Sharing a ground with automotive circuit. Any precautions?

The goal of the project is to use an RTD temp sensor to measure coolant temp.

I was planning on initially using a 2-wire RTD sensor to keep the Arduino and Automotive circuits isolated.
The setup was supposed to be as pictured below:

The 2-wire RTD sensor didn't fit in thermostat so now I am forced to use a 1-wire RTD that grounds into the engine circuit.
The setup will be as pictured below:

I am worried about the shared ground. What precautions should I take so any transients from the automotive ground won't fry the arduino?

I read that an automotive ground can have a reverse polarity voltage spike. Is a diode before the RTD all I need. Any insight will be appreciated. I want to learn how.

(It does seem that even during a reverse polarity voltage spike on the automotive circuit, the arduino circuit will not flow any current as the 5V side on the Arduino has no return path to a lower potential.)

As long as the only connection to the automotive ground is the sensor itself (and there are no other connections to the automotive electrical system) you should be OK.

However, the thing on the right of your diagram suggests that you will be drawing power from the 12V automotive electrical system. If so, that gizmo must be protected against spikes of > 125V, as well as polarity reversals. And it will be connected elsewhere to the automotive ground, leading to the possibility of ground loops or faulty connections between the sensor ground and the power supply ground, and capacitively coupled spikes getting into the 5V supply.

The shared ground will be engine block + frame + battery negative.

Almost EVERYTHING on the car uses that shared ground as a return path to the battery ground, so it won't only be current from RTD sensor traversing that shared ground (e.g. radio, starter, etc will all be traveling back to the battery negative via the shared ground).

The 12v -> 5v gizmo is a typical car cell phone charger.

They are designed for automotive transients on the 12v side and do produce a clean output on the 5v side.

This #%^! shared ground on the 5v side on the circuit just has me going stupid as to what will be happening.

As long as the only 5V "automotive" ground connection is the sensor itself, you should be OK. But unless the gizmo guarantees complete 5V circuit isolation, I would check for continuity between the gizmo 5V negative and 12V input negative.

jremington:
As long as the only 5V "automotive" ground connection is the sensor itself, you should be OK. But unless the gizmo guarantees complete 5V circuit isolation, I would check for continuity between the gizmo 5V negative and 12V input negative.

Ill measure continuity and cross my fingers. I have a couple different branded chargers.

Out of curiosity, if I have the:

  1. arduino
  2. two 5v relays
  3. rtd temperature sensor
  4. 16x2 lcd screen

all powered with the cell phone charger, which will now have its 5v ground connected to the 12v engine ground, what makes this configuration not compatible?

(Once the ground from the RTD is connected to the engine block, all the 4 components above are grounded to the automotive ground as well)

If the charger is connected to the automotive ground, there is the possibility that "ground loops" in the measuring circuit will interfere with the readings. You won't know until you try.

jremington:
If the charger is connected to the automotive ground, there is the possibility that "ground loops" in the measuring circuit will interfere with the readings. You won't know until you try.

Can something be done to create isolation in the circuit if the 5v negative and 12v negative are NOT currently isolated in the cell phone charger to account for potential ground loops?

No. But there may not be a ground loop problem.

jremington:
No. But there may not be a ground loop problem.

You say no, but what about this setup.

Use:

  1. 12V DC -> 120V AC inverter then
    2)) connect 5V wall wart charger to inverter output

Wouldn't the the resulting 5V ground from the wallwart and original 12v ground from the battery be well isolated after the 12V DC -> 120V AC -> 5V DC conversion?

See reply #1 and #3.

I would not worry about it. The original proposal seems like the best. Unless you do something stupid like removing the ground strap between engine and frame, there won't be any significant ground loop current to worry about.

You may find that the reading goes a little wonky while cranking the starter but normal running should be fine. Check with the headlights on and off to see if you get different readings.