Hi,
That would certainly be enough to drive me crazy. LOL Did this just start after it sat? I do have another question. Please understand that I am watching a video, so I could be completely wrong.
I didn't like the alternator output. Before starting, it was at 12v. When it started, it hardly went up at all. I don't know if you had the electrical system loaded, but it should have been approximately 14v running.
I'm suggesting this because the tachometer is controlled by the instrument cluster circuit board based upon cluster programming and electronic messages received by the cluster from the Engine Control Module (ECM) on vehicles equipped with a diesel engine over the Controller Area Network (CAN) data bus. I ask because the electronics on these vehicles are very sensitive to low voltage. It's just a thought.
Also, since this is likely a CAN-related issue or the cluster itself is failing, you should consider having the can-bus scanned for codes.
Here is a link that explains how that is done:
https://www.2carpros.com/articles/can-scan-controller-area-network-easy
If you haven't, you may want to just check if the alternator is putting out near 14v when the engine is running.
Here is a link that shows how it's done:
https://www.2carpros.com/articles/how-to-check-a-car-alternator
I have a few other things to check. Check the ABS module fuse (10 amp fuse, #28 in the TIPM). Make sure the fuse is tight and there isn't a corrosion issue. Also, check the connectors at the module to make sure there isn't an issue. If you have a live data scan tool, check to see if you can communicate with the ABS module when the tachometer drops.
Let me know.
Joe
Thursday, December 29th, 2022 AT 9:25 PM