Dual Voltage Alternator Over-Voltage

[dlr2112]

Greetings,
I have an issue with my charging system that happened suddenly. I stopped to fuel up, and when I restarted the truck, my voltage hit 35-40vdc (depending on RPM). It seems that most people say the regulator is the most common issue, but using the test procedures from C.E. Niehoff, the resistance test has me measuring between pin A and B+ stud on the alternator that should be between 1.8 and 2.2 ohms. I'm at 2.5 ohms and was wondering if anyone knows if there is a buffer for this or is that a hard failure because it is out of spec. I sometimes see +- that is acceptable. The alternator is a 200 amp N1225-1 and the regulator is dual voltage N3117. I was told that the newer regulator is N3135. Thanks for the input if any!
Dave/

 

[Mogman]

I think you are probably OK at 2.5 ohms, probably a bad regulator.
Many of the digital meters will show .2 ohms or so when the leads are shorted, why I love my old Simpson 260, we both look like crap cuz we have both been around the block a few thousand times but the meter still works perfectly (unlike me)

 

[Coug]

Did you check to see what the resistance of your leads and wires are before touching them to the alternator? It's not uncommon to have up to an ohm of resistance just in the leads and connections.
I'd say it's most likely a failed regulator.

 

[87cr250r]

If you have batteries your voltage can't shoot up. They will delay the rise and possibly have other issues before you get to 40 volts. I would be suspicious of your gauge first.

 

[dlr2112]

I think you are probably OK at 2.5 ohms, probably a bad regulator.
Many of the digital meters will show .2 ohms or so when the leads are shorted, why I love my old Simpson 260, we both look like crap cuz we have both been around the block a few thousand times but the meter still works perfectly (unlike me)

Hi Mogman. Great Saying (which is what I always say).. It is Always Something! I've been using a Fluke 87, which has been around me for a couple decades now. I have the same trust with the old meter as you have with yours (some thing do just "mostly" work). Some things never let you down.. That said, I don't think I rechecked the calibrations and zero'd the meter, but I'll recheck it. It's a good thought. Thank you for the input.

 

[dlr2112]

Did you check to see what the resistance of your leads and wires are before touching them to the alternator? It's not uncommon to have up to an ohm of resistance just in the leads and connections.
I'd say it's most likely a failed regulator.

Hi Coug. I'll recheck the calibration of the meter and double check the resistance. I hope the regulator is bad. It's a much more affordable alternative. The alternators are hitting $2K these days. Yikes! Thank you for the response. I'll report back with an answer.

 

[dlr2112]

If you have batteries your voltage can't shoot up. They will delay the rise and possibly have other issues before you get to 40 volts. I would be suspicious of your gauge first.

Hello 87cr250r- So I do have (2) AGM batteries that seem to be fine. My "Dash" gauge spiked and I have a digital gauge on my USB pod (measuring both 24vdc and 12vdc) that also confirmed the voltage independently. It was interesting because there was not regulation of the voltage and seemed to be directly proportional to the engine RPM. My digital gauge maxed at 40vdc, so I couldn't see how high the voltage actually got. Before I could stop the truck, the batteries did absorb some of that power because they did get warm. When I review the TMs, there was a step in the diagnostics that said if voltage exceeds 32vdc, replace regulator. I'm trying to be as sure as possible because of the costs. A regulator is $500-800 and an alternator is $1.2-1.9K . Frightfully expensive. I appreciate your feedback, and I'll recheck my resistance checks and try and source a regulator and report back with the solution. Cheers-

 

[Coug]

Hi Coug. I'll recheck the calibration of the meter and double check the resistance. I hope the regulator is bad. It's a much more affordable alternative. The alternators are hitting $2K these days. Yikes! Thank you for the response. I'll report back with an answer.

When I had to replace my regulator a few years ago, the general thoughts about it are that the alternator itself is pretty bulletproof; it's almost always the regulator that fails.
Since it's putting out power at high levels, it's extremely unlikely that the alternator failed because a failed alternator would almost always produce less voltage or none at all. The regulator failing is about the only way you can get high voltages.

 

[dlr2112]

When I had to replace my regulator a few years ago, the general thoughts about it are that the alternator itself is pretty bulletproof; it's almost always the regulator that fails.
Since it's putting out power at high levels, it's extremely unlikely that the alternator failed because a failed alternator would almost always produce less voltage or none at all. The regulator failing is about the only way you can get high voltages.

When I was originally restoring the motor and removed the alternator, I was amazed by the mass of the alternator. I thought the same thing, this is literally bulletproof. Here is a little update. As suggested, I retested the alternator using the "Static" tests after checking the meters' cable resistance. After checking that, I was more like .5-1 ohm within tolerance. I appreciated that advice.
But, I wanted to start the truck with everything connected to see if I can tell if the high voltage was only from the 28vdc source or also from the 14vdc stud. And to my surprise, The darn thing was working perfectly. It must've been the alternator to regulator connectors (high resistance) as they were still sealed with the silicone from the factory?. Those were assembled 23 years ago, so it's very possible?. I took it apart, cleaned the connectors and put dielectric grease on it and reassembled. It seems to be working fine. I'm a little embarrassed I didn't try to do that earlier, but I didn't want to damage anything more than it might have already been due to being overvoltage by 75%.
Thank You for taking the time to respond to my question. This forum is such a great forum! Take Care-

 

[Mogman]

When I was originally restoring the motor and removed the alternator, I was amazed by the mass of the alternator. I thought the same thing, this is literally bulletproof. Here is a little update. As suggested, I retested the alternator using the "Static" tests after checking the meters' cable resistance. After checking that, I was more like .5-1 ohm within tolerance. I appreciated that advice.
But, I wanted to start the truck with everything connected to see if I can tell if the high voltage was only from the 28vdc source or also from the 14vdc stud. And to my surprise, The darn thing was working perfectly. It must've been the alternator to regulator connectors (high resistance) as they were still sealed with the silicone from the factory?. Those were assembled 23 years ago, so it's very possible?. I took it apart, cleaned the connectors and put dielectric grease on it and reassembled. It seems to be working fine. I'm a little embarrassed I didn't try to do that earlier, but I didn't want to damage anything more than it might have already been due to being overvoltage by 75%.
Thank You for taking the time to respond to my question. This forum is such a great forum! Take Care-

Just keep an eye one it (like I need to say that) it may fail when it gets really warmed up.

 

[Coug]

When I was originally restoring the motor and removed the alternator, I was amazed by the mass of the alternator. I thought the same thing, this is literally bulletproof. Here is a little update. As suggested, I retested the alternator using the "Static" tests after checking the meters' cable resistance. After checking that, I was more like .5-1 ohm within tolerance. I appreciated that advice.
But, I wanted to start the truck with everything connected to see if I can tell if the high voltage was only from the 28vdc source or also from the 14vdc stud. And to my surprise, The darn thing was working perfectly. It must've been the alternator to regulator connectors (high resistance) as they were still sealed with the silicone from the factory?. Those were assembled 23 years ago, so it's very possible?. I took it apart, cleaned the connectors and put dielectric grease on it and reassembled. It seems to be working fine. I'm a little embarrassed I didn't try to do that earlier, but I didn't want to damage anything more than it might have already been due to being overvoltage by 75%.
Thank You for taking the time to respond to my question. This forum is such a great forum! Take Care-

When mine was failing, it would work for a while, then not work. I managed to limp it along by turning off the truck, tapping on the regulator with my closed pocket knife a few times, then starting up again and it might or might not work.
Just keep an eye on the voltage gauge. If it starts overcharging again then time for a new regulator.
These trucks will run for a pretty long time off just the batteries. I'm not sure what the simplest method to prevent alternator output to the batteries is, whether it's disconnect the main leads or if there is a smaller wire you can pull, but having the tools and a lot of electrical tape might save you from cooking a set of batteries while still getting home. (other option is having AAA plus towing; I paid for the 100 miles of towing just in case my truck breaks down)

 

[Mogman]

You can pull the IGN lead off of the regulator with the truck not running.