ST60 log only reads when engine is running

[DHV90]

Hi guys,

I have a ST60 problem which isn't thermistor related, surprisingly.

St60 log, thermistor had failed so soldered in resistor, log now works but will only read a speed when the engine is running. Doesn't matter if its in gear, or in neutral ticking over, the minute you kill the engine the speed reading drops to zero and won't read again until you start the engine, even if its not driving the prop. Very confused. Batteries are in good condition with good voltage, 1/2/both makes no difference.

Other than that the wiring is really simple. 12v goes through the main switch panel to a Seatalk box which sends power out on way via sea talk and with one other spur which goes to the compass bulb, other than that it's basically battery - switch - seatalk - display.

Very confused as to what difference the engine running makes, charging on idle isn't very much so on a well charged battery the voltage doesn't go up much. There is nothing else running off the battery so nothing drawing much current.

Any ideas very welcome!
Thanks

 

[sarabande]

Have you checked the voltage of the feed wiring to the display with the engine OFF, then ON ? ISTR there's a minimum voltage for ST kit.

 

[DHV90]

Good shout I will have a look at what the instrument is actually receiving. The bulb added in is in parallel so shouldn't effect it but I will check nothing's being lost through the cable run.

 

[greeny]

What happens if you have shore power connected and the charger is on? You can remove the log and spin it to simulate boat speed of course. If the symptoms are the same it will give you a clue whether it's voltage drop related or not.

 

[DHV90]

Ah yeah, another interesting aspect i forgot to mention - With the log removed, engine not running, spinning the wheel gives a reading up to 6 ish knots so seems to work fine, but when fitted and moving through the water is when the issue shows up, so possibly not voltage related? I spun the wheel yesterday before fitting, will try it again tomorrow to see if I can force it to do the same thing again.

plus the ST60 depth works fine all the time and is running off the same source, just daisy chained with a seatalk wire off the log readout.

Batteries at rest are a good 12.7v and with the engine at idle hovering around 13.1v, no shore power so can't try a charger unfortunately.

 

[Just_sayin']

Ah yeah, another interesting aspect i forgot to mention - With the log removed, engine not running, spinning the wheel gives a reading up to 6 ish knots so seems to work fine, but when fitted and moving through the water is when the issue shows up, so possibly not voltage related? I spun the wheel yesterday before fitting, will try it again tomorrow to see if I can force it to do the same thing again.

I'm ignoring all the electrical stuff as it confuses me.

Are you absolutely sure that the wheel spins TOTALLY freely? I once apparently over-antifouled mine and my finger could flick it round and get a reading on the head but water pressure wasn't enough to move it.

Also, do you have an arrow on the transducer showing which way it should go in? I have no idea why it should make a difference but mine did have an arrow and just because it did I always put it in facing the relevant way.

It's probably electrical :encouragement:

 

[duncan99210]

Just a thought. If you get a reading when you manually spin the paddle wheel even when the engine is off, then perhaps the paddle wheel is jamming somewhat and the vibration of the engine, even at tickover, is enough to allow the wheel to spin?

 

[DHV90]

the paddle wheel and shaft are both new, and spin if i blow air at the flats of the paddle wheel, and is aligned to the arrow. The jamming idea might have some merit still though, the 3gm shakes the whole boat on idle. Is there meant to be any kind of washer to stop the paddle wheel catching at each end? perhaps it gets pinned to the side when sailing and cant spin?


Thanks for all the input everyone, lots of ideas to try!

 

[DHV90]

So, I checked the voltages and 12.8 volts on the batteries equated to 12.79 measured at the end of the seatalk plug which powers the log, without the engine running, so I can't see there being a problem there?

Is it possible that the impeller could be gripping the sides of the transducer somehow??

 

[earlybird]

If you can blow the impeller to rotate it, I can't see it being stuck in any way. The only suggestion that I can think of is the impeller is badly aligned with the local water-flow resulting in it jamming, with the engine vibration shaking it free to turn. I can't say that I find that theory very convincing though.

 

[scrambledegg]

Let's retrace steps to what is likely to go wrong with electrical connections in a boat: corroded or otherwise dodgy connections.
Perhaps with the engine running the vibration is allowing one or more of those connections to the log to make adequate connection, which isn't the case in the low-vibration sailing case?
Your removal of log to spin by hand may also have a similar effect.

 

[greeny]

Hi guys,

I have a ST60 problem which isn't thermistor related, surprisingly.

St60 log, thermistor had failed so soldered in resistor, log now works but will only read a speed when the engine is running. Doesn't matter if its in gear, or in neutral ticking over, the minute you kill the engine the speed reading drops to zero and won't read again until you start the engine, even if its not driving the prop. Very confused. Batteries are in good condition with good voltage, 1/2/both makes no difference.

Other than that the wiring is really simple. 12v goes through the main switch panel to a Seatalk box which sends power out on way via sea talk and with one other spur which goes to the compass bulb, other than that it's basically battery - switch - seatalk - display.

Very confused as to what difference the engine running makes, charging on idle isn't very much so on a well charged battery the voltage doesn't go up much. There is nothing else running off the battery so nothing drawing much current.

Any ideas very welcome!
Thanks

Lets go back to basics. I assume (dangerous) that the log worked before the thermistor failed. You soldered a resistor in instead. Now it only works part time. Only difference is the resistor instead of the thermistor. How or why this is a problem I don't know but it's the only thing changed.

 

[Vegable]

Two thoughts from me.
Have you made a dry joint with your soldering? It should look nice and shiny. If it's dull then you've got a dry joint which isn't making a good connection. Engine vibration could be making the contact.
Secondly, has solder bridged any contacts? or is there any solder splatter hovering about?
I'm assuming you've soldered onto the contacts at the back of the instrument not on the log itself.
After that I'm stuck
Mike

 

[ghostlymoron]

Why do you say it's not thermistor related but the follow that by saying that the thermistor has failed and you've replaced it with a resistor. I don't actually know what a thermistor is but suggest you replace the resister with a thermister of the same spec and see if it works.

Hi guys,

I have a ST60 problem which isn't thermistor related, surprisingly.

St60 log, thermistor had failed so soldered in resistor, log now works but will only read a speed when the engine is running. Doesn't matter if its in gear, or in neutral ticking over, the minute you kill the engine the speed reading drops to zero and won't read again until you start the engine, even if its not driving the prop. Very confused. Batteries are in good condition with good voltage, 1/2/both makes no difference.

Other than that the wiring is really simple. 12v goes through the main switch panel to a Seatalk box which sends power out on way via sea talk and with one other spur which goes to the compass bulb, other than that it's basically battery - switch - seatalk - display.

Very confused as to what difference the engine running makes, charging on idle isn't very much so on a well charged battery the voltage doesn't go up much. There is nothing else running off the battery so nothing drawing much current.

Any ideas very welcome!
Thanks

 

[RichardS]

Why do you say it's not thermistor related but the follow that by saying that the thermistor has failed and you've replaced it with a resistor. I don't actually know what a thermistor is but suggest you replace the resister with a thermister of the same spec and see if it works.

Replacing the ST60/Airmar temperature sensor with a 10K resistor is a standard practice and many have done it, including me. It can't be that substitution, in itself, that is causing the problem.

Richard

 

[neil_s]

I think Duncan (post 7) has it right, here. Not an electrical problem - you have a very small barnacle just on the lip of your thru-hull tube that needs to be cleaned off. Have this problem with my ST30 log every now and then.

 

[DHV90]

The soldering is all good, no worries there, but I am considering that there might be a break somewhere in the wire potentially, or even inside the transducer thats got the same result. Think it might be new transducer time!

To clarify, the transducer has never worked in the time ive had the boat, never bothered to do anything about it to start with but when the boat came out for a refit 2 years ago I measured the thermistor which had failed so soldered the resistor in, and tested off the boat and it seemed to work and left it at that, so I have no guarantee that it was just the thermistor that had failed.

 

[pvb]

The soldering is all good, no worries there, but I am considering that there might be a break somewhere in the wire potentially, or even inside the transducer thats got the same result. Think it might be new transducer time!

To clarify, the transducer has never worked in the time ive had the boat, never bothered to do anything about it to start with but when the boat came out for a refit 2 years ago I measured the thermistor which had failed so soldered the resistor in, and tested off the boat and it seemed to work and left it at that, so I have no guarantee that it was just the thermistor that had failed.

I seem to recall reading once that the temperature sensing has to work otherwise the log won't work. However, if you've added the 10K resistor properly, it should work. Could you swap the display head temporarily with a working one from a friend's boat, that would at least allow you to narrow down where the fault lies.

 

[Alex_Blackwood]

Hi guys,

I have a ST60 problem which isn't thermistor related, surprisingly.

log now works but will only read a speed when the engine is running. Doesn't matter if its in gear, or in neutral ticking over, the minute you kill the engine the speed reading drops to zero and won't read again until you start the engine,

When you say reading drops to zero do you mean 000 or that the screen goes dead? If it goes dead then probably power related if 000 then it is working but not getting a speed signal.

 

[DHV90]

The screen drops to 000, but gradually as it always does, which indicates no signal from the transducer.

Thermistor (thermal + resistor) is a variable resistor, replacing it with a 10k resistor just tricks the screen into thinking the water is 27 degrees in the absence of a proper reading. Only reason for saying its not thermistor related is that is a common problem well documented on this forum so didn't want anyone thinking I hadn't had a search and a google before writing basically.

The boat has only just gone back in the water smooth, barnacle free etc. Also, surely a barnacle would interfere with readings all the time, rather than being a variable issue? The log does register our speed when motoring, seemingly accurately, but that said we motor at about 3 knots to get off the mooring then the engines off so it might need some more exploration under motor which Ill do as soon as I get the chance!

Just to clarify, the other day when the log went down to 0, I started the engine in neutral and the speed immediately climbed up to 4 or 5 knots, I killed the engine then the speed started dropping to 0 straight away, and I think its not just us being rubbish sailors? Just to show how pronounced the correlation with the engine running is!