage jumping around as long as you have 14 volt. its not like your alternator is going to put out extra power when it is not needed.

 

If you are not sure, take it in and have it tested. Auto Zone and others can test it on the truck.

 

took alternator apart and installed new brushes (old ones were half worn)bench tested alternator.puts out steady voltage and amps.installed alternator and hooked up wires.steady volts(14.5) but amps still all over the place.took dash apart,fixed amp gauge.the needle bounces all over the plus side.not the discharge side.i just don't get it!

 

sheesh

What we're trying to tell you is that the alternator is supposed to hold a steady voltage. The amps is the draw, or rather the difference between the alternator output and the load, either absorbed or made up by the battery.

If the alternator is putting out 100 amps, and the load is 100 amps, the ammeter reads 0.

The alternator is fine. Something on the truck is varying the load. Depending on what you have going on (500 watt bass speakers for instance), it could be normal.

 

Doesn't a 89 have a volt meter, not an ammeter?

 

stock gauges suck especially the shunt type amp gauge that is used in the older trucks. there is a reason they finally replaced it with a voltage gauge. my stock oil pressure gauge barely moves but my mechanical gauge will move about 30psi.

 

j.martin,thanks,now i think i get it (sometimes i am hard of thinking).has shunt type amp gauge.any ideas what could be making that odd up and down amp draw? it's an 89 with no extra electrical stuff.doesn't even have a radio! no grid heaters.no ac.just nothing. turn on the lights or heater fan or both,same fluctuation. do you think i should try some type of single wire regulator if i can find one to cut out the factory wires? i don't know if anyone makes one.

 

i would just ignore the gauge. some gauge move more than others. the amp gauge in my 81 seemed to move a lot less when it was wet out than it did on a warm sunny day. even some days the gauge would jump to D every time the turn signal was on. best thing to do is install a voltage gauge so that you know what is going on. now if you see a lot of fluctuation in a voltage gauge then i would be worried but not the amp gauge.

 

You still haven't gotten it.

Have you tried replacing the shunt, or the meter. The shunt is a low resistance, often with several solder or braze joints in it that can fail. If one is intermittent, it'll cause the gauge to jump up and down.

The location of wires on the shunt is also critical. The power wires go under their screws, and the meter wires go under separate screws in the same heavy blocks. If you put power and meter wires under the same screw, crazy things happen. It has to do with the high currents and extremely low voltages involved. (millivolts to the meter)

Or the gauge itself may be defective.

Or you may have a frayed wire hitting the frame somewhere.

Or there may be an internal connection problem with the battery

Your alternator is good

The thing is over 20 years old. If the erratic gauge bothers you, why don't you just put a piece of masking tape over it?

 

it's not just the gauge jumping,i can live with that.if you put a load tester on the alternator wire the test meter jumps amps under load.tried another battery,same thing.unpluged light plug at switch.unpluged several plugs under dash.same thing.while i had dash apart i replaced printed circut board and added a ground to it.has anyone ever had an alternator bench test ok,but on the truck act like this? i know you all are getting sick of this thread,but i just can't figure this one out.

 

i would not worry about it. just install a voltage gauge. as long as you got about 13-14volts you are fine.

 

What "tester things" are you using?

How did you hook the tester-thing to the alternator?

I have a feeling you are seeing the output of the alternator as a result of the field's on-off switching.