Miller Roughneck 2e won't weld consistently

[WELD81]

I have a Miller Roughneck 2e serial# HH027668. It is one of the older ones that has 115v aux. power and welding up to 225 AMP, AC, CC, only. This Machine when running will produce 115 vac as it should but when I switch the machine to weld and idle up the engine it fails to produce any welding current. If I try to strike an arc in the engine idle and power mode there is current and if I quickly switch to weld and throttle up the engine, the machine will weld. If I stop welding for a few minutes and try again it won't weld. If I continue to weld without pausing for to long it keeps welding just fine. When I put a volt meter on the field winding to check for exciter voltage there is always voltage present in the power mode but I tend to loose the field voltage in weld mode. With all wires disconnected I have checked for continuity on all sets of windings for both the stator and rotor windings. I have checked for a fault to ground on all windings. I have checked every component on the exciter control board. I have checked the brushes, the ground terminals, and the capacitors. When the machine is running there is voltage present from the charge winding for the battery in power mode but not in weld mode when the machine isn't welding. When I put the volt meter on the charge winding to monitor the machines performance it looses voltage when switched to weld mode unless I try to strike an arc in power mode and switch to weld mode. If I don't switch the machine to weld mode in a short period of time and start welding I can watch the voltage taper off, almost as if I am loosing the magnetic field that produces the current. As long as I try to strike and arc in power mode and then immediately switch to weld mode and start welding the machine will work. the only reason I tried to weld in power mode was to see if the weld windings were producing current in the power mode while there was clearly a field voltage present whenever I was in power mode. I wanted to make sure the weld windings weren't burned somewhere in the stator housing. Any suggestion would be greatly appreciated. I have to the best of my ability described what I already tried and will let you know when given any suggestions if I have already tried that as well. I am able to read schematic diagrams and understand electrical terminology so don't worry about trying to simplify any explanations, I'm hoping someone with a lot more experience than me has seen this problem before and was able to diagnose what was going on. The schematic for my machine is in the roughneck 1E manual listed as electric start model, circuit diagram No. CB-900 286-1Q1. My machine is wired exactly the same.

[cruizer]

Boy thats long.... might have dirty slip rings, or more likely the power/weld switch is bad or has loose wiring

[WELD81]

Sorry for the long post it is hard to explain everything I tried to fix the problem. I have replaced the switch and cleaned+tested all wire connections. I believe the old switch was good, as a continuity meeter showed it working every time. I will try and clean the slip rings and let you know if it works.
Thanks!

[WELD81]

Ok, the slip rings have always looked clean and the welder would produced voltage in power mode so they were not suspect in my opinion, but I took the carbon brushes out and cleaned them with carb cleaner. I also lightly polished the slip rings and wiped them down with carb cleaner. For the first time in two years this machine seems to be working right. The only other problem right now is the battery charge winding is producing 18volts at the proper operating rpm. I might build a power regulator to solve this problem. For all the people out there who think they know it all, it is good to ask questions. this is why I appreciate someone with experience answering my question. Thanks for the good advise. I'll be posting more to this thread if the machine gives me any more grief.

Thanks!

[duaneb55]

18V out of the battery charging windings at WELD and POWER speeds? The throttle switch changes winding tap leads to maintain proper charge output at each speed.

Battery connected or not?

Charge state of battery at the time?

[cruizer]

Yep either your power /weld switch is wired wrong, or you've got something wrong with your battery.

[WELD81]

The battery I'm using is a sealed lead acid part # EVX12340. It is similar to the original battery that would have been on the machine except that it is a deeper cycling battery with a lower maximum amp load rating. It is More that sufficient to crank the starter for this machine. I charged and load tested this battery before I put it into service. I am getting about 13 or 14 volts in the idle down/power mode. I am getting 18 volts in the high RPM welding mode. The charge winding has a coil tap that cuts the #of windings in half at high rpms. This coil tap is selected by a micro switch on the throttle control and the switch is working fine. I also checked the diode in the charging circuit and it is ok. I am not sure what the problem is.

[cruizer]

You don't really want to put a deep cycle battery on a machine that wants to charge it all the time. The deep cycles are really for Boat engines without the capability to charge. They are designed to completely discarge then recharge

[WELD81]

I realize the purpose for a deep cycle bat (correct me if I'm wrong) but I also believe that they can be cycled frequently with a small load, such as a small engine starter, and kept on a float charger. I wanted to build a power reg. to keep the battery float charged while the machine was running. Say about 13.5 volts. I just can't figure out the 18 volt issue. Even a standard flooded lead acid bat would suffer plate degradation from such a heavy charge.
I also got the battery for free because it had to be changed out by regulation, which was a plus.

[cruizer]

nope, they are designed to be drained completely + your not really putting a small charge into it so your going to crash that battery in no time.