So one issue I've been experiencing is that of running the engine too hot. (This happens because I spliced together the GM and mercedes radiator hoses using plumbing fittings which drastically reduce the coolant flow).
My "temperature gauge" up until now has been an overflow tube which I ran from the radiator up to the windshield. When the radiator starts to boil over, I see liquid start to burble out of the tube, and I ease up on the gas pedal.
I've been guessing that I've been running somewhere between 212F and 250F (I'm not sure exactly how much antifreeze raises the boiling point of water given my 7psi radiator cap). I'm not really concerned with the engine running at that temperature, but what I am concerned about is the fact that the transmission cooler runs inside my radiator. And if the coldest point in my transmission system is the radiator which is at least 212F, then its likely I'm cooking my tranny.
So I decided to make some temperature gauges so I could get some data on just how hot I've been running things. YAY SCIENCE!!!
I decided to use LM35AH temperature sensors. Basically, they are little chips inside of metal cans which spit out a voltage which is proportional to their temperature. So you can use these to, ie, drive an analog needle gauge.
I used some jb weld to affix one each to my radiator, oil cooler, and supply line for the transmission cooler. jb weld takes 4 hours to set up, so I held them in place using rubber bands.
For a wiring harness I strung CAT3 phone cable between the sensors, in a daisy-chain manner. CAT3 has 8 conductors, so at each sensor I spliced open the cable sheath and soldered a different wire to each sensor. Then I ran that cable into the cab and connected it to the gauges.
For the gauges, I used some "AsiaEngineer" analog gauges which I bought on ebay last year (they were $5 at the time, but now they are a little more expensive).
These are 30 volt gauges, and consist of a coil in series with a resistor. With gauges like this, it turns out their range is entirely determined by the resistor. In my case, the temperature sensors output a maximum of 1.5v (at 150C), so I needed to replace my resistors and turn my 30 volt gauges into 1.5 volt gauges.
Back when I figured all of this out last year, I put up a little webpage about it:
http://jason.pepas.com/wiki/index.ph...EngineerGauges
So, it turns out the needle reaches full deflection at 1mA of current. So I want to size my resistor such that 1.5 volts will drive 1mA through the gauge. V = IR, so 1.5 volts = 1mA * 1500 ohms. So I need a 1.5k resistor. However, the resistance of the coil itself is 159 ohms, so actually I need a 1341 ohm resistor. I combined a 150 ohm and 1.2k resistor in series to get 1350 ohms. close enough for now
so the last step is replacing the gauge face plates. I wrote some software (for you free software geeks, I used python and cairo) to do this for me (see the above link to my website to download the software):
The next step is to take it out on the highway and push it hard until it just starts to boil. Then I'll know how hot I've been running it.