voltage drop testing specs for alternator positive circuit

[Jedidiahwiebe]

Hey all! Just had my alternator rebuilt by Roko in Vernon BC. They told me that the alternator overheated from working to hard and this was the cause of failure. It's relatively new - Installed 12 months ago - although it had spent some time on my parts van before it became a parts van. I've been wracking my head trying to figure out what might have caused the failure. Can not see any visible damage to the cable that goes from alternator positive to battery pos so I have performed a couple voltage drop tests. 1. Before installing the battery isolator and my secondary deep cycle battery: was a voltage drop of .21v. 2. After installing the isolator I am getting 1.2v drop from the pos lead on alternator to the pos on my cranking battery, and .28v from pos lead on alternator to the other end of that cable where it joins up with the battery isolator.
Does anyone know the specs for the l300? Anybody hazard a guess if my setup is okay or dangerous for the alternator?

Thanks!

[Growlerbearnz]

I don't have a specific answer, but I have a vague idea brewing:

Not sure what kind of isolator you have, but I had trouble getting a diode-type isolator to work and eventually switched to a Voltage Sensitive Relay (this one: http://amzn.com/B0108LNWT6). A diode type isolator has a small voltage drop across the diode (0.2-0.7V depending on the type of diode) whereas a VSR is just a relay- no issues with voltage drop.

The alternator monitors battery voltage through the Sense wire (White/red stripe in the same plug as the lamp wire). If the sense wire has a voltage drop, corroded connections, or is measuring from the wrong side of a diode isolator, could that cause the alternator to work harder than necessary?

The Engine Electrical workshop manual (http://www.delica.ca/forum/viewtopic.php?f=17&t=7333) specifies a voltage drop of no more than 0.3V at 30A on the main alternator output lead, and has a bunch of other tests that sound like they might help.

[Jedidiahwiebe]

Wow, thanks for the great answer! Could you possibly expand on what you mean by ' had trouble getting a diode-type isolator to work'?

[Growlerbearnz]

It was a long time ago (in a galaxy not actually that far away?) so I may be misremembering, but I started with normal isolator wiring- alternator output to the Common/"A" terminal of the isolator, one battery on each of the other output terminals. I left the sense wire connected directly to the main battery. The alternator wouldn't start charging. I moved the sense wire to the "A" terminal of the isolator and the alternator started charging, but the battery kept going flat. I couldn't figure it out at the time, so used it as an excuse to buy a voltage sensing relay, which worked perfectly (and still does).

It's possible my isolator was faulty- it really should have worked in the standard configuration as long as the sense wire is connected directly to the battery, the whole point of a sense wire is to compensate for any voltage drop, be it from corrosion or from a diode in the line.

Regarding overheating: I recently overheated my alternator, had the dreaded three lights of doom on the dashboard, but that was from hauling a heavy caravan up a steep hill for about 10 minutes at 20kph in first gear. Our alternators don't get much airflow but are right below the turbo: I think the combination of driving slowly and working the turbo hard was probably a bit much. Once I sped up the three lights flickered a bit and eventually went out, and I haven't had any trouble since. So far.

[Jedidiahwiebe]

3 lights of doom? I didn't realize we had that many! The rest of your saga.... really interesting. I've had no such troubles with three different brands of isolator. Other than one dying of course.

[Growlerbearnz]

Mitsubishi were too cheap to build a self-test timer for the A/T Temp and "Water in Fuel Filter" warning lights. Instead, they used diodes to wire them into the alternator warning light. Great for checking your warning lights work before starting the engine, a little confusing when your alternator stops working and all 3 lights come on at once. (Because of the diodes, the A/T temp and Fuel Filter lights can come on individually without also powering the alternator light, but not the other way around).

[Growlerbearnz]

I've just remembered that you had an issue with the alternator putting out 50V when your previous isolator failed. I wonder if that episode damaged your alternator and caused the premature failure?

[Jedidiahwiebe]

Holy crap you have a good memory. That's a good point. I wonder if that was the cause. Thanks for reminding me. I had completely forgotten about that episode.