Temperature sender and gauge doing my head in.

[scrunch]

Hope someone can help me with this my temperature gauge went right up to the top even with the engine cold, I have worked on loads of engines but all car based, so I did a few tests and thought it was the sender which I have replaced, but unlike the cars I have done there are 2 wires from the sender one I think is an earth the other one appears to be live.

the rear of the gauge there are 3 spades one with a blue wire one with 2 black wires and one with 2 white wires, blue wire goes to the sender which with the ignition on shows 12v on a test meter. One of the 2 black wires goes to the sender and again with a tester shows common earth. The white wires are live with the ignition on.

.

I really cannot get my head round how this works all the other senders I have replaced are just a gradual earth as the engine gets hot it grounds more, increasing the needle on the temp gauge.

I have tested all the wires and there is no break in any of them the only thing that is confusing me is not being able to understand how it works and why is the blue wire showing as being live.

Please put an old man out of his misery and even if I am being thick I will not be offended in the least.

[Sir Nibble]

This is a bi-coil type gauge. One coil within the instrument is powered directly from the ignition and earthed, the other is similarly supplied but earthed through the sender. This is so that both coils are equally effected by voltage variations so no voltage stabiliser is fitted. The symptoms you describe are due to an open circuit in the sensing side and will either be due to an open circuit sender, wiring or coil within the instrument. The 12V you are seeing is read through the coil and is a good demonstration of the difference between pd and emf, if you connect the sender it should reduce to a lower value, if not the sender wire is open.

[MtB]

This is a bi-coil type gauge. One coil within the instrument is powered directly from the ignition and earthed, the other is similarly supplied but earthed through the sender. This is so that both coils are equally effected by voltage variations so no voltage stabiliser is fitted. The symptoms you describe are due to an open circuit in the sensing side and will either be due to an open circuit sender, wiring or coil within the instrument. The 12V you are seeing is read through the coil and is a good demonstration of the difference between pd and emf, if you connect the sender it should reduce to a lower value, if not the sender wire is open.

 

Well that was interesting, I never knew that. Thanks Snibbs, what an elegant design!

 

 

MtB

[scrunch]

I am forever in your debt Sir Nibble that makes sense now, I am thinking now as I have tested for breaks in the wire and it is a new sender, must be the gauge that has packed in.

[Sir Nibble]

If you can see 12V through the coil then it's not open. Suggest you carefully check the earth connection to the gauge. Don't rely on volt meters to tell you anything unless the circuit is complete, checking voltage on a disconnected cable will allow a massive resistance fault to go unnoticed.

[scrunch]

Thanks again I shall check the earth as you suggest.

[scrunch]

Stlii doing my head in although the temp gauge now doesn't move at all even with the engine hot . I have checked all the wires no breaks , does anyone know how you test the actual gauge itself . Pleas keep it simple convinced now anything to do with electrics belong to the dark arts ? any help much appreciated

[MtB]

I'm no expert even having read and understood Snibble's explanation of how this gauge works. I still see no reason for the two wires on the sender. I'd have expected one, with the sender body being the 'earth' connection.

 

How many wires on the meter? And are any of them connected to 12Vdc?

 

Edit to add a picture. Is your wiring like this?

 

 

And do you have a multimeter? What is the resistance across the two wires on the temp sender (disconnect it first!)

 

MtB

Edited May 4, 2014 by Mike the Boilerman

[scrunch]

Very similar but there is a single blue that with the ignition on shows 12v that goes to sender , then the other wire from the sender goes to the gauge and is connected to a spade fitting with another black wire that shows as being earth . There are 2 more live white wires connected to a 3 rd spade. Cheers for the reply much appreciated

[MtB]

Very similar but there is a single blue that with the ignition on shows 12v that goes to sender , then the other wire from the sender goes to the gauge and is connected to a spade fitting with another black wire that shows as being earth . There are 2 more live white wires connected to a 3 rd spade. Cheers for the reply much appreciated

 

Hmmmm ok, let's go back the beginning. How do we know it is actually wired up correctly in the first place?

 

Did it ever work correctly, or are we trying to get something going with no history of knowing it is wired up right?

 

 

MtB

[matty40s]

I ran a boat this week that showed 100 deg c plus with a mile of tickover cruising, and then rose to 120-30 with gentle canal cruising.

After 16 hours, was the same, probably topped 135 deg c after a mile of aggressive river cruising just to test the fault. Gauge was max 150deg c. Fault is either the sender, or the gauge itself, as no coolant overheating/loss was experienced, and no smell or raise in header tank either.

[scrunch]

Yes it did used to work coming into a lock happened to look at temp gauge and it was right up , stopped the engine checked it but was at normal temp . Did about another 30 hours to get back to my mooring engine still normal temp but gauge still right up .

When I get to work I will try and post some pics .

[MtB]

Yes it did used to work coming into a lock happened to look at temp gauge and it was right up , stopped the engine checked it but was at normal temp . Did about another 30 hours to get back to my mooring engine still normal temp but gauge still right up .

When I get to work I will try and post some pics .

 

Sounds like sender failure to me.

 

Oh....

 

 

MtB

[nicknorman]

I'm no expert even having read and understood Snibble's explanation of how this gauge works. I still see no reason for the two wires on the sender. I'd have expected one, with the sender body being the 'earth' connection.

 

 

No, this type of ratiometric sender has 2 wires, a third connection being the grounding of the sensor through the engine. The 2 wires are 12v and the sensor output which will be somewhere between 0 and 12v depending on the temperature. Inside will probably be a potentiometer - ie wiper running over resistance wire connected across the supply.

 

The point of this is that the output is a percentage of the supply voltage so that, as snibbs says, it is not sensitive to supply fluctuations. Some mid temperature is alway 50% of the supply, so the same temperature would output 6v with a 12v supply, 7v with a 14v supply etc.

 

Within the meter are 2 coils at 90degrees to each other, thus producing magnetic fields in 2 directions. One coil is powered from the supply, the other from the sender. Of course in reality these two magnetic fields merge into one with a direction dependant on the relative strengths of the two fields. The needle is a bar magnet that aligns itself with the direction of the resultant magnetic field. The outcome of all that is that the reading will be constant for a range of supply voltages, since the strengths of both magnetic fields are equally affected by supply variations.

 

This sort of gauge can be identified by a tendency for the needle to float somewhere in the middle when unpowered, rather than being spring loaded to the bottom of the scale.

 

Anyway, I suggest you check the voltage on the 2 sensor wires with it powered up and at a mid-temperature. One should be the supply voltage (say 12.5v) and the other should be in the mid range somewhere around the 6v region. If that is the case, nothing wrong with the sender but check those voltages are also present at the gauge. If so, the gauge is duff.

Edited May 5, 2014 by nicknorman

[nicknorman]

Ps if the gauge is faulty it might well be repairable - if you are good at microsurgery!

 

Inside there will be 2 coils with very fine wires attached to solder terminals. Chances are one of these wires has fractured at the point where it is soldered, due to vibration. It might be possible to reattach the wire. The wire will be coated in lacquer so you might have to remove a bit of that to get a good solder connection. Scrape it with a scalpel blade

[scrunch]

Ps if the gauge is faulty it might well be repairable - if you are good at microsurgery!

 

Inside there will be 2 coils with very fine wires attached to solder terminals. Chances are one of these wires has fractured at the point where it is soldered, due to vibration. It might be possible to reattach the wire. The wire will be coated in lacquer so you might have to remove a bit of that to get a good solder connection. Scrape it with a scalpel blade

Cheers for that makes a lot more sense now , 1 terminal was showing 12v the other showing 3.6v so I have taken the gauge off to have a look.

Thank you very muchly.