MEP-016B Engine Control board issues

[Sparky Kilowatt]

Rebuilding an -016B and came across a problem I have never seen. Brief history - 1st no compression. repaired dropped intake valve and engine started right up. 2nd - AVR1 missing. Bought SX460 and installed. With a field flash it excites and powers properly with the following exception - The field won't flash at start-up. If I manually flash the field all is good, but suddenly the starter kicks in and grinds. If I start the generator then disconnect the starter, then flash the field the generator provides stable power perfectly. After doing this I meter out the starter solonoid line and find it to be hot. I have combed through the schmetics and see there is a start disconnect circuit that seems to interlock with the field flashing circuit, but I can't find any reason for the starter not to kick back in until the generator is excited.

I'm at a loss. Any thoughts would be greatly appreciated!​

 

[ETN550]

Rebuilding an -016B and came across a problem I have never seen. Brief history - 1st no compression. repaired dropped intake valve and engine started right up. 2nd - AVR1 missing. Bought SX460 and installed. With a field flash it excites and powers properly with the following exception - The field won't flash at start-up. If I manually flash the field all is good, but suddenly the starter kicks in and grinds. If I start the generator then disconnect the starter, then flash the field the generator provides stable power perfectly. After doing this I meter out the starter solonoid line and find it to be hot. I have combed through the schmetics and see there is a start disconnect circuit that seems to interlock with the field flashing circuit, but I can't find any reason for the starter not to kick back in until the generator is excited.

I'm at a loss. Any thoughts would be greatly appreciated!​

In the original configuration the field should flash when the starter is engaged. I think this is the reason the starter engages when you mannually flash the field. Field flash current is backfeeding the starter solenoid. Probably want to keep this feature, flashing during cranking.

I would check over all of the wiring carefully. It seems like something is mis-connected somewhere. You might try measuring the field when cranking. Possibly it is flashing but not high enough voltage for the SX460 to pick it up. Check all grounds too.

Can you bypass the existing wiring and flash the field on with the crank signal? Caution, you might need a resistor in the circuit, I've never looked into this.

Do you know what caused the valve to drop?

 

[Sparky Kilowatt]

Thanks for getting back to me so fast! When I pulled the valve cover off I found the exhaust retainer split in half and laying on either side under the cover. Not sure why, it appears it may have been a defective retainer and the reason the genset was mothballed. PM list on the gen. stated the last time it had been run was '86. There was no damage. I flushed the engine, installed a new retainer and keepers, and the gen. fired right up. I have used it many times since and the engine runs fine.
In the words of Paul Harvey I should give you "the rest of the story" on the regulator and cabinet issues. The AVR was missing when I got the gen. I built a new one which worked fairly well, but did not compensate for voltage droop well enough under high load. I read about installing the 460 on this site and got one off of E-bay. It worked great! Generator started, the fields excited, and all was well for a while. I had dead batteries one day (two 12's in series). I charged the first, but in my impatience put a jumper pack on the second. The generator started right up and ran fine as it usually does. I pulled the jumper pack off the dead battery thinking the charging system would take over. I head a "pop" from the cabinet and the generator shut down. After that I fully charged both batteries and restarted it. it cranked right up, but did not excite. I hit the start/flash on the materswitch again. The starter did not kick in again, but the field did not excite either. I opened the cabinet and flashed F+/F- with a 9v battery and quickly removed the battery. The generator excited and powered properly. Shortly after, I heard a grinding and found the starter engaging. Now, consistently, if I start it and don't flash the field the generator runs fine but does not provide power. As soon as I flash the field the starter kicks in after the generator excites and the flash battery is removed. I am pretty confident on the install of the sx460 considering the generator worked properly for some time. I have not modified any of the origional harness, just connected the two field wires, two 120v wires, and two voltage control resistor wires. I have checked all grounds. I will check the flashing voltage tonight. That's a good idea.
View attachment 443908

 

[ETN550]

To go any further I need to get up to speed on the flash / crank circuit. The pop makes me wonder if the left hand control board has suffered one or more diode failures. I think there are a bunch of current reversal protection diodes that act as fuses if the battery is connected backwards. Maybe one of the diodes failed in a manner that has somehow caused the manual flash to hold the starter relay or backfeed the starter relay.

Unplug the starter relay on the board or disconnect its output at the terminal strip and jump the starter solenoid on the engine to crank it up. Then flash your field and make some measurements with the unit running. With the starter relay out on the board it should be impossible for it to engage the starter by itself.

I can see how the sudden change in state of the battery voltage when disconnecting the jumper pack could have initiated the problem.

The fact that it is not flashing on start up leads me to believe that circuit has been damaged. And somehow it is backfeeding the starter relay on the board. Since there is a small starter relay on the left hand board feeding a bigger starter relay on the rear of the box feeding the main starter solenoid it would take very little voltage or power on the small relay coil to initiate the whole starting process. Although the diodes are supposed to fail open that may not always be the case. Without studying the wiring diagram I'm not sure if a normal diode failure would initiate your symptoms. maybe...

P.S. I have an SX460 waiting for a unit with a bad regulator. Can you use the existing voltage adjustment pot on the panel? And if so what range of voltage does it min / max at say when on the 120 single phase setting?

 

[Sparky Kilowatt]

Did not get a chance to do any testing last night. Briefly this morning I was able to at least start it, pop the solenoid wire off and take readings on both sides of AVR2, the charging regulator. I found 35 V on the AC side and 28 V on the DC side. I am pleasantly surprised by this. I expected that my jumper pack blunder took out the regulator. I will pull the A3K1 disconnect relay, excite the field and take more readings tonight. Hopefully this isolates it to the board. If I can find the bad component I am sure I can soldier in a compatible replacement and fix it. I am still stumped on how exciting the main generator field could cause anything to change state on the engine control board. They seem to be completely isolated from each other.

I used the existing pot on the front panel with the sx460. The resistance of the panel pot is not the same as what is recommended for the sx460. I centered the pot then set the voltage with the pot on the 460. When I checked the range on the front panel I had significantly reduced adjustment. About +/- 5 or 6 volts. Strangely the direction of adjustment was reversed. when I turned the pot clockwise the voltage dropped. This could be remedied by reversing the pot connections, but I'm not that concerned about it. The 460 has great response. I am impressed with how well it works. 460 manual recommends a 1K 1 watt Pot to give you +/- 7 percent

 

[Sparky Kilowatt]

The plot thickens on this. I was able to get some more testing done and am more confused than when I started. I removed relay A3K1 (start disconnect relay) on the engine control board. As expected when I turn the master switch to start nothing happens. I insert the relay, start the engine, then pull the relay. The generator is not excited at this point. I checked my charging circuit voltages and they are all good. I also check the coil voltage across A1K1 main panel aux start relay and found zero volts as there should be. To be safe I removed the wire to the starter soloniod in case the A1K1 relay engages for some reason as it had recently. I excite the F+/F- with a 9 volt battery. The generator powers up perfectly and provide good stable current. I notice a buzzing that should not be there and decided to check across A1K1 coil again. To my surprise I am getting 120VAC to the coil and it is engaged. Fortunatly the starter was disconnected. I have checked every connection in the generator and cannot find any reason to get that kind of voltage there, especially when it should only see 24vdc when engaged. Aside from the 6 wires that went to the main AVR I have never removed another wire in the cabinet. Any thoughts?

 

[ETN550]

The plot thickens on this. I was able to get some more testing done and am more confused than when I started. I removed relay A3K1 (start disconnect relay) on the engine control board. As expected when I turn the master switch to start nothing happens. I insert the relay, start the engine, then pull the relay. The generator is not excited at this point. I checked my charging circuit voltages and they are all good. I also check the coil voltage across A1K1 main panel aux start relay and found zero volts as there should be. To be safe I removed the wire to the starter soloniod in case the A1K1 relay engages for some reason as it had recently. I excite the F+/F- with a 9 volt battery. The generator powers up perfectly and provide good stable current. I notice a buzzing that should not be there and decided to check across A1K1 coil again. To my surprise I am getting 120VAC to the coil and it is engaged. Fortunatly the starter was disconnected. I have checked every connection in the generator and cannot find any reason to get that kind of voltage there, especially when it should only see 24vdc when engaged. Aside from the 6 wires that went to the main AVR I have never removed another wire in the cabinet. Any thoughts?

Well, I think the A3CR8 diode may be bad on the control board. If it failed to a short or leaky condition then that would explain the backfeed to A1K1. Was it possible that you were measuring running field voltage at the A1K1? If A3CR8 was shorted then the field would have to go quite high because there is a resistor to go through as well as the A1K1 coil in addition to getting enough on the field to make the gen A/C.

Additionally, a leaky A3CR8 diode could be bleeding some of the flashing voltage which is preventing the AVR from picking up and taking over at cranking.

Look at the diagram, lower right area, page 4-95 of the 288 page TM. The Army TM number is TM 5-6115-615-12.

You could also swap the A3K1 relay with A3K2 (glow plug preheat) to make sure the A3K1 relay is okay.

I think it would be pretty important to have the f+ and f- wires from the new AVR on the correct terminals as well.

One thing about the 460 or any other substitute regulator is that there could be trouble during the handoff from forced field flash to self regulated operation. The forced field flash ends when the a/c output from the dc charging system builds enough voltage to pass the zener diode thus allowing the A3K1 to stop the flash and lockout the starter. Meanwhile, and all along the AVR is begining to see A/C voltage build up in the generator. At some point it is taking over. Hopefully, all of that occurs without stressing the diode. I did not look further on the diagram to determine if the A/C AVR is held off line until the flash is terminated. maybe a good modification with a substitute regulator. Because the next question, if a failed diode is why did it fail?

I suggest pulling the control board and looking it over carefully.

 

[ETN550]

Did not get a chance to do any testing last night. Briefly this morning I was able to at least start it, pop the solenoid wire off and take readings on both sides of AVR2, the charging regulator. I found 35 V on the AC side and 28 V on the DC side. I am pleasantly surprised by this. I expected that my jumper pack blunder took out the regulator. I will pull the A3K1 disconnect relay, excite the field and take more readings tonight. Hopefully this isolates it to the board. If I can find the bad component I am sure I can soldier in a compatible replacement and fix it. I am still stumped on how exciting the main generator field could cause anything to change state on the engine control board. They seem to be completely isolated from each other.

I used the existing pot on the front panel with the sx460. The resistance of the panel pot is not the same as what is recommended for the sx460. I centered the pot then set the voltage with the pot on the 460. When I checked the range on the front panel I had significantly reduced adjustment. About +/- 5 or 6 volts. Strangely the direction of adjustment was reversed. when I turned the pot clockwise the voltage dropped. This could be remedied by reversing the pot connections, but I'm not that concerned about it. The 460 has great response. I am impressed with how well it works. 460 manual recommends a 1K 1 watt Pot to give you +/- 7 percent

BTW thank you for reporting the voltage output of the A/C side of the dc regulator. I'm working on making a substitute dc regulator to make it more robust and about a third the cost of the bad mil spec unit that is subject to failure. I replace one last weekend and i could see where the components got hot underneathe the translucent urethane bedding and just smoked the urethane and created a failure. I have also noted that the exhaust pipe and muffler give off a lot of heat to the control box especially in the corner that the regulator is in. maybe some exhaust wrap on the tailpipe would help the regulator!

 

[Isaac-1]

ETN550 there was a seller a while back that was selling the updated design 24V dc regulator (southern automotive drop in replacement?) on ebay a while back for around $100, I bought one, looks completely different than the one grooved heat sink style, but screws line up.

Ike

 

[Sparky Kilowatt]

Great information! A3CR8 does seem bad. Good call! I have monitored field voltage at F+/F- and it never went outside of the range of 5.5-6.0 VDC during operation all the while the control board was throwing 120VAC to the A1K1 relay. I am sure that bad diode has something to do with it, but cannot imagine where the 120 volts is coming from. I even removed the lead connections from A3TB1 9 and 14 where A1K1 connects. I was thinking maybe I had a rub-through on the harness somewhere and field voltage was back-feeding to the board. When I meter out those points directly off of the board I get the 120 VAC. This proves it is definitely board related. It still only happens when the set is excited. The only place I can see field voltage present on this board is at the current monitor resistors for the three phases. I am wondering if whatever caused the diode failure has caused a short to these resistors allowing AC to pass. I agree pulling the board is the next step. I also verified AVR output polarity relative to F+/F- and it is correct.

I completely agree about the heat from the exhaust on the back side of the cabinet. It does seem to introduce a lot of heat into an area where you are trying to remove it. I was planning on addressing it after I got the get set fully functional. I wish the electrical’s worked as well as that Onan diesel. I have never had a diesel engine crank up as nicely and consistently as that one does.

Thanks for the help and I will keep you posted on the board.

 

[Sparky Kilowatt]

Issac-1 - Do you have more info on the retro-fit 24V regulator? I did some searching and have not come up with anything.

 

[skinnyR1]

I bought one of those 24V regs. It was a Libby regulator, 84-13183; I have it in my purchase history. Libby Power Systems Regulator Battery Charging 84-13183. However, I am unable to find any for sale on the web currently.