1980 300td -- Alternator diagnosis ??'s
 

Hi there: Thanks in advance for help!!

Pretty sure from observations that lack of charge from the alt., and/ or power draw from the failing alt. are draining my battery.

Will clean all terminals as a matter of course, and pursue testing of the alt. output, but I was hoping I could get some clarifications / advice re: alt. diagnosis.

Keep in mind my understanding of the system is rudimentary, so simplicity and explanations would be appreciated.

My understanding is that replacement of the regulator or the brushes may all that is required, and might be simpler and less expensive. Any thoughts? Is brush replacement likely to be within my abilities?

The manual calls for testing the alt. output at the battery, but I've been informally advised that testing right at the alt itself might be a better way to go. Is this true? and if so, how exactly would I go about it?

Thanks!

Replacing the voltage regulator is pretty easy. There are two phillips screws holding the regulator into the alternator. Remove the two screws and the regulator will come right out. Your brushes should be equal in length and more than a 1/4" long. If they aren't you need to replace it. The pic below is of a new regulator. To check you voltage output put your voltmeter probes on the battery and with the car running you should get 13volts or so. If you are getting close to 12v or lower then your alternator is bad or the regulator is bad. But check the regulator before you condem the alternator. Also make sure the alternator belt is tight. A loose belt can cause poor charging.
http://www.pelicanparts.com/catalog/...021545806A.jpg

Don't know where your located, but most of the auto parts stores such as Autozone or Advanced Auto will be happy to check out your alternator at no cost. Good luck

Checking at your battery is more than good. Afterall, if you are having charging issues, voltage at the battery is most important.

Connect the negative of your meter to the battery negative, the positive meter lead to the battery positive.

At 1k + rpm's you should be reading about 13.5 or more. I use the 20 volt DC scale.

I just went thru this on my 82D and it turned out to be a bad glow plug relay so please check that before you pull your alternator if its output is below 13.5 at running rpm's.

I have a Black & Decker electronic battery charger (10A max) I got cheap at Big Lots which has a "alternator test" function. I understand it just looks for a typical voltage signature across the battery leads that a functioning alternator would give (sinusoidal wave or such), so is more of a screening tool than a full test. To measure actual current output, I use an F.W. Bell clamp-on DC probe I got cheap on ebay. I would never pull an alternator before testing it in the car and maybe replace the Vreg & brushes first.

Many thanks for the advice!!

Sorry to neglect this for so long. With busy holiday times, and the merciful abillity to drive during daylight hours despite my car's problems, I've been having trouble getting down to business.

I think I will have Autozone check out the alternator, as observation has seemed to indicate at least some deficiency.

I also almost certainly have a current leak while the car is sitting, not running.

In case it's really a smoking gun, one question before I start testing the alt and pulling fused looking for the leak: COULD A PROBLEM WITH THE ALT / REGULATOR / BRUSHES CAUSE BOTH POOR CHARGING AND A CURRENT LEAK??

Thanks,

Bradley

I just pulled my voltage regulator. I have had poor charging lately. Anyways my brushes are at 1/8" and 3/16", and my alternator is reading 12.75 at no load which is a fail. Sourced out a new voltage regulator for $16 CAN and will swap it out and report back here soon. The shop that tested my alternator wanted $421 CAN for the re& re. YIKES!
If that doesn't fix it, then I can get an alternator from the wreckers for $95, or a refurbished from an auto parts store for $180.
I am not sure if the alternator re&re is a hard job or not.

Some of the aftermarket odd brand voltage regulators of this type seem to malfunction early in their life. Keep you bill for it. Lots of alternator and starter replacement parts are junk out there.

Yes, an alternator can cause battery drain if it has a bad diode.

bradley6,

Also after the engine IS running, try disconnecting the MAIN wiring connector from the glow relay, and then see if the alternator is actually working.
Many times the glow relay sticks ON & that will take all of the available power from system.

Sticks on despite the dash light turning off?

Yes, the gp relay can and will stick on with the gp dash light out.

Easy check is to connect a meter negative lead to the battery negative, connect the meter positive lead to any gp with the meter on the 20 volt DC scale or any scale over 12 volts.

Turn your ignition key to glow. Meter should read for a few seconds and go to zero.

If it does not, your gp relay is sticking on.

Checked the glow plug relay and it seems to be fine.
Installed the new voltage regulator, battery reads 12.5 off, 13.5 running, when I introduce load (lights and fan) it drops down to about 12.5 and doesn't seem to recover.
Could this be because the brushes on the new Voltage Regulator are new and not yet curved to the commutator in the alternator, or do I still have a faulty alternator? Thanks.

Wheatfield (or anyone else, as well):

Could a bad alternator diode cause battery drain when the engine is NOT running?

Thanks to everyone for the GP relay advice; when I get around to testing the alt I will unplug and see if there's any improvement.

If not, set your meter to Volts AC and you should have a reading of ~100mVAC.
If the VAC is too high, then look for a rectifying diode failure.

I have not read all of the threads so this could be a reapeat of inf.
I don't recall you saying your Charging Light was on when you Battery is not charging.

If when you first turn your Key on your Charging Warning Light (all of the warning Lights are suppsed to turn on) dors not go on there is a chance that the Warning Light Bulb is burned out.
Without a 2 Watt Light Bulb in the circuit to excite the Alternator to charge it is not going to work right.