MarkPan

So.. when i have everything on, headlights, rear defogger, A/C, the system voltage drops from 14 to ~11v.

I changed the alt, still no dice.

so i'm thinking maybe its the exciter wire.. i've heard there is a connector on it, up by the charging post.. but I've got a lot of stuff there, can't see it. Is it tied to the frame like the charging post? or?


apart from that i'm out of ideas pretty much.

scruffie

Mark have you checked your voltage at the jump terminal??[ Loose Belt] Also check votage at the Batt . Maybe its that splice in pass footwell > also if your gettin 14 v from alternator with no load & it drops under load Maybe check out the alternator for O/P on a test bench . Just some thoughts S

RussB

Is your car turbocharged? What year/model? Most of the 5000s had 90 Amp alternators. Many (not all) 200s and 100s had 110 Amp alternators. If the afterrun coolant pump and fans operate frequently, they'll drain the battery. I replaced my 88 5000CSTQ's 90 Amp alternator with a reman Bosch 110 Amp alternator, and low battery problems disappeared.

A car alternator can "partially" fail. It's a three phase AC alternator with a pair of diodes comprising a full wave rectifier for each phase (making six diodes total). If only one phase has failed (shorted or open winding or an open diode), the alternator will produce some direct current -- just not enough.

When you changed the alternator, did you install a new or supposedly remanufactured one? The only remanufactured Bosch alternator that I trust is one that's been done by Bosch, comes in a Bosch box, and carries their warranty. Even then, I'm still a bit skeptical.

If you have a turbo car, you must make certain that the 2 inch diameter air cooling hose from beneath the bumper to the alternator is in place, or the voltage regulator or one or more of the alternator's diodes will fail due to the high ambient temperature.

AFAIK, the only time that current's drawn through the exciter wire is at zero or very low RPM. Does the ALT lamp light with engine stopped, when you first turn the ign key to on? If so, the exciter winding's drawing current through the exciter wire, as it should.

Is the voltage regulator brand new and is it the correct part? What condition are its brushes?

MarkPan

Yeah, it is a turbo car. 90A alt.

first I changed the reg on the existing alt, then i changed the alt with one from my parts stash, new regulator. all the same deal. check engine light is lit before starting.

AC seems to bog it down the most.

RussB

The Check Engine Light (CEL) is irrelevant to this problem. The ALT light is a separate light (red in color, IIRC). Refer to your owner's manual and/or shop manual for its location/description. It must light with the ignition in the "on" position before the engine is turning.

I recommend an inexpensive clamp-on digital DC ammeter from Sears. Clamp it on either battery lead to watch current flow in and out of it, or clamp it around the alternator output lead to see how much current it's producing. Or, I suppose, clamp it around the small exciter wire to the alternator to see if the exciter winding is sucking any current before things begin turning.

Have a read in this thread:<ul><li><a href="https://forums.audiworld.com/v8/msgs/91909.phtml">https://forums.audiworld.com/v8/msgs/91909.phtml</a</li></ul>

MarkPan

i'll check the other thread, thanks

MrGoodbytes

I agree. Voltage output of the alternator could be good but resistance in the line could cause a drop. I pulled my hair out trying to find troubles on a 944 until I found some questionable spots on the battery cable. Alternator off the car tested 14.5V, on the car it was 13.2V until the headlights and radio and rad fan and everything else was on. Then it dropped to 12.6. Corrision of terminals caused resistance in the line.

I'd check resistance between the battery's positive post and the plug on the back of the alternator. You can leave all cables bolted on for this test. Just set your DMM to ohms and set one lead on the B+ terminal of alternator and one on the positive post of the battery. You may need a second set of hands to hold one of the test leads.

RussB

<center><img src="http://www.metercenter.com/biddle/DLRO10.jpg"></center><p>
Accurately measuring resistances of a couple of Ohms and less is almost impossible with our usual multimeters. Reason: Ohm's Law: E=I*R. The small 9 Volt battery in a DMM isn't capable of producing enough current to result in a significant voltage drop across a very low resistance. Another problem is that the test leads and probes carry both the test current and the measuring current, leading to more inaccuracy.

There are specialized low resistance instruments available: the Ducter and DLRO (illustrated) are two. Most are four terminal instruments: a set of heavy test leads carries the test current while a much lighter set of leads is used to measure voltage drop across the resistance that's being measured. Because with very low resistances, the test current must be quite high. most modern low resistance testers pulse the test current.

So, don't count on your measurement of resistance of battery cable being accurate or repeatable if you use a DMM.

In this situation, in situ measurements of voltage drops while sucking measured tens of Amps through the cable will be more repeatable than DMM measurement of resistance. This is where a clamp-on DC ammeter is handy, together with a plain Jane voltmeter to measure Voltage drop. Then if you want to know cable resistance in Ohms, use Ohm's law to find R=E/I.<ul><li><a href="http://www.metercenter.com/cgi-bin/webshop.cgi?config=en-dlro">http://www.metercenter.com/cgi-bin/webshop.cgi?config=en-dlro</a</li></ul>

MarkPan

guess I can try it. seems to be pretty good with low resistances.

just fixed it up a little while ago. old RCA, from when they used to make electronics here.

RussB

It's good that you can zero out test lead resistance, but for accurate measurement of low resistance, that isn't sufficient. You need to pump some serious current, on the order of tens of Amps, through the test piece, and most Ohmmeters, including those built into old RCA Voltanalyst VTVMS, can't do it. Typically they might pump out a few hundred milliAmps maximum.

As these instruments pump out more current, the resistance of their test current leads changes due to heating effects, so zeroing out lead resistance no longer works. The four terminal idea -- a pair of test current leads and a pair of Voltage measuring leads -- nicely eliminates this source of inaccuracy.

Without a specialized low resistance Ohmmeter such as the Ducter or DLRO, you'll have the best results using the car's battery and alternator to supply test current, and measuring Voltage and current yourself.

Do you live near Camden or Harrison, NJ, headquarters of David Sarnoff's empire?

Another decent DLRO:<ul><li><a href="http://www.megohmmeterstore.com/index.php?cPath=109&amp;products_id=484">http://www.megohmmeterstore.com/index.php?cPath=109&amp;products_id=484</a</li></ul>