I have a 2 terminal coolant level sensor that goes to a little control box which is powered by 5V. The control box outputs either 1.27 V when it's submerged in coolant, or 3.64 when not submerged. I'm trying to figure out a way to get a signal of approx. 150 Ohms when submerged and 2.7 kOhms when not submerged. I'm trying to do this without an Arduino, just some circuitry, transistors, etc.
And how well is it working? Why are asking us to crystal ball your mystery device since no Arduino is involved?
If you are at all serious about getting help, include some documentation on your mystery device. If it is part of some automobile, then at least include a copy of the service manual relating to the device.
Paul
Not that it makes sense to me, but if you want that solution:
Get a relay or relay module that's ON at 3.64V and OFF at 1.27V. Then connect the intended resistors to the contacts.
Hammdy:
I'm trying to figure out a way to get a signal of approx. 150 Ohms when submerged and 2.7 kOhms when not submerged. I'm trying to do this without an Arduino, just some circuitry, transistors, etc.
What do you mean by that?
Thanks.. Tom.. 
Sorry I guess I need more detail.
I have a vehicle that uses 2 low coolant sensors which are the same. One of them warns the driver that it's getting low, the second one shuts down the engine. For the sake of this conversation the fact that there are 2 is not important. When submerged in coolant it has approximately 150 Ohms which is read by the engine control unit. When not submerged it's about 2.7 kOhm. These sensors are very unreliable and fail all the time. The schematic for this is "ISL Sensor".
I have another vehicle that uses a different sensor. This sensor is connected to a small module and when submerged in coolant it outputs 1.27 V to the engine control unit, and when not submerged it's 3.74 V. This setup is very reliable so I'm trying to figure out a way to use it on the other vehicle which looks for a specific change in resistance instead of voltage. The schematic for this is "ISX sensor".
OK, I'll take pity.
ISX Sensor
It makes no sense to attempt to emulate a resistance. You need to know what voltages are detected in the target application; you can do this by using a variable resistor to match the resistance and see what voltage results in each case when connected to the ECU, then you arrange an op-amp (an LM386 might suit) to generate those voltages from the more reliable sensor.
This has of course, nothing to do with Arduinox.
Other option - a relay to switch two resistance values. Use a SPST to add a resistor in parallel to emulate the lower value, this avoids an open circuit.
If this was my vehicle, I would replace the ISL sensors with equivalent 1/2 watt resistors and permanently fix the problem. Why is the coolant level going down should be resolved.
Paul
It's not that, it just simplifies needing to look at the coolant reservoir.
Like all the other modern automations.
Self-driving cars should then be able to take themselves to the service shop. 
It's always tricky working with something unknown and it's risky making "random" connections to your car's "computer" because they are expensive!
I'd guess both circuits are [u]voltage dividers[/u]. You might try disconnecting the "voltage sensor" to see if it's resistance is changing.
You could use a relay to switch between two different resistors, or to connect a resistor in parallel with another (for the low-resistance value).
You can use a [u]comparator[/u] to check if a voltage is above or below any particular reference. (Often, a fixed voltage divider and regulator are used to set the reference.)
Voltage Controlled Resistor
There are digital potentiometers but you need a microcontroller.
Self-driving cars should then be able to take themselves to the service shop.
I'm sure that day is coming... "No Dave, we're going to the shop now."
Actually, that would be a great convenience... It's always a pain getting a ride to-and-from the shop...
It would also be handy if the car could drop you off and then go find a parking place, or if you're in "the big city" and there's not much parking, it could just drive around 'till you're ready for it to pick you up! 