Rev Counter Problem

[Richard Martin]

Our rev counter connected to a Shire 1200 diesel engine has suddenly started giving incorrect values.

 

We used to cruise along at about 1100-1200 rpm now the same speed reads about 500rpm.

 

Any suggestions as to what is wrong, could it be a faulty connection at the engine (not sure where that would be) or is the unit on it's way out.

 

Any suggestions gratefully received.

[John Orentas]

Our rev counter connected to a Shire 1200 diesel engine has suddenly started giving incorrect values.

 

We used to cruise along at about 1100-1200 rpm now the same speed reads about 500rpm.

 

Any suggestions as to what is wrong, could it be a faulty connection at the engine (not sure where that would be) or is the unit on it's way out.

 

Any suggestions gratefully received.

 

 

Your drive belt needs tightening!

[Guest]

Your drive belt needs tightening!

Echo, our Vetus does the same. Very useful piece of multitasking instrumentation.

[DHutch]

Yeah, sounds quite plausable.

- The rev counter is driven of the alternator, so if the alternator belt is sliping, you get the odd rev readings. Also, you batterys end up flat!

 

 

Daniel

[bottle]

 

 

Your drive belt needs tightening!

 

Agreed but do not overtighten as that would cause damage to the bearings.

[Richard Martin]

Many thanks for the replies.

 

I will check out the tension at the weekend.

[Richard Martin]

Tested the fan belt last night and the tension is ok does not seem to be the problem.

 

Cannot check the battery level as I (stupid idiot) left my test gauge on last time I used it and the battery on that is flat.

 

There is a white lead which is not connected to anything but not sure if it should be, no obvious places for it to connect, if this was the case the rev counter would not function anyway

[Chris Lingwood]

Maybe the supply voltage to the counter is low. Might be due to corrosion. That would be my guess. Or corrosion on the sensor lead can do strange things. I know on MGBs it makes the needle wave around, it could perhaps lower the reading if it was very specificaly corroded...looks round for more straws to clutch at

[John Orentas]

Rev counters work on digital principles, high resistance or low voltages should not affect it, though a bad contact on the sensor wire could in an erratic sort of way. I still think that the problem is caused by the alternator not running at the proper speed, perhaps being artificially slowed down.

[Serendipity]

When I took my boat on, the erratic operation of the rev counter while under way was a source of considerable annoyance, Sometime working correctly, most of the time reading at around half true, other times not reading at all.

 

Quite separately, when asked to check the state of my batteries, the marina diagnosed a faulty alternator (but i don't recall quite why) and replaced six months ago. Since then the rev counter has been absolutely fine apart from the occasion a couple of weeks ago when it started under reading again miles from anywhere. I thought here we go again, but three minutes later the belt shredded. I did wonder why it had gone so quickly, but fortunately I had asked them to put a spare on board when they replaced the alternator.

[John Orentas]

This business of premature failure of drive belts comes up regularly on the forum. In my view there are two possible causes.

 

In the nature of things alternators on boats work much harder and put more load on the belts than the road-going equivalents.

 

Alternators and their drive systems in boats tend to run very hot as ventilation is often very poor.

[Chris Lingwood]

Rev counters work on digital principles, high resistance or low voltages should not affect it, though a bad contact on the sensor wire could in an erratic sort of way. I still think that the problem is caused by the alternator not running at the proper speed, perhaps being artificially slowed down.

 

 

No they don't. They might nowadays but they certainly didn't. Purely analogue, all be it with a PDM input.

Also if the contact is bad the input signal will be lower which does cause problems (I've seen it happen, replaced a connection and it was fixed).

 

Each time a pulse comes the needle is deflected. More pulses more deflection, less pulse less deflection. Put a scale on it and there you go. It never really counts anything as such.

 

Depending of course on whether the instrument designer compensated for this its quite possible that either the input voltage or supply voltage could change the deflection. Lower input signal means less deflection, also lower supply potential means less amplification so lower deflection.

 

Of of this is of course potentialy irrelavent if its a newish counter

 

I would check it. Check the resistance of the sensor wire. Then check the voltage at the back of the instrument. If the resistance is nigh on 0 and the voltage is around 12v then its the signal. Its free to check. And possibly pence to fix.

Edited September 29, 2006 by Chris Lingwood

[Hobart]

A rev counter. Ahhh, that would be nice. Oh, and while I'm dreaming - a morse control throttle/gear, or even neutral - what luxury! And one day, maybe even a weed-hatch!

 

Our boat is so weird. Half the time I have no idea what you are all talking about. Until recently, I thought all narrowboat engines were cooled with a car-type radiator!

 

No really, we love our boat. Just would be nice to be able to afford to update it with 'all the mod-cons'.

 

Neil

[Guest]

This business of premature failure of drive belts comes up regularly on the forum. In my view there are two possible causes.

 

In the nature of things alternators on boats work much harder and put more load on the belts than the road-going equivalents.

 

Alternators and their drive systems in boats tend to run very hot as ventilation is often very poor.

All of those plus bigger and bigger alts are being fitted on ridiculously narrow single v belt drives. Not nearly good enough.

 

A rev counter. Ahhh, that would be nice. Oh, and while I'm dreaming - a morse control throttle/gear, or even neutral - what luxury! And one day, maybe even a weed-hatch!

 

Our boat is so weird. Half the time I have no idea what you are all talking about. Until recently, I thought all narrowboat engines were cooled with a car-type radiator!

 

No really, we love our boat. Just would be nice to be able to afford to update it with 'all the mod-cons'.

 

Neil

must admit one of our previous boats just had a voltmeter and a couple of idiot lights. That was it. Never needed anything else.

[DHutch]

A rev counter. Ahhh, that would be nice. Oh, and while I'm dreaming - a morse control throttle/gear, or even neutral - what luxury! And one day, maybe even a weed-hatch!

We dont even have a gearbox!

[Ernie]

When I took my boat on, the erratic operation of the rev counter while under way was a source of considerable annoyance, Sometime working correctly, most of the time reading at around half true, other times not reading at all.

 

Quite separately, when asked to check the state of my batteries, the marina diagnosed a faulty alternator (but i don't recall quite why) and replaced six months ago. Since then the rev counter has been absolutely fine apart from the occasion a couple of weeks ago when it started under reading again miles from anywhere. I thought here we go again, but three minutes later the belt shredded. I did wonder why it had gone so quickly, but fortunately I had asked them to put a spare on board when they replaced the alternator.

 

I recently had a similar problem. Rev counter all over the place and sometimes no reading at all. Also, battery charging was also fluctuating, especially when batteries near full charge. New alternator cured the problem.