Bukh DV36 Alternator issue

[petedg]

The “Amp” light on our Bukh DV36 engine does not switch off. I checked the 3 wires on the alternator and noticed that the yellow wire had come off the spade terminal. I made a new connection and assumed that would fix the problem but the “Amp” light is still on with the engine running. I also noticed the tachometer seems to be more unsteady than usual Or even not working at all.
I installed a new alternator today which seems to work fine, the “amp” warning light goes out when the engine is started. However, the tachometer and pressure gauge remain on zero and the water temp reads full scale in the red. I’m wondering if there is an earth issue? Any thoughts would be appreciated.
Thanks in advance for your help.

 

[robmcg]

Sounds like your new alternator is an isolated earth one. The original fit dv36 alternator was a non isolated earth. It causes the symptoms you describe on the panel. You can confirm this by using a wire from the -ve terminal of the alternator to the engine block. Touching it to the block will result in the tacho coming back to life as it should. I had the same issue when changing to a 70amp alternator on a dv36 a few years back. I'm pretty sure the 'permanent' cure was an extra wire from the -ve terminal on the alternator back to a suitable bolt on the engine.

 

[Boater Sam]

If you take just the insulating washer off the -B terminal on the alternator it should then ground the -B terminal to the frame and hence to the engine block without having a wire.

 

[VicS]

Sounds like your new alternator is an isolated earth one. The original fit dv36 alternator was a non isolated earth. It causes the symptoms you describe on the panel. You can confirm this by using a wire from the -ve terminal of the alternator to the engine block. Touching it to the block will result in the tacho coming back to life as it should. I had the same issue when changing to a 70amp alternator on a dv36 a few years back. I'm pretty sure the 'permanent' cure was an extra wire from the -ve terminal on the alternator back to a suitable bolt on the engine.

If there was no negative connectionto the alternator the warning light would not come on initially.

I dont see why any problem with the alternator will cause the oil pressure gauge to read zero and the temperature gauge to go full scale but I must find the schematic for diode board that links thec warning lights and buzzer

 

[LittleSister]

 

[petedg]

Thanks for all the comments, they give me a few things to try next time I go down to the boat. Where is the diode board located, maybe behind the instrument panel?

 

[VicS]

This post on the Canalworld Forum shows the connections to the diode plate and describes its operation. This is the later type. There was an earlier type which Google will find.

Engine and devices grounded to hull?? This looks weird.

I notice by Tony Brooks who many will remember for his TB Training boat electrical notes

Thanks for all the comments, they give me a few things to try next time I go down to the boat. Where is the diode board located, maybe behind the instrument panel?

Behind the switch panel but I don't think relevant to your problem.

Check the negative connection to the instrument panel.. I notice that the engine wiring schematic shows the battery -ve is earthed ( to the engine ) and that the panel negative is taken from the B- connection on the alternator therefore relying on the alternator earth connection to the engine..

 

[LittleSister]

Where is the diode board located, maybe behind the instrument panel?

I know Vic suggests that the diode board isn't your issue, but for future reference (it's one of the Bukh's weak points) mine is located on the back of the start switch and rev counter instruments panel. Looks like this -

This post on the Canalworld Forum shows the connections to the diode plate and describes its operation. This is the later type. There was an earlier type which Google will find.

That Canalworld post is a great find Vic, thanks for the link.

I'm not certain, but vaguely recall someone suggesting in a previous thread about the Bukh diode board that the only difference between the old and new boards was that the diodes were Zenner type in the later boards. I also vaguely recall, and not certain, that the change in the diode board was required because of the different alternator type (non-Paris-Rhone, perhaps?) on later Bukh's - the newer alternator with the old diode board meant the alarm would sometimes sound when it shouldn't.

 

[VicS]

This is the old diode plate. Resistors are I beleive 270 ohm and the diodes 1N4002

 

[petedg]

Sounds like your new alternator is an isolated earth one. The original fit dv36 alternator was a non isolated earth. It causes the symptoms you describe on the panel. You can confirm this by using a wire from the -ve terminal of the alternator to the engine block. Touching it to the block will result in the tacho coming back to life as it should. I had the same issue when changing to a 70amp alternator on a dv36 a few years back. I'm pretty sure the 'permanent' cure was an extra wire from the -ve terminal on the alternator back to a suitable bolt on the engine.

Thanks Robmcg your memory served you well, the above info was spot on. I attached a wire from the Alternator B- to a ground connector on the engine and all 3 gauges sprang to life. The alternator now also seems to be working fine and delivering 14+ volts when measured at the battery bank.
Thanks to all the other contributors as I’ve also learned something along the way and no doubt I will find the diode board when necessary.