Rev counter doesn't work until engine revved

[Dazzajohm]

Morning all,

Just picked up a new to me boat so want to go through and try sort some niggly little bits. My port side rev counter will not work on starting until I rev the engine past about 2000rpm.

Any ideas what could be causing this?

Cheers
D

 

[Leighb]

Check for a poor electrical connection somewhere.

 

[CLB]

Loose belt could also cause this

 

[[2068]]

Does the battery voltage “jump” to around 14v at the same time the rev counter comes to life, or not?
Either way, I would start by checking the alternator connections.

 

[rafiki_]

Yep, I’m with Flowerpower on this.

 

[gordmac]

Normally the tacho signal on diesels comes from the alternator, if it isn't charging no signal. Often the alternator needs to be sped up a bit to generate. No idea why but mine have been like that forever with no problems.

 

[BruceK]

What engines? Not all tacho's take their signal from the alternator.
If they are taking a signal from alternator check the W terminal is clean and all subsequent connections to tacho.
Also check the alternator excites and starts charging. If the batteries have been on charger or on charger (now fully charged) switch on a few lights etc and see if that makes a difference. I had to replace my signal wire to the tacho as the fault was somewhere within the wiring loom and completely hidden and inaccessible

 

[Dazzajohm]

Thanks guys, so many replies. Will try answer the questions.

Engines are TAMD 41 B's. When I picked up the boat a couple of days ago it was working fine but I do leave her plugged in to the shore supply and hence the battery charger as per Brucek questions and shore supply is normally the last thing I connect before letting go so hence battery charger connected when engine started.

Belts all seem fine to me.

I haven't noticed the voltage and whether it jumps but will check tomorrow.

Cheers

 

[kashurst]

if all the belts and connections stuff doesn't work, have a look at the brush assembly.
Sometimes they stick or have worn too much and loose contact

 

[Greg2]

Try giving the engines a quick blip on the throttle in neutral - in my experience sometimes required to ‘excite’ the alaternator to get things working.

 

[Assassin]

You need to establish what type of system is in use as some are alternator triggered, some are sensor triggered using a sensor on either the camshaft or crankshaft or some simply tap into existing sensors for their signal such as the crank angle sensor or fuel injector pulse on one of the injectors.

I would suggest finding out which system it is and work from there and if possible run a diagnostic check as this would be a starting point to start from.

 

[PCUK]

As said, it is definitely the alternator needing a few more revs to start it charging. A very common issue and nothing to worry about.

 

[Dazzajohm]

Try giving the engines a quick blip on the throttle in neutral - in my experience sometimes required to ‘excite’ the alaternator to get things working.

Yes that’s exactly what I do. Stbd engine is fine and the counter works fine on start up. Just the port one.

 

[BruceK]

Yes that’s exactly what I do. Stbd engine is fine and the counter works fine on start up. Just the port one.

Do your alternators charge different battery banks? i.e. port does starter battery and starboard does house batteries or vice versa or do you have a split charging system? To me it sounds more and more like a dirty connection and the alternator is simply not exciting at low revs

 

[Dazzajohm]

Do your alternators charge different battery banks? i.e. port does starter battery and starboard does house batteries or vice versa or do you have a split charging system? To me it sounds more and more like a dirty connection and the alternator is simply not exciting at low revs

I would have to get down below and have a proper look but there are three batteries on board, two for the engines and one for the domestics. I'm sure I was told that the two for the engines are charged from one alternator and the domestic battery is charged from the other alternator. I also noticed a "battery" switch on the dash and when I queried this was told it is a bridging battery switch - bridging the battery banks. There are two battery banks, you press the button and a solenoid switch links the two together – so if one battery bank were to be ‘down’ you could use this switch to get the power from the other battery.

 

[luptonj]

If both alternators were charging both battery banks you could get interference between them.
that is the setup I have and since I changed one alternator I sometimes get one of the rev counters dropping to 0 between certain rev bands. Once I rev up enough it comes back to life again.

 

[VicS]

You need to establish what type of system is in use as some are alternator triggered, some are sensor triggered using a sensor on either the camshaft or crankshaft or some simply tap into existing sensors for their signal such as the crank angle sensor or fuel injector pulse on one of the injectors.

I would suggest finding out which system it is and work from there and if possible run a diagnostic check as this would be a starting point to start from.

The wiring diagram in the owners manual indicates that the tacho takes its signal from the alternator "W" terminal