Help! Cant find the gremlin
Fleet Owner
Joined: Oct 2005
Posts: 20,789
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From: Kentucky
The voltmeter has nothing to do with the alternator exciter circuit. Power flows from the Ignition Switch when it is in the Start or Run position. From there it goes to the Charge Indicator bulb in the cluster. There is a shunt resistor in parallel with the bulb in case it opens. This signal leaves the cluster then makes it's way to the alternator. I suspect the alternator has a bad diode which causes the output to have significant harmonics which is affecting the computer.
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5th Wheeling
Joined: Aug 2012
Posts: 49
Likes: 3
It goes through the voltage light on the dummy lights in order to pass through the 194 bulb, which acts as a resistor, this resisted ignition is what is used to excite the alternator to charge. if you put an LED in the place of the incandescent it will cause the same issue, for example.
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5th Wheeling
Joined: Aug 2012
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Likes: 3
The voltmeter has nothing to do with the alternator exciter circuit. Power flows from the Ignition Switch when it is in the Start or Run position. From there it goes to the Charge Indicator bulb in the cluster. There is a shunt resistor in parallel with the bulb in case it opens. This signal leaves the cluster then makes it's way to the alternator. I suspect the alternator has a bad diode which causes the output to have significant harmonics which is affecting the computer.
Fleet Owner
Joined: Oct 2005
Posts: 20,789
Likes: 1,753
From: Kentucky
The Light Green/Red wire is the supply voltage to turn on the alternator. Not sure how the Charge Indicator would turn on with the connector to the alternator removed.
As far as testing the alternator goes most places do not test for bad diodes,/excessive ripple on the output anymore.
As far as testing the alternator goes most places do not test for bad diodes,/excessive ripple on the output anymore.
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5th Wheeling
Joined: Aug 2012
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Likes: 3
SO, here’s where I’m at. I replaced the volt gauge since I had already ordered it but it made no change. I unplugged the green/red wire at the connector by the battery and checked the voltage. It’s getting 11.8 volts with the truck running. I plugged that back in and checked voltage at the green/red terminal going to the alternator and it had the same voltage. This tells me the alternator exciter is sending power to the alternator just fine. However when I plug the alternator in the violent shuddering immediately begins. I did change out the alternator again but it made no difference. I also made sure all the lights in the cluster are good so they shouldn’t be an issue.
I just don’t know what else to check. The problem seems to be occurring when the alternator is on and charging the battery. What could it be?
I just don’t know what else to check. The problem seems to be occurring when the alternator is on and charging the battery. What could it be?
Fleet Owner
Joined: Oct 2005
Posts: 20,789
Likes: 1,753
From: Kentucky
I unplugged the green/red wire at the connector by the battery and checked the voltage. It’s getting 11.8 volts with the truck running. I plugged that back in and checked voltage at the green/red terminal going to the alternator and it had the same voltage. This tells me the alternator exciter is sending power to the alternator just fine.
I am not following you here. Your previous test was done with the alternator output not connected to the battery?
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5th Wheeling
Joined: Aug 2012
Posts: 49
Likes: 3
Fleet Owner
Joined: Oct 2005
Posts: 20,789
Likes: 1,753
From: Kentucky
With the alternator "turned on" the alternator charges at a higher voltage than static battery voltage. I would expect something greater than 13 VDC or a little higher. You stated the voltage measured is the same. To me the alternator is not charging or the fuse link between the alternator and battery is open. Which would not explain why having the alternator running as designed causes a stumbling problem if it is not actually getting to the battery. If the voltage was a lot higher than static battery voltage that would lead me to believe the voltage regulator was bad.
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5th Wheeling
Joined: Aug 2012
Posts: 49
Likes: 3
With the alternator "turned on" the alternator charges at a higher voltage than static battery voltage. I would expect something greater than 13 VDC or a little higher. You stated the voltage measured is the same. To me the alternator is not charging or the fuse link between the alternator and battery is open. Which would not explain why having the alternator running as designed causes a stumbling problem if it is not actually getting to the battery. If the voltage was a lot higher than static battery voltage that would lead me to believe the voltage regulator was bad.
something is happening when I plug up the exciter line that is causing the stumble.
Fleet Owner
Joined: Oct 2005
Posts: 20,789
Likes: 1,753
From: Kentucky
That makes a little more sense I suppose.
Since you have installed more than one "new or rebuilt?" alternator we can cast a less leery eye towards that. One check I would do is use a jumper cable with one end attached to the negative battery terminal, then attach the other end to the alternator bracket. Start the engine then see if the stumble remains with everything connected. Doing this will help eliminate a poor ground to the engine. Usually you would get slow starting, but this needs to be eliminated.
If that is good try running a jumper lead from the battery (+) or fused source directly to the alternator end of the Light Green/Red wire. Then see if the stumbling remains. This will eliminate the exciter circuit as the cause.
Since you have installed more than one "new or rebuilt?" alternator we can cast a less leery eye towards that. One check I would do is use a jumper cable with one end attached to the negative battery terminal, then attach the other end to the alternator bracket. Start the engine then see if the stumble remains with everything connected. Doing this will help eliminate a poor ground to the engine. Usually you would get slow starting, but this needs to be eliminated.
If that is good try running a jumper lead from the battery (+) or fused source directly to the alternator end of the Light Green/Red wire. Then see if the stumbling remains. This will eliminate the exciter circuit as the cause.
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5th Wheeling
Joined: Aug 2012
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Likes: 3
That makes a little more sense I suppose.
Since you have installed more than one "new or rebuilt?" alternator we can cast a less leery eye towards that. One check I would do is use a jumper cable with one end attached to the negative battery terminal, then attach the other end to the alternator bracket. Start the engine then see if the stumble remains with everything connected. Doing this will help eliminate a poor ground to the engine. Usually you would get slow starting, but this needs to be eliminated.
If that is good try running a jumper lead from the battery (+) or fused source directly to the alternator end of the Light Green/Red wire. Then see if the stumbling remains. This will eliminate the exciter circuit as the cause.
Since you have installed more than one "new or rebuilt?" alternator we can cast a less leery eye towards that. One check I would do is use a jumper cable with one end attached to the negative battery terminal, then attach the other end to the alternator bracket. Start the engine then see if the stumble remains with everything connected. Doing this will help eliminate a poor ground to the engine. Usually you would get slow starting, but this needs to be eliminated.
If that is good try running a jumper lead from the battery (+) or fused source directly to the alternator end of the Light Green/Red wire. Then see if the stumbling remains. This will eliminate the exciter circuit as the cause.
I talked to a local guy that deals with these trucks a good bit and he suggested trying a new ECM. I had discounted this because the truck ran fine without the alternator. I figured if it was the ECM then it would act up all the time. In his opinion the reason it stumbles when it gets the full 14v is there's something wrong with the ECM. at 12v it's fine but when it has full power it tries to compensate for whatever is wrong and stars causing problems. So, I guess at this point I will order a ECM and see what happens. If that doesn't fix it I guess I'll just part it out and start over. Hopefully I can get most of my investment back selling parts.
Lead Driver
Joined: Jun 2021
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I had already run an extra ground, which did nothing. I unplugged the exciter line from the dash and fed 12v straight from the battery and the stumble cam back.
I talked to a local guy that deals with these trucks a good bit and he suggested trying a new ECM. I had discounted this because the truck ran fine without the alternator. I figured if it was the ECM then it would act up all the time. In his opinion the reason it stumbles when it gets the full 14v is there's something wrong with the ECM. at 12v it's fine but when it has full power it tries to compensate for whatever is wrong and stars causing problems. So, I guess at this point I will order a ECM and see what happens. If that doesn't fix it I guess I'll just part it out and start over. Hopefully I can get most of my investment back selling parts.
I talked to a local guy that deals with these trucks a good bit and he suggested trying a new ECM. I had discounted this because the truck ran fine without the alternator. I figured if it was the ECM then it would act up all the time. In his opinion the reason it stumbles when it gets the full 14v is there's something wrong with the ECM. at 12v it's fine but when it has full power it tries to compensate for whatever is wrong and stars causing problems. So, I guess at this point I will order a ECM and see what happens. If that doesn't fix it I guess I'll just part it out and start over. Hopefully I can get most of my investment back selling parts.
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5th Wheeling
Joined: Aug 2012
Posts: 49
Likes: 3
It’s the only reasoning I have left. I have replaced and or tested every thing else it could possibly be. The only thing left is the ECM.
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5th Wheeling
Joined: Aug 2012
Posts: 49
Likes: 3