Fan Motor amp draw 75 280z

[Dave WM]

The fuse cover over the AC fan motor fuse was distorted (lightly melted). I replaced the cover and used the correct 20amp fuse.

I installed a reman alternator a few months back after the orig unit shorted thru the diode pack, taking out the fuse link with in and killing the entire electrical system.

With the reman alt in place everything works BUT with the lights and fan (AC) on at idle (800-1000 rpm) the voltage at the alternator is 12.5v and the amp meter shows a slight discharge. If I get above 1500 it will just keep up.

I am pretty sure its not the regulator as I have jumped the field coil to the bat term (idle speed only to avoid over voltage) and I get the same voltage  results, so I presume the reg is working (with rpm it limits to about 14.2v).

I pulled the reman out and took it to 3 diff auto parts store to test, it "passed" and I assume that is under load.

Thinking back to the melted fuse cover and the fact that the prob really only presents with the blower motor on makes me wonder if there could be an excessive amp draw, not enough to blow the 20 amp fuse but enough to get hot and perhaps draw down the voltage while at idle.

I have no idea what the correct running current should be on the fan motor, but I am guessing the fuse is rated at twice the normal current, if that is the case then I would expect the max current thru the fuse at about 10 amps.

I don't have an amp meter that goes over that (I have an old simpson 260 and a more modern digital but 10a is the max IIRC).

so with that in mind I will try measuring the resistance of the fuse and check for a voltage drop across it, use ohms law to calc the current. The fuse may be to low a resistance to get a good reading on, if that happens I will look for a length of wire that I can get at least a .1 ohm reading on and use that.

 

 

 

 

[Dave WM]

ha went out to do some testing and it clearly says 160w yea! so at 12v 13amps, now I have something to compare to.

I was getting about 130mv across the fuse, so that would indicate a .01 resistance if it is indeed 13 amps. I just don't have a meter that can read that low so I need to either get an amp meter that can go higher or try the wire trick or maybe I can google the typical resistance of a 20 amp buss fuse.

 

 

Edited March 17, 2016 by Dave WM

[Zed Head]

Don't forget the clutch magnet on the AC compressor.  I had a reman alternator that had a similar problem at low RPM.  It just didn't put out enough current.  I couldn't use the wipers at idle without them getting stuck halfway across the windshield.  Luckily for me, the quality of the reman was so low that it died after about eight months.  I replaced it with an old Nissan original junkyard alternator and things improved dramatically.  Cleaning up numerous electrical connections helped a bit more, and I don't even think about it anymore, except for when I realize that it's probably over 33 years old and won't last forever.  I still have the replacement lifetime warranty reman in the box, waiting, but I took the pulley off of it to make a 90 amp Maxima alternator conversion.

The short story is, the reman alts just aren't very good.  Maybe ask for an exchange anyway, just to see if you get a better one.

[Dave WM]

I think I will try taking the whole car down so they can test on the car (my guess is they read the voltage) then will get replacement. I almost talked them into in but one of the other counter guys insisted it was not the alt but the regulator. I know that is not the issue as stated in the 1st post but did not want to belabor the point. I may just replace the diode pack, would have done that in the 1st place but they did not have them stocked (back when I got the replacement alt).

[Dave WM]

Another Idea, today I decided to check the total amp draw before starting, don't know why I have not done this already.

KO (to energize the ignition), AC on (clutch/fan), head lights on, puts me on the scale of about 40 amps IF the amp meter in the car is accurate. This was just a guess since all I have is 0 to -60 amp, but base on the about 2/3 deflection I figure 40 amps. Given  that kind of draw I guess its not surprising the alternator is struggling to keep up at idle. So is 40 amps the typical amp max amp draw? please chime in if anyone can do a like test with the built in amp meter.

oh and just to eliminate the possibility of a VR problem I ordered a new one to try, but I am pretty sure the old one is ok as I went thru it and carefully set it up and based on the test using the field coil energized by a jumper with no real improvement at idle I don't think its the issue. When it come it I will try the new one anyway.

 

Edited March 18, 2016 by Dave WM

[Zed Head]

The ammeter's aren't accurate.  Different alternators have different output curves.  They're all much lower at idle though.  That's the basic problem.  A 60 amp and a 90 amp alternator can have the same output at low RPM.  Added a picture of some GM alternator curves, same model but still different curves.

You can waste a ton of time with reman alternators.  Most people describe them as "crap". in general, after having several fail.  On the plus side though, you are learning quite a bit output your charging system which will help you in the long run.  One problem you can run in to is that while working on the charging system, the battery gets run down, then the system is using  significant extra amps to recharge the battery.  So the battery load maxes out the available current at idle.  Starting with a fully charged battery can get you back on top.

[Dave WM]

I think the battery is a good one, NAPA looks new, checks about 12.8v

here is how it goes, starts quick everytime, amp meter swings to the pos for a few seconds then kicks down to just about the 0 (actually a bit positive since with everything off (bat disconnected) its sits just a bit to the neg so no centered). Voltage while running is around 14v drops down some as the AAR closes up but will hang in there at about 13.8 with no load, limits to about 14.5 when reved up. On come lights and AC, drops down to about 12.5v

I took it back today in the car, they put a voltmeter on it, and as expected it does the same. So now I will get a replacement, just have to pull it out again.

If that does not do the trick then I will see about replacing the diode pack on my old one. I don't recall having this issue with the orig alt, and that was with a dying diode pack (one of the diodes literally was out of the circuit, it had come loose but was not shorted, I guess the extra load on the others caused them to short out). The only down side to that plan is I don't know if the windings were smoked, hopefully not if the fuse link did its job right.