I think I am having a grounding problem.

My tachometer needle is bouncing, my radio is making noise, and I can no longer connect my scan tool to the car while running.

When I connect the tool it just says waiting for data, until I shut off the car, and just turn the key on, wait for it to connect, and then sometimes I can start the car and get data, sometimes it freezes.

I recently had to run a new grounding strap from the firewall, as the old one broke, but it was running fine after I did so.

Any idea where to start?

 

Check all the other straps including the ground cable also make sure on the one you replaced the ends are clean on both surfaces.

You may also want to measure the battery voltage while running and make sure it is steady

 

ECM?