My volt meter had been reading 14-16 volts, now it is reading 18 at idle and pegs at any engine speed above idle. if i turn on my lights, HVAC vent to high (A/C w/o freon) and tun up the radio, it will move just barely when i use the turn signal or windows. Went ot AutoZone and they hooked a tester to the battery while running. It reported "Failed Diodes"?

What are "diodes" as it relates to the alternator? Is that part of the internal voltage regulator and if so, can it be replaced while in or out of the car?

Or, Do I need to replace the alternator?
If so, i read that I should upgrade to the 88 style, but I can't remember the tread that discusses it (with the link to the non-forum how-to page)

I just got the pully kit from Dogerunner and am waiting to put it on until I figure out what to do about the "failed diodes"

Thanks.

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Fiero SE 86 auto...on a mission to a bring her to a modern level of performance and comfort.

Diodes
As the alternator is running, it produces current that passes through the diodes to the battery connection on the alternator. The current then travels to the battery through the positive battery cable. As long as there is a good connection to the battery, the battery will be maintained in a fully charged condition and the electrical system will function properly. A small amount of current also flows through the circuit for the alternator light on the instrument panel to provide and indication of the alternator's health to the driver. Properly functioning diodes also protect all of the sensitive electronic equipment in the vehicle from reverse current flow.

Causes of Failure
When there is a defective connection between the battery and the alternator, charging current will be forced to find an alternate route to flow out toward the battery. A defective connection may be caused by corrosion or an open circuit from a break in several battery cable stands. The alternate route the current finds usually leads to excessive current flow, causing the diodes to overheat and fail. Diode overheating also occurs when the alternator is used to bring an undercharged battery up to a fully charged condition. As the vehicle is driven to bring an undercharged battery up to the correct voltage, the excessive current flow can overheat the diodes, leading to failure.

Human Error
Jump-starting a car battery Diodes can be damaged when there is an attempt to swap out a battery while the engine is running. As soon as the battery cables are disconnected, the current will seek out another route to flow, causing the diodes to overheat and fail. Diodes can also be damaged when there is an attempt to jump start a battery, as an excessive surge of power can burn out the diodes on either vehicle. Additionally, diodes can be damaged if a battery charger that is set at a high amperage is connected to a battery in reverse polarity while the battery is still connected to the alternator.

you'll need to take the alternator out and have it serviced or replaced... there are several diodes (fiero number ?), very common for a few to "burn"... an alternator/starter re-builder shop would be your best reference... my 87 nissan truck's battery cable loosened while running and three out of six diodes (memory) were burned... $125 (included cleaning, paint and new bearings/bushings) at our local shop and all's well...............

chasgas

as i posted this, the prior post showed...

[This message has been edited by chasgas (edited 10-19-2012).]

quote Originally posted by Knight: My volt meter had been reading 14-16 volts, now it is reading 18 at idle and pegs at any engine speed above idle.

That is a classic regulator failure. Bad diodes or not, you need to replace the alternator.

The function of the diodes (a.k.a. rectifiers) is to convert the AC (alternating current) generated by the alternator to the DC (direct current) required by the car. There are usually 6 such diodes in a typical alternator. Bad diodes, by themselves, will not cause an overvoltage condition.

[This message has been edited by Marvin McInnis (edited 10-19-2012).]

When ever I had a diode go out in my alternator, the charge gauge on the dash,will either read way low or way high, and ocasionally may go up and down with the engine RPM. Not good for the battery, because you cauld,over or under charge the battery,killing your battery.Your check engine light may allso come on, and you may hear static over the radio.You might be able to find a GM alternator rebuild kit on the internet (no sodering,the complete electronic internal components). For some unknown reason, most autoparts stores dont carry those kits anymore.Probubley because they want to sell you a new alternator.Ive got a few tips on, alternator assembly if you need help,send me a PM.

[This message has been edited by James Bond 007 (edited 10-19-2012).]

most shop will test for free...

See my Cave, Rabid-Wombat's CS-130 Conversion for 85-87 V6.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

So thats why the radio is either static or very spotty with static. Did I kill my radio?

Really looking into the CS-130 conversion. That's the plan anyway. just have to get the parts together. Does anybody make a plug adapter? If not I'll have to make one myself, but then I have to get the parts and borrow some tools.

Any suggestions?

PS: Thanks for all the info. This is tthe best place ever for automotive info!

yes, great time to do the CS-130 upgrade. much better alternator.
and, most stores carry the connector, few carry a full on adapter - but they do exist. more likely at older "speed shops".
its an easy enough wiring job.

with the CS-130, you get alot more output at idle, so your blinkers & wipers dont slow down when you come to stops.


edit: anyways - yes, the diodes are part of the voltage regulator, which in GMs, is built into the alternator, while servicable on its own - usually not worth the effort. eassier to replace the whole unit.

[This message has been edited by Pyrthian (edited 11-14-2012).]

The increased voltage output can play havoc with your electrical system. And yes, it can damage stuff. I would avoid running the engine until the alternator is replaced. Because right now, you run the risk of frying the ECM.

Fry Alt can Fry everything else... That said...
Old Delco radios are, well, getting old and can have problem dirt, electrolytic capacitors drying out, etc.
One cap is filters power at the big plug. This cap get power anytime the key is on so it will be damage be a bad alt but just old age will degrade it too.

I have never replaced an alternator. for about $4 you can buy the 1 diode and install it. it's not rocket science. the only time to replace the alt is for Major failure, armature, etc. Why pay for what you don't need ?

quote Originally posted by Lou6t4gto: I have never replaced an alternator. for about $4 you can buy the 1 diode and install it. it's not rocket science. the only time to replace the alt is for Major failure, armature, etc. Why pay for what you don't need ?

For $20, you can buy a kit at Oreilly's and replace the voltage regulator, brushes, and bearings. I did that route on my '84, because I was short on money at the time, and it seemed to work fine for the 3,000 miles I used it before the engine died (blown head or shredded timing).

Autozone ah a replacement voltage regulator for th alternator, but listed it as in internal regulator. Is that the one I need and how do i replace it if so? That would save me so much time and money. especially if it can be done with alternator still in the car.

to rebuild the alt.. you should get
the diode tro
the voltage gegulator
rectifier
bushes
and bushings/bearings..
you can get kits to do the rebuild
victory lap makes them amoung others..
most autoparts shores either carry it in stock or can get them..

[This message has been edited by E.Furgal (edited 11-16-2012).]

Can i just pull off the back cover from the alternator annd replace the regulator? Are the diodes in the regulator?
Will that then fix my alternator? Since it is producing too much voltage.

the voltage reg is not part of the rectifier assembly. if you must rebuild it....


http://alternatorparts.com/index.htm#gm-type

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'97 Ram SS/T
'87 Mustang GT- 7730 w/S_AUJP V4
'86 Fiero SE- 7730 swap in progress
'85 S10 Blazer- ARI 3.4; GT3582R

the hardest part of rebuilding an alt is: the put the new brushes (or old ones) you just use a "straightened out paper clip" inserted in the hole to keep the brushes "apart "untill you slip the case back on.

Phonedawgz, Thanks for taking the time to post the video.hopefully someone learned something.