Amp and Temp meters full deflection

[Lundylad]

A friend has asked me to look at his boat as the Amp and Temperature meters go to full deflection when turned on, the engine is cold, so electrical issue.
Will look at earth connections and check what current is being drawn etc.
Any suggestions would be of help.
Thanks

 

[padge]

check the ammeter shunt hasn't gone open circuit or has a bad connection .

 

[William_H]

I find it difficult to imagine how the 2 could be related. Temp guage probably means a bad wiring connection somewhere.
The amp meter is usually a different kind of circuit in the wiring of the battery and alternator output. The amp meter may be showing high current froma short circuit but this would be accompanied by a rapidly declinign battery voltage and smoke. As said the shunt if it has an external shunt is the likely cause of high reading. A shunt is just a carefully calibrated piece of bar of a slightly electrically resistive metal. iron or stainlesss steel or nichel/ chrome. The current passes through the bar with terminals at each end. Ideally the take off to the meter should be separate screw terminals onto the bar. If they use the same terminals then any bad connection to the bar will increase the resistance and so the reading for a given normal current. Other amp meters do not have a shunmt but rather feed alll the current through the meter (case) I high reading here would be unusual buit would mean replacing the ampmeter. I don't know if any of this helps good luck olewill

 

[VicS]

A friend has asked me to look at his boat as the Amp and Temperature meters go to full deflection when turned on, the engine is cold, so electrical issue.
Will look at earth connections and check what current is being drawn etc.
Any suggestions would be of help.
Thanks

Like William I am finding it difficult to think of a single fault that would explain both gauges going full scale.

What engine is it? Finding a wiring diagram would at least be a starting point for the "little grey cells"

Disconnect the temp sensor and see what happens. If temp still goes full scale look for a short to earth somewhere between the gauge and the sensor.

Ammeter going full scale most likely, as suggested , a problem with the shunt unless a genuine reading but if the shunt is o/c I'd expect the ammeter itself to burn out pretty soon, but if genuine ??????

 

[cimo]

possibly your friend has had some panel re-wire work done?
chase all connections to each guage - check for positive/negative mix up(s).

 

[chewi]

Take the guages out of circuit and test them in an isolated environment, make sure they work before scouring the boat for faults that might not exist. #

Did the temp guage ever read Ok? maybe it wrong for the sensor and might need its own shunt? if so gat the gauge, battery and sensor and set it up around a boiling pan to get the shunt value right. Start with matching the resistance of the coil, taht will go from full scale deflection to half scale.

As said for the ammeter, it has to be the shunt. A real short would have manifested in a blown fuse, dead battery or fire.

 

[Lundylad]

Thanks for the replies.
William and Vic, yes both faulty together is odd?.
I know a little of the layout of the boat so it will take a little time to familiarise myself.
Yes Chewi checking the gauges first is a good shout. Will post the results of what we find which may help someone else who has the same issue.

 

[cimo]

Thanks for the replies.
William and Vic, yes both faulty together is odd?.
.

reversed polarity at the gauge power inputs would be a likely common cause.

 

[paul-essex]

if the output from the temp gauge go's down to earth it could draw a lot of amps and the amp gauge show a actual reading so sender unit might be us or wire from temp gauge to sender unit melted / rubbed / trapped earth point

 

[TQA]

Any chance there are warning buzzers involved if so worth checking those out of circuit.

 

[nigelmercier]

Looks like a missing ground wire to me.