Westerbeke 8.0 BTDA kicking my arse

It wouldn't take a dead short one one side of the diode to fry it, just more current than it's rated for. If it's a 4 Amp diode and you're drawing 4 Amps though it, you're running on borrowed time. If half of the current flowing through the fuse was going through that diode, then the diode would fail around the same rate as the fuse would.

The other thing that can fry a diode is "back EMF" from something like a relay coil. When the current flow to a relay coil is interrupted, the magnetic field around the coil collapses. When this happens a spike of voltage is sent out both ends of the relay coil. It's like a micro-sized ignition coil and can cause a spike of hundreds or even perhaps a thousand volts. If the diode is not rated to handle this spike, then it will fail when the spike arrives at the diode's leads. A small capacitor to ground can eliminate this spike before it becomes a problem.
All of the years and hours on the generators and? I would think that the manuf. has considered all of these issues wouldn't you? However, if the WB relay has a typical suppression diode integrated and someone installed a relay without then I could see the issue being real. The Only device on that diode is one small relay.
 
All of the years and hours on the generators and? I would think that the manuf. has considered all of these issues wouldn't you? However, if the WB relay has a typical suppression diode integrated and someone installed a relay without then I could see the issue being real. The Only device on that diode is one small relay.
Well, it worked just fine for ~1200 hrs, so I think the manuf did something right!!

How do you explain both the diode failing and the fuse blowing?
 
Well, it worked just fine for ~1200 hrs, so I think the manuf did something right!!

How do you explain both the diode failing and the fuse blowing?
Yea that's a good question. Unless something did a dead short on the relay side of the diode and as a component of the diode failure the fuse let go I don't know. And they could be completely independent issues also....
 
We lost power on the dock. Dock transformer box was left open in the rain. Everything wet. So I need my generator. Replacement parts haven’t arrived. Decided just to try to start it with the preheat solenoid disconnected. Started right up. I was surprised. Thought I’d have to find a way to heat the intake air.
 
We lost power on the dock. Dock transformer box was left open in the rain. Everything wet. So I need my generator. Replacement parts haven’t arrived. Decided just to try to start it with the preheat solenoid disconnected. Started right up. I was surprised. Thought I’d have to find a way to heat the intake air.
The big thing pressing the preheat button does is getting fuel pressure up and energizing the fuel solenoid to get the thing running until the oil pressure is up. In very cold weather the fuel needs to be heated which is the glow plugs and pre-heat solenoid.
 
The big thing pressing the preheat button does is getting fuel pressure up and energizing the fuel solenoid to get the thing running until the oil pressure is up. In very cold weather the fuel needs to be heated which is the glow plugs and pre-heat solenoid.
Air temp in the engine room was about 70. Ran great for several hours. The dock master plugged the blow dryer into my boat to dry out the wet breakers in the dock electrical panel. Everything is dry and back on shore power.
 
So I studied the schematic 1000 times to try to figure out what happened. As part of that it appears to me that with the positive wire to the preheat solenoid disconnected, the diode is serving no protection purpose. It could be a direct connection from the preheat switch to the run relay in that case. It seems the purpose of the diode is to prevent the preheat solenoid from getting energized via a path through the shutdown switches when preheat is released after starting.
 
So I studied the schematic 1000 times to try to figure out what happened. As part of that it appears to me that with the positive wire to the preheat solenoid disconnected, the diode is serving no protection purpose. It could be a direct connection from the preheat switch to the run relay in that case. It seems the purpose of the diode is to prevent the preheat solenoid from getting energized via a path through the shutdown switches when preheat is released after starting.
Yes, that's the purpose, but it's not from the shutdown switches, it's from the path that provides power to the fuel solenoid. This gets power through several SAFETY switches which are designed to cut power to the fuel solenoid if any of the conditions put the genset at risk... high engine temp, low oil, or high exhaust temp. The shutdown switches do provide power to that circuit though.

Now, that diode is there really for one-button convenience. You could add a push-button to supply power directly to the preheat solenoid and just hold that down prior to starting the genset too.
 
Yes, that's the purpose, but it's not from the shutdown switches, it's from the path that provides power to the fuel solenoid. This gets power through several SAFETY switches which are designed to cut power to the fuel solenoid if any of the conditions put the genset at risk... high engine temp, low oil, or high exhaust temp. The shutdown switches do provide power to that circuit though.

Now, that diode is there really for one-button convenience. You could add a push-button to supply power directly to the preheat solenoid and just hold that down prior to starting the genset too.
Not quite, it still will not start. That preheat button not only closes the glow plug solenoid but more importantly also provides +12VDC through the diode to the K2 relay coil until oil pressure is up and THEN +12VDC is routed through the safety switches to keep relay K2 energized when the preheat switch is released. The crux of all this is that Oil Pressure switch is open until the generator is running. The K2 relay must be energized closed at all times when starting and when running.
The K2 relay contacts provides +12VDC from the 20 amp circuit breaker to:
  • Fuel Pump
  • Fuel Solenoid
  • Alternator Field
  • Gauge power
So, if that diode was replaced with a length of wire then the glow plugs would be activated all of the time the generator is being started and when it is running which would not be too good.
 
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Maybe I wasn't as clear as I thought. You could remove the diode and have a separate (momentary) switch that JUST powered the glowplug circuit. You'd have to hold that one down for a few seconds prior to pushing the START switch which would provide power to the K2 relay. The start switch would be wired to where the diode connected to the K2 coil.
 
Maybe I wasn't as clear as I thought. You could remove the diode and have a separate (momentary) switch that JUST powered the glowplug circuit. You'd have to hold that one down for a few seconds prior to pushing the START switch which would provide power to the K2 relay. The start switch would be wired to where the diode connected to the K2 coil.
Roger. And I wasn’t clear I meant “safety shutdown switches” when I wrote “shutdown switches”.
 
Maybe I wasn't as clear as I thought. You could remove the diode and have a separate (momentary) switch that JUST powered the glowplug circuit. You'd have to hold that one down for a few seconds prior to pushing the START switch which would provide power to the K2 relay. The start switch would be wired to where the diode connected to the K2 coil.
But then the starter would run all of the time once the OP switch closed right?. Maybe I'm not getting the issue with the diode??
Plus it takes time to build the fuel pressure so to expect the fuel pump to initialize when cranking would take time. The last thing you want any generator to do is to excessively crank; water ends up in the cylinders.
 
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My last post on this (until the next failure). My diode failed closed (I tested it after I removed it). So it end up acting just like a wire. Thus, it became a path for the preheat solenoid to constantly be energized when the generator was running. It could have been like that for a long time. Who knows. Eventually the solenoid started acting up and drawing more than the 8 amp limit, blowing the fuse. So disconnecting the wire from the solenoid eliminates this situation. Since I don’t really need the preheat to start the generator in the climate I boat, I may never reconnect that wire to take the diode out of play. I’ve seen that diode fail too many times on multiple generators.
 
Bill - I would try and talk you out of this action - Yes that will enable the generator to start but the starter motor switch then remains active when the unit is running and if accidently pressed will damage the starter motor and/or the ring gear.
 
But then the starter would run all of the time once the OP switch closed right?. Maybe I'm not getting the issue with the diode??
Plus it takes time to build the fuel pressure so to expect the fuel pump to initialize when cranking would take time. The last thing you want any generator to do is to excessively crank; water ends up in the cylinders.
OK, so now I see the flaw in my 'plan'! The diode isolates the preheat solenoid AND the start solenoid from the 'run' circuit, so you'd either still need the diode in place to isolate the start solenoid (for one-button) operation, or you'd have to do something else (like add a another button for the starter). Adding a separate "preheat" button but retaining the diode to isolate the start circuit might be worth considering, especially when you might rarely need the preheat function. You'd only push that button when the temps were low enough that the engine might need that boost to start.

I think you could solve the diode-reliability problem just up up-rating it to a size larger than the fuse. I don't really see a downside to that. Much easier to source fuses than it is diodes when you're on a trip.
 
OK, so now I see the flaw in my 'plan'! The diode isolates the preheat solenoid AND the start solenoid from the 'run' circuit, so you'd either still need the diode in place to isolate the start solenoid (for one-button) operation, or you'd have to do something else (like add a another button for the starter). Adding a separate "preheat" button but retaining the diode to isolate the start circuit might be worth considering, especially when you might rarely need the preheat function. You'd only push that button when the temps were low enough that the engine might need that boost to start.

I think you could solve the diode-reliability problem just up up-rating it to a size larger than the fuse. I don't really see a downside to that. Much easier to source fuses than it is diodes when you're on a trip.
https://www.amazon.com/MCIGICM-Rect...ywords=rectifier+diodes&qid=1642005763&sr=8-1
 
Bill - I would try and talk you out of this action - Yes that will enable the generator to start but the starter motor switch then remains active when the unit is running and if accidently pressed will damage the starter motor and/or the ring gear.
I still have the functioning diode in there. I’m not removing it. I’m just removing the connection to the preheat solenoid. But you are right. Might be kinda pointless to do that. I’d eliminate one item to signal me that that diode might be sick. A fault light on the anode side of the diode would have been nice. If it lit up with the engine running, time to replace the diode.
 
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I still have the functioning diode in there. I’m not removing it. I’m just removing the connection to the preheat solenoid. But you are right. Might be kinda pointless to do that. I’d eliminate one item to signal me that that diode might be sick. A fault light on the anode side of the diode would have been nice. If it lit up with the engine running, time to replace the diode.
I think you would always know the diode is functional as when you press the Preheat button the fuel pump will run and you should hear the fuel solenoid click. If the diode was open (fried) then nothing but the preheat solenoid would operate with the preheat button pressed.
 
Good news today... Gen oil analysis.

MARK: No problems stand out for this generator. Universal averages show typical wear after about 100 hours of oil use, and these results look great after a 160-hour run. Your generator actually produced less metal overall, which is always good news. There isn't any sign of trouble in this data, and there isn't any fuel, water, or coolant contamination to point out. Low silicon and insolubles show the air and oil filters are both working well. Just check back to start building trends. All is well from our end at 1,009 total hours!
 

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