Diode rectifier problem?


#1

1990 Cherokee 4 L str6cyl…again :slight_smile:

this winter I had a problem with battery draining after 2-3 days of not running.
when the temp dropped to near 0 degrees F , it would drain overnight.
since it has warmed up this problem has disappeared, or at least has slowed to the point of not being noticeable.

I am leaning toward the problem being in the alt., leaking current due to bad diodes, not another parasitic draw.
I understand that this happens with Cherokees due to alt. tending to get wet frequently due to location.

my question is…, does the fact that this happens more quickly, depending on outside temp being colder, lead you all to lean toward the diodes in the alt. causing this drain?


#2

There’s generally a small temperature effect on diode junction leakages that is in the other direction. Being hotter might cause a slight bit more leakage. I had that problem once with leaky alternator diodes and if there was a temperature effect I didn’t notice because I was pretty uneducated in those days.


#3

Nope. The diodes in the alternator are part of a bridge rectifier that basically turns the AC alternator (more correctly “generator”) output into DC. If the car is off, the diodes are not in a circuit and cannot drain the battery. Leakage of the dielectric in a diode will only cause ripples in the alternator output.

Your parasitic drain is from another source. Perhaps even the battery itself. Remember that the battery must be able to keep the excess electrons on the negative plates from moving through the electrolyte to the electron-deficient positive plates. Bad/contaminated electrolyte can become a high-resistance path that slowly allows electron flow between plates.

Have you had the battery tested? How old is the battery? Have you measured the drain with the engine off? What’d you measure?


#4

I have read, heard and seen, from many sources that bad diodes can discharge a battery…


#5

Bad diodes can prevent an alternator from maintaining a battery charge, but not drain one in a static system (a sitting car). For a battery to drain in a static system there must be a circuit. A system that’s off does not leave an active circuit through the rectifier.


#6

I wish I knew how to post links. if you google “diode rectifier battery draining” you will see many reputable sources that say leaking diodes can drain a battery at rest


#7

From wiki.answers, for what that’s worth:

----Bad diodes in the alternator can cause the battery to go dead when the car is off. The diode bridge (six diodes) is directly connected to the battery, and is used to convert three phase delta AC from the alternator’s stator to DC for the battery. Normally, if the engine is not running, the diodes are reverse biased and do not conduct current. If one of the high side diodes is shorted then the diode trio (another set of three diodes supplying the regulator/rotor) could conduct and drain the battery. Often, a diode failure is a short or an open. A short in a high side diode along with a short in a low side diode would result in high current from the battery, and that would probably blow the fusible link. An open would result in poor performance of the alternator.----


#8

The diode bridge output is wired directly to the output line of the alternator, which is wired directly (through a fuse) to the battery.
There are no switches or relays to disconnect the diode output from the battery when the car is off.
A switch or relay would have to handle the 100 amps or so that a modern alternator can output, that would be bulky and costly; and serve no purpose because the diodes prevent backflow (when they work properly).

Take a look at this wiring diagram:

There’s nothing there to stop backflow if a diode is leaky.


#9

www.freeasestudyguides.com/electrical-rectifier-bridge.htlm


#10

@circuitsmith‌ , that is my understanding as well. is there anything in your experience that indicates very cold weather making this drain the battery charge more quickly?


#11

My feeling is that you have a parasitic draw unrelated to the alternator. Even a new battery can be suspect and some have been known to develop an internal short.
Disconnect the alternator, connect a VOM set on the milliamperes scale, and see what happens.

Just from personal experience anyway, I’ve never seen a diode issue cause a current draw; even on the older Subarus which were prone to diode pack problems.


#12

Silicon diode leakage normally increases exponentially with temperature, meaning it about doubles for each 10 deg. C increase.
That’s maybe a few microamps at low temps and a couple milliamps at 100C.
A defective diode might behave very differently.


#13

apparently it is not uncommon on Cherokees from the jeep guys I ve talked to.

it is not the battery. I tried it with three batteries last year and no change

I have not done a detailed search for other parasitic draws yet.

I am having no problems since spring but I want to get it taken care of before winter. I don t want to have to get out the charger on cold mornings again…


#14

it was the weather making a difference that led me to suspect that the diodes or another part of alt. was expanding and contracting somehow due to temp., thus causing them to short circuit


#15

I agree that a bad diode pack will drain a battery. I had one go bad in my 89 Mustang GT and with the engine off there was a 5 amp draw from the battery into the alternator which would kill a 50AH battery in a vehicle in less than 10 hours.When I disconnected the wire from on back of the alternator to the battery all the draw was gone. Replaced alt and no draw.


#16

Circuitsmith, I confess to seeing in your schematic a path through the common ground. I retract my earlier contention. Now, if you’ll excuse me, I have to retract my foot… again.


#17

A good way to check is to disconnect the big red wire at the alternator when you park for those days and connect it before starting the engine. It’s hot so keep it away from metal!


#18

The ground path is through the alternator housing, so it would seem the best way to check would be to connect an ammeter to monitor current drain, dismount the alternator, and see if it drops. I’d wear leather gloves, as I always do when doing this type of work, to prevent any burns from possible arcing, although there should not be any. You could also drape a leather over the battery to prevent it from any arc (batteries vent hydrogen… remember the Hindenburg?). And always wear safety glasses. That’s just good sense.


#19

No need to retract your foot because I didn’t understand a word you said as my eyes glazed over. All I know is that a diode is a one way switch so if they allow current both ways, it is bad. When I would overhaul my GM alternators on a PM schedule, I’d replace the brushes, bearings, regulator, and diodes. The whole thing would cost about $20. I think the diode trio was about $5. That’s why I’m just a parts replacement guy, 'cause I’m no good at the diagnostics. So if you feel strongly about it, why not just replace them?


#20

Thanks, but when I make a mistake I try to learn from it. Fortunately, there are enough technically astute people her to catch me when I get it wrong. I would far, far, far rather have it that way.

I’ve been accused of making people’s eyes glaze over. Mostly by my kids, but from others too. Just today I had a young fella ask me why cars use more gas in cold weather. The kid is a good kid, a neighbor, and he really wanted to know. I talked about the combustion process, what heat actually is, how temperature affects combustion, how that affects the power available to run the engine and drive the car, and ultimately why more fuel is required to get the same amount of necessary power in the cold. I even got into what temperature actually is, a measure of atomic activity, using the Kelvin scale to illustrate. The kid, a young college student, has a good grounding in physics, he was really interested, and he really understood me. If I’d just said “the engine has to run richer” he would have left with absolutely no idea why. He left really understanding why an engine runs rich in the cold. But anybody without the physic grounding would have glazed over quickly. Of course, I would have avoided the dissertation for anybody without that grounding… or at least I would have tried to.