Bill, nice big shop, is that a hydraulic tubing bender and big blast cabinet I see?
I think I am going to go with your original chart for now, it is somewhat close to the newest one. Although the newer one gives across the board lower resistance the curve is similar enough. And since we are working with theatrical gauges right now anyway it should be close enough.
main reason being the original corroborates CDans statement that the beginning of the flat spot is around 140F, after a lot of assumptions about this circuit on my part that makes for the beginning of the balance point of the circuit at 75 ohms on the thermistor, your original chart puts it at 135F for 75 ohms witch is very close, also that chart is very wide with temps from 60 F to 270 F making it useful for calculations.
That adds another piece to the puzzle, with the zener diode removed and using the original 75 ohm R1 resistor the gauge would center @ 140 F and travel towards hot from there, the value of the resistor R1 will have to be brought down. Possibly somewhere around 15 -25 ohms.
I think for my testing of the gauge I will get a cheap electric hot plate, they are generally weak and may be more controllable than a propane stove, I think I will heat a pot full of ATF (cheap low viscosity oil), the added mass should slow things down some also, agitation would be best to mix cooler and warmer layers of oil but not sure how to easily do that besides the occasional stir, I like the idea of the plywood, it makes a place to mount everything and keeps heat in witch should help accuracy. Wood also does not conduct much heat nor electricity.
I was originally thinking of trying to use 50/50 mix of coolant and distilled water as it is the medium it will use when installed in the engine, also coolant would circulate and conduct heat better than oil but its boiling point at one atmosphere is between 223 to 227 F depending on who you listen to. And somewhat less at my 900 foot elevation. not high enough to put the gauge though its paces, I cannot think of an easy method for raising its pressure and still getting the sender in there, a pressurized vessel full of hot liquid and a cobbled together garage test just does not sound safe. Also coolant will change concentration over the testing period as the water evaporates
.
In the FSM:
2. INSPECT RADIATOR CAP
………..Using a radiator cap tester, pump the tester and measure the
relief valve opening pressure.
Standard opening pressure:
74 – 103 kPa (0.75 – 1.05 kgf/cm2, 10.7 – 14.9 psi)
Minimum opening pressure:
59 kPa (0.6 kgf/cm2, 8.5 psi)
HINT:
Use the tester’s maximum reading as the opening pressure.
If the opening pressure is less than minimum, replace the radiator
cap.
so average is 12.8 PSI, at that pressure 50/50 coolant boils at…..umm I don’t know, but it is less than 265 F that it boils at under 15 PSI of pressure, so the max I would even think about looking at is somewhere around ~ 260 F as the sender will not read coolant temperatures past that because the coolant will be gone.
Also depending on what chart you look at the sender is getting less and less sensitive as temperature increases resistance is in the single digits.
“Basically, from 175-200 F the temp changes 2 degrees per ohm. 200-210 3 degrees per ohm. 210-220 4 degrees per ohm. 220-230 5 degrees per ohm. 230-245 6. 245-260 7. From 28 ohms at 175F to 5 ohms at 260F “
cary said:
Concerning the range of the guage, I would say a wide range. This would keep the guage from bouncing like a pogo stick every time you speed up or slow down, but would show when the motor starts to heat up.
semlin said:
great work guys, this is fun to read...
concerning the average temp, I suspect that will fluctuate somewhat between trucks. So many possible variables in the vehicle to affect temp and so many miles on them. I would suggest instead targeting the standard thermostat opening temp at say 1/3 or 1/4 of the dial.
I vote for a wide range that would include all possible operating temps. I think that once we have the flat spot eliminated, a wide range will have enough sensitivity for what we need. A significant gauge bounce every time you went up a hill or stopped at a traffic light would be distracting.
Maybe I am being overly optimistic here but I am hoping that Toyota has used good materials in this gauge that would be fairly consistent over years of use, the Wheatstone bridge circuit itself is very accurate and not bothered by problems such as fluctuating source voltage like other circuits. The air core motor also within reason does not wear out, it has a permanent magnet rotor, even major degradation of the magnet would not affect reading until it did not have the strength to move the needle. The stators are just windings of copper wire, as long as there is a continuous connection and no shorts between winds they will produce a consistent field for any given amperage. As long as all the components are in good shape it should be reasonably repeatable truck to truck, I have no idea’s on year differences yet but have not seen any indication that they are different yet so for now will assume they are the same.
For fluctuating temperatures if they are fluctuating personally I would like to see that, I would think (probably optimism again) that as I cruise down the highway at a consistent speed on flat or on the gently rolling hills around here the temperature would be fairly constant.
Gauge said:
Actually I don't mind watching the gauge rise when I am climbing a hill and go down when I am descending - at least I know that my gauge is working. And if my engine gets hot, even if it is for a short time, I don't mind knowing that.
Cary Semlin and Gauge, any chance putting #’s to those ranges? Maybe we are thinking more alike than we think. One though I had was making the middle the normal operating temp (still don’t know exactly what that is but it is around 200 F) and perhaps the bottom of the red area be around 226, that # was sinifigant to Toyota for some reason as that is where the AC kicks off at. total WAG but that would give 245 or better where the gauge pegs up over the “H”, does anybody want to know any more than 245 F? For me no I need to shut down before then. This would not show much of the warm up, another WAG maybe first movement off cold at 155F, again anything below that is just “not close to operating temp yet” to me in a northern climate I could see where you may want to see more because there is “not warmed up yet” and below that “really not warmed up yet”
Anyway any #’s helpful, what average operating temp is being seen and also what temperatures you would like to be able to see.
The Miata mod includes several different ranges people can select for their mod, maybe I can do that also
Turbocruiser, no offence taken, discussion gets to the bottom of things. Boils them down to truths. I see some of what you are saying, a marked wide range gauge is quite very nice. It Gives real #’s to work with almost down to the degree, but a modified stock gauge is enough to tell me enough about engine temperature to make decision on its operation. With testing of the circuit both “on the stove” and in the truck with comparison to OBD2 readings translations of needle position to temperature can be made. This is all I want and is good enough for me. I would say that I do want to do this.
Rookie2 said:
Awesome. Got one on the way. This is perfect, learn a little about electronics, and have another resource for enhancing the GA knowledge.
Rookie2
I forgot besides being an engineer you are also a private pilot, I hope you like : )
Once we have a working gauge, It would also be great to have an overlay sticker to apply directly to the gauge face (not the bezel) with a few data points. That seems like a job for a graphic design person to come up with something that looked reasonably oem.
Another thought I have had on the LED alarm issue is that the turbo diesel 80's have an LED light in the tachometer instrument face to indicate when the turbo is on. The tach is otherwise identical so I wonder if the 80 gas series tachs might already have a ready made mounting point and punchout hole for an LED that could be drilled to allow for this as a temp alarm.
I’ll look next time I am in the cluster. I am pretty sure the face card of the temp gauge could be drilled to accommodate an LED, or better yet just back light the gauge red, it does pass light in the white areas. Might be a little dim in the day. I am wondering if an LED could be integrated into the gauge without needing a switch, possibly even using the removed zener diode as the trigger. Tapping another device in the circuit would change its readings some, the smaller the current the less change. Hmmm getting more complicated.
This is getting long, Going to need a recap soon.
. http://www.egauges.com/vdo_mult.asp?Type=Elec_Engine_Temp&Series=EVA-R&Cart= I didn't do much research, just checking around summit and a few other sites.
