Contour Enthusiasts Group Archives Forums General Troubleshooting Alternator diode question

Hey all,
I recently took got my battery changed because it is heading into its 6th winter (no problems or anything yet), and I didn't want to get stuck somewhere. The installer guy at sears said that my alternator was bad and needed replacing. I took it to autozone for an alternator check, and they said I had a bad diode, but it was still driving current into the battery. I'm wondering if I actually need to replace the alternator. It seems unlikely that it would have broken only a day or two before I happened to take it in for replacement, is there any chance the alternator could work with a bad diode? I'm not sure how the circuit works, is it a full bridge rectifier and maybe I only lost half of it? If so, I would imagine I am ok. I'm guessing I'm probably screwed, but since alternator replacement is so damn expensive, and it sounds like I can't do it myself (I don't have a lift) I thought I'd ask.

Thanks in advance,
Dave
'99 silver frost svt

There are nine diodes in an alternator. Six are configured as a three phase full wave bridge rectifier. The other three form a half wave bridge to supply the field winding via the regulator, and rarely go bad.

If any one diode in the full wave bridge rectifier is open, output current available will drop by 37%. This usually shows up at idle with the alternator light coming on if the headlights are on, or at least the headlights will dim at idle. The headlights will brighten as the engine speeds up and the voltage comes up to the point where the battery is charging again. This type of problem will leave you stranded on a rainy night.

If a diode is shorted or leaky, the battery will drain while parked.

Thanks, that was quite informative. I was thinking of ac coupling the the battery terminals to a scope to see if I can see half a phase missing. Also, do you know of any way to tell if I have an open or shorted diode? It sounds like if I have an open diode, I don't need to replace the alternator, but I probably should if it's a short.

Thanks again,
Dave
'99 silver frost svt

Don't these diodes reside in the voltage regulator ?
I might be wrong but I'm afraid you have to remove the alternator in order to pull it out. Don't remember exactly how much access you have with the alt. installed.

Btw when I replaced my alt. the whole unit was cca $200 and the regulator about $40.

Tiv

The diodes are in the alternator. They are not part of the regulator. The diodes change the alternating current of the alternator to the direct current that the battery and car system uses. The diodes are also known as rectifiers for their role in this conversion process. Also the plate that the diodes are mounted on is called a rectifier plate.

Diodes are a one way device. Current only flows one way through a diode.

Thanks for all the responses. Bottom line: can I get away with not replacing the alternator?

Thanks,
Dave
'99 silver frost svt

If the diodes are failing, no. You may want to test it again to confirm their diagnosis.

just get a new alternator and do yourself a favor.........
the diodes let electricity run one way.. from the alternator to the battery.. if you have a bad diode then the juice will run out of the battery back to the alternator and your battery will go bad.. if you have to ask
questions like this tben you dont know enough to work inside an alternator.. and where are you gonna get diodes and will they be the right kind??? some diodes that they were using in the 80's ford cars had to be high temp. diodes.. the remanufacturing places were using just regular diodes and the replacement parts would not last............

I understand what a diode is. I'm fairly sure I could figure out the circuitry of an alternator, I do have a masters in electrical engineering. I also confident that I have an open, not shorted, diode. So my battery should not be draining, I'm just missing part of the pseudo-dc waveform. My question is if my battery will be charged adequately and not damaged by the less than 100% duty cycle.

Dave
'99 silver frost svt

If the diode is open and will stay open you will not probably hurt much in the short run. Sometimes an open diode becomes a shorted diode depending on how the pieces got blown out on the rectifier plate. Sometimes they kind of move from open to shorted according to temperature or load.

If you have a blown diode on an alternator, it is unwise to continue to depend on the alternator.

I understand your question. Basically you want to know if the alternator will function well enough in the short run at only 2/3 capacity. That mostly depends on if the alternator will continue to function at 2/3 capacity and if you keep the electrical load light. No lights, no A/C, no radio, etc., etc.