mclarenross Posted May 1, 2015 Share Posted May 1, 2015 Guys, I took my Supra out for a blast last night as its finally back on the road, but I think its running a little hot. around 115 degrees... here is my set up; New alloy radiator Toyota red coolant just been drained and refilled, (have run her upto temperature twice to bleed the system) new thermostat installed External overfill bottle (does this need to be vaccum sealed?) Recently rebuilt head, with new Gaskets all round including HG now the dial on the dashboard is perfect, sits exactly where it should in the middle of the gauge. However I've fitted an aftermarket sensor in the upper pipe as it goes back into the radiator and last night it was showing temperatures of around 115 D, which I know will be hotter than the OEM sensor gauge as its at the hottest point the coolant would be, but I believe this is hotter than it should be...? Should I be worried? should I drain and try again? could it still be an airlock? Quote Link to comment Share on other sites More sharing options...
Dnk Posted May 1, 2015 Share Posted May 1, 2015 The expansion tank should not be sealed, as the hot coolant goes in the air inside needs to go out through a vent hole, then as it cools the coolant gets drawn back into the rad and air needs to get into the expansion tank Is your new gauge correct ? What temp is the stat rated at, what pressure rating is your rad cap ? Quote Link to comment Share on other sites More sharing options...
marc_p Posted May 1, 2015 Share Posted May 1, 2015 Is it a proper gauge and sensor? (i.e. Stack, Defi, etc, etc,) Or an ebay jobbie? Secondly, as you say, that is the hottest place, so I'm not sure what temps are supposed to read there, so could be normal. Quote Link to comment Share on other sites More sharing options...
mclarenross Posted May 1, 2015 Author Share Posted May 1, 2015 Great, the expansion tank isn't vacuum sealed so that's good news, I'm hoping its potentially the gauge, but it wasn't a cheap one( can't remeber the make at the minute) and the cap is 1.1 bar Quote Link to comment Share on other sites More sharing options...
chrispot Posted May 1, 2015 Share Posted May 1, 2015 i would say it's the gauge / sensor. what i would do to confirm is take sensor out place in container with a known good temp gauge and check what gauge in car reads in relation, try to get water hot as possible like a just boiled kettle?. if car gauge is out you could then put a small adjustable resistor inline with it to trim gauge (not ideal but it works) so both temp gauges read the same. the resistor would have little effect and lower temps and more at higher but gauge would still show a higher temp. regards chris Quote Link to comment Share on other sites More sharing options...
mclarenross Posted May 1, 2015 Author Share Posted May 1, 2015 Great I'll test the sensor this evening, is it common for the sensors to be so far out? Quote Link to comment Share on other sites More sharing options...
chrispot Posted May 1, 2015 Share Posted May 1, 2015 i don't know about sensors being far out, not had many to check, but like everything else there are tolerances, gauge can be out by 5% one way the the sensor by 5% ( not actual figures just made) so could show for example 90 to 110 when temp is 100. i had to mod my cheap ebay gauges to read right (but i like them). my temp gauge read about 7 degrees higher. same problem with oil and fuel pressure, but egt reads right but egt sensor work in a different way, not resistance as most temperature car sensors use but instead uses millivolts produced from the sensor. let me know what happens and can advise on what to buy and how to wire etc. thats the trouble with gauges you would have been happy with the gauge in the car and everything shows normal. To coin a phrase "worry gauges" Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted May 2, 2015 Share Posted May 2, 2015 Trust the stock sensor and gauge. Most El Cheapo aftermarket ones can be miles out. I find the Defi stuff particularly terrible, in quality and accuracy. Quote Link to comment Share on other sites More sharing options...
mclarenross Posted May 8, 2015 Author Share Posted May 8, 2015 i don't know about sensors being far out, not had many to check, but like everything else there are tolerances, gauge can be out by 5% one way the the sensor by 5% ( not actual figures just made) so could show for example 90 to 110 when temp is 100. i had to mod my cheap ebay gauges to read right (but i like them). my temp gauge read about 7 degrees higher. same problem with oil and fuel pressure, but egt reads right but egt sensor work in a different way, not resistance as most temperature car sensors use but instead uses millivolts produced from the sensor. let me know what happens and can advise on what to buy and how to wire etc. thats the trouble with gauges you would have been happy with the gauge in the car and everything shows normal. To coin a phrase "worry gauges" So managed to check the thermometer sensor today and it's about 18-20 degrees out, could you let me know what I need to buy/ wire to correct it? thanks Quote Link to comment Share on other sites More sharing options...
chrispot Posted May 8, 2015 Share Posted May 8, 2015 yes i can do, have you got a digital multimeter? if yes try to measure the sensor at about 25c as this is the standard by which thermistors have there resistance stated at. just so you know and others that are reading this, a thermistor is a temperature dependant resistor. which means as temperature changes so does the resistance. there are two types of thermistors neg temp coefficient, which means as the temp rises the resistance falls which is the most common used. i let you work out the positive coefficient type? there are different types of resistance to temperature curves, but for the most part they all follow roughly the same change. ( i not a thermistor expert so please don't slag me down) because your gauge is reading hi and uses a neg temp coefficient sensor, we need to make it read lower, so what's needed is to place a resistor inline with the thermistor ( temp sensor) which will increase the total resistance and the gauge will think thats it at a lower temp ( higher resistance). a value of about 200 ohms should be enough but would need to know what your sensor reads to give a better guess at what resistance to use. because of the range that a sensor goes through it could be say 10.000 ohms at 20c and 500 ohms at 100c ( made up figures) and you may be aware adding a 200 ohm resistor would make very little difference when cold. this is just a rough guide. i don't have time now but will show diagram of how to wire up and use. let me know what the sensor reads cold (25c) regards chris Quote Link to comment Share on other sites More sharing options...
mclarenross Posted May 9, 2015 Author Share Posted May 9, 2015 I broke my last multimeter, so I'll need to buy another one., could I just buy one of these instead? http://www.ebay.co.uk/itm/US1-Universal-Water-Oil-Temperature-Sensor-1-8-NPT-Thread-For-Aftermarket-Gauge-/121577654560?pt=LH_DefaultDomain_3&hash=item1c4e97c920 Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted May 9, 2015 Share Posted May 9, 2015 No, you MUST use the correct sender!! Buy another cheap meter, far less costly than "trying" a new sender. Quote Link to comment Share on other sites More sharing options...
chrispot Posted May 9, 2015 Share Posted May 9, 2015 personally i would not. it could make the gauge miles out. if what i have wrote sounds complicated don't worry, i was trying to give you a understanding how temp sensors work. i will post a drawing of how to fit variable resistor, which is easy to do and give a parts list of items needed the reason for finding your sensor resistance is because to trim gauge reading you will be using a rotary dial. and using the wrong part could it harder to adjust. for instance it be just a quarter turn that moves your gauge from full to to showing the lowest temp rather than using the full range of the dial. i will give details a bit later as i using my tablet to post this, and when i can i will use my pc äs i find it much easier to type and draw etc. have you got a soldering iron? regards chris Quote Link to comment Share on other sites More sharing options...
mclarenross Posted May 9, 2015 Author Share Posted May 9, 2015 Thanks, yeah I've got a soldering iron. Cheers Quote Link to comment Share on other sites More sharing options...
chrispot Posted May 9, 2015 Share Posted May 9, 2015 No, you MUST use the correct sender!! Buy another cheap meter, far less costly than "trying" a new sender. as chris says buy a cheap digital multimeter, because it could come in very handy if you repair you own car. or if your not sure on using it then someone helping you repair car could use it? buy the way chris i like your name. regards chris. Quote Link to comment Share on other sites More sharing options...
chrispot Posted May 9, 2015 Share Posted May 9, 2015 good news on soldering iron. i will check later if i have got a trim pot you could use, more than likely i have which you can have for nothing and i can post to you? if not they are only pence to buy. regards chris Quote Link to comment Share on other sites More sharing options...
chrispot Posted May 9, 2015 Share Posted May 9, 2015 (edited) sorry mclarenross been very busy today sorting daughters car, then over to see me old man, only just got back home not had dinner yet. Anyway good news iv'e got a pot that you could use for gauge. but i will not have the time tonight to show wiring etc. as said before it's very simple to do, just fits in between the gauge and sensor. i will post up sun a diagram on sunday, and before i waffle on about thermistors etc do you want to know or just sort gauge? regards chris. ps. i will not be offended if you don't care about sensor and just want gauge sorted? Edited May 9, 2015 by chrispot (see edit history) Quote Link to comment Share on other sites More sharing options...
mclarenross Posted May 9, 2015 Author Share Posted May 9, 2015 Hey Chris, thanks for all your help, I'm still trying to get my head around what a trim pot is... :-) so I take it that you solder it in on the sensor wire somewhere between the gauge and the sensor and that reduces the reading? I may be miles off, I never got my head around electrical problems ohms / volts etc... Had a nightmare when I was trying to wire in a resistor pack for the injectors! Quote Link to comment Share on other sites More sharing options...
chrispot Posted May 9, 2015 Share Posted May 9, 2015 i will show you in a basic terms to get a rough understanding of it all. don't worry, this mod is very easy to do. and yes you are right where it goes and what it does. anyway must get out of bath now, i have cleaned me nuts enough now:D. now going to have something to eat. regards chris Quote Link to comment Share on other sites More sharing options...
chrispot Posted May 10, 2015 Share Posted May 10, 2015 as promised Fig A if two resistors are placed across a voltage the output will depend on resistors values Fig B & C you can see the voltages output for different values used Fig D is a resistor connected to a thermistor (temp sensor) and as you know a thermistor changes it's resistance value according the temperature. this resistance change will change the output Fig E is a variable resistor sometime called a pot which is short for a potentiometer. voltage and also be expressed as a potential difference, and we can use this variable resistor to split it up as in figs A, B, and C or to meter voltage hence name potentiometer. Fig F if the pot is connected as shown it resistance will range from 0 to the value of the pot. Fig G is your existing setup which reads too high and we need to increase resistance to lower the temp shown on gauge. also have shown resistor and output in gauge. Fig H is what you need to do, allowing you to adjust gauge ( can only make gauge read lower). once the value is found it could be replaced with a single resistor if needed? for anyone that knows this stuff please don't pull it to bits as it's a rough guide to help someone. Thanks please note the voltage will decrease with increasing temperature but gauge will show a higher temp. Quote Link to comment Share on other sites More sharing options...
chrispot Posted May 10, 2015 Share Posted May 10, 2015 because i don't know the value of your sensor i would use a 1k pot. it could be a trim pot as this http://www.ebay.co.uk/itm/X-2-pcs-finger-adjust-1k-potentiometer-horizontal-preset-trim-pot-trimmer-/251211605363?pt=LH_DefaultDomain_3&hash=item3a7d60e573 Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted May 10, 2015 Share Posted May 10, 2015 Was your forum user name chosen because you are potential divider aficionado? Quote Link to comment Share on other sites More sharing options...
chrispot Posted May 10, 2015 Share Posted May 10, 2015 (edited) Was your forum user name chosen because you are potential divider aficionado? NO! it's because I was once a drug baron than knows a bit of electronics . Spy buys, pinhole cameras where used to catch my dealings i would then half of what they saw and heard with my knowledge of pots. (they would only prosecute if over 1kG). Eventually got caught when i got into op-amps, made the mistake of amplifying what they saw. got done for 10 ton of the stuff! Edited May 10, 2015 by chrispot (see edit history) Quote Link to comment Share on other sites More sharing options...
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