.150 V on engine block?

Have been working on a vehicle with intermittent no communication with the tcm.

Today, while doing power and ground checks on a TCM on a 2003 chrysler town and country 3.8L, I found the grounds to the tcm to be .100-.150 V. Knowing this can cause problems with vehicle computers, I began doing voltage drop checks on the grounds. I found voltage drops of .100-.150 between various points on the block, the alternator housing, grounding points, etc.

At this point, I unplugged the control wires to the alternator and the voltage drop disappeared. My main question is, is it possible for an alternator to be internally shorted, causing it to send voltage into the casing and block. Also, are there any other points that I may need to look into to further diagnose this issue before calling a bad tcm.

Other points:

-Transmission range sensor codes and transmission temp sensor codes when I can communicate with tcm

-voltage drop tests on battery ground cable were good from battery post to ground point (voltage drop from chassis to block grounds is what is high)

-voltage drop is only when vehicle is running and alternator control is plugged in

Thank you in advance for any and all input.

.150 v on a block ground isn’t high in my opinion. It’s not even.2 volts if it were me I’d check powers and grounds data lines at tcm.

Do you lose communication with any other modules?? Or tcm the only one

Tcm is the only one that I am aware of. I actually didnt try communication with bcm or any other modules. Engine appears to start and run fine, as far as idling. It seems that I have more chance of gaining communication with the tcm when the engine is cool (which was only the first bit I was at the customers house). When the engine warmed up, while I was checking my theory on the alternator) is when I lost communication with the tcm again.

The grounds at the tcm were around .150 and data lines had activity at the time I checked them. If I remember correctly, the b+ at tcm was at battery voltage (12+v)

Tcm gets its ground on block above starter on this vehicle according to wiring diagram.

Where does pcm ground at? What else is on that ground?? If you are worried about that ground runs a redundant to battery negative and see if it still does it. If I were there I’d try a hair drier on the tcm or a light tap on it see if it loses communication

I’m thinking if it were the alternator it’d effect more than just 1 module. If it happens again I’d see if I could communicate with anything else

Looks as though the pcm shares the same ground. Was just wanting to make sure there wasnt anything else that could rule out a new tcm, but it appears that would be the issue. Hopefully the transmission did not get ruined in the process. Thank you for your input, cheryl.

Any other ideas or suggestions? I tested several other vehicles for voltage drop between chassis and block and all had less than 25 mV (.025 V). I'm still thinking .140-.150 is too high for a tcm ground. No?

Try running a redundant ground from the tcm to the battery negative temporarily? Then see if it still does it only other thing I can think of

Attached ac ripple from alternator, old alternator and one I put on this evening. Also brought the engine grounds down to .090 V. Still having tcm issues.

Sound like a bad tcm

Sure does. Now that I have a ground I am comfortable with, I feel more comfortable calling the bad computer. I'll follow up as the work continues on.