Low voltage at idle, alternator at 14+ volts

[TripleJ]

I'm having a low voltage condition while idling in gear. I'm not exactly sure how to diagnose it.

It started with two indicators. While idling in gear the volt gauge will sink to the low 1/4 of the gauge. I've tested voltage at the distributor and it shows ~12.5V. When I rev the engine the gauge always increases to the normal range.

The wierd part about this situation is if I test voltage AT the alternator output to ground, it shows over 14v, I've seen up to 14.9v. The alternator also seems to be whining as if it is full-fielding, pretty much all the time...

My rig uses a lestek alternator and external regulator. I'm not certain how having an isolator in the mix affects how to diagnose this situation.

Any ideas, or a place to start???

[legomybago]

Whats your voltage when measured at the back of the gauge?

[Rickf1985]

Check every ground you can find and then look for more. You have a voltage drop somewhere and I will bet it is a dirty ground. Also be sure the battery cables are clean, ie, clean them to be sure. The plug on the back of the dash that goes into the ribbon gets dirty and sometimes the connections burn. Something else to check if the other stuff does not fix it.

[ClydesdaleKevin]

On our 89 Holiday Rambler, all of the dash and gauge grounds are interconnected on the back of the gauge bracket screws inside the dash (accessible through cutouts in the top of the dash on ours, so hopefully yours is similar) ...and all of ours were loose (REALLY loose), producing similar symptoms to what you are describing.  Get in there and tighten all of them, and see if that helps.  It fixed ours completely (except the headlights, which is a WHOLE other story). 

Kev

[TripleJ]

Ok I'm still working on this. I have a question about battery isolators.

Is there any reason for an isolator to have a reduced voltage read at the 1 and 2 posts? I started the engine today, and at fast idle the A post to ground read 15.2v. The 1 and 2 posts both showed 14.4v.

Is the isolator suppose to reduce the voltage from the alternator?   

[Elandan2]

The isolator is not "supposed" to lower the voltage, but the diodes inside the isolator cause the small drop that you have measured.  Rick

[TripleJ]

Thanks! That is just what I have been reading. Older diode battery isolators will lose between .3V and 1V from what the alternator is producing. That is a difference of up to 1V between the A post and the bat1 or bat2 posts when measured from common ground. Seems kinda shabby doesn't it, on these rigs where power is at a premium?

That means in order to feed your batteries the needed 13.9V while driving, your alternator must be producing up to 14.9V or more depending on your electrical load.

Now just I need to find where the remaining ~1v is going. 

[DaveVA78Chieftain]

I take it you have a 4 wire Diode isolator.
1 from alternator B+
1 to chassis battery
1 to House battery
1 "sense" terminal

The sense terminal is feedback to the alternator regulator which is telling it to output the 15.2v so the diode loss is accounted for.

Dave

[TripleJ]

That's a negative. Mine is a three wire. The regulator sense is connected to a wire under the doghouse but I'm not yet certain where it goes.

Mebbe to that bus bar setup right above the isolator?



This pic is old, it's not quite as crusty looking at the moment...

[Rickf1985]

My first question would be where are you getting 15.2 volts from? You should not have voltage that high anywhere in the system.

[legomybago]

Sounds like your regulator in having issues. And if your alt is whining, maybe you have a bearing going bad causing some belt slip? Nothing sounds consistant with your voltage readings. Hm?

[Rickf1985]

Alternator is probably whining from running full load. You need to address that before you cook your battery.

[TripleJ]

Actually, as I learn more about how this system works I'm finding that the alternator and regulator are doing their job correctly. The regulator sense line (S) is reading a low voltage from somewhere, and commanding the alternator to ramp the voltage up until it gets the ~14V AT THE (S)that it requires.

The ~15V is ONLY at the alternator output/A terminal at the isolator. Nowhere else in the vehicle is there 15V available. In other words the batteries are not getting the 15.2V that I read the other day. They are seeing about 14.4V due to the diode isolator. I think this is still a bit too high tho?

Regardless, the vehicle electronics are not functioning optimally at ~12.5V which is all I'm seeing at the distributor B+ wire and the voltage regulator (S) line.

The isolator outputs (terminal 1 and 2) are where start and house battery voltages are available.  What I need to find is why the voltage at isolator terminal 1 (start battery) is not the same voltage I'm getting at the regulator sense line (S). They should be about the same.


Also, when I report a voltage from a source, I usually try to get it using 3 different grounds if possible. The three are engine, frame, and body. So far I haven't seen much, if any, variation between the three ground sources. I believe this indicates that my grounds are generally in pretty good shape... When I am able to, I still plan on cleaning the grounds tho. To be sure.

[Rickf1985]

Always check your grounds at the battery, that is where it all comes back to. What you need to do to find your differences is voltage drop tests across all of the different components. I would start right off with the battery to the engine, battery to the body and battery to the alternator case. I will bet you find a drop in one of those tests, probably the battery to body. Then you need to find and clean that ground, or add a new one. Always check and repair and ground issues first and then move on to the power side.

[DaveVA78Chieftain]

Sense wire (red wire) goes from the 2 wire plug on the alternator down to the starter solenoid B+ terminal via a fusible link at the solenoid.

Dave