'86 DL240 wagon

Phew. Survived the onslought of Xmas carols. Now back to the alternator. I've checked for 'parasitic drains' and found the radio was shorting and disconnected it completely. Checked resistance on main battery ground cables and put in a new ground wire from alternator to engine block. Checked entire charging circuit - battery to starter to alternator; D+ wire from alternator to junction box to key to charging light to fuse. Warning light goes off after engine starts and also grounding the D+ wire causes warning light to remain on - meaning it's functioning correctly. And . . . alternator, tested at Auto Zone as good, still doesn't charge. Checked diodes on alt and they show a flow in only one direction - with Ohm reading at about 1500 for 'live' side. (someone here said that number should be 500-800 but videos on YT show #'s in the thousands like mine)
So! What else could it be? One video on YT said that the air intake throttle can get gunked up and throw the alternator off. Mine's pretty dirty so am about to clean that up. This alternator HAS reached 14 volts once. It climbed up there one decimal point at a time but then went back down and now all voltmeter readings only show the present state of the battery charge - 12.3 car off - 12.1 - 12.2 running - and going down as the car uses up battery juice to run. Very frustrated and parked in a bad place to boot! What else -besides possibly the throttle setting, could be preventing a good alternator from charging? Could the air mass sensor cause a similar problem by mis-informing the regulator? Is the ECU involved in all this?

 

i had a 'liftetime' warranty alternator for a vw bought from autozone would not charge. took it back tested good on their bench. took to advance auto store across street tested bad lol. brought back to autozone changed for different one no questions asked (at least not many!) all good.

 

Do we assume the engine is grounded to the chassis of the car. Smells like the alternator is bad. Have it tested someplace else.

 

Good idea. I think I will as it's still out of the car. Thx.

 

You're the second person replying to this post suggesting I test the alt somewhere else. Think I will. Thx.

 

When the guy is testing the unit tell him you want to see that the unit is putting out between 13.5V -14.2V. Let him show you that and not just get a good/bad reading.

 

I'll do that. All their tests do is say 'testing' and 'tested' as proof that it passed.

 

I just use a volt meter to test my alternators. a fully charged battery thats been resting a couple hours should be around 12.6V, and when the engine is running the alternator should be outputting about 13.8-14.2 volts, or about 1.2-1.6 volts higher than the battery rest voltage. switching on all the lights, fan on high, etc, with the engine idling, the voltage shouldn't drop more than about 0.1 volts from the elevated charging voltage, and with even a slight rev, should bounce right back.

if I *don't* see that elevated charging voltage, I start looking for ground differentials. measure volts from alternator ground post to the battery - post, should be very close to zero volts. measure from alternator ground post to the engine block, ditto, close to zero V. measure voltage from the alternator B+ large terminal to battery +, should be very close to zero volts. engine switched off, ignition switch on, D+ terminal should be like 5-9V relative to any ground.