[918Stealth2SS]

I have had my car in for over a month with a bouncing tach needle. 3rd or 4th time it has been in for this. My warranty went out by time while its been in dealing with this issue. Now with the car sitting for 3 weeks its not acting up now. But I know it will aftwr a few days of picking it up. It always does. Last time they put in a new battery. 2 days the bouncing needle is back. I have called Chevy. Got a case number. I need to get it to another shop but cant close the ticket due to warranty dates. Do I just need to wait for Chevy to call me back and just rock the loaner? They are wanting me to get my car cause now its not doing it. I know it will in a week from me getting it.

[918Stealth2SS]

Update. So finally they see it and notice a ground issue. No car for over 2 months. Car is completely torn down from how it sounds. Have a tech case in now as well. Hope car is the same when put back together.

[Shorty45]

Mine did this after some work done. Ground wire on back of heads both sides were loose.

[918Stealth2SS]

Well picked car up....had it 2 days. Problem is still there. This is getting beyond crazy now.

[Cechk01]

I had this same problem ended up getting a new cluster, alternator, several new ground wires. I personally think it was a bad wire from the negative jump lug to the engine block. I had to take my car back 3 times over a month and a half. Techs put over around 500 miles on it test driving it.

The tsb says it can be cause by Electromagnetic interference from electronics. Try removing your diode dynamics stuff and see if it works. On the 2015 mustangs the diode dynamics leds interfered the the touch to unlock circuit on the door handles.

[918Stealth2SS]

When i originally took it in about a year ago for this they blamed my sidemarkers. I removed them went stock and it did it still. Would the interference happen with lights off? No power going to them to cause an issue? Cause this does happen with diodes in but lights off. Ill try pulling them again in the morning just to check again. Ill let you know.

[Cechk01]

The markers could be on in a low state ie not visible.if its not a lot of hassle I'd unplug them and drive around with them unplugged and see if the issue happens again.

take note of your charging system voltage when the issue is happening. Mine would only happen under 14v most of the time. Several things can trigger the charging system to bump the voltage, battery needing charge, wipers on for a few seconds, driving over 90, the engine fan goes into high powermode, the HVAC sometimes requests extra voltage.

Mine was also effected by the ambient temp outside, it would only do it on warmer days and after the engine was warmed up.

you need to do your homework and identify all the variables that trigger the tach to bounce, the dealer are pretty clueless about electrical/computer issues. GMs tech hotline is pretty useless too. all they told my dealer to do was to check grounds over and over.

GM needs to fix the issue because it was preexisting and positively identified while your car was still under warranty, it isn't your fault they didn't fix it while it was under warranty

[918Stealth2SS]

Warmer days definitely cause an issue for sure. They said the found a bad ground ...paperwork not in front of me but ipc6 or icp6 something like that.

I do watch the voltage meter. Its when it gets to around 12.4ish that I can attempt to accelerate. Not aggressive by any means, enough to pass another vehicle and the needle will bounce. Did it 3 times on way home yesterday. And 1 while just cruising. And i might be crazy. But after the last voltage issue on way home. I swear my ac started blowing harder.