Okay, so I have no less than 3 ports in my water pump occupied by sensors or senders, and am thinking of a way to clean them up.

Does anyone know if the resistance curve of a GM coolant temp sensor (the one the FAST uses) would work with an Autometer electric gauge? I was thinking about giving the idea a whirl with the old "boiling pot of water" trick, but wanted to see if anyone is doing this currently.

Anyway, if someone has tried this, post your experiences. It would be nice to have one do-all sensor instead of a mess of wires and fittings.

 

It wouldn't matter if they were all the same you could only read a sensor with one device at a time.

 

Well, I could send the signal to my FAST and split it to read the gauge, no?

 

Is it the sensor with a black and a yellow wire coming out of it? If so, don't even bother the impedances are way different.

 

There is no signal and that is the problem. A three wire device such as a MAP sensor has a power, ground and signal. It is an active device that generates a signal. This signal is a voltage output that many things at one could potentially read. A temperature sensor is a two wire device and a passive device. It is simply a resistor that changes resistance with temperature and the two pins are the two sides of the resistor. A passive device is measured by the ECM, gauge, etc. by injecting a current and measuring the voltage that current produces. Two ECMs, gauges, etc. trying to do this to the same device at the same time would produce an unuseable result.

In general a three wire device produces a signal and a two wire device is measured.

 

Not without making internal mods to the FAST unless it has a specific "share senders" option built in, which I'm sure it doesn't. The sensor works by the ECU using it as a voltage divider - there's a resistor in the ECU, and a resistor in the sensor. They're arranged in a circuit so that the ECU gets a voltage reading. Trying to share the sensor adds extra resistors to the circuit and throws the math all out of whack, so neither the gauge nor the ECU would read correctly. In the Megasquirt world, some people modify the ECU's internal circuit (it usually means removing just one resistor - it probably would on a FAST, too) and recalibrate it. The math is a bit involved, but it's possible. I don't think FAST's tech support line would like that idea, but it wouldn't hurt to ask.

 

Thanks for the replies, guys.....

I understand the difference between the two - the GM sensor is a plain old thermocouple, and so is the sender for a plain vanilla Autometer electronic gauge (not the full sweep one), but if the resistance curves are different, then I won't bother.

 

You missed the whole point of what Matt was saying.

It doesn't matter if they use the same resistance curve, you can't hook up this type of sensor to more than 1 device unless you modify the bias resistor value in both devices (meaning take apart and modify the automater guage and the ECU).

 

Actually, it's possible to modify only one of the devices, probably the ECU as I doubt the gauge can be recalibrated. (Not being all that familiar with FAST, it's possible that the ECU may not be something you can recalibrate either.)

The temperature sensor is a thermistor, a kind of resistor that changes its resistance with temperature. A thermocouple is a different sort of temperature sensor, one that gives a voltage as a function of temperature. Sharing a thermocouple between two devices is, in theory at least, a bit easier than sharing a thermistor.

 

True... I suppose you could completely remove the bias resistor from 1 of the devices.

 

Oh, okay okay.....yeah, then I guess I was not all that familiar with what's going on.

Thermistor! That's the word I was looking for.

I am going to try this when I sell my stupid FAST and put an MSII V3 into the car....thanks for the advice everyone.

 

There may be a simpler solution to this with the FAST.

There is an .ini file that specifies temperature vs. A to D counts. If you establish a temp curve with the sensor already hooked up to both devices, you could probably come up with another set of A to D values that would make the FAST read correctly.

Oh hell, never mind. Even if this made the FAST read right, the gauge will still read wrong when the FAST gets hooked up. Can't be done. Just thinking out loud! Carry on.

 

If you go MS it will be easy to remove the pull up resistor and read the device at the same time as another box or gauge. You will simply program the voltage vrs temperature curve of the other device into MS and let it read it as a pure analog input.