Voltage all over the place

[SuperSix]

Now - I would like to preface this that I had this issue BEFORE I installed Mike's 200A alternator upgrade.

I was piddling with the Caddy yesterday, and thought it was high time to fire up the TR and let it run for awhile.

I remembered the wonky voltages, so once it was running, I checked the voltage at the back of the alternator - 14.5v.

After it warmed up, the voltage was all over the place - varying from 13.2 - 14.3.

I don't remember what all was in the kit, but as I said - it did it before.

What could be the cause?

[ULYCYC]

Could be normal. Load, rpm's, temps and battery condition effect alt output. Need more info

[SuperSix]

Could be normal. Load, rpm's, temps and battery condition effect alt output. Need more info

What additional info do you need?

I have to assume the battery was fully charged - it's on a battery tender at all time. (Disconnected when engine running)
It was completely warmed up.
No load other than the running motor, guages, etc. AC/Fan off, no radio, etc.
RPM was idle - ~750 rpm or so.

What other tests can I do to isolate the issue?

[ULYCYC]

You gave me the info and you didn't even know it :rolleyes:
Should be 13.9 to 14.5 at cold startup. Don't buy parts or tear it apart because it's 14.3. Values are just an average and do vary. Once warmed up lets say 20 minutes it will drop down to around 13.5. Same rule above applies. Could be 13.3 or 13.7.
The charge state of the battery has a lot to do with it. If you cooked all the electrolyte out with the battery tender and replaced it with Florida swamp water you can have these issues.

[Steve Wood]

I believe it is normal to see a fluctuation if one uses a digital meter with one probe on the output terminal and the other on the case.  I see changes on mine any time I read it.

As Ed stated, it should charge high when cold, or first started, and lower as it warms up.

It takes more voltage to charge a colder battery as the internal resistance rises the colder the battery.  Alternators are pretty smart when it comes to internal electronics.

[phil_long]

......and to think that I was about to go changing out stuff because of the variances in the voltage of my car.  this helped a ton and saved me some money.  good thread

[Steve Wood]

A few things to note...

One is that the scanmaster very often reads lower than the back of the alternator and it may vary differently than readings at the back of the alternator...t he moral being to check voltage at the alternator as was done above.

Secondly, using a digital meter will show variations that will not be seen on an analog meter.  Whether these variations are absolutely correct or, are due to the sampling rate of the meter and the switching going on inside the regulator circuit of the alternator, I am not sure.  I don't think they are important as long as the voltage does not fall for prolonged periods of time under wide open throttle, or at idle with the lights on, etc.

Given the age of our cars, it is not uncommon to find some fairly large voltage drops in the circuitry.

Remember the alternator is grounded by the case to the accessory mount.  The mount to the engine by bolts and the engine to the ground cable.  With age, there can be problems.

If you put your meter across the alternator housing with a good contact point and the other end to the ground terminal of the battery and read 0.5v on the meter, then you know you have a bad ground path that is losing a half volt before you even get started....the above with the engine running, of course.

If you do the same for the positive terminal on the back of the alternator to the battery plus terminal and see a significant problem, then you know you have a problem in the plus side of the circuit.

I learned this long ago with a big drop across the cable to the battery from the alternator, and later, after I powdercoated the alternator which caused a drop on the ground side of the circuit.

We always want to change parts rather than do a little trouble shooting for simple things....fort unately we don't go to jail for such or I would be way up the river

[phil_long]

Im gone have to try that.  One probe on the alternator and one on the neg post on battery.

[Steve Wood]

yep, meter on volts, of course....be sure you are making good contact on the case.  Any voltage on the meter will be the result of the improved path thru the meter making up for a loss in the normal circuit.

Do the same thing between the battery terminal on the back of the alternator and to the positive battery terminal.

[challengermike]

so lets say you turn on the lights and maybe the heater.Should'nt the voltage kick up some.My car will drop to 11.9 at idle when doing this.I have 2 alts that both tested good and it still does this with either one.I know it maybe alittle lower at idle but this just seems to much of a drop.I think of this a discharging the battery at idle correct?

[Steve Wood]

Are you reading the voltage at the alternator?

It should not necessarily kick up the voltage, but, it should hold it.  Anything under about 12.5 is discharging the battery.  For brief moments, it is okay, but it really should be mid 13s.

Read the voltage across the alternator.  (terminal on back and the case and compare that to what you read across the battery as a first step.

If there is a big difference, then do the voltage drop checks I suggested above.

[Pyro6]

so lets say you turn on the lights and maybe the heater.Should'nt the voltage kick up some.My car will drop to 11.9 at idle when doing this.I have 2 alts that both tested good and it still does this with either one.I know it maybe alittle lower at idle but this just seems to much of a drop.I think of this a discharging the battery at idle correct?

Your measurement isn't voltage drop, it is more a reading of your system not able to keep up with voltage usage. Voltage drop on an active circuit is a difference in voltage between to points, which should be minimum.

[challengermike]

I have the engine out of the car right now but when i get it back in i will check it out.It has'nt caused me any problems but its just bugging me.

[Steve Wood]

definitely not right...these days it is hard to count on a rebuilt...or even some news ones to work properly.

If you are in an area with good junk yards, go find an Impala style alternator from 94-96.  140 amp CS144.  All the crossover numbers are on my site.  They are pretty close to the GN unit in price new, but they are better built all the way around

[challengermike]

That is the other alt i have and tried with the same results.The place i took the alt to have it tested said this alt (cs144)was great,put alot of amps out at a low rpm.It was by far better than my stock 1.He seemed like he was shocked how well it worked,lol