macnorton
regular
Reged: 11/12/2006
Posts: 240
Loc: East Anglia
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Whwen i turn my 12v fridge on the engine temp guage jumps by 10 degree's, it returns to normal when i turn the fridge off.
anyone had similar? i have a few ideas why but what do you think~? 
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"The world is full of kings and queen's, who blind your eyes and steal your dream's"
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Shuggy
regular
Reged: 10/03/2006
Posts: 274
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Sounds like an earthing problem somewhere. Have both got clean connections to -ve?
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Tafna's for sale
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lw395
regular
Reged: 16/05/2007
Posts: 610
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Could possibly be interference, as the fridge works by inverting its own ac to drive the motor.
Try a capacitor across the fridge supply terminals?
The earthing problem is probably more feasible, you may be able to measure a voltage between the control panel earth and the engine whch changes as the fridge switches on. Try running a return wire direct to the engine block if it currently goes back to a busbar?
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William_H
regular
Reged: 28/07/2003
Posts: 3499
Loc: West Australia
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Temporarily wire the fridge directly to the battery (both pos and neg) or better still to another battery not connected to the engine. This will isolate any AC interference which I think unlikely.
From there try using the normal +ve supply but substitute another heavy wire to the neg battery terminal. And then in turn try substuting the positive supply wire. This procedure should isolate any common voltage drop which I suspect is causing the problem.
Some temp gauges work in a manner opposite to what you might think such that lower voltage will increase the reading. Most cars have a voltage regulator to stabilise the supply voltage to the gauges.( don't know about boat engines) just a few ideas good luck olewill
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lenseman
regular
Reged: 03/06/2006
Posts: 1190
Loc: South East Coast - United King...
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OK, your engine temperature meter effectively works as a voltmeter and the 'voltage' that it reads is from the temperature sensor attached to the engine block (or radiator).
This temperature sensor is what is known as a NTC sensor (different colour ends, black, brown, blue, red, green etc) depict different operating parameters. NTC stands for 'Negative Temperature Coefficient and the resistance goes down towards zero as the sensor warms up. Sometimes and rarely PTC (Positive Temperature Coefficient) sensors are used but these are not so common. They work by an increase in resistance with a rise in temperature.
From what you say, the temperature gauge is reading a drop in resistance but as the NTC sensor is not dropping, it therefore appears that when you turn the fridge on, the temperature gauge then reads both the NTC sensor and also there seems to be a change in resistance when using the fridge.
You will possibly find that one of the fridge wires is wired via the engine block or that one of the fridge wired is causing an effect on the wiring to the engine block and this can include the engine earthing or bonding straps.
A lesser problem that causes this type of problem is the state of the electrics and wiring.
With the engine warm and running or not, use an AVO meter or DVM to read the voltage AT the battery terminals. Now turn on the fridge. Does the voltage at the battery fall or remain stable. Hopefully, it should be stable. If the battery voltage falls then your batteries need charging, servicing or replacing?
Finally, with the AVO or DVM, read the voltage between the ENGINE BLOCK (-ve meter lead) and BATTERY TERMINAL POSITIVE (+ve meter lead). Turn on the fridge as in the last test. Again, the voltage should remain stable.
If any of these two tests show a marked drop in voltage (more than say a tenth of a volt) you possibly have a high-resistance (HR) connection in the electrical circuit. This needs to be corrected. This would then point towards a loose connection or a corroded connection or terminal. Both can easily be corrected without spending too much money. 
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regards David
"Away Sea-boats crew - Special Sea Dutymen, Close-up"
Skype: lenseman-uk
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macnorton
regular
Reged: 11/12/2006
Posts: 240
Loc: East Anglia
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Thanks for that, the sender i have shows a higher resistance as the temp increases,
The fridge is wired to the distribution board (both +ve and -ve).
The batteries (700ah each) are both new.
I will have to get the meter out and try to find out what is happening, hopefully it is just a poor conection that can be found without to much drama as I dont want to put to sea with unreliable instruments...
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"The world is full of kings and queen's, who blind your eyes and steal your dream's"
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lenseman
regular
Reged: 03/06/2006
Posts: 1190
Loc: South East Coast - United King...
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Where in East Anglia are you based?
I have to go up to Felixstowe Ferry this afternoon to urgently fix some electrical problems on a yacht moored out on the Deben if this is of any use at all? I am leaving West Sussex in 15 minutes ( at 12 noon).
Give me a shout on my mobile number which is listed in my biog. 
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regards David
"Away Sea-boats crew - Special Sea Dutymen, Close-up"
Skype: lenseman-uk
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macnorton
regular
Reged: 11/12/2006
Posts: 240
Loc: East Anglia
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thanks for the offer, will have a further look this weekend and see how i get on.
Rick
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"The world is full of kings and queen's, who blind your eyes and steal your dream's"
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