7.6BTD Blowing 8A Fuse Part 3 - Bad Diode?

dtfeld

dtfeld

Water Contrails
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Jun 5, 2016
5,545
Milton, GA
Boat Info
410 Sundancer
2001
12" Axiom and 9" Axiom+ MFD
Engines
Cat 3126 V-Drives
Calling all Electrical Guru's

Found the diode that I think feeds the K2 and K1 relays to be damaged. I think what was producing the acrid electrical burn smell. The generator will preheat and start up no problem, but the fuel lift pump is not operating when the preheat switch is pressed.

The diode is illegible and the schematic doesn't get into the level of detail needed to order some new ones. I have an email into Westerbeke, but thought I'd post this up and see if I can get the right part on order.

Dave

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Got a nice reply from Westerbeke. There are 6 of these total in the unit and they can be had for the low one time price of $31!

Not a bad markup for 5¢ diode!! I'd like to order from Mouser, but if I cant find the spec, I'll bite the bullet and order these.

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I received the Westerbeke version, it is a basic BY255 diode. I confirmed in another thread that any BY255 diode will work. Additionally, I could only find a similar diode locally, but it fixed the issue.

I ended up using a NTE 5809 with a spec of 3A and 1000V resistance.

5D51E417-E16B-40C5-B006-46A86598A60D.jpeg


Generator back to 100% operational.:)
 
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So this is my first time posting, but I’ve been reading a lot of threads on here especially having to do with this 8 amp fuse blowing on my 7.6 btd. So this is how it went down, generator shut down while I was underway. Got to the dock and realized When I would switch the remote start switch to the first click the 8 amp fuse would blow. If I hit the preheat switch before pressing the start/run switch the fuel pump would run but as soon as I hit the run/ start button the fuse would blow. I read all these threads, figured out and replaced the water temp switch and that fixed it. I also happened to replace the fuel lift pump at the same time. My original lift pump failed a few weeks prior, I replaced it then with a cheaper version I got off eBay, but the fittings didn’t line up and leaked so I was just replacing it a second time with a actual Westerbeke pump. So new water temp switch and new fuel pump installed and everything’s running fine when the engine shuts down. I immediately realize I forgot to open the fuel valve after replacing the pump. I open the fuel valve, run the preheat switch to prime it up and it fires back up but only runs for a couple minutes or so then shuts down and now it’s the 8 amp fuse again. But this time the 8 amp fuse blows when I press the preheat button and not the start/ run switch. So it’s opposite to before. I checked all the switches and they are all good. I hear the pump run for a split second then the fuse blows. What’s going on? Any ideas? Sorry for the long drawn out story but I just feel like the details matter with this electrical trouble shooting stuff.
 
John Kemper said:
So this is my first time posting, but I’ve been reading a lot of threads on here especially having to do with this 8 amp fuse blowing on my 7.6 btd. So this is how it went down, generator shut down while I was underway. Got to the dock and realized When I would switch the remote start switch to the first click the 8 amp fuse would blow. If I hit the preheat switch before pressing the start/run switch the fuel pump would run but as soon as I hit the run/ start button the fuse would blow. I read all these threads, figured out and replaced the water temp switch and that fixed it. I also happened to replace the fuel lift pump at the same time. My original lift pump failed a few weeks prior, I replaced it then with a cheaper version I got off eBay, but the fittings didn’t line up and leaked so I was just replacing it a second time with a actual Westerbeke pump. So new water temp switch and new fuel pump installed and everything’s running fine when the engine shuts down. I immediately realize I forgot to open the fuel valve after replacing the pump. I open the fuel valve, run the preheat switch to prime it up and it fires back up but only runs for a couple minutes or so then shuts down and now it’s the 8 amp fuse again. But this time the 8 amp fuse blows when I press the preheat button and not the start/ run switch. So it’s opposite to before. I checked all the switches and they are all good. I hear the pump run for a split second then the fuse blows. What’s going on? Any ideas? Sorry for the long drawn out story but I just feel like the details matter with this electrical trouble shooting stuff.
Click to expand...

P.S. I went through a LOT of 8amp fuses trouble shooting all this.
 
Thanks for the feedback y’all! I just checked the (tiny little diode) it looks so much bigger in the pictures. The diode is still in tact and doesn’t appear to be damaged. Does that mean it’s still good??? Or could it still be the culprit? I never smelled the acrid burning electric smell that someone described. And also the 8 amp fuse blows regardless if I use preheat at the remote station or at the panel on the generator, not sure if that’s a clue or not.
From what I’ve read the usual culprit is one of the three switches but I think the main detail is that it only blows when I hit the preheat, I can “first” click the run switch and get the oil pressure alarm but then the preheat switch is what blows the fuse. Beforewhen I had this problem my fuse would blow at the “first” click of the run/start switch, is this an important detail that’s being looked over?
Thanks for the help y’all!!!
 
Just bumping this because I think some of the info MIGHT vary. I have a 7.6 BTD with this same busted diode problem and was originally using the same wiring schematic to trouble shoot but that diagram (one above and the diode # was off a bit).

On mine, the diode (Part number 041081) runs on the TB2 buss between #2 & #6 as in that wiring diagram (which you can see it also runs on #2 & #6) but there's no wire on #6 as there is on mine.

My wiring diagram is #44121 and on that you see a wire on #6 as well and will give you a different Diode part # and is only $5.10 each. That part # isn't listed anywhere that I could find so had to call Westerbeke to get it.

Just in case that helps anyone.

Pg 38
https://www.westerbeke.com/operator's manual/40457_rev2_7.6btd_operator_man.pdf

The 041081 is much smaller than 035931 listed above as you can see in attached pic. I originally ordered 035931 (which is new one in pic) and was going to put in, until I saw the difference and I researched more, which I posted above.

Good luck.
 

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Pg 38
https://www.westerbeke.com/operator's manual/40457_rev2_7.6btd_operator_man.pdf

The 041081 is much smaller than 035931 listed above as you can see in attached pic. I originally ordered 035931 (which is new one in pic) and was going to put in, until I saw the difference and I researched more, which I posted above.

Good luck.[/QUOTE]
so looks like your diode is in two pieces....mine still looks intact. How do you test this with a regular multi-meter?
 
EKM said:
Pg 38
https://www.westerbeke.com/operator's manual/40457_rev2_7.6btd_operator_man.pdf

The 041081 is much smaller than 035931 listed above as you can see in attached pic. I originally ordered 035931 (which is new one in pic) and was going to put in, until I saw the difference and I researched more, which I posted above.

Good luck.
Click to expand...
so looks like your diode is in two pieces....mine still looks intact. How do you test this with a regular multi-meter?[/QUOTE]

Can’t exactly recall but I think you use the ohms setting on your multimeter and you will see it only allows electricity to flow in one direction. So depending on which side you have the positive or negative you’ll either get a reading or zero. But there is a link somewhere but how exactly to measure a diode.

That link is on one of the threads if you search Westerbeke on this forum.

Good luck.
 
Couple questions please.

Narrowing the issues down but one (and I’m no electrician so I appreciate the help).

When I check the hot side of the preheat switch it reads 13.8v however when I click the switch the solenoid just clicks away and the voltage on the switched side of the switch and the post on the solenoid reads 11.8

Not sure what to make of that. Does that indicate a bad switch?

Also, if my remote panel is hooked up to the panel on the box on the generator nothing happens when I press the Preheat switch and it just blows the 8 amp fuse. Thoughts on that fun issue are appreciated :).

This is it disconnected, of course.

Thank you in advance.
 

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I think what’s happening is you are turning on the preheat circuit. That turns on the fuel lift pump (that’s the clicking sound) and all the glow plugs. Of the top of my head, each one pulls about 10 amps, so your putting a big load on the battery, dropping the voltage.

The circuit also bypasses the low oil pressure switch...
 
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HP12Sea said:
Couple questions please.

Narrowing the issues down but one (and I’m no electrician so I appreciate the help).

When I check the hot side of the preheat switch it reads 13.8v however when I click the switch the solenoid just clicks away and the voltage on the switched side of the switch and the post on the solenoid reads 11.8

Not sure what to make of that. Does that indicate a bad switch?

Also, if my remote panel is hooked up to the panel on the box on the generator nothing happens when I press the Preheat switch and it just blows the 8 amp fuse. Thoughts on that fun issue are appreciated :).

This is it disconnected, of course.

Thank you in advance.
Click to expand...
When mine did that exact failure it was because of the diode. I replaced it and the remote switches worked normally after that. Looks from a previous post you replaced the diode. Could you have installed it backwards?
 
dtfeld said:
I think what’s happening is you are turning on the preheat circuit. That turns on the fuel lift pump (that’s the clicking sound) and all the glow plugs. Of the top of my head, each one pulls about 10 amps, so your putting a big load on the battery, dropping the voltage.

The circuit also bypasses the low oil pressure switch...
Click to expand...

Thanks. When it was running fine, there would be a thud when flipping on the preheat switch.

Now there's no thud and just clicking, like when a car battery is half dead. I also put my fingers against the solenoid and it's def that making the clicking noise. For some reason, it's not getting enough umph.
 
ZZ13 said:
When mine did that exact failure it was because of the diode. I replaced it and the remote switches worked normally after that. Looks from a previous post you replaced the diode. Could you have installed it backwards?
Click to expand...

Thanks, I'll have a look when I'm back and anything is possible for sure.
 
The clicking noise is normal; that is the fuel lift pump operating. You should also hear/feel the glow plug solenoid actuate and the fuel solenoid actuate when the Preheat switch is depressed. When that glow plug solenoid closes you should see a drop in voltage as that is a large current draw.
All the diode installed in TB2 does is allow the power to bypass the oil pressure switch when Preheat is on to actuate relay K2 which operates the fuel lift pump and fuel solenoid. Once oil pressure is up and that oil pressure switch closes power to the lift pump and fuel solenoid relay (K2) is supplied through the oil pressure switch.
So, if the lift pump starts to "click" when you press Preheat then that diode is OK and relay K2 is closing.
Now, if the diode is burning up and/or the 8 amp fuse if opening then there is a short to ground somewhere between the oil pressure switch / fuel shutoff solenoid and relay K2. The current through that diode (which occurs only when Preheat is depressed) is normally very low as it is only used to close relay K2.
If, for example, that diode has failed then the glow plugs will operate when preheat is depressed and the generator will crank. It will however, not start because the oil pressure switch has not closed and consequently the fuel lift pump and fuel shutoff solenoid are not operated.
A good test would be to lift the two wires off the oil pressure switch and connect them together. If the generator starts and runs normally then the issue becomes isolated to the oil pressure switch or that bypass circuit.
 
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