My house is 20+ years old. While working in the main electrical panel, I've noticed the A/C wiring for the inside and outside units have their ground wire cut off at the entry to
The main panel, and therefore not connected to the ground bar. After further examining
The inside and outside units, I found that the ground wire was cut off at the entry
To the unit and not connected. The power to these units are 240v, therefore they
Have HOT: Red&Black, Neutral: White wires. I shut the power to these units,
And measured the resistance from the white wire to the chassis, it reads close to zero.
At this point I can only assume that since 20 years ago, there was no code for
Grounding the 240v units, the white wire is doing the job of both Neutral & Ground.

Question: By code Neutral & Ground are bonded together in the main panel. If
I try to add a ground wire from the main panel to these A/C units, how do I
Wire it in so that, there would be no bonding between Neutral & Ground
At the unit?

I am assuming that bonding Neutral & Ground any place other than main panel is
Against code, am right?

At this point I am not clear how these A/C units are wired internally for the
Neutral wire. Any idea how 240v A/C units are wired to handle the ground
And Neutral wire so that they won't be bonded together at the unit. Can I
Rewire the inside of these units to fix the Neutral & Ground issue
Or is this a task for the manufacturer of these units
And they should a have provided terminals for the Red, Black, White, Green.
These units are 20+ years old, and NEC was different back then.

Your going to have to fill in a lot of blanks. Not sure what kind of furnace you have: oil, gas, electric, heatpump

For typical outside units, only L1, L2 and ground are required for 240 operation.

Indoor units are primarily 120 V except when electric heat.

Is it possible that someone used the white as a ground, but didn't label it properly?

The inside unit is only electric. It only has the cooling modules in it. NO heating.
The job of the inside unit is to blow air on the cooling elements that is
Copper piped to the outside condenser unit.

As far as the inside unit wiring in the main panel goes, there is a dual pole breaker
With a common trip. Each of the breaker is marked as 15A. Red wire goes to
One leg, and Black wire goes to the other leg. The white is connected to the
Ground bar, and the Ground bare copper wire is cut off at the entry to the main panel.

Is it possible that, the inside unit is 120v, and each of the dual-pole breaker
Is only supplying 120v to a circuit within the inside unit? Since they used
Dual-pole with a common trip, I assumed it was a 240v circuit.

I think the outside units were almost wired the same way, with the following
Exceptions: inside the main panel, the white wire goes to Neutral bus bar, and
The dual-pole breaker with common trip is marked as 40A each.

So, it appears that they used white for ground. If so, it should be taped green. In the main panel ground and neutral are usually tied together, so white can either be a ground or a neutral. 240 stuff does not always require a neutral. It does require a ground.

Please bare with me as I am trying to understand how 240v circuits need to be wired to code.
So for 240v circuits, the return current (by code) is done over
The ground wire rather than the white Neutral wire?

It seems odd and dangerous to me that code would allow this type of installation.

Even though service Neutral and Ground wire are bonded
Together in the main panel, I always thought that for safety
(and by code), we wire everything to return current back
To the main panel on the white wire, and ground everything.

If I understand you correctly, the inside and outside A/C units
Are almost wired correctly, and all I have to do is to either
Place a green tape on the white wire and/or remove the white
Wire from the Neutral bus bar and place it under the ground bar.