Howdy folks!

I have an '89 Ford E350 (short school bus), in which I keep burning diodes on my alternator.

History:
I burned out diodes once or twice on the stock 1G alternator (5-6 years ago), which prompted me to do a 3G upgrade (4-5 years ago). I did it nicely, with large gauge wire, nice fuse, etc. I thought that would solve my diode burning problem, but I was wrong. Over the past few years since the conversion, I continue to burn them up.

About the bus: New batteries two years ago (Interstates), all upgraded heavy duty 2/0 cables from batteries to ground, to starter, to other battery, etc.

Heavy loads: I have an ACR (Automatic Charge Relay) that I occasionally use to connect my starting batteries with my house (solar) batteries. I do this sometimes to charge the house batteries when there's no sun by idling or driving the bus. When charging, it often initially jumps to 60 amps of charge being fed into the house batteries (I monitor this with a Tri-Metric display), but this tapers off pretty quick down to 35 or less within a minute or so. I also connect the battery banks sometimes to run a centrifuge (for cleaning waste vegetable oil). This load can vary between 55 and 75 amps.

What causes a diode to burn out?
- Poor wire connections - All my connections seem tight and clean. I was thinking that perhaps it was an oil leak that was getting onto a battery to engine ground wire, but I've cleaned this up and still had alternators go bad.

- Excessive loads - Recently, I put in a new alternator and drove around a bit with it. After a couple weeks of it being fine (not a daily driver though), I decided to connect the battery banks to charge my house batteries. Later that day I noticed the alternator voltage fluctuating (sign of burned diode). After replacing the alternator with one from the junkyard (tested prior to install), I thought I was good to go. It worked fine until a couple weeks later when I ended up using some LEDs running off my starting battery (probably not more that 30 amp/hrs). As soon as I started her up, I noticed that I had fluctuating voltage again. Dang!

I know that these alternators come with a piece of paper that says that they're not for charging a discharged battery. Would such a low amount of discharge cause a diode to burn out?

Also, many people tout the "90 amps at idle" aspect of the 3G alternator. This seems to be counter to the idea that excessive loads can burn out diodes.

Also, it seems like the voltage doesn't always fluctuate. I can have a good day of driving where there doesn't seem to be a diode out. Also, when in higher RPM, there usually doesn't seem any voltage fluctuation. However, I am aware that since I monitor using a digital voltmeter (built into my brake controller) that the fluctuation is so quick that it isn't registered by the voltmeter.

Solutions: I have tried many things.
- Doing a high-idle mod (making it so I can bump the idle higher while idling for better alternator output and reduce the chance of wet-stacking). Although, I haven't had the chance to use this while having the battery banks connected.
- Cleaning all battery/wire connections.
- Adding a 2/0 ground cable from the block (where the battery connects to the block) to the negative terminal of the house batteries.
- Purchasing a reman alternator from Autozone with a lifetime warranty (swapped it out twice now)
- Changing the loose-ish original belt with a shorter one (still a little wobbly, but not as bad).
- Replacing the single wire connector on the alternator (the old one had lost the clip for holding it in and I thought it might be loose).

I feel like I've exhausted what I can do. I was expecting this alternator to perform as people had said (90 amps at idle!, etc). I feel like it burns up diodes even when a small demand is put on it (recharging 30 amp/hrs, or charging at 35 amps or less. Even when a bigger demand is put on it (75 amps at idle), is that too much? How about with the high-idle mod (bumping the engine from 700 to 1100 RPMs)?

Are there any other reasons that an alternator diode can go bad? How can I test how many amps are flowing out of the alternator at any given time (I can only test the house battery amp input/output now via a hardwired shunt/Tri-Metric display)?

Does anyone know the duty cycle of a 3G alternator at full output?

Thanks in advance!
Guisepi

 

I've done a little research and what I would do is get yourself a larger rectifier pack than your alternator can put out - say 130A rated for a 95A alternator.
The problem is that an alternator(or motor) is a bunch of wires and magnets. It follows E=IR, or Ohm's law. If you drop the output voltage(as into a dead battery or bigger load), the current will increase significantly. Unfortunately, the rectifiers are solid-state devices and have a very finite current limit; go above that, even for an instant and they will fry.
You might also want to install a 90A(or even lower; 75A) circuit breaker as added protection -- like http://www.ebay.com/itm/DC-12V-24V-Car-Auto-Circuit-Breaker-Fuse-Reset-Inverter-Durable-US-BIC-/191582873187?pt=LH_DefaultDomain_0&var=&hash=item2c9b3aa663


One other option would be to just modify your alternator and put the rectifier externally.
Using the instructions for this external unit: Alternator USA Built Quicktifier 420 Heavy Duty External Rectifier Alternator | eBay

and just get a massive 3-phase bridge rectifier instead: http://www.ebay.com/itm/MDS200-16-3-Phase-Diode-Bridge-Rectifier-200A-Amp-1600V-FKS-/221743147189?pt=LH_DefaultDomain_0&hash=item33a0ebe8b5You'd need to put it on some large heatsink area to keep it cool(below 85C/158F for full power)

 

I would think you should be looking for a generator. that's a lot of load to be applied to an alternator continuously. I have had no problems with my 3g swap in running my plow, but hats short burts of high amp draw not continuous high loading. more battires may also help you, but id be looking for a generator. and a rectifier, or find a jump start unit that produces 12 or 24 vdc

 

Thanks for the replies, fellas.

I'll be looking into putting an external rectifier on my alternator, but that still doesn't answer a few of my questions:

- Why would an alternator fail with very minimal strain (charging 30 amp/hrs of discharge)? Do you think that the engine compartment of an e350 with a 7.3 IDI is too hot?
- What is the duty cycle of a 3G alternator?
- Could there be some sort of ghost load that I don't know of? How to test for this? (Looking in to installing an ammeter for my alternator to see how much juice it's putting out)

I won't be using a generator (Don't have the space for one, and not too partial to fossil fuels - been running biodiesel and WVO 95% of the time for the past 7 years in this rig. ) - thanks for the suggestion, though.

Thanks!!!

 

ive had a used 3g on my 88 f250 for around 3 years, bought a used one with the intention to make sure this swap was going to work then replace it with a new one, but it still charges good so I never messed with it. as for my truck, I run a meyer e47 pump, im running dual interstate black tops, around 800cca I think. between the plow pump and lights that puts a pretty heavy load on it I have around an hour drive from work, high beams on the whole way running a/c and blower motor. so id say you might have another issue. you might need to switching to deep cycle batteries may help.


what brand of alternator are you buying?

 

Hm, here's an idea: If you had too much resistance in the alternator power wire(the heavy one going to the battery), the controller might increase the output voltage too much to compensate. IIRC, the diodes on these alternators are rated at 48V or less, so if you had too high a voltage going through them...
Really, though, I've got no clue. I've never seen or heard of this type of constant failure, though I do know that a lot of modern alternators are just plain crap and fail due to that.

 

i am not sure where you get your 60 amp load when recharging your batteries. A discharged battery will take all it can get. It is called in rush current and that is the reason for not using an automobile alternator to charge dead batteries. Here is a link to a site that has alternators big enough to do what you want to do. http://alternatorparts.com/ford-3g.html
I worked on large UPS systems and have seen the in rush current high enough to trip out 1000 amp breakers. Also Autozone alternators aren't known for being high quality.

 

i am not sure where you get your 60 amp load when recharging your batteries. A discharged battery will take all it can get. It is called in rush current and that is the reason for not using an automobile alternator to charge dead batteries. Here is a link to a site that has alternators big enough to do what you want to do. http://alternatorparts.com/ford-3g.html
I worked on large UPS systems and have seen the in rush current high enough to trip out 1000 amp breakers. Also Autozone alternators aren't known for being high quality.

 

Alternators are not designed to charge but to replace and manitain. By that I mean replace the voltage lost in a short high draw event such as staring, when the voltage is back to full it should maintain that voltage

 

What's /supposed/ to happen is that the voltage will drop because the current draw(resistance) is so low -- the alternator cannot put out any more current than it has magnetic potential.
If the diodes can't handle this current, though...

Edit: Also, alternators are supposed to supply all the running current(easily adding up to 10s of amps) needed for the lights, fans etc.

 

The problem being some alternators are capable of producing more current then there diodes are designed for. In a short burst they can handle this higher current but will not handle it for long durations. Bottom line you are asking a standard alternator to do more than it was designed to do.

 

I think we are all saying the samething. It not meant to charge a bank of batteries for a long time. Perhaps on option would be to get the ambulance alternator and find a way to mount it as a second alternator just for those batteries

 

I'm running into the same charging problem with my 7.3L PSD I had before my alternator change. I thought I had solved the battery drain with all the work I have done so far, but today after my last stop to pick up E-free gas for our mower I noticed the voltmeter not charging but going down. It was foggy coming home but because of the low voltage problem I didn't want to turn on the lights. Now I'm pissed!!! I stopped my system from draining juice through the glow plugs by undoing Diesel by Dummiez & now it's as though being haunted by a ghost that won't go away!!!! My alternator light ain't coming on. I got a Napa Re-built Alternator with the Lifetime Warranty @ 130 amps...wondering if I gotta go higher.
Aloha,
Chief