Outer Limits 4x4 Board

Im wondering if anyone knows about the temperature sensor circuit. When i measure the wire coming from the zook, into the temp sensor, then into ground thru sensor, the voltage is all over the shop, even with the sensor disconnected. It seems to vary constantly and randomly between about 3.5volt to 0volt, and sometimes open circuit, is this normal? Shouldnt it be a stable voltage according to temperature?

Also, 2nd question, does anyone know if the temperature sensor rises in voltage as the engine gets hot, or does it drop in voltage when the engine gets hot?

The first bit sounds quite bizarre, not sure whats going on there..
The second bit... I don't think there is any voltage change... It's a resistance thing isn't it?

it is a voltage and resistance type of thing , generally the ecu will input between 4-5 volt into the sensor , as temp increases resistance gets higher so less voltage input is fed back to the ecu.

If the input voltage varies dramatically , you have an issue.

suzimad wrote:it is a voltage and resistance type of thing , generally the ecu will input between 4-5 volt into the sensor , as temp increases resistance gets higher so less voltage input is fed back to the ecu.

If the input voltage varies dramatically , you have an issue.

ECU??? I didn't realise we were talking about a car equipped with a computer... Disregard my post if this is the case.

Whilst not a specific zook answer the instrument regulators on vehicles of Sierra ( and Vitara ?? ) age are usually bimetalic switching types, they switch on and off to maintain a constant power to the temperature measuring circuit. As the temperature gauge is also bimetalic it all averages out over time and you get a constant indication that averages out the regulator switching.

If the regulator wasn't there you would get indication errors caused by battery voltage changes.

Use a multimeter on the circuit and you wonder what the hells going on....

P.S. same for the fuel gauge system.

Damn, so it looks like i wont be able to use a voltage sensing circuit like the one from Jaycar to control my thermo fan.
I might just go with my original plan of removing the inlet manifold, drilling out the temp/vacuum switch on other side of thermostat housing from dash gauge sensor, and retapping with metric thread, then fitting a thermo switch, the ones that switch on at 95 and off at 90. Is on at 95 too high, should i get one that switches on at slightly lower temp?

erm if you dont have a ecu , wheres the voltage coming from , i assumed since you were talking voltages to sensors that there was an ecu involved , now i am confused ....

There is no ecu, the power must come from the gauge i reckon and grounds through the sensor

Ah - that's what your up to....

No the operational principle of the system is different as stated. You could probably modify the voltage switch to do the job with a thermistor sensor connected in a voltage divider arrangement. If you do "redesign", make sure the voltage divider picks up its supply post regulator otherwise your operating points will vary with battery voltage.

The other option is just to use the " simple temperature switch kit ".

You MAY get away with fixing the thermistor to the outside of the engine water discharge but I recycled another sensor housing and inserted it into an existing threaded hole ( not zook ).

The relay in the kit is way too small so it will need to control an automotive power relay that then switches the fan.

It was a good idea however... you just didn't know a few things.

Yeh i was going to run a larger relay i knew that lol.
Im thinkin, could i fit another standard cheap temp sensor like the one thats already in it, then use something like the 78L05 to regulate the voltage to 5volt for supply to the sensor (if the battery voltage drops below 5volts and starts effecting sensor readings, it doesnt matter as i would be screwed anyway), so then i have a stable supply to the sensor.
Im not sure if that will work though, as the device would keep regulating the voltage to 5volt i think as the sensor changes resistance.

Unless theres another similar way to that, i think i might just go with the caveman temp switch as i mentioned earlier.

If you want to use another temp sensor and a regulator then you need to run the reg o/p through a resistor before feeding it to the temp sensor. Then the voltage drop at the sensor will vary. You might need to experiment as to what resistor value to use.

Hey thats not a half bad idea, would i need a 5watt resistor heatsinked you reckon? Thinkin about it a standard 1/2w would do i think, as the 5v regulator is 100ma so it couldnt put out that much juice.

Connect Vin to the 8 volt regulator output.

Buy a thermistor - keep the value up to minimise the current drain and self heating. With a 10k thermistor and a matching 10k fixed resistor the current drawn from the eight volt supply by the divider chain is 4 microamps - SFA.

Remove either of the 1M voltage divider resistors on the input and connect the thermistor there via flying leads ( thats your temp sensor ).

Replace the other input divider resistor with a fixed resistor about the same resistance at room temperature as the thermistor.

Depending on which divider resistor you remove and whether its a PTC or NTC thermistor you will need to swap the L/H - H/L link.

Done - enjoy.

Mightymouse youve lost me, what 8volt output? Where are these resistors, which kit you talkin about?

I didnt really want to use a thermistor as i would have to find somewhere to put the damn thing.

I'm talking about the "simple voltage switch" kit - which is what I assume you have.

Look at the schematic, you will see its powered by an 7808 eight volt regulator ( REG 1 ) so there's your eight volt regulated supply. For example you could tap off the + side of the 10microfard / 16 v electro.

Resistors - look in the top LHS of the schematic, follow Vin and the first components you will see are two 1M resistors - that's them.

Oh and I forgot if you have to change H/L - L/H you need to reverse D3 as stated in the instructions.

As for thermistor mount but your original idea isnt going to work so your just going to have to find another option.

Ok i understand u now, so the thermal sensor i was planning on using would sink too much current would it, is that why you dont think i can use it?
Thanks for youre help with this too, its appreciated.

The "thermal sensor" you have is almost certainly a thermistor - just like we are discussing.

Check its resistance with a multimeter, it should change resistance as you heat it. Its almost ceretainly going to be a NTC thermistor - its resistance will fall as its temperature increases but it doesn't really matter as long as it changes.

Use Ohms law to figure out if its going to draw excessive current.

Remember if its say 500R then the other resistor in the circuit could be 500R - giving you a grand total of 1K ( series )

I=V/R........8/1000 = 0.008A, ie 8mA not a problem for the circuit....

I measured the temp sensor at cold (room temp) and it read something really high like 530k ohms, then when the engine started warming it jumped to 350ohms and gradually fell as the engine warmed to hot to 47ohms, that would be ok wouldnt it?

PJ.zook wrote:Damn, so it looks like i wont be able to use a voltage sensing circuit like the one from Jaycar to control my thermo fan.
I might just go with my original plan of removing the inlet manifold, drilling out the temp/vacuum switch on other side of thermostat housing from dash gauge sensor, and retapping with metric thread, then fitting a thermo switch, the ones that switch on at 95 and off at 90. Is on at 95 too high, should i get one that switches on at slightly lower temp?

Fit a sigma or 929 thermo housing and you get an extra temp sensor so the original temp circuit remains 'factory'

Note in the pic below the housing is angled different to a zook and you may need to frig with hose a bit

Its a bit lower than ideal, if the second resistors around the same value - ie 94R total divider resistance then the currents around 85mA. For the regulator without a heatsink its a bit higher than I'd like.

Assuming its a sensor where the case forms the second electrical connection - ie its grounded through the case.....

The sensor need to replace the 1M resistor to ground and use say 220R for the first resistor. This gives a current of 30 mA. A small heatsink bolted to the regulator tab would keep things safe - just don't make it heavy otherwise the off road vibration might be too much for the regulator leads to handle causing them to snap off or add some mechanical support to the heatsing/regulator.

This system will result in the electronic seeing a voltage that falls as the engine heats up - so your going to have to set the system up for High - Low transition

If of course you use a different sensor you will need to check its resistance as this does matter.

OK thanks for that Mightymouse, its a great help. I will let you know how it goes.

No problems, remote diagnosis / advice is never easy - hope it works.


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Lots of good trips, knowledgeable members and first hand help

Don't davis craig make a adjustable sensor that clamps into radiator hose to control there fans , uses a piece of extra rubber to seal arround probe sesor wire , Cheers Paul.

Yes they do.

I and several others have had problems with them under off road conditions. In my case the bulb in the top hose actually broke off, and wasn't much fun to fish out again. Have also seen leaks around the rubber insert required for the capillary tube - easily fixed with some silicone but it makes the hose difficult to remove in future

There are other vehicles with better mechanical thermostats IMO, but its a bit of a lottery as to which one suits particular applications as unlike the Davis Craig units they have a fixed operating temperature and require a more permanent installation.

I used a Honda Civic one for years on a road car and it proved very reliable, but it was selected mostly because it screwed straight into an existing plug on the thermostat housing.

I'm not advocating any particular solution - just trying to help PJ.zook solve his problem.

Davies craig stuff is higly over rated. I bought a DC electric water pump for my commy and nearly fell over when I got a closer look at it. I sold that on ebay for more then I bought it for and bought a CVR.