bruce7

I'm curious how to check the state of health of the alternator.

I decided to measure the voltage at the battery on my car today as I
progressively added electrical load at idle.

First let me say that my battery is about 2 years old and the car and alternator
have 76k miles on them. I used a Fluke 73 III digital volt meter to measure.
I drove the car about 80 miles in the morning and then did the test.

I started the car and after 3 minutes the voltage = 13.38v
I added the A/C on low, fan speed 3 = 13.24v
Added headlights = 12.9v
Added radio = 12.7v
Added fog lights = 12.3v
Turned off A/C, added rear defroster = 11.98v

What should an alternator be putting out that is in good health at idle?
At what point would you rebuild the alternator?

-bruce

P.S. I'm going to post this and then go take a break. I'll check in later.

KNS

Your first reading at 13.38 at idle sounds good. Naturally you are going to bring down the charge rate as you load up the system with all those consumers.

See if you can check the charge at 2500-3000 rpm, you should get anywhere from 13.5 to 14.7

bruce7

In the winter I replaced the alternator at 103k miles.
It is the standard 3 v-belt arrangement for fan, alternator and A/C.
I'm now at 112k miles and did an alternator charging test
that I recently came across.

The following video by Delco Remy on YouTube describes the procedure to
check battery voltage drop. It covers testing the alternator and starter
circuits too but I didn't go that far due to the inaccessible test points.



The test procedure summarized is:

1. Key Off Engine Off
2. Voltmeter connected to battery
3. Record base voltage
4. a. If <12.4, charge battery and repeat.
4. b. If >12.6, remove surface charge. Turn on headlights and blower
motor for 1 minute. Turn off devices and wait 2 minutes.
4. c. If now 12.4 -12.6v, start engine and warm-up to operating conditions.
5. Run engine at 1500 - 2000 rpm and turn on all electrical loads.
6. Charging voltage should be at least .5v above base voltage.
If not check for excessive resistance. (Remainder of the tests cover
voltage drop testing the alternator and starter circuits.)

My base voltage when first connected was right on 12.5 but by the time
I took the picture it was 12.47v.




I started the engine and at idle the charging voltage was 13.97v.




I ran the idle up to 2000 rpm.




and turned on the following electrical loads:

Air Conditioning Max
Radio
Radar Detector (Valentine V1)
GPS (Garmin)
Low Beam Headlights
Foglights

My charging voltage was now 13.55v. This is about 1v above the base
voltage which is very good. Twice the voltage that Delco Remy says is
the minimum. I can rest assured that with the maximum electrical loads
switched on I am recharging the battery at a high rate.





I switched off all the electrical loads and returned to idle and the charging
voltage settled at 14.01v for the moment.




I just completed a nearly 4k mile roundtrip driving vacation over the July 4th
holiday and ran these exact same electrical loads while driving. So I would
consider these to be a typical load for a cross country trip.

Other loads I could have switched on were the electrical defoggers but I
rarely use those and just for a short time unlike these other loads. I am
also not running the auxiliary rear blower as I removed it and I'm running
the RS heater bypass setup (which I really love and by the way had plenty
of heat to ward off fogging or condensation in the cabin while driving in
4 days of rain on the return leg. I used the air conditioner to dry the air
with just a little heat added.)

Let me know if you try this test, especially if you are running the RS pulley
setup or the Clewett setup. Would be interesting to compare.

-bruce

AOW162435

Bruce,
Nice write-up. I'll try to get some readings soon with my Clewett setup.

Also, you may wish to install a new battery ground cable, as I've read a few comments regarding corrosion between the lead clamp & copper braid. Inexpensive item for additional piece of mind as these cars get on in years.



Andreas

bruce7

Andreas, thanks for the tip on the ground cable. I'll check it out. -bruce

M. Schneider

Bruce -
Very nice write-up. With the recent audio amplifier addition (stacked in spare tire) as above and beyond consumables ... looking forward to test alternator/battery arrangement.

Kool-aide acid test !

http://www.clewett.com/index.php?mai...index&cPath=60 Clewett Engineering - serpentine belt modification for 964/993.

IXLR8

Well the voltage at my battery with the engine idling measured 14.2V. That is with everything off that can be turned off.

The voltage regulator Vset is 14.5V according the the Bosch specs that I found. That is at the alternator.

The voltage will decrease as the engine temperature in the engine compartment increases.

IainM

V=I*R

The battery is at the end of a long cable so you need to consider where the current is flowing to judge voltage readings on the battery.

If you want to test the alternator, you need to measure the voltage at the alternator terminals, not the battery terminals. Or maybe the voltage regulator output - I don't know the details of the electrical system.

Bottom line is, the quality of your ground & battery cable terminations will affect the volt drop across them vs current drawn and thus battery terminal voltage measured.

I think if your battery is getting charged to ~14V after a good run with no accessories, then the charging system is OK.

IXLR8

What really matters is the voltage at the battery if you want long battery life.

Even with accessories. The alternator is rated at 115A at 14V at some RPM (not idle).

bruce7

Andreas,

I did a voltage drop test today on the ground strap and measured 0.001 V
with the engine running so I think my connection is good and corrosion free.

-bruce