Aux channel used for Cooling Fan

NITROPIXIE · Post by **NITROPIXIE** » Sun Sep 28, 2008 4:41 pm

Hi all

I'm trying to find away of cooling the engine (for a mini), well operating the radiator fan to be exact. I've tried using the fan switch built into the radiator but to no avail. It isn't the original radiator for the car (off a fiat cinqucento).Currently i am running a temperature gauge (smiths), off 12v, to the sensor in the cylinder head.

If i used the megajolt to control the fan operation by monitoring the voltage between the gauge and sensor, would this be ok?? and am i correct in that i have to make an alteration to the circuit board by removing a track and then set up the gauge in the software so it will read the correct temp approx.

Ryan

Post by **brentp** » Tue Sep 30, 2008 6:19 am

Hi Ryan,

If you are using a resistive temperature sensor (known as a "thermistor") then you can usually connect that directly to the input using this diagram.

http://www.autosportlabs.net/MJLJ_V4_ve ... iary_Input

You would not need to perform the hardware modification reserved for 0-5v input.

Tell us more on the temperature sensor in your cylinder head. Does it have one lead or two? If it has one lead, then that lead would go to the Aux input of the MJLJ; the other lead is the sensor body, which goes to ground.

Once you have this hooked up you should see the aux input gauge vary based on temperature. It won't be 'calibrated' to actual degrees, yet; this can be performed as a next step.

Regards-

NITROPIXIE · Post by **NITROPIXIE** » Tue Sep 30, 2008 11:41 am

Hi Brent

Yeah i am using a single wire thermistor sensor. The body of the sensor does go to ground. What i am really concerned with is can i run my Smiths temperature gauge aswell as splice the wire between the thermistor and the gauge to the megajolt unit to control the cooling fan.

I'm thinking the Smiths Gauge works as a current sensing device so would the megajolt see a difference in Voltage as temperature increased??

Thanks

Post by **brentp** » Tue Sep 30, 2008 1:45 pm

Ryan,

Your gauge most likely measures the variable resistance of the sensor and translates that to a movement of the needle on the gauge.

If you have a volt/ohm/amp meter, you can set it to the ohm setting and measure the resistance between the sensor wire and ground on the engine, with the gauge disconnected.

You should see one reading when the engine is cold, and see it change as the cylinder head temperature goes up. The temperature will go up or down depending on the type of sensor.

The MJLJ will measure the sensor in a similar way. The next question is, will connecting the MJLJ and gauge to the same sensor throw off the reading on the dashboard gauge. It will require a bit of testing and experimentation on your part. For example, See if your gauge jumps or moves significantly if you see If you connect the both the gauge and the MJLJ to the same sensor.

In the end, you may be able to piggy-back both the gauge and the MJLJ off the same sensor. If not, you will need to find a spot for a secondary sensor on your engine (or maybe your radiator, if there's a fitting/plug for such a thing!)

Let us know what you find out!

NITROPIXIE · Post by **NITROPIXIE** » Tue Sep 30, 2008 2:29 pm

I will give it a go then. Can't do it anytime soon though as i have just chopped the end of my finger off in the fan belt last Thursday. Stupid me, others be-very-ware!!!!

Maybe the best way would be to leave the engine running with the thermistor wire connected to the temp gauge and not the megajolt. Then using a fluke (multi) meter see if there is a variance in potential difference across the thermistor. If there is a change in pd what then?? Does the MJLJ have an input resistance similar to a fluke as in Mega Ohm?? I would expect the thermistor for an old A series engine to be a few hundred Ohms so wouldn't affect the signal much at all. would i then have to break the track on the circuit board as it will be a 0-5v signal?? well really 0-12v

I think i will browse the web since i cant get to the workshop, but i suspect the Smiths Temp Gauge is a current measuring device, to measure the variance in resistance and may cause a problem but will see. Wish i could draw my diagrams on these stupid computers, maybe oneday i will figure out how to, makes things so much easier to describe.

Post by **brentp** » Tue Sep 30, 2008 6:10 pm

Hi Ryan,

Sorry to hear of your finger! I hope you didn't loose too much- did you have it sewn back on???? Yikes!

Keep in mind 'current sensing' is how this type of sensor is read: As the sensor resistance changes, so does the amount of current flowing through it.

The MJLJ has a high input impedance on the aux input channel, but likely not as high as the meter. As before, careful testing will lead you in the right direction. As always, post more info, questions, even pictures if you need- we're here to help.

Regards,

spetom · Post by **spetom** » Sat Oct 04, 2008 7:00 pm

Sorry - all I seem to do nowadays is butt into your threads, Ryan.

As I have a similar engine / MJ setup, I thought you might like to know the results of my experiment with an A-Series single terminal temperature sender (exact model unknown) and the Aux Input of the MJ. Although 'experiment' isn't really the right word... since I simply connected everything up and crossed my fingers.

Splitting the sender wire before it reaches the gauge (also a Smiths), I found no discernible difference on the gauge's reading.
However, the MJ results weren't what I was expecting - the reading, although stable when the engine wasn't running, became completely unreadable once it started to warm up - it jumped continuously from 0 to seemingly random amounts.

I haven't done much more experimentation because although it's fitted and tested, I'm not running the MJ full-time until I can get a basic tune up sorted out. Perhaps a good earth source, using a two terminal sensor might make a difference?

Ken555 · Post by **Ken555** » Sat Oct 04, 2008 8:49 pm

spetom wrote: it jumped continuously from 0 to seemingly random amounts.

Remember that older Smiths gauges, are fed from a very basic 12V regulator, that relies on a bi-metal strip to break the circuit, and then remake it, to reduce the 13.8V to 12V.

You will never get a steady reading from it.

NITROPIXIE · Post by **NITROPIXIE** » Sat Oct 04, 2008 9:59 pm

Spetom, don't worry about butting in, its good to share ideas. I've not had chance to experiment yet with trying to get megajolt to read temperature , as i've not managed to try to start her with a partly missing finger. I fear from what you have said that the Smiths temperature gauge is current sensing (operates like an ammeter) which is what i am afraid of. There are 3 options here.

Option 1 - Don't use megajolt to sense temperature, just leave it to the smiths gauge.
Option 2 - Don't use the Smiths Gauge and get megajolt to read temperature instead, although not really an option as i would like to see if my engine is over heating when driving
Option 3 - Find a resistor of similar value to the thermistor when its cold, hook it in series to the thermistor, to 12v. This should then achieve a better voltage signal than with using a smiths gauge in series with it, and should be more stable. Then make a simple bargraph led display to indicate temperature.

If your readings were all over the place it could be from the alternator not producing a smooth wave and the sampling rate of megajolt means it will take voltage readings that look all over the place. or something strange with the gauge and thermistor setup. Will try to investigate more whne i can.

Ken555 - Now i maybe very wrong but when i have hooked mine up, as i converted from later dials to early, the 12v supply in the haynes comes from the ignition switch side of things than the reguator on the clocks. I wondered if it work better with the regulator, probably be alot more stable? I was also playing with the idea of introducing a more modern semiconductor voltage regulator but i think that maybe a lil overkill.

mwman · Post by **mwman** » Mon Oct 06, 2008 6:51 pm

Two thoughts on this, one install a 10 volt three terminal regulator in place of the bi-metal regulator.
Using two Shockey diodes, one in the path from the gauge to the sensor, one from the megajolt to the sensor.
I have not tried it myself but I face the same problem when I get my setup installed, this was my way of trying to solve the problem!

Simon

NITROPIXIE · Post by **NITROPIXIE** » Mon Oct 06, 2008 7:15 pm

Ken555 - had another look in the old haynes and you were right the temperture gauge does run from the 12v regulator, so that was something i have corrected.

mwman - some modern regulator would be a good idea although 10v might give you a low reading on the gauges, not so good if its water temperature, what about taking one from a more modern car?? surely these would be usable and reliable?? Will see what i can find next time i go down scrapyard.

I see where your going with the diode business, but i'm not overly convinced. There are some electronic experts on the base i work at, so will have a chat with them as they know what happens with voltages, currents and a like but i have a feeling it wont be that simple. Somewhere along the lines of if a thermistor is at a steady resistance then current will be constant provided voltage is. Now since the smiths gauge is current sensing then all current has to go through gauge for it to function correctly. If a current comes from another source ie megajolt then the gauge wont work properly as it will have less or no current. Current will always take path of least resistance

Post by **brentp** » Mon Oct 06, 2008 8:16 pm

Another thing to base-line your experimentation is to get a second sensor and get it to work with that, then work backwards toward your intended goal.

Or, you can just run two sensors in parallel, if you can find a spot on the cylinder head to mount it.

spetom · Post by **spetom** » Mon Oct 06, 2008 9:00 pm

I may be wrong, but I think the standard voltage stabiliser is supposed to put out 10V, not 12.

You can get fairly cheap semiconductor-based replacements for classic stabilisers that output a constant 10 rather than the on-off of the bi-metallic type. I'll look into getting one and switching them.

NITROPIXIE · Post by **NITROPIXIE** » Mon Oct 06, 2008 9:06 pm

I'll have a check tomorrow with my regulator on the car as it is. Could be onto a nice reliability mod here for older cars that usually gets overlooked. Mind you do they go wrong that often??

Alexander_Monday · Post by **Alexander_Monday** » Tue Oct 07, 2008 1:45 am

According to http://home.mindspring.com/~purlawson/f ... ilizer.pdf the Smith's
does indeed approximate 10 volts. In that PDF, it shows replacement with a 10V regulator.

This is similar to the VW VDO fuel gauge "regulator". The VDO puts out a square wave with a little
less than 45% duty cycle at 12V with the duty cycle increasing as the voltage drops. I was installing
a capacitive style sender in a custom tank, and the square wave was wreaking havoc with the digital
chip in the sender. I replaced the mechanical regulator with a 7805 regulator.

The square wave would indeed make it impossible to accurately sample with the aux input without
good low pass signal conditioning. If you do replace the regulator with a 7810 or similar, something
else to be concerned about would be if you need to make a voltage divider circuit and or zener or
crowbar protection to prevent a possible over voltage of the microprocessor input pin?