Darien

While driving with headlights on, if I notice the lights dim and the volt gauge fluctuate with the turn signal or window switch engaged, is it the battery or alternator?

Darien

If it's the Alternator, which one do I need? I've seen 75-130 AMP??

the flyin' scotsman

Darien...........you can perform a simplistic alternator check by putting a volt meter on the battery engine running and record the reading at idle. Rev the engine and observe the change.......a good alternator will raise the reading by 1.5-2 volts. You can confirm the condition of the battery by having it load tested..........you'll need a special tester to perform this op.

If you need an alternator the part # is 928 603 011 AV.

User 4221

To a certain extent, this behavior is normal. The best way to check what is happening is to connect an analog volt meter to the battery and operate the lights like you mention above. If the volt-meter fluctuates like the dash unit, then it's probably and alternator.

The reason this happens is the connection to the dash voltmeter has about 6 connections between it and the battery. Each connection will grow in impedance over time and cause fluctuations when a higher current item demands power.

Think of it like a drinking straw and garden hose. Both items are connected to the same water source. The dash voltmeter is fed by the drinking straw and the lights from the garden hose.
When you turn on the garden hose, it will take precedence over the straw due to current flow.

So, connect a voltmeter to the battery and see what's happening.

HTH,
Scott

Darien

Thanks... I'll get the meter on it. My in dash volt meter shows just under 12 volts with the lights on and drops when I select any interior switch.

karl ruiter

Which lights dim? Is the the headlights or just the dash lights? Some of this is normal, especially the volt meter movement and dash light dimming, but if you have more than a normal amount of this I would start by looking at electrical connections: Ground points, fuse contacts, battery cables, etc.

Darien

All lights in and out, should the volt gauge be at 12 or 14?

Garth S

The battery alone should be 12.3 to 12.6v: with the engine running, the alternator should put out at least 13.8v - and hold this with lights, etc. on and revs above idle (~2000 rpm).

Darien

Well it looks like my Bat is low, only putting out around 10V Should the alternator still maintain 12 plus volts after the engine is running?

John Struthers

Chicken or the egg?
I've had the shop check, and both alt and battery demonstrated proper function.
Driving at night, lights on, brakes on at a light or stop sign the voltage would drop to the point that the engine was stumbling at idle. This got so bad I had to do a heel and toe for reasons other than racing.
I've bought several replacement alt's from Advance and or O'Reilly's, some were bad alt units,
At one point I had three rebuilt units -all special order- finally swapped regulators around till I got one unit to put out proper voltage.
In any case If you buy a rebuilt have them test it right there!!! They had trashed the adapter and couldn't do the test for me.
Option.
Take the unit to a alt/elect.shop and see if they will test for bad regulator. A bit cheaper.
Personally, when I get off my lazy butt I'm planning on installing a single wire High output unit on the Weissach and the White 81 with the bracket, direction and pully snout being the only forseeable problems.

Garth S

Have you tried a full recharge of the battery? If so, and it does not hold for long, then ....
Sounds as if there is a dead cell in the battery. It may charge up to show 12v, but only hold what is termed a 'surface charge' ( no significant amperage capability). This is hard on the alternator, as it works overtime to recharge a poor battery.
Pull the battery and take it to your favourite AutoZone/NAPA/ ... for a CCA test (free ). That is easiest to do - and will let you know if the alternator is off the hook ....
* CCA = cold cranking amps: battery spec is ~750 new - when they drop to 1/2 that, or show a cell 'surface charging' - replace.

John Speake

If there's a seperate plug for each cell, you can use a cheap battery hydrometer to identify if a cell is down, compared to the others.

the flyin' scotsman

Wow..........how quickly we forget! John (must be all that -48v experience) is correct in that you can check the condition of the battery acid with the hydrometer if you have caps on each individual cell.

If you have a maintenance free battery and if the electrical tests indicate a failing power source then not much else to do but replace. I run a AC Delco which operates perfectly YMMV.

Dennis Wilson

Sounds like a shorted cell in the battery which is stealing amps from the alternator.

Dennis

Brett Matthews

Yes, go get the Battery load tested. I am fortunate to have an Interstate Battery wharehouse located close to me, and they have two bays to service drive-ins! They've already replaced a couple batteries for me, under warrantee! Once for Rosy, and once in my van. NO CHARGE!
I've had lousy experience with Diehards..... Pro-rated to the point of the cost being almost a new batteries' cost.