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I had been having issues with what appeared to be the alternator. Alternator died on me last month. Replaced it with Denso Refurb 100 amp (I should have gotten 130 but I was not aware). When installing the new alternator, I noticed that the battery cable that goes to the alternator was rusty and the cap that protects it was somewhat burned too. I cleaned the connector and repaired part of the cable. After installing the new alternator I noticed the alternator output was 14.0 (at the bolt, where the issue was), but the battery was only receiving 13.3-13.6 volts. I again checked the cables and alternator the next day, and for the last 4 weeks or so, my alternator output has been between 13.3-13.6 when driving (based on OBD at the module), and output at the bolt is about the same. When vehicle idles, voltage fluctuates to as low as 12.4 volts, usually when I have a gear and make a stop.

Today I purchased the GM diode hack (GM Part 12135037) and alternator output is now higher in between 13.6-14.00 volts when running, and it stays around there, but at idle it still goes down to 12.4 volts and now it fluctuates more (or greater than before) from 12.4-13.7.

Any pointers as to where to check for other cable issues? Maybe a ground issue? The needle on the vehicle also jumps when at idle. And lastly, does the change in voltage has the potential to damage up my red top optima, should I go back and remove the diode?

I appreciate any help!

1. Volts prior to diode
Volts prior to Diode


2. Volts after diode
Voltage after diode


3. GM Part number
1151932
 

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Two things jump out at me:
1) "the cap that protects it was somewhat burned" - that is an alarming statement. Why is the cap burned? Something has overheated, and at a major electrical joint that indicates either corrosion in the joint, which you hopefully cleaned off thoroughly before re-assembly, or the cable is bad (if a joint, or piece of wire has an internal resistance, that turns it into an electric heater / fire starter).

2) "After installing the new alternator I noticed the alternator output was 14.0 (at the bolt, where the issue was), but the battery was only receiving 13.3-13.6 volts.", "I again checked the cables" - how did you "check" the cables? Did you take an ohm meter and measure the resistance in it, when both ends were disconnected from the vehicle? It doesn't seem right for the V at one end of the cable to be 14 and at the other end to be 13.5, it sounds like a bad cable could be your issue.

I'm not sure if a bad cable can make an alternator fail, but it is an interesting coincidence that you have this evidence of a bad cable/bad joint and your alternator failed.
By the way, how many miles were on your alternator when it failed?


Norm
 

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Discussion Starter #5
Thanks! I have spend about an hour reading through this. You were one of the two with the issue, did you end up finding the issue? There is a lot of info that I can try on my own.

Thanks again!


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Two things jump out at me:
1) "the cap that protects it was somewhat burned" - that is an alarming statement. Why is the cap burned? Something has overheated, and at a major electrical joint that indicates either corrosion in the joint, which you hopefully cleaned off thoroughly before re-assembly, or the cable is bad (if a joint, or piece of wire has an internal resistance, that turns it into an electric heater / fire starter).

2) "After installing the new alternator I noticed the alternator output was 14.0 (at the bolt, where the issue was), but the battery was only receiving 13.3-13.6 volts.", "I again checked the cables" - how did you "check" the cables? Did you take an ohm meter and measure the resistance in it, when both ends were disconnected from the vehicle? It doesn't seem right for the V at one end of the cable to be 14 and at the other end to be 13.5, it sounds like a bad cable could be your issue.

I'm not sure if a bad cable can make an alternator fail, but it is an interesting coincidence that you have this evidence of a bad cable/bad joint and your alternator failed.
By the way, how many miles were on your alternator when it failed?


Norm
1. True. I am not sure why the cap burned, but I credited it to the failed alternator that I was replacing. There was corrosion at the joint. I removed all corrosion,I made a cut about half inch of the material protecting the cable to inspect the wire, cleaned and brushed everything to bring bright connector. I did not check resistance, but will start there.

2. I actually inspected the cables, as a visual inspection looking for more corrosion. I, again, did not check for resistance. Also, the vehicle might have been at cold idle and revving higher when I got the 14 volts, and it might have decreased when I check the battery, this was about a month ago when I replaced the alternator, so I am not 100% sure how the different measures happened... But I can confirm that for the past 2 weeks, that I have check voltage at both the alternator and battery, I get the same voltage.

The original alternator had 150k on it. For the past year, the FJ was not my primary vehicle, and due to covid issued at the county's office, I could not get it registered... So the FJ was parked with battery disconnected for about 4 month. On December I sold my primary vehicle after I got it registered and stated using the FJ everyday until alternator failed about a week after I stared using it.

I too think is a cable, I am not sure just where to start, but based on you post, resistance is a good place.

Thanks again for your time!


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Thanks! I have spend about an hour reading through this. You were one of the two with the issue, did you end up finding the issue? There is a lot of info that I can try on my own.

Thanks again!


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Nope, two and a half years later, voltage is still fluctuating at idle. Seems like some of these reman Densos come with bad voltage regulators. Where did you buy yours?
 

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Nope, two and a half years later, voltage is still fluctuating at idle. Seems like some of these reman Densos come with bad voltage regulators. Where did you buy yours?
I purchased it from a local part distributor, XL Parts in Houston. I liked that it was Denso and refurbished in the US. Yesterday I monitored the voltage while driving around. With the diode I was constantly at 14.00 volts while driving, and at stopes would go down to 13.6 which is not as bad as before, however, sometimes it would touch 12.4 or so but not often.
 

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That seems low for having the diode in the system. I have the HBK and at idle I am right around 14.0v and under heavy loads (cold weather) it is at 14.8- 14.9v

I wonder if it is related to the GM diode is rated for 1A and the HBK is ratred at 7A - just spitballing
 

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That seems low for having the diode in the system. I have the HBK and at idle I am right around 14.0v and under heavy loads (cold weather) it is at 14.8- 14.9v

I wonder if it is related to the GM diode is rated for 1A and the HBK is ratred at 7A - just spitballing
Based on the brief research I did on this diode, it provides about 1 extra volt on 4runners. In my case, it went from my high of around 13.6 without it, to as high as 14.2 with it, but more like 14.0 constant.

I am waiting for the weekend to remove the terminals and clean them well.


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That seems low for having the diode in the system. I have the HBK and at idle I am right around 14.0v and under heavy loads (cold weather) it is at 14.8- 14.9v

I wonder if it is related to the GM diode is rated for 1A and the HBK is ratred at 7A - just spitballing
It's not related to the diode's current rating, but to the 'forward voltage drop', one of many electrical parameters that define a silicon diode. The forward voltage drop can vary from diode to diode, along with the method that is being used to measure the charging system's voltage, and exactly where the voltage is being measured. Since the alternator's voltage regulator is temperature compensated, alternator temperature also enters into the equation.

Best practice would be a high-quality digital multimeter (like a Fluke), measuring directly at the battery terminals.

With a Bluetooth OBD module and Torque Pro, there are TWO DIFFERENT battery voltages that can be displayed: the battery voltage at the B+ pin of the OBD connector (labeled ADAPTER ), and the digitized battery voltage (labeled 'CONTROL MODULE') as reported by the engine ECM. There is always some delta between these two voltages, probably due to voltage drop in the wiring and to analog-to-digital conversion somewhere in the measurement process.
 

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That seems low for having the diode in the system. I have the HBK and at idle I am right around 14.0v and under heavy loads (cold weather) it is at 14.8- 14.9v
After I installed the Dirty Parts voltage booster my voltage at idle went from about 14.25 to 14.77 at the battery.

I have a 2010 if that matters.
 

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So does this Diode work? I see over the 4-Runner and Taco forums that it provides the necessary voltage increase. I purchased one myself and hoping to see my battery light go away. Anyone else use this GM part successfully?
 

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So does this Diode work? I see over the 4-Runner and Taco forums that it provides the necessary voltage increase. I purchased one myself and hoping to see my battery light go away. Anyone else use this GM part successfully?
Yes, it works. It is giving me about 1 extra volt compared to the regular fuse.

Let us know your outcome once you are able to measure the difference.


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So does this Diode work? I see over the 4-Runner and Taco forums that it provides the necessary voltage increase. I purchased one myself and hoping to see my battery light go away. Anyone else use this GM part successfully?
If your charging light is on, you have a problem that a diode won't fix.

First thing is to verify that your 7.5A ALT fuse isn't blown.

How many miles on the odometer? Somewhere around 120-150K miles the alternator brushes will wear out.

A replacement brush module is around $15, but to replace it you need to pull the alternator..
 

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My truck is at 140K so it seems right in the sweet spot of failure.

As I am learning more about this issue, I didn't quite understand what i should be seeing at the battery terminals. This morning, I saw that my battery, while the car was running, was reporting a voltage of ~12V. I believe this indicates that my alternator is not outputting anything, as it should be MORE than that, to provide a charging voltage to the battery. Essentially, it appears that my alternator isn't functioning.

Are my assumptions correct?
 

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If your alternator wasn't providing any output current, the battery voltage would likely have been less than 12V after a cold start, and will continue to slowly drop as the engine runs.

Go back and measure battery voltage at idle, and then increase engine speed to 2,000 RPM. If the voltage climbs at all, your alternator is providing at least some output.

Then, turn on major power consuming accessories (headlights, HVAC blower motor, etc.) when engine is idling and measure voltage, then bring RPM back up to 2,000 and let us know what you see.

Also, what kind of meter are you using to measure battery voltage? You really want a fairly good quality digital multimeter capable of resolving tenths of a volt, not a 30 year old Radio Shack analog meter.
 
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