760's eating alternators - Page 8 - The Combine Forum
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post #71 of 88 (permalink) Old 12-10-2018, 06:30 PM
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Any findings on this??? Or did you pull it in the shed?!


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post #72 of 88 (permalink) Old 12-12-2018, 10:18 PM Thread Starter
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Any findings on this??? Or did you pull it in the shed?!
Yeah, been waiting to get some other stuff out of the shop so I can get it in there.

However, I did figure out I missed something. I'm not sure how.

When I pull the comm wire off the alternator, the wire still has 8.3V. Apparently the turn-on wire for the relay is being fed from somewhere else. With the wire unhooked, the terminal on the alternator goes to 13.5V (engine running). With the relay plugged in and the wire off, the wire is at 8.3V. With the relay out and the wire off, the wire goes to 13.5V.

The terminal on the alternator seems to have no effect on the wire. The comm wire measures the same whether it's hooked up or unhooked.

I figured this out when I was checking the function of the idiot light. With the relay in, the light is out no matter whether the comm wire is on the alternator or disconnected. As soon as you pull the relay out, the idiot light comes on.

Now I'm puzzled.

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post #73 of 88 (permalink) Old 12-13-2018, 08:45 AM
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Where does the 8.3 volt come from when the comm wire is unhooked?

Last edited by 1934 a; 12-13-2018 at 08:47 AM. Reason: Grammar
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post #74 of 88 (permalink) Old 12-13-2018, 12:09 PM
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Jeff posted what i call a generic drawing on the first page. Generic because to me it doesn't properly show the connection of the idiot light. I suspect it connects to the battery is some way because most idiot lights are connections between battery and alternator with the battery as the power supply, not the alternator.
I have a few theories but really need detailed drawings of the circuits as well as internal drawing of the alternator.
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post #75 of 88 (permalink) Old 12-13-2018, 01:19 PM
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Well that wire on a traditional alternator sinks current so your not really looking for voltage there.

Like above, a test light hooked to positive and that post should turn on when not turning, off when turning and charging.
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post #76 of 88 (permalink) Old 12-13-2018, 06:53 PM
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According to the schematics, That wire is hooked to only 3 things: the Idiot light, the relay coil, and the alternator. Unless you have the wire shorting to another voltage source, that voltage is coming is coming from the idiot light...as expected.

As mentioned, a typical idiot light circuit is 12V IGN-->Idiot light--> alternator. When the alternator is not charging, the alternator control wire sinks current thru the light, turning it on. When the alternator is charging, the control wire goes up to 12V, so the Light has 12V on both sides and is off (no current flow thru light).

In this case a relay coil is added "sampling" the circuit. With the alternator disconnected, the circuit is 12V IGN--> light-->control wire--> relay coil--> ground. The voltage on the control wire is "in the middle"....dependent the internal resistances of the Light and the relay coil. If your relay coil is 120 ohms, and the light is 65 ohms, your voltage would be 8.3 between the two. The relay is apparently pulling enough current thru the light by itself to turn it on --is it dim??

TO test if the 8.3V power is coming from the light, pull Fuse F058: 3A fuse for A074 (indicator module) (Late model combines may have different fuse #s). The Light should go out and the voltage should disappear.

My remaining question is if you hook up the control wire to the alternator, key ON, but motor NOT running (or belt off), is the voltage on the control wire lower that 8.3V? Ie--does the alternator actually pull that wire down to ground when not charging. You need to test this.

As long as the alternator pulls the wire low, I think using a SS relay should still work. This should not change the basic function of the operation we discussed---the alternator should pull the the control wire low (turning on the Light) or push it high (turning on the Relay). What would not work is the light would not come on if the control wire is disconnected.

Whats really confusing me is your last statement....pulling the relay turns the light ON with the alternator wire disconnected.... if that is true, then either my theory of operation is all wrong, the schematic is wrong, or .... something is really strange. Or the A074 indicator light module is was smarter than I thought..

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post #77 of 88 (permalink) Old 12-14-2018, 12:22 PM
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Jeff, i assume you are looking at other schematics to know that the idiot light is connected to another 12v source. If so i follow your thinking. And pulling the relay out turning on light tripped me up yesterday also so i have nothing more to add - there are unknowns yet. Likely that the idiot light is more than a simple 2 wire unit.



Did the voltage to ground checks get done? Measure k061 terminals 85 and 86 to ground with relay installed.



One thing to help with theory for others. Not good to consider something as pulling current. Everything is result of voltage, the load is a resistance and therefore reacts to the applied voltage. Its like saying a hose is letting or pulling water through. Its isn't letting any water through if there is no pressure from the pump. The more pressure at inlet the higher the water let or pulled through. As you can see "let/pulled" is not a correct term. The more water "will" flow through is better. In fact electrical theory can be more easily understood if you take the idea of fluids (current) driven by a pump (voltage source) pushed through a resistance. Even series/parallel circuits can be understood more easily if thinking of fluids.

So, with the idiot light, the light is on because there is 12v from battery, through light and to ground through alternator winding when alternator not charging. This is your current flow. What stops the current flow is more difficult to understand until you think in terms of voltage. When alternator is charging there is voltage in the windings so in fluid terms you have one pump (12v) pushing against another pump (14v) with a resistance (light) in between.

Not charging: 12v > light > 0v ground = 12v across light and it turns on
Charging 12v > light > 14v generated = 2v across light, not enough voltage to turn on
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post #78 of 88 (permalink) Old 12-17-2018, 11:02 AM
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Ken: yes, there is a schematic showing the "idiot Light" Module having a 12V source. What it does in the module....is best guess. Could be anything from a dumb light connected as we surmise to a microcontroller system sampling current thru a internal load.....

Thanks for clarifying my sometimes confusing thought process. I find the water flow analogy helpful as well. I was trying to teach electronic theory to a daughter a while back, and was discussing voltage, current flow to ground, resistance....when she looks at me "But Dad, I thought you said it was really electrons flowing from ground to the higher voltage..." Well YES....but convention says.... Agggh.

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post #79 of 88 (permalink) Old 12-18-2018, 11:35 AM
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The water analogy works 100%, same principles as electricity but easy to visualize. I need to throw it in more often when helping guys troubleshoot.

With this alternator issue i am sure there is something being missed. I need to be on the machine with the drawings to get a handle on things. I have tried troubleshooting problems over the phone many times and often i went to site got an entirely new approach that honed in on the problem. I suspect this here is the case. The solid state relay may work and is worth a try. But i am quite certain we don't have an overloaded power supply issue in the alternator, nor the 12v source via the idiot light. The voltage divider circuit as i drew out a few pages back may be on track. So the solid state relay will change the circuit and may work around the problem. The nagging thing is changing the alternator seems to fix the issue and without seeing things first hand then everything else is best guess solutions. Everyone will be very disappointed if it is simply brushes hanging up in the holder after 80 posts.
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post #80 of 88 (permalink) Old 12-18-2018, 10:02 PM
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One thing to help with theory for others. Not good to consider something as pulling current. Everything is result of voltage, the load is a resistance and therefore reacts to the applied voltage. Its like saying a hose is letting or pulling water through. Its isn't letting any water through if there is no pressure from the pump. The more pressure at inlet the higher the water let or pulled through. As you can see "let/pulled" is not a correct term. The more water "will" flow through is better. In fact electrical theory can be more easily understood if you take the idea of fluids (current) driven by a pump (voltage source) pushed through a resistance. Even series/parallel circuits can be understood more easily if thinking of fluids.
Even using water flow as a model the “source” (pump) is just as important as the “sink” drain/sump.

And without question your lost in electronic troubleshooting if you cannot differentiate between a current source and a current sink when it comes to logic and switching.

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