Rev counter over reading today.. What to check?

[wipe_out]

Started up the boat today and the rev counter is over reading.. So at what sounded like around 1200-1500rpm it was reading ~3000-3500 on the rev counter.. I switched off and on a again but it was the same.. Late this afternoon when I started up to come back in the rev counter was still over reading buy not by as much.. At idle it was reading about 1200-1400rpm and at about 3200rpm (by ear and approximate speed) the rev counter was showing a little over 4000rpm..

The engine is a 5.7L V8 Carburetted Petrol with electronic ignition..

What would cause this?

Anyone have any ideas what/where to check?

Thanks..

 

[Duster]

Not joking, mine used to do similar when it had not been run for a while..............give the gauge a sharp smack with a screwdriver handle..........worked every time!

 

[lynall]

Ive had damp in mine, a couple of nights in the airing cupboard fixed that problem.


Lynall

 

[wipe_out]

Hmmm.. Interesting.. I used the boat last weekend and it was fine and we have had much hotter/drier weather this week than usual so I doubt its damp.. Maybe it's a UK gauge and doesn't like the heat..

 

[cryan]

Check your tacho connection to the alternator and all your ground connections. Although these normally cause a bouncy needle as opposed to a constant high.
A constant high could indicate a zero error in the calibration.
I would never recommend, "Technical Taps," as your more likely to damage it than fix it.

 

[Duster]

I would never recommend, "Technical Taps," as your more likely to damage it than fix it.

"Impact maintainance" is an effective an well recognised technique, dating back to B&W TVs...........first thing our computor lady used to do if you took her a machine with an intermitant electrical fault........Bang it down on her desk. very often it cured it, occasionally it caused the fault to become permenant so it could then be traced.

Remember the old adage, "the skill is not in hitting it - it's knowing where to hit it!

 

[wipe_out]

Check your tacho connection to the alternator and all your ground connections. Although these normally cause a bouncy needle as opposed to a constant high.
A constant high could indicate a zero error in the calibration.
I would never recommend, "Technical Taps," as your more likely to damage it than fix it.

That,s what threw me.. A faulty connection would have a intermittent or low reading rather than a high reading..

What's a zero error and what calibration is there on a non-EFI engine? I thought it just picked up a trigger/pulse signal from somewhere..

 

[cryan]

That,s what threw me.. A faulty connection would have a intermittent or low reading rather than a high reading..

What's a zero error and what calibration is there on a non-EFI engine? I thought it just picked up a trigger/pulse signal from somewhere..

There are several types of error on instrumentation. A zero error is where the zero point is off. If you ploted a curve on a graph of the guage range the curve would match the true curve but the zero point would be higher or lower and so the curve would sit higher or lower that true. A multiplication error is where the zero point is the same as true but that as the value rises the curves seperate to form two different curves. This comes from mechanical gauges where the was a multiplication quadrant but now means that there is a fault with the computers sums.
The Tacho will pick up a pluse from somewhere.(usually the "W" post on the alternator) But it will then need to be calibrated to that specific engine as unless it takes a feed direct from the crankshaft there will always be a speed difference between the crankshaft and the pickup point. An alternator may spin 2.5 times faster than the crankshaft and so the tacho needs to be calibrated for this. This is usually a once only calibration but any faults in the circuit could throw off the calibration including power surges etc. Sometimes this is done simply by adjusting the calibration screw to the actual engine revs which can be obtained by a handheld tacho. However some are far more complicated requiring some serious video recorder programming type skills!(ie hold button for two second then spin three time reciting the Lords Prayer backwards in Gaelic before pressing button two for five seconds then button one and three together while hopping up and down for six seconds!)

 

[wipe_out]

There are several types of error on instrumentation. A zero error is where the zero point is off. If you ploted a curve on a graph of the guage range the curve would match the true curve but the zero point would be higher or lower and so the curve would sit higher or lower that true. A multiplication error is where the zero point is the same as true but that as the value rises the curves seperate to form two different curves. This comes from mechanical gauges where the was a multiplication quadrant but now means that there is a fault with the computers sums.
The Tacho will pick up a pluse from somewhere.(usually the "W" post on the alternator) But it will then need to be calibrated to that specific engine as unless it takes a feed direct from the crankshaft there will always be a speed difference between the crankshaft and the pickup point. An alternator may spin 2.5 times faster than the crankshaft and so the tacho needs to be calibrated for this. This is usually a once only calibration but any faults in the circuit could throw off the calibration including power surges etc. Sometimes this is done simply by adjusting the calibration screw to the actual engine revs which can be obtained by a handheld tacho. However some are far more complicated requiring some serious video recorder programming type skills!(ie hold button for two second then spin three time reciting the Lords Prayer backwards in Gaelic before pressing button two for five seconds then button one and three together while hopping up and down for six seconds!)

I see.. I have no buttons on the rev counter so perhaps there is a screw on the back..

Didn't know it picked up from the alternator.. I always assumed it was from the coil or distributor..

 

[cryan]

Didn't know it picked up from the alternator.. I always assumed it was from the coil or distributor..

It might do on a petrol. I'm more used to diesel which is usually the alternator. The calibration buttons/screws etc will usually be on the back so as to avoid it being played with by mistake. Either way you will need to get a hold of a handheld tacho to get an idea of what the actual revs are. I prefer laser versions as then your hands are a bit safer from spinning belts and pulleys.

 

[rbcoomer]

I'd have thought the tacho feed would be from the alternator as most vehicles are. Over-reading could be faulty gauge, failed rectifier pack in alternator or even incorrect belt path/pulley. As presumably the latter hasn't been changed since last run, I'd go with gauge or rectifiers. What's the 12V looking like? Do you have access to an oscilloscope - if so you can look at the signal and work out the rpm... Noise on the signal could trick the gauge into counting extra pulses/revolutions, but although a loose connection also could, I'd say unlikely as it would likely be sporadic and thus the gauge would bounce about a bit! A failed rectifier could create a consistent 'noise'.

P.S. warm weather increases the chances of damp as any water in the gauge gets evaporated and can then move about! (It's unlikely to find a way out)