Alternator diode heat

[dave110]

And my 95 PSD that I had for 14 years and 165k miles only replaced 1 alternator because it quit charging. Never even heard of AC noise back then and no ECM, PCM, or other computer related issues at all. And those glow plugs take alot of juice too. Used to pull the volt meter down as far as these Cummins grids do. Now it ate glow plug relays for breakfast due to over heating but never an alternator diode problem.

[JAG1]

On 10/18/2018 at 6:05 PM, dripley said:

Just never saw it or had to clean mine.

Dave110, every year dripley coats his whole engine in chicken grease.

 

 

Just a thought about the two power leads to the grids..... Is it possible the insulating washers have deteriorated or are dirty enough to cause a degree of 'earth leak' creating a higher load than when new?

 

I have seen dirt on a battery top cause a mild short or draw between the posts.

Edited October 31, 2018 by JAG1

[ofelas]

As Haggar posted above, malfunctioning/high resistance grids (would still be less draw than a correctly functioning starter) would not affect the alternator diodes, as long as the battery is in good working order & able to sustain a temporary, heavy, normal amperage load.

 

I have a few 26 year + grid elements/solenoids on their original engines/cabling/terminal ends, and they all function rather reliably. 

 

No engine oil coated terminals either; just a light coating of Ox-Gard on metal to metal connections.

 

I would point to bad diodes and/or substandard cabling/ends.

 

 

[Tractorman]

12 hours ago, Mopar1973Man said:

What's different? Electrical loads... Like the diodes get rather hot to the touch just as the grid heater hits. As the grid heater shut off you can feel them cool down from what air movement the alternator can create.

 

Like I've got the Quadzilla set for a 170*F warm-up mode and 15-second delay so if the coolant is below 170*F and no moving for 15-seconds it jumps to 1,200 RPM. What I'm trying to figure out is reducing the diode heat. Once the grid heaters shut off the diodes cool right off. With the Quadzilla setup, this way helps because the alternator fan keeps the diodes cooler. Still a band-aid to the problem. If I disconnect the grid heater power supply outright the diodes never really get warm. Here I'm back to circling the grid heater again... 

 

I think there are some things going on with the grid heater operation that are normal, but have not been mentioned so far.  Also, I don’t think that the grid heaters are the problem (as in defective). To illustrate, consider a 136 amp alternator charging a circuit that is electrically sound maintaining 14.0 volts with engine running.

• Alternators typically put out their maximum rating at around 6,000 rpm (alternator speed)

• Engine pulley to alternator pulley ratio for diesel engines is approximately  3:1 (6.7 Cummins 3.22:1, not sure what 5.9 is)

• A diesel engine at idle – alternator is not capable of delivering rated output amperage.

• A diesel engine at high idle (1200 rpm) – alternator  will deliver higher amperage, but probably not rated amperage.

• With engine running the alternator will deliver up to its rated output (dependent on alternator speed) if battery voltage at sensing location is pulled down by one or two tenths of a volt and held there.

Example:  If an electrical load pulls battery voltage down to 13.9  volts, the voltage regulator (PCM) will command alternator output, but only enough output to bring battery voltage back to 14.0.

 If a large electrical load, such a two grid heaters pulling 200 amps total load, is placed on the electrical system, the battery voltage will fall rapidly even if the alternator is delivering its rated output of 136 amps (136 amps being delivered – 200 amps being consumed = a 64 amp deficit).   Battery voltage will continue to fall until combined battery amperage and alternator amperage is equal to the load amperage being consumed.   If the alternator speed is less than rated speed, the alternator amperage to the batteries could be considerably be less, especially with an idling engine.

• Diodes by design generate a voltage drop, about .6 volts.  This voltage drop generates very localized heat.  The example below assumes 40 amps are passing through a diode.

   

  Example:  40 amps x .6 volts = 24 watts (think of a 25 watt soldering gun)

So, ( just my opinion) but I think that after the grid heaters are cycled , the starter is cranked, and finally the post grid heater cycles begin, that the diodes being hot to the touch on any alternator would be a normal condition.

Another thought. 

     An idling engine with grid heaters cycling would generate a specific amount heat in the diodes with a specific amount of air flowing past the diodes for cooling. 

     An engine fast idling under the same conditions would generate more heat in the diodes, but would have more air flowing by the diodes for cooling.   Would the diodes actually be cooler than the above example?  Maybe, maybe not.  Maybe the diode heatsink is inefficient or there could be other factors like what Haggar mentioned – quality control with diode manufacturing.

One way that grid heaters could affect the life of an alternator could be in controlling the post heat cycle event – how long each cycle is activated and how long each cycle is deactivated.  Also, are both grid heaters activated, or just one?  I am not sure how that is supposed to work.

I know that this information does not provide a definitive answer as to why your original alternator performed so well, but I just wanted to share what I have learned over the years and maybe some of the information can possibly help with diagnosing your issue.   

My method of avoiding repeated high alternator current output conditions has been to re-wire the grid heater control relays with a momentary switch so that I control when and how long the grid heater cycles and to avoid the post heat cycles completely.  I know that this method sets a couple of codes, but I know what the codes mean and the codes have no impact on driveability.

 

- John

Edited November 1, 2018 by Tractorman
another error