More help required with vdo rudder indicator problem

[RIN]

Further to my recent post about vdo rudder indicator problems,

http://www.ybw.com/forums/showthread.php?456515-help-with-siemens-VDO-rudder-sender

I have traced all the wiring and found that there is no 12v+ at the upper helm indicator. In fact a dedicated upper helm vdo gauge had blown. I replaced it with a 5A fuse and this time a 7.5A fuse nearer to the battery blew. I replaced it with a 2A fuse and again the dedicated fuse blew.

After a bit of an investigation, it turns out the fuse blows when either engine is switched off.

I have also bought a new gauge and tried it in both helm positions so it's not the gauge unless two have developed the same fault which would be very unlikely.

What is going on? Can anyone help or suggest what I can check next?

Many thanks

 

[volvopaul]

The fuse blowing is overload, so maybe the power supply to the gauge is also feeding something else, switching off the engine baffles me that the fuse blows.
Is the gauge power fed from the engine ignition?

 

[RIN]

No the gauges are on when the keys are turned to the right but not all the way to ignition. There must be some sort of spike as the ignition goes off. I can start and switch off from the upper helm tomorrow and see if switching off from there also blows the fuse. The only other thing I can think of is some fault in the sender unit when the engines are switched off but can't think of why that would happen. The dedicated fuse that is blowing only serves the upper helm gauge. With a 5 amp fuse fitted, it doesn't blow but a fuse upstream (7.5A)of it does.
I am really confused.

 

[jimmy_the_builder]

Where does the -ve for the gauge go?

Edit: I've just re-read your prev thread. What happened when you measured the sender resistance?

 

[RIN]

From a few ohms to about 100 as I recall. so not far off what it should be.

Just thinking about this, the only thing I have done to the boat which might match with this problem starting is fitting a galvanic isolator but I can't think that would cause this problem as that will be 240V. I can bypass it and see if the problem still occurs

 

[jimmy_the_builder]

It's hard to see an obvious link between the gi and a 12v instrument circuit blowing. The gi should be interrupting your mains earth as close to the incoming shorepower supply as possible.

So just to recap - the sender is ok, but you're still getting no reading on the gauge at both helms, is that correct? And something is blowing the fuse protecting the 12v supply to the gauges when you turn the engines off?

What happens when you disconnect the sender wire from the gauges - does the fuse still blow?

 

[RIN]

I the sender is probably ok unless somehow it is shorting when the an engine turns off.

Both gauges are now reading perfectly until I switch an engine off at which point the dedicated upper helm gauge fuse pops, or if I put 5A fuse in it, another fuse 7.5A upstream of it pops.
I replace fuse, restart engines and everything works, until I switch an engine off and a fuse goes again

 

[jimmy_the_builder]

What happens to the fuse when you disconnect the sender wire from the gauge and turn the engines off? At the risk of stating the obvious, you need to try and isolate the problem further. I'd also want to know where the -ve from the gauge is going, and how good that connection is.

 

[RIN]

Continuity checked ground from sender to both gauges, -ve from battery to both gauges and +ve from battery to both gauges. All good until fuse blows.

Will try removing upper helm gauge and see if fuse blows when engines switched off tomorrow if weather good. Currently raining

 

[RIN]

Removed upper helm gauge, switched off engines and fuse did not pop
Replaced gauge, retried and fuse did not pop either. Tried this 6 times and fuse OK.

So it's now an intermittent problem.

The rudder gauges and the fuel tank gauges are not ignition dependant but come on whenever the engine circuit breaker is switched on ( two other breakers, domestic and negative) so I reckon if I always switch off the engine breaker before switching off the engine I have a work around.

I would still like to get to the bottom of this though.