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Alternator Question

Justin Fleming

Justin Fleming

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Location
Oxford MI
I have been chasing this one for a bit. The alternator regulator seems to go on and off quite often to the point when driving down the road at night the lights are kinda low, then spike up to brighter when the regulator comes on and it will do this continuously. I can watch with volt meter where is is running at about 13v then spikes up to 14.4ish of which would be considered charging at both levels. sometime you will see it get down in the 12.3ish range then jump right up to 14.4ish . It passes an alternator bench test, the batteries both pass bench test. I have gone through all hots and grounds and cleaned all connections. Do you think this is a regulator getting ready to go out?

I guess it could be working correctly (as that is what a regulator is supposed to do) and I never paid close enough attention, however in modern cars, the alternator puts out 13.8 and just stays there so I feel like something is up..
 
I should add, this is a dual battery set up for plowing and I have a modified 200amp alternator on it from some alternator hop up shop in the area.
 
I believe that the regulator would be the suspect, I would also verify that the alternator case is grounded well. Paint or corrosion where the alternator bracket bolts to the engine block could be causing ground issues. I am assuming that you have serpentine belt drive because I remember you having an '89. This is why I would check the ground, the aluminum bracket can oxidize.
Maybe try checking for a voltage drop between the alternator case and the battery ground. Or install a jumper ground wire and see if anything changes.
 
I have a dual battery set up, when my aux battery is low, the charging system voltage gets pulled down below 13.5. my smart relay opens. Since the main battery is not low the system voltage quickly returns to above 13.5 and the relay closes. At night I can see this happening in the head light brightness. I only have a 105 amp alt.
 
Yea I have a big ground cable going directly to the battery from the back of the alternator housing. This just might be something that I had not noticed previously and sense getting older and getting eye glasses maybe I am more attentive....
 
And for what its worth in case others are reading and might have an idea. It is repeatable that I can watch the volts come down under normal acceleration, and if i instantly lift from the pedal the regulator kicks in and it jumps up to the 14.4ish volts. This was about 90% of the time repeatable. Also notice almost every time sitting at a light it is sitting at 14.4ish volts
 
Voltage drop follow up

driver side battery

from neg battery post to frame 5mv
from neg battery post to back half of alternator case where I have a ground cable hooked 20mv
from neg battery post to front half of alternator case 12mv
From neg battery post to ground cable going up to intake bolt (passenger side battery) 20mv

Check from passenger side battery
from neg battery post to frame (driver side battery) 16mv
from neg battery post to back half of alternator case where I have a ground cable hooked 36mv
from neg battery post to front half of alternator case 30mv

Not sure what all this tells me, but maybe "6872xtc" you can tell me. being it is mill-volts I am not sure it has much affect?

​

 
Justin Fleming said:
Voltage drop follow up

driver side battery

from neg battery post to frame 5mv
from neg battery post to back half of alternator case where I have a ground cable hooked 20mv
from neg battery post to front half of alternator case 12mv
From neg battery post to ground cable going up to intake bolt (passenger side battery) 20mv

Check from passenger side battery
from neg battery post to frame (driver side battery) 16mv
from neg battery post to back half of alternator case where I have a ground cable hooked 36mv
from neg battery post to front half of alternator case 30mv

Not sure what all this tells me, but maybe "6872xtc" you can tell me. being it is mill-volts I am not sure it has much affect?

​

Click to expand...
Those numbers are nothing to worry about. Now if it was in the 0.5V (500mV) or in the volts, then that’d be something to look into.
 
Justin Fleming said:
Voltage drop follow up

driver side battery

from neg battery post to frame 5mv
from neg battery post to back half of alternator case where I have a ground cable hooked 20mv
from neg battery post to front half of alternator case 12mv
From neg battery post to ground cable going up to intake bolt (passenger side battery) 20mv

Check from passenger side battery
from neg battery post to frame (driver side battery) 16mv
from neg battery post to back half of alternator case where I have a ground cable hooked 36mv
from neg battery post to front half of alternator case 30mv

Not sure what all this tells me, but maybe "6872xtc" you can tell me. being it is mill-volts I am not sure it has much affect?

​

Click to expand...
Ya, I wouldn't be concerned about those readings.
The RPM and throttle influence is interesting. Could this be a sign of a mechanical problem? I wouldn't think that belt slippage would be the problem.
 
Could very well be belt slippage. Since happen under power.
The rotor might be opening under rpm and dropping a loop or shorting a loop
 
Not that this would be related but just checking - when you upgraded to a high amp alternator did you also install a larger gauge charging wire to handle the increased current? The stock wire won’t be sufficient.
Also, since you have dual batteries, what kind of battery isolator did you install if any?
Have you always had this problem with the dual battery setup or has it been working without issue and just started happening?
 
Yea, all cables were upgraded to I believe 2awg or 4awg

I do not have a battery isolator.... is this required

I am not 100% sure if this has always been a issue or new. The truck only gets driven maybe 800 miles a year so its hard to tell..
 
If one battery charges/discharges at a different rate than the other then it will drain the other battery. The batteries would still test fine but they’d have different charge/discharge rates. This is especially true if they are different types (AGM and lead acid for example), from different manufacturers, sizes, manufacture dates, etc. Not sure if it would cause your issue but I could see a condition where if one gets charged quicker it would shut off the alt, then after the other battery drains the charged battery it’d turn the alt on again. It’s just a shot in the dark.

What you could do is disconnect one of the batteries and take it for a spin to see if the problem still exists. Then disconnect that battery and connect the other one and drive again. Just make sure to leave them in their original locations so the wiring isn’t a factor (i.e. don’t swap one battery in place of the other).
 
Sounds good I will entertain this approach this weekend. One problem I see is that the alternator output goes to 1 battery, so I would have to make shift this connection for testing purposes.

What is the battery Isolator gizmo you were referencing... do I need to be concern with that?
 
Do that test and we’ll see if you need one. They basically do what the name implies - isolates the batteries from each other.
Are your batteries different types, manufactures, dates, sizes, etc?
 
Should work fine with charging going to one battery and the other battery hooked positive to positive and negative to any good ground or to the first battery also. In my opinion I would say your alternator I'd not working correctly. How bout @500$k5 what do you think on this?
 
I had a small thought, in theory I guess it should not matter, but I have the batteries paralleled, that being said I would call the passenger battery the primary as all the loads etc. connect on this side, and the driver side the secondary battery. That being said I have the alternator output going to the secondary battery, in theory it shouldn't matter......but.....maybe a thought that the alternator output should be landing on the primary battery?
 
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