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Dart Temperature Gauge

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heh101353
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Re: Dart Temperature Gauge

Post by heh101353 »

@syd
clear as mud.

I already checked if Ohms law applies to my Dart as well. Did not take long. Result positive. Despite positive earthed ... ;) ;)
Fiddling with the wiring has to wait.


@John
I guess I would have dropped dead on the spot running into you, by coincidence, somewhere along the route of "Grand Tour of Switzerland".
A spectacular run where you get a thorough look at the beauty of the country.
Best time to do it is either mid summer (your're not freezing to death up on the Alpine passes) or early autumn as weather is mostly pleasant and stable and still not too cold up there.
But you'll never know!

Spa 2021 has to take place no matter what! :twisted:

Hans
:o owner since 2015 :mrgreen:
https://www.wyhe.ch/

daimlersteve
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Re: Dart Temperature Gauge

Post by daimlersteve »

Re temp sender ---- Whats the thread in the manifold ?

daimlersteve
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Re: Dart Temperature Gauge

Post by daimlersteve »

found it

Ian Slade
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Re: Dart Temperature Gauge

Post by Ian Slade »

You have a bad earth on the instrument circuit, or your battery is dying if the engine is not running with headlights on or any other load and the ammeter is showing balance or charge the instruments readings shouldn't change.
Owner since the 70's, Genghis is slightly to my left.

Petelang
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Re: Dart Temperature Gauge

Post by Petelang »

Simple test to get to the bottom of this conundrum is to do a volt drop test.
Take a voltmeter. Connect positive to the earth point that is on the instrument case and negative to battery new post. If your wire isn't long enough use a jump lead to bridge the gap (it offers negligible or zero resistance due to the volume of cable).
Switch on all loads, wipers, heater, fans etc.
If you get a reading then that is the amount of volt drop (aka resistance) over the earth path.
It should be almost zero.
If it's not, you need a thicker core cable to the earth point, or from earth point to the battery negative.
If your car is positive earth, reverse meter connections.
You can try cleaning up the earth connection and see if it makes any appreciable difference.
The longer the cable route, the thicker the wire should be, or the more current passing.
It's always good to quantify this to know if there's an induced fault (like a corroded terminal) or a built in problem due to wrong cable sizing.
Peter
Peter Langridge
Cloud Nine Classic Weddings, Nottingham.

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heh101353
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Re: Dart Temperature Gauge

Post by heh101353 »

I investigated a bit further
The described phenomenon (needle hitting the limit stop) only occurs when both conditions below are met
  • coolant temperature >=160°
  • engine is running
It never occurs when the engine is off (dynamo not running) , ignition on of course. In that case coolant temperature does not matter at all.

I did a small experiment. Instead of hooking the fan earth directly to an earth point on the chassis I hooked it up to the temperature sender at the thermostat housing.

Image
I omitted the relay in the diagram for clarity.

Closed the switch on the dashboard. Fan not running.
Went for a run and observed the Temp. Gauge as best as I could.
As soon as the needle on the gauge was halfway up to the curved line I stopped. Fan not running. Temperature of the thermostat housing 150°.
Engine idling for a few minutes the fan came on. Just a small twitch of the needle was all there was to be seen. Temperature of the thermostat housing 163°.
So why is there such a spike when the fan is actuated by the dashboard switch?

Hans
:o owner since 2015 :mrgreen:
https://www.wyhe.ch/

Ian Slade
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Re: Dart Temperature Gauge

Post by Ian Slade »

Your dash switch is switching the live as in the case of+ve earth the -ve, what is the sender switching? It would normally switch the earth as the casing is normally earthed, try switching the dash board switch on the earth line.
Owner since the 70's, Genghis is slightly to my left.

Brian-H
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Re: Dart Temperature Gauge

Post by Brian-H »

There's a "bad" connection between the thermostat housing and the battery +ve (earth). It could be anywhere, most probably between engine and chassis or possibly between chassis and battery +ve.

As before, the high current causes a voltage drop. It's actually "described" by Thévenin's theorem https://en.wikipedia.org/wiki/Th%C3%A9venin%27s_theorem

In this instance, in your circuit diagram 2, you need to insert a "fictitious" resistance between the common "T" point and the ground symbol. It's possible that the resistance will decrease as the fan current flows, causing a changeable voltage drop (this resistance can be viewed as a Thevenin source in the ground circuit, which varies in voltage).

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heh101353
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Re: Dart Temperature Gauge

Post by heh101353 »

Brian-H wrote: Thu Jul 16, 2020 7:41 pm There's a "bad" connection between the thermostat housing and the battery +ve (earth).

As before, the high current causes a voltage drop. It's actually "described" by Thévenin's theorem https://en.wikipedia.org/wiki/Th%C3%A9venin%27s_theorem
You are absolutely right. According to the theorem it think it does not matter at which side (live, earth) the switch sits.
The experiment with the temp sender was just adding a new unknown to the equation. Silly me.
Besides there was a severe voltage drop when switching the fan on.

To prove the point I ran a 2.5mm2 wire from the earthing point of the gauges directly to the kill switch which sits between the battery and the earthing point of the chassis.
Both temp gauge and fuel gauge don't alter the reading at all now when adding a new load to the circuit.

To those who opted for bad earth connection "Thumbs Up".
Thanks to all of you sharing your advice!

Hans
:o owner since 2015 :mrgreen:
https://www.wyhe.ch/

Brian-H
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Re: Dart Temperature Gauge

Post by Brian-H »

Just a bit more.

As well as being able to convert a network with multiple voltage sources and resistors to a single voltage source, where there is actually a single voltage source in a network with multiple resistors, it is also possible to transform" some of the resistors to voltage sources. In effect this is one way of imagining what a bad earth does.

Note also that, as well as Thevenin's Theorem, there's also Norton's Theorem https://en.wikipedia.org/wiki/Norton%27s_theorem which says the same thing but for current sources instead of voltage sources. Furthermore, it is also possible to "transform" voltage sources to current sources and vice versa https://en.wikipedia.org/wiki/Source_transformation . I've never been on a automotive electrics course, but I'd think that if these theorems are not taught explicitly, they would be implicit. From the very limited exposure that I've had to heavy electrical engineering (power stations and power grid etc) this is frequently done in modelling the power grid network but using impedance and reactance instead of just pure resistance.

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